Tag Archives: heating

Hydronic Radiant Heating

Hydronic radiant heating, or more specifically under-floor heating, can be accomplished in several ways.

Using a natural gas-fired tankless water heater for both domestic hot water and space heating is one alternative. This is the method I have used in several houses. It as been very successful

hydronic radiant heating

Schematic of a recently installed heating system

n example of Hydronic radiant heating

An actual 3 zone system, nearly complete.

This plan was for heating three separate areas of the house, main floor, basement, and garage.  A separate thermostat controls each 24 volt zone valve.  The zone valves in turn control the pumps with line voltage end switches.  The diagram shows 3 circuits on each manifold but there could be more or less, depending on the area heated. The maximum length of a circuit should be less than 250 feet.

The three-way valve are for flushing the heater when needed. The pumps are Armstrong three speed with check valves. Their capacity is 10 gallons per minute.

The zone valves are Honeywell 3/4 inch, motorized, with end switches.

A pressure only relief valve is used. Maximum temperatures are ensured by integral limit switches in the water heater.

The heat exchanger is a single wall plate type. It is oversized for maximum heat transfer at lower temperatures. I am not sure this was necessary, but the cost difference was not significant.

Wiring zone valves and pumpsfor hydronic radiant heating

Wiring zone valves and pumps

 Wire the zone valves and pumps as per the above diagram. Place a switch in the live 120 line for ease in servicing.  Provide grounds to all 120 volt components.  Provide a grounded outlet for connecting the water heater.

No extra controls are necessary for the heater as it will turn on as soon as flow is initiated by the primary circulating pump.

There should be a separate, dedicated, electrical circuit for the system.

Another way to distribute hydronic radiant heating

An alternate method of heat distribution

 An alternate method involves using a circulation pump for each manifold.  The pumps can be controlled with line voltage thermostats, or you could use low voltage thermostats and relays.

This is the system I use, when I only have one area to heat, but it would work as well for multiple areas.

I used the zone valves as I found them for under $75 each. The pumps were about $150 each.

If you use a boiler, the configuration would be different.  Probably, something like the next illustration.

In this case you would likely use a heat exchanger and a tank for supplying domestic hot water.

wiring for hydronic radiant heat using a boiler

Boiler wiring

When plumbing all the parts together, you can do a very neat job with copper lines and fittings.  It is, however, more costly than pex and will radiate considerably more heat into the mechanical room.

I place a the radiant heating lines under the floor at a maximum spacing of 16 inches.  In a conventional floor, that normally means one line in each of the joist spaces.  It doesn’t seem to matter if there are two lines in a occassional joist space if that is necessary for things to work out.

In a concrete floor slab, place 10 mm rebar in a 16 inch grid. Tie the pex piping to the rebar before pouring the slab.

The distance from the floor surface to the pipe circuit does not seem to be important. Place it where it is least susceptible to mechanical damage.

It is a good idea to put more than one radiant heating circuit in parallel under a slab in case a leak develops.  I have never heard of this happening but that does not mean it is impossible.

Place insulation under the pipe in a basement ceiling. R14 fiberglass works well and is inexpensive.  You might also place a layer of reflective material under or over the insulation. Ordinary foil food wrap placed shiny side up is a low-cost choice.

If you have an area where it is difficult to place radiant heating coils in the floor, it is possible to use a fan coil for heat. You lose the benefits of under-floor radiant heating, but it is easier than installing second source of supplementary heat.

Many tankless water heaters can be run at temperatures up to about 185 degrees fahrenheit.  If you choose to use higher temperatures, be sure to install a mixing valve in the domestic water supply, to reduce the risk of scalds.

If you require more capacity than can be provided by one tankless heater, it is possible to install two or more in tandem.

The approximate cost of material, for the system illustrated by the first diagram, was $3000.00. This compares very favorably to a forced air system and a hot water tank.

Hydronic heating systems can be expanded to include other uses besides space heating.

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Heating with a Hydronic Radiant System

Hydronic heating is a system which uses water or steam  as a heat transfer medium.  Radiant heating is a method of heating objects by heat radiated from a warmed surface.

