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
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.
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.
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.
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.