Background: I am installing a hydronic radiant floor heating system myself and getting all the information required is a combination of many different websites. It doesn't seem like there is an all-in-one site for what is needed.

I have an existing tankless (on-demand) water heater as the hot water supply for the house. I am looking to use a heat exchanger off of that to warm the water. I have 3 zones: upstairs master suite (1 manifold/2 loops); downstairs bedroom (1 manifold/1 loop); main living (living room 1 manifold/2 loops, kitchen&bathroom 1 manifold/2 loops).

Question I have created this diagram on how I am planning on hooking everything up. My question is everything in the correct location? Am I missing any components? The only thing I don't have in the diagram is the electrical wiring and the thermostats. Don't assume there is anything I am installing, but just didn't put in the diagram. Everything I know is on that diagram. Radiant Heating Diagram

  • Does your tankless heater have sufficient capacity to provide both the hydronic system and your hot water usage? Will there be sufficient hot water pressure to the house when the circulating pump is flowing water to the heat exchanger? – jwh20 Oct 27 '20 at 11:22
  • I found radiantcompany.com pretty comprehensive, just a customer, built some parts myself as I can solder and found their prices on soldered assemblies a bit steep. But they had lots of information in one place, as well as being in the selling it business. – Ecnerwal Oct 27 '20 at 14:00
  • My first thought is that your domestic hot water heater won't supply enough hot water at a high enough temp for the floor without it being too high for taking a shower. – FreeMan Oct 27 '20 at 15:29
  • Floor heat actually runs rather low temperature, normally - indeed, it's pretty common to use a thermostatic valve to keep the feed temperature LOW enough, though that could also be managed by how the heat exchanger is run in this case. Likewise, the intersection of Legionella and showers tends to favor thermostatic valves for the feed to showers and sinks anyway. When it's hot enough to kill them, it's hot enough to need tempering before use. – Ecnerwal Oct 27 '20 at 15:39

This isn't an "answer" but a caution. I have a hydronic, in floor heating system in my house and let me tell you, there is a LOT of design and engineering to do to get this right. Even the tubing layout in the floor has design criteria (pattern should be that half way thru the loop, the tubing should parallel the first part for more even heat distribution. There are sooo many options out there. Are you providing DHW off the boiler? or is the "boiler" a water heater? You also need to do heat loss calculations to determine if the BTUs supplied by your boiler or water heater are sufficient.

You didn't say, but I would guess that your radiant floor heating system will be installed in light weight concrete? If so, that has a lot of thermal mass. A thermostat that isn't too bright would leave the heat on until it reaches set point, then the residual heat in the slab could easily "overshoot" the setpoint by several degrees, making the room too hot and wasting energy. So at a minimum you'd want a tstat with adaptive intelligent recovery. But even that said, in floor radiant systems are "slow" so you can't expect to set the tstat for 60 at night and 68 during the day to be effective.

I know this is a DIY site, and you could probably do much of the install yourself, but I'd strongly suggest getting in touch with a pro for advice and design. Many supply houses have designers on staff that will offer their services for little or no cost, as long as you buy your materials there.

There are just too many opportunities for things to go wrong here so consulting a pro for design and advice is my strong suggestion.


You might want thermometers and/or flowmeters and valves for balancing the system, as I assume the valves you show are just the full on/full off thermostatic valves. i.e. if all zones are calling for heat, balancing makes it even, not balancing may mean one zone "wins" and the others "lose."

Whether you are happy slapping on some temporary thermometers once, adjusting, and calling it a day or whether you want them there all the time is more about your mindset than any "one true way."

Likewise, you need

  • fill, drain, & bleed valves on the heating loop
  • and usually an air scoop + auto-bleed goes with the expansion tank.
  • You need BOTH fill and drain since the usual way to fill that closed loop involves a pump connected to one, a bucket, and the hose from the other running back into the bucket (if antifreeze)
  • otherwise there's a whole other set of parts to fill, isolate/backflow-prevent and what-not for filling with plain water (but you get to skip the pump.)
  • And pressure relief (at least) with pressure-temperature relief advisable, even if you never expect the temperature part to come into play.
  • And a pressure gauge (which can be a pressure-temperature gauge) on the heating loop.

While it may be "just how you drew it" normally you want to have the hot feed at one "end" and the cold return at the "other" - so rather than connecting both at zone 1, with 2 & 3 beyond, you would connect one to zone 1, and the other to the far end of zone 3. That helps with balance, before you get into restricting flow with valves.

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