Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

tankless gas water heater question

bob eck
bob eck Member Posts: 930
Most tankless gas water heaters require 140,000 up to 199,000 BTU on residential models. In a lot of homes the gas line is 3/4". I believe on most jobs when totaling up all BTU needed to run all gas appliances the 3/4" line will not be big enough so if the home owner wants to have a tankless gas water heater installed they need to up grade the gas line and that is a major expense when trying to sell TGWH. Does any tankless gas water heater manufacturer have the capability of making the TGWH have priority over the gas furnace or gas boiler. When the TGWH turns on the power from the thermostat to the gas furnace or boiler would be turned off this way the original gas line in the house would be big enough to supply the TGWH and you the contractor would not have to up grade the gas line to a larger size


  • Ironman
    Ironman Member Posts: 7,379
    edited January 2012
    A Simpler Solution


    The only thing that I know of would be using on of the various models of combi units. To try to do what you're suggesting would void the certification of any of the existing appliances or cause the existing boiler to lockout if something was installed prior to the appliance that shut the gas off. Not a bad concept, though.

    A more practical approach is to have the gas co. increase the service to 2 or 5 psi. and put step down regulators at each appliance. That way the line sizes do not have to be increased.


    After looking further at your post, I see that you're suggesting interrupting the thermostat signal on a domestic call. Sounds like a good idea, but the tankless manufacturer would have to incorporate a set of dry contacts in their unit and have it approved as a viable system. The weak link, of course, would be the installer. A plumber that doesn't understand controls could make a mess of a multi-zone system while trying to install the water heater.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Tim McElwain
    Tim McElwain Member Posts: 4,628
    edited January 2012
    Off the top of my head

    without really sitting down and thinking more about this why not a multizone panel with a priority zone and make the on demand operate just like we do with an indirect on an call for domestic the priority zone (On demand heater) has priority. Then operate the furnace or boiler off another relay contact.This could be looked at as a diversified load  I would really have to look at the wiring on this but again just jumping in and thinking without any research

    Now another question, what about the other equipment in the house cooking stove, dryer etc what do we do with those loads. Again the issue is the high BTU for the on demand, that is one of the reasons I do not like them I have others.

    I wish Jack from Rinnai was still around I am sure he may have run into this at some time.
This discussion has been closed.