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.
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.