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Hot radiators in the heat of summer

dtmaine Member Posts: 15
Hi all, I have a 6-unit apartment building with an approximately 7 year old forced hot water heating system.

We have a high efficiency triangle tube natural gas wall mount unit that provides both the domestic hot water for the whole building and provides the hot water for forced hot water radiators throughout the building.

I bought the building in November and it heated the whole building with great efficiency through a very cold Maine winter.

However I now have several tenants complaining that their radiators are still emitting heat in the middle of this latest heat wave.

All the radiators have two pipes, one with a shutoff going in, and on the top of the outflow pipe there is a white thermostatic valve which can be adjusted from 0 to 7.

I know that the system is equipped with an outdoor reset, but I don’t know how it works. My understanding was that it was supposed to turn