Tankless water heater used for radiant heat

I have a question about the viability of using tank less water heaters as a means to heat water for radiant heating systems. The specific concerns are as follows.

  1. Does using the tank less water heater for both domestic and radiant needs shorten the life expectancy and or void the warranty.
  2. Normally a boiler closed radiant system is between 12-20 psi. The domestic system is generally 40-80 psi. Does than mean with this type of configuration the radiant now has the same psi as the domestic. Is that a problem?
  3. Boilers normally heat water to 165-170 degrees for the radiant heat. The tank less water heater heats to only 120 becuase there is no separation between the heat and domestic systems. This must mean it has to run longer and or take longer to heat the same sq. footage.
    I am concerned that while these systems are labeled “green” and are much less expensive that they are actually not sufficient to do a good job in a cold weather climate like Northwest Colorado.
    Thanks in advance for your time and assistance.


I have read those articles. The manufactures seem to thing their product is great for this application, however if you read other forums plumbers and some installers state the opposite. I will likely call a couple of local HVAC guys but was wondering if anyone here had spoken with any installers or had clients which considered these systems insufficient under normal living conditions.

Unless you use one of these.


Depends. I did this when my son and I built his house. Use an on demand for the radiant, but it is a separate closed system. Can raise the temp higher since the DHW was separate. Works great.
Have a client that has the same thing and all hell is breaking loose. Noritz claims it voids the warranty and their system is not designed for radiant heat. So client may sue seller to have it replaced with a boiler.
To tell the truth, altho it seems to work, it might be easier to just put in a boiler. An on demand runs $1500-1800, not installed. You can get a LAARS Mini-Therm boiler (75k BTU) for about $1700. Since the price is comparable, why not just use a boiler that everybody understands and forget the hassle.
My 2 cents.