radiant floor heat/domestic hot water units

crosscountry's picture

 I failed with a longer post I tried to submit, so will keep this short and expound on it if warranted. Basically, I just wanted to know if a radiant floor/domestic hot water combination unit was more or less efficient in actual practice than having stand alone boiler and hot water heater. I live in northern Illinois, and this concerns new construction.  My thinking is that trying to make one boiler do two purposes means the unit will not be at peak efficiency most of the time...but then again maybe he difference in working temperatures might mean better recovery of heat in the combination unit (?)  Any pertinent comments? Thanks.


Two separate systems is my (post #206988, reply #1 of 5)

Two separate systems is my recommendation without knowing more about the size of the house, number of occupants, etc.

Depending on the heat load, a 2nd domestic WH can be dedicated to just the radiant heat, and although its efficiency is not equal to some of the boilers, it is far cheaper, and much of its lost heat goes into the house anyway, and it is far less complex. 

A separate system also eliminates the complications that result from the code requirements to keep the potable water from being contaminated by the radiant system because the radiant system can be a closed-loop system, meaning that it's not connected to the potable system. This also means it can operated at a low pressure, and any leak would be very small since the volume of system water available would be very small.

it depends.   If you mean a (post #206988, reply #2 of 5)

it depends.  

If you mean a boiler with an indirect fired water heater, heated from the boiler, the answer is that it is horrendously inefficient in the summer UNLESS you have a "purge" function which dumps unused heat from the boiler into the tank when the call is done.  Or, a very low mass boiler, but even then purging is good.

If you mean a boiler with a built in coil, then I would say that the problem is that you are usually oversizing the boiler.  most combi units can only produce enough hot water for one shower, but have outputs more suitable for homes that have 3 or 4 baths in them (much larger homes).  they could be 3x or 4x to big for the heating load, and so will be less efficient in that case.  If you have a larger home that only needs to run one shower at a time though, they can match nicely.

Generally the best bet would be a sealed combustion high efficiency (or electric) well insulated tank, or a boiler with indirect and purge control.  In unoccupied conditions an on demand may make more sense, but pretty much ONLY in that situation.


Rockport Mechanical

HVAC Design and Installation in beautiful Rockport Maine


Thanks for the insights. I (post #206988, reply #3 of 5)

Thanks for the insights. I think I'll go with separate systems, partly because I can locate the water heater next to the two bathrooms (located at one end of the house), but the radiant boilers (primary heat for the house) will be central to the floors being heated by using two seperate mechanical rooms. Any thoughts on hyro-air combination units? I was thinking about using a split system (Mitsuibishi) as backup heat and airconditioning, but now that I'll have to meet the new code requirements of having a air exchange unit I'll have ducting anyway.

im starting a project and was (post #206988, reply #4 of 5)

im starting a project and was wondering the same thing and kept reading about the diakin altherma unit, very interested in hearing more about it. seems like a complicated system to get to work flawlessly especially if also hooked up to solar like they show but if you could afford the initail cost and it did what they say... seems pretty awesome!

we are using the Altherma to (post #206988, reply #5 of 5)

we are using the Altherma to heat/cool our shop here in Gardiner Maine.  we love it and are a distributor for the units now to trained professionals.

typically if you can use 120 deg water or less, and you have a 35,000 BTU load or less, in a cold climate you can expect to run at about half the cost of propane or oil.  close to natural gas and pellets.

great for radiant floors, ceilings, panel radiators.  I am not a booster of the solar integration but I do think doing DHW with it is a no-brainer.


Rockport Mechanical

HVAC Design and Installation in beautiful Rockport Maine