Unlike forced air heating, or gravity furnaces, there is little, or no reliance on convection.  In other words, heat is not distributed by moving heated air.  The heat may be radiated from floors, walls, or overhead ceiling panel

The heating system I am going to discuss uses the floor as a radiator.  This is the system I have used in several houses, either partially or as the total heating system.  It is the sole source of heat in the home I have just built.  The same source of hot hater is utilized for domestic use.  I have just installed a more complicated, three zone system, in my sister and nieces new house.  I had also installed a similar system in my sister’s previous home which has operated very well and trouble-free for the last six years.  This system utilizes piping, run under the floor surfaces, to distribute the heat.

This climate reqires a robust heating system

Our heating system has to deal with this climate. This photo was taken in mid November.

While my research and experience is not sufficient to qualify me as an expert, I have certainly gained enough insight for competent comment.  I am going to use a question and answer format, Some of these questions have been  asked of me, and many are questions that I have researched  for myself.

Q/  What about cost?  Isn’t hydronic heating more costly to install?

Not necessarily.  utilizing a single heat source for both domestic hot water and heating reduces cost considerably.  A high-efficiency tankless water heater, and pex pipe, can reduce the cost to less than a conventional forced air system and separate water heater.

Q/  Is Hydronic radiant heat more efficient and have lower operating costs?

I can’t be certain of that without a controlled experiment, but logic would indicate that it is, and my experience seems to reinforce it.  Comfort seems to be achieved with a lower air temperature. As there is little air movement it seems that less heat is lost when a door is opened.  It is ,however a little more difficult to lower the temperature for short periods, such as during sleeping or work hours.

Q/ Doesn’t the floor get uncomfortably warm when the system is working?

A warm floor with hydronic heating.

Toasty toes on a warm floor. It looks as if I should trim my toenails

No. In fact the floor remains at an almost constant temperature, which is only slightly above the room temperature, and always comfortable.  So comfortable, in fact, that I hardly ever wear slippers.

Q/ Under slab heating has become common in garages.  Is it necessary to have supplementary heat for fast recovery when large garage doors have been opened?

Actually, recovery seems to be faster than with conventional fan forced heat.  This is likely because less heat is lost through the loss of circulating heated air, and because of the large heat sink of the floor.

Q/  Do we need to place insulation under the floor?

Yes and no.  In the case of an upper level floor, the heating will need to be isolated from the space below with some degree of insulation.  R12 fiberglass seems to work well, and it is low-cost.   A reflective surface above or below the insulation may also be beneficial, but likely not critical.  Ordinary foil food wrap placed shiny side up is an economical approach. Some jurisdictions have building codes which require insulation under heated slabs.  Experience has shown me that this is largely a waste of money except in certain circumstances.  If the water table is within a couple of feet of the underside of the floor, then insulation may be beneficial.  It is, arguably, also beneficial if the slab rests on solid rock.  While reasonably dry dirt is not a good insulator, it is nevertheless an insulator, and you have an almost unlimited depth.  The difference in temperature between the earth and the slab is also not great.  In most places it is only about 20 to 30  degrees fahrenheit (11 to 22 degrees celsius.)  Of course, if you are building on permafrost it must be protected from the heat of the floor.  The extra heat sink provided by the soil can actually be beneficial, by aiding in recovery, and in case of system failure.  It is important that the foundation walls are well insulated to at least 24 inches (.6 meters) below the surface.

Q/  Can this or a similar system be installed in an existing home?

Yes, there are several ways it can be done.  It is not likely that it is cost-effective though.  You would, probably, be better advised to spend money on increasing insulation, and on sealing air leaks.  Even replacing windows with more efficient ones may be more cost-effective.

Q/ Could this be a DIY project to install?

Yes, if you have basic plumbing skills.  Electrical skills would also be useful.

Q/  What is the best heat source?

My preference is a gas-fired, high-efficiency, tankless water heater, although almost any source can be used.  The difference is largely in the control mechanisms.  Your choice would depend on what sources of energy are available, the level of heating required, and cost.  Because heating requires so little of the capacity of many tankless heaters, it should be used for domestic hot water as well.  This will ensure that it, at least occasionally, runs at full capacity and should reduce maintenance issues.

Q/  How large should a tankless water heater be.

In our climate, with incoming water temperatures at about 40 degrees F. (about 4.5 degrees C.), a tankless heater needs to be about 200,000 BTU per hour input. This will supply two or three hot water outlets at once.  The space heating needs of a modern well-built home, should require only a portion of this.  Since hot water needs are normally only for short periods, there should be plenty of excess capacity for this purpose.

Q/  Are there any situations where you would not use a tankless?

Some water can damage, or quickly reduce the efficiency, of a tankless water heater.  The water heater,  in this case, could be isolated by using heat exchangers and tanks,  which are less costly to replace.  It may be easier to use a boiler in a closed system. Water softeners,  Filters or other water purifying systems may be in order.  Water heaters are available with stainless steel heat exchangers, which may be a little less susceptible to corrosion, than copper.

Q/  Is a high-efficiency, condensing, water heater worth the extra cost.

If you use high volumes of hot water, I am sure it is. I am not sure the efficiency is as great for heating, as the heater will run at far below capacity, when in use for heating alone. The cost difference is not so much that I would be discouraged from going with a condensing model.

Q/  What effect has the choice of flooring have?

It doesn’t seem to make much difference.  My preference is for wood, laminate, tile or vinyl flooring. Carpet is not an advantage when the floor is always warm.  I am writing at my desk this morning, in a house coat, and with bare feet on a laminate floor.  It is minus 25 celsius outside with a minus 30 wind chill.

Q/  Can I heat multiple floors or separate areas with the same system?

Heating casn be controlled with a line voltage thermostat.

A simple line voltage thermostat.

Yes you can.  It may necessitate  more complicated system utilizing several zone valves or circulating pumps.  Separate pumps can be controlled with line voltage thermostats or with low voltage thermostats and relays.  Low voltage thermostats can be used to control zone valves which in turn will control pumps through line voltage end switches.  I will detail these systems in a my next post.

Q/ Where can I find parts and supplies?

Most of what you need can be found or ordered at local hardware stores at a reasonable price.  The more uncommon parts are available from plumbing supply and electrical supply sources.  You can often find the more unusual parts through the internet at dramatically lower cost.

Q/  How much piping will be needed?

I like to space pipe runs at 16 inches or less.  If more than 250 ft. of pipe is required, then manifolds should be used to provide several circuits. This is using oxygen impervious pex pipe designed specifically for hydronic heating.  Pex, or cross linked polyethylene, is not very conductive and is not efficient at transferring heat.  This does not affect the efficiency of the system at all, but it does mean that more piping is needed than with metal pipe.  More piping does mean a more even distribution of heat. Plumbers will sometimes tell you that glycol is needed for efficient heat transfer.  Once again, this does not effect the overall efficiency of the system at all and only adds extra cost and an element of risk.  Glycol is poison, and I do not want it anywhere near my water system.

Q/  Is placement of the pipe in relation to the floor surface important?

It doesn’t seem to be.  If in a slab, the depth of concrete over top of the pipe seems to be insignificant.  If in a joist space, it doesn’t seem to matter as long as the pipe is between the insulation and the under surface of the floor.  Expensive metal plates for heat distribution seem like a unecessary expense.  Simply fasten the pipe where it is convenient, and the least susceptible to damage.

Q/  Is this an environmentally friendly system?

As a rule of thumb, the most environment friendly system is the most energy-efficient.  Secondly the least use of material and whether that material is recyclable is important.  This system compares favorably with other methods. the use of natural gas as fuel releases the least pollutants to the atmosphere.

Q/  What about building codes?

If using a water heater for both domestic hot water and space heating, the most recent Canada codes I am familiar with, requires a circulation system to replace the water in the heating portion periodically. An alternative is to separate the heating and domestic with a heat exchanger and this is required in some jurisdictions. These are bigger concerns if you are using untreated water. Some authorities will even require a double wall heat exchanger so that a leak between will be visible. A double wall heat exchanger is much less efficient at heat transfer, and requires a much larger and more expensive unit.  North American codes in relation to this type of heat are still in flux due to unfamiliarity.  The biggest concern seems to be with legionnaires disease although I cannot find a single case that has been traced to single family residential heating . It is possible to route all water use through the heating system so stagnation is not possible.  I have used heat exchangers in mine and my sisters houses.  Do your own homework, in regards to codes, for your locality.

Q/  How hot should the water be?

I have found 140 degrees fahrenheit (60 degrees celsius) to give efficient heat transfer. This temperature also keeps bacteria growth low.  This temperature is easily regulated with a tankless. Most boilers need to operate at higher temperature for efficiency.  If using a boiler, mixing valves will be required to lower the water temperatures.  You may want to use a mixing valve, to lower the temperature for domestic use, if you have small children.

Q/  Are there maintenance issues?

If your water is hard, you will need to flush a tankless water heater with vinegar periodically, to remove scale.  This is a simple procedure that you can perform yourself with some garden hose lengths, a small submersible pump, and a pail with about 4 gallons of vinegar. How often this needs to be done depends on the water.  There is at least one in line water filter which may need to be cleaned if flow slows down.   With soft water at my last house, the heater required no maintenance in 6 years.  A forced air system requires filter changes several times a year at the least.  In case of failure, the various parts of a tankless, are replaceable with no need to replace the entire unit. Scale build up in a conventional tank type water heater usually means that the entire unit will need to be replaced.

Q/  What about safety?

A simple hydronic heating system.

The simple hydronic system in my house.

This is a low pressure, low temperature system that poses very little danger of burns or explosion. Temperatures are just high enough to restrict bacteria growth.  By using a direct vent system for the water heater, the chance of carbon monoxide release into the house is practically eliminated. There is no duct system to facilitate the spread of fire, smoke, or airborne pathogens.   No chemicals are used.  I believe this is one of the safest heating systems.

Q/  What are the space requirements?

From 4 to 6 ft. of a ouside wall should be sufficient.  A chimney is not required. The equipment can be located in a basement or upstairs mechanical room or in a crawl space.  In warmer climates it can even be located outdoors with minimal protection.

Q/  What happens if the system is meeting space heating requirements when you need hot water?

Domestic hot water can be given priority simply by taking hot water before it reaches the heating system.  I have never noticed a significant water temperature drop if the heating kicks in while I am showering.

Q/  What are the major advantages of this system?

No air movement to stir up dust and carry it around the house. The system is almost totally silent. Comfortably warm floors.  Using a tankless means you never run out of hot water.

Q/  what about disadvantages?

It is difficult to make quick temperature changes.  It may be difficult to find a plumber familiar with hydronic systems, or tankless heaters, if repairs are needed that you cannot perform yourself.

If you have any questions that have not been answered here, I will reply to comments. I will go into detail and include some drawings in my next post. I will discuss a couple of specific systems I have installed that are in use. Until next time

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A Nearly Finished Home

Our nearly finished home

Our home

Gage, the super, is checking my every move.

We now have a nearly finished home. It has been a bit of a strain on this old body but we were able to move in before winter. There is still quite a bit to do but the pressure is off.

Changing seasons

The scenery changing with the seasons

For those of you who live in more moderate climes, we had our first snowfall a few days ago (Oct.28). The temperatures at night are fairly consistently below freezing, with the lowest to date about minus 14 celsius. That is about 7 degrees fahrenheit. Our somewhat unconventional heating system worked perfectly through this first test. Of course, the house is super tight, and super insulated, so it requires very little heat. In fact, we were heating with only a small electric heater until we had our gas hooked up, less than 2 weeks ago. It was always comfortably warm.


We have a bed room

Our bedroom is nearly finished. We went outside the box a bit, and installed a chandelier.

We had one of our unforeseen delays in acquiring our gas service. Apparently, the gas company had a major outage and were short of staff. It took nearly two weeks, from the time I requested service, until the meter was installed. It was fortunate that everything worked when we started it up.

I promised to report on the results of my ceiling stippling job. I thinned drywall mud to the consistency of thick paint, and applied it in about 5 ft squares with a long nap roller. Stippling was done by pushing a special brush into the mud. After a few days drying, I primed and painted. The end result was satisfactory but not as perfect as I would have liked. The drywall joints should have been finished just a little better. The  joints are slightly visible when some lights are on, and should have been finished to nearly the same degree as required for painting. It is obvious that this is not a method to hide large flaws in your finish. It does, however, avoid the problems inherent in painting a large flat ceiling. Overall I am happy with the results, of this, my first attempt

A stipple ceiling

My stipple job

Stomping brush used for stipple

Stomping brush used for stipple

The brush used for stippling is sometimes known as a stomping brush. They are available in a few different configurations, such as rosebud or crowfoot, which can be double or single. I had a little difficulty finding one in Canada, as this is a method seldom used in this area.  Lowes was the only retailer I could find that stocked one. I was able to order via the internet. They may be more available from drywall distributors.

Most of my final inspections have been done. Everything passed with only a few minor changes.

I had originally planned on two water heaters for potable and heating, but research, and a few other concerns, convinced me to use one heater with a heat exchanger instead. Since the Navien NP-240(A) I am using is rated at an input BTU of about 200,000, there is no shortage of capacity.Because of a lack of experience, I chose to oversize the heat exchanger. The extra cost was insignificant. Unlike other types of heating, the over sizing of components has an insignificant  effect on fuel efficiency. I will dedicate a future post exclusively to this type of heating. I will add that I am sitting at my computer this morning with bare feet. Very comfortable.

Heating and hot water

This provide all our heat and hot water

I installed a circulator in my potable hot water system. This provides almost instant hot water to sinks and showers. This allowed me to use 3/4 inch water lines to all branch offs. This means that if you are in the shower, and somebody opens a faucet, there will not be a shocking change in water temperature. Although there is some energy cost ( reduced by insulating all hot water lines), there are significant reductions in water wastage.

Heat exchanger and watercirculators

A look at part of our system

Gage has a girlfriend who comes to visit and to go for a walk everyday. That’s her staring at the camera. Her name is Lulu, and the only things she likes better than Gage, is her “Mom” and food.



Gage the supers girlfriend



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Raising Walls


We have progressed as far as raising the walls.

Heating coils

With help from son Wayne and wife Bobbi, I was able to run the heating coils for the floor.. This is a much larger, and more difficult, job than it appears.

You will notice that I used cross bracing, instead of blocking,  between the floor joists. This avoids pulling pipe through holes drilled in the blocking. When installing this type of bracing, fasten the top only, until after the floor deck is down. This allows you to adjust joist spacing as you fasten the sub-floor. We used construction glue (PL400) and spiral nails to fasten the deck. Neighbor Bill helped me and this was only a short days work.

When framing, I use two air nailers. A coil framing nailer holds more 2 inch nails for applying sheathing. It saves time by not having to change nails in your gun. I have a Paslode coil nailer that I purchased used for 50 dollars. My Mastercraft nailer is a new one purchased at a yard sale for 40 dollars. I am pretty certain that I can get my money back when the project is finished, if I so desire.

Subfloor installed

Sub-floor done and ready for walls

One of our next steps was to apply waterproofing to the foundation. In this case it was likely not really necessary, considering soil conditions and the type of foundation, but it is always better to err on the side of caution. This is my son Wayne and myself preparing for the job. Needless to say, I let  Wayne get splattered with the tarry stuff.

Foundation waterproofing

Let the tarring begin

The rubber boots I am wearing has been a wardrobe necessity lately. With the historic floods occurring in Alberta right now, I am glad we are high and and a little drier.

I am beginning to need a haircut rather badly, but hate to take the time. I guess I am lucky to have hair at my age.

Once the waterproofing was done we began building walls

Building a wall

Building a wall

The two longest walls have been built and I am preparing to raise one. Notice that I am installing stops to prevent pushing the wall too far. Necessary when you have little help.

The bottom of the wall is toe nailed to the floor to prevent it sliding off. Just a few nails are necessary.

Preparing to raise a wall

Preparing to raise a wall

Nieghbor Bill helped to raise these two longer walls. With jacks, it is possible for one person to acomplish this but it is faster with two.

A wall is raised

A wall is raised

Don’t skimp on bracing. I have several times seen walls blown down, due to insufficient bracing.

These are the jacks that make raising a wall possible without help.

Tools to raise a wall

One person can raise a wall with these

The two side walls are up.

Raising walls

Side walls up and laying out the end walls

The headers over the window and door are two 2 x 10 and a 2 x 6 with insulation in the middle. These walls bear the weight of the roof, so adequate headers are essential. A bit of overkill doesn’t hurt here.

Window header in bearing wall

Window header in bearing wall

The end walls going up. You may note that the end walls are not completely sheathed. This allows for sheating to tie the walls together. Considerable strength is added. Sheathing is also applied to allow for tying the wall to the top foundation plate. Just a little more wind resistance.

Raising an end wall

End wall going up

Raising an end wall

Raising an end wall

The toe nails holding the wall were mostly pulled out and this wall was still threathening to slide so I put in a few more for insurance.

We acquired a supervisor this week.

Our cute dog helping out

Our new supervisor

Oh,oh, It is too hot out here. I think I will find some damp floor in the shade.

Puppy in the shade

It is hot. Must find shade

Headers are not really required in non bearing walls. I do like to add some strength without providing too much thermal bridging. This has been done here by ripping 2 x 6 to 5 inches and making a header box faced with 1/2 inch OSB. This allows for full insulation with the minimum of thermal bridges.

Header in non bearing wall

Header in non bearing wall

The walls are up and Bobbi is surveying her living room

Living room

Bobbi in her living room


A view from our bedroom window

By using our imagination, we can now get a little feel for what our house will be like.

I love this type of work. The results are so obvious.

I would like to do the backfill but it has been too wet to use equipment on site. I have not even been able to get the foundation drainage to inspection stage. Well, it is bound to quit raining eventually. There are always things that can be done in spite of the weather. This is one of the advantages of doing all your own work. You are seldom stalled for long. You are not likely to get any breaks at all, unless you just arbitrarily take one

Overall things have been going very well. I did fight with one of my nailers for half a day, until I realized it worked much better with the right brand of nails. No other real problems were encountered.

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Easy Efficient Radiant Heating

This is an easy efficient radiant heating system, of my own design, that I will be using in our new house. I have used this system in the past. It has worked very well, both for in slab radiant and under floor heat. I am not specifically recommending this system. It simply seemed like the best choice for our house.

In the past I have used the tankless water heater for both heating and potable water. Current codes,as I understand them, do not allow for that without a heat exchanger to prevent mixing of heating and potable water. The concern seems to be for bacteria growth in heating coils during periods of low usage.

easy efficient radiant heating

Underfloor Heating

My own belief is that using temperatures of 140 F. (60 C.) or higher will minimize this risk. Codes may require a mixing valve so that potable water is delivered to faucets at lower temperatures. However, it is better to go with safety over economy, so I would suggest that completely separate systems are a safer choice.

It is difficult to understand how a heating circuit could be any riskier than a garden hose left out with a nozzle, or municipal water lines dead ending in vacationers houses, or waterlines to an unused bathroom.

Of course, if glycol, or other chemicals, are to be used in heating circuits, then there must be no connection with the potable water system.

A simple solution may be to run all water use through the heating coils, so that the water has no time to stagnate, and municipal treatments are kept to the highest level possible. Another method is to use a timed valves to periodically flush the coil contents to a drain. This should replace the heating water with freshly treated water from the municipal system. The remaining levels of chlorine or other treatment would have to be sufficient in order for this to be effective.

When designing a heating system, it is useful to forget everything but your goals. My goal is to provide a comfortable living space, at the lowest initial cost, and the least ongoing fuel cost, with the least environmental impact and with maximum safety. Compromise between these goals may be necessary but safety comes first.

The first consideration is an energy efficient building envelope. Given that, providing efficient heating (or cooling,) becomes much easier.

In our climate and location, natural gas is the least expensive fuel. I will be using a Navien 240A tankless water heater, mostly because they provide the highest efficiencies for heating water. I was also able to obtain one very inexpensively. It has features that are unnecessary for this use that will be disabled.

At about 199,000 BTU, this unit is much larger than needed, but in this case, that will have little effect on efficiency. The point is that a much smaller unit could be used. The only relevant factor is the efficiency of the unit and the cost of the fuel used. Other water heating methods such as solar could be used if climate or location allow. Conversion in case of changing market conditions is also relatively easy. You only have to change the heat source.

In my case heating coils will be spaced at 16 inches, or one run between each set of floor joists. That provides about 750 ft. of coil for the entire house. Past experience has indicated that this will be sufficient, but it could easily be doubled if necessary. The length of each coil is reduced by using manifold to split the flow into three. In a larger house you may need more. Balancing is done with valves to control flow. Another option would be zone valves to control heating in separate areas.

Since we are building on a conditioned crawl space we are using only R.12 insulation under the floor as per the diagram. This should be suitable if you are on a heated basement as well. If building on an unconditioned crawl space, much more insulation would be appropriate. I would suggest R.34. Be sure to allow for an airspace. If your coils are in slab, no insulation is needed underneath but perimeter insulation to at least 24 inches below ground level should be provided. The exception would be if groundwater is present near surface. Then under slab insulation may be needed.

I will be placing aluminum foil on top of the insulation. and placing both in the bottom portion of the joist space, leaving about a 5 inch air space. My goal is to provide even heating of the floor above. I believe that heating the air underneath to a few degrees above room temperature is the best way to accomplish this. Some volume and free circulation is necessary for this.

On one similar installation over a heated basement (with in slab radiant,) we found it necessary to turn basement coils practically off to prevent the basement from overheating. Zone valves may have been a better option.

You will hear all types of arguments related to the efficiency of a system like this. Most will be based on assumptions that only vaguely relate to the actual operation.

I have been told that the heat transfer from PEX is very inefficient, and that glycol in the system is the best way to improve that. The fact is that the efficiency of the heat transfer matters not at all, as long as enough is transferred to keep your house comfortable. None of this has any effect on the overall efficiency of the system. For a system to be inefficient there has to be a energy loss. There are only two areas of heat energy loss in your house, through the exterior walls and ceiling, or through the chimney (vent).

Choosing a high efficiency heat source addresses the chimney or vent loss. Choosing to build an energy efficient house addresses the other. The environment is best served by this approach as well. That only leaves comfort and safety as concerns. In other words, will the system work without blowing up, burning down your house, or making everyone sick.

With any heat source the dangers are fire or carbon monoxide. Although tankless water heaters have excellent safety features, there is no such thing as perfect security. Install and maintain smoke and CO1 detectors for peace of mind. If a heat source can be installed as direct vent, be sure to do so, for extra safety and slightly better efficiency.

It is safer to keep heating water and potable water completely separate or to purchase an appliance specifically designed for combination use. My own decision is to use separate water heaters mostly because I was able to keep the costs very low.

I will be setting the water temp at 140 degrees Fahrenheit for good heat transfer to the air space. I don,t expect air space or floor temperatures to exceed 75 degrees. A living area thermostat will control a water pump. Integral flow controls and temperature controls in the heater will control firing and water temperatures.

Experience has convinced me that this system will work well. Extra heating capacity could be achieved by adding more conductive surface between the air space and the water. In other words, by adding more coil length. This would have no effect on efficiency of the overall system.

There are several advantages to a hydronic radiant heating system. There is virtually no noise if the heater and pump are isolated from the living space. There is no fan forced air to stir up dust and allergens. It can be highly efficient. With no fan, electrical use is minimal. With no air filters, maintenance is reduced. With less air movement over walls and windows, conductive heat loss should be reduced, a tiny bit at least.

There is only one disadvantage that I can think of. Because of heat stored in floors that act as a heat sink, it may not be practical to use for occasional heat needed in the summer months. This would mostly be a problem with in slab coils, A  small supplementary heat source may be a good idea. Probably a good idea in any case or with any system.

There are some outrageous claims made by advocates of hydronic heating, Many will claim that huge savings in fuel can be realized. The fact, as I see it, is that a well designed house uses very little fuel, no matter the heating system. You may see some savings but they will not be that large. If doing a retrofit, the money might be better spent to improve the house envelope. Spending large sums on state of the art hydronic heating, may not be cost effective, Many gimmicks and fancy installations probably do nothing at all to improve efficiencies. They may only serve to burn up money. Keep in mind that maximum efficiency comes from minimum heat loss. Anything that does not reduce energy loss is probably a waste of money as far as energy efficiency is concerned.

One final caution, using a water heater for this purpose may void the warranty. Contact the manufacturer if this concerns you.

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Heating systems commonly used in homes today

There are three basic concepts, for distribution of heat, in home heating systems used today.

A forced air ducted system is the most common in much of the country. 

Air conditioning and humidifiers are incorporated into these  heating systems quite easily, and it is commonly done. Filters are added to the system for some degree of air cleaning. Filters are usually the only regular maintenance required besides a seasonal check.

High efficiency heating system

High efficiency forced air furnace in crawl space

Heating for this system may be provided in several ways, but a furnace fueled by gas or fuel oil is the most common. These are available today with efficiencies of over 90 %. The highly efficient furnaces require a drain to dispose of water condensed from flue gases, and they are often called condensing furnaces. They can be vented directly out a wall with PVC pipe. All combustion air can be drawn from outside, so there is no source of ignition that is not isolated from the air in the home. If you use a direct vent or forced vent water heater as well, it means no chimney would be required.

I believe that direct vent heating equipment is safer than other types. The lack of a chimney lowers costs on new construction. It frees up space, and there is no roof penetration as a source of possible leaks.

An air handler or fan coil unit can be used, as well. With this method, heat can be sourced in different ways. A boiler or other hot water heater can be used. Solar heating can be used by itself, or as a supplement to other methods. Heat pumps can provide the heat, which can be drawn from the air, the earth or groundwater. In rural areas an outdoor furnace may be used, which can burn coal, wood, or other biomass. (An outdoor furnace may produce unacceptable levels of air pollution in some areas). Other sources of heat may be available locally.

Care must be taken to balance these heating systems for even heat distribution. If you are using a furnac,e then proper sizing is important to ensure comfort, and to prevent short cycling. Too large a furnace will run for very short periods at a time causing inefficiencies and maintenance problems. You will need to do a heat load analysis to determine the size of furnace needed. If your home is small, super insulated, sealed, and utilizing passive solar gain, it could be difficult to find a furnace small enough.

In floor heating system

Manifolds to heating coils for garage floor

Alternately, if your home is going to be large and complicated it could be difficult to balance a forced air system, resulting in uneven heating.

Be aware of the difference between input and output when sizing a furnace. A furnace of 100,000 BTUH input has an output of 60,000 at 60%, but produces 90,000 at 90%. Simple but easy to miss.

BTUH is British Thermal Units per Hour, and is still commonly used in the U.S.. The H may be dropped on rating plates. The metric equivalent of a BTU is 1055 joules. Although Canada officially uses the metric system, we often end up using a confusing mix of the two because of the influence of our large friend and neighbor, who also often uses a confusing mix of the two. I often use British units when writing because many of my readers will be American. I am familiar with both, as are many Canadians.

Radiant heating is the next most common heating system.

It uses water, or a mixture of glycol and water to distribute the heat, in most cases. Sometimes called hydronic heating as a reference to water. Electric heat is also distributed as radiant. Sometimes with a fan assist. Fireplaces are also a form of radiant heat, but are commonly fan assisted these days. As an aid to understanding radiant heat, you can think of the sun. It heats the earth and the objects on it with radiant heat. The warmth passes through the air without heating it. The air is heated in turn by contact with the earth.

This illustrates the purpose of a low E coating on windows. It slows the loss of radiant heat from the house in winter, and slows the gain of radiant heat from outside in summer. It has little effect on energy of shorter wave lengths, such as light.

Radiant heat can be very efficient. There are water heaters with efficiencies up to 97%. Check your codes, and with the manufacturers, for suitability before planning to use them. Extra efficiency is gained because the air is not heated directly, but rather objects (including yourself) are heated. You will be comfortable at lower temperature settings, and heat loss through ceilings and walls is reduced.


Possible to use a water heater in a heating system

A tankless water heater installed in crawl space

Hydronic radiant heat is distributed (who would have thought it) by radiators, or by incorporating radiant pipes or panels in walls, ceilings or floors. A common and very good method used today, is to place pex piping in concrete slabs, or directly under a framed wood floor. I have used both, and even very simple heating systems work very well. The warm floors are a real luxury.

Different rooms or portions of the home can be heated at different temperatures by using thermostats connected to zone valves. This also makes it possible to maintain an even heat if some areas of the home have different heating demands. It is even possible to use it to melt the snow on your driveway and walks if you use glycol in that portion of the system.

There as as many sources or combinations of sources that can be used to heat the water for a hydronic distribution system

You may be able to use one source to provide both domestic hot water and space heating. Plumbing codes, however, have been getting more restrictive for dual use in recent years. If your water is hard, it may also cause problems with mineral deposits. A water softener may be necessary to prevent undue maintenance issues.

One advantage of hydronic heating is the ease of adding solar as a supplemental source of heat. Another is that there may be less suspended particles in the air, since there is no fan blowing air around. This could be important if anyone in your family has respiratory problems.

Although it is not impossible to use a hydronic heating system for cooling, it is not possible to connect a conventional air conditioner to the system. An air conditioner does as the name implies. It conditions the air by cooling and removing humidity. (Once again, who would have thought it.) This uses different principles than a hydronic heating system.

A fairly recent innovation in heating and cooling is known as the Mini Split.

This system heats each room or area individually. Small units are placed in each area to provide heat or cooling, and are normally sourced from a heat pump.

That is about the limit of my knowledge about the system ,and I am not even sure how accurate that is. I will provide some links here, which I hope can explain it a little better.

Ductless, Mini-split Heat Pumps 

Ductless, Mini-split Systems

Ductless Mini-Split Air Conditioning (and Heating) Systems You tube


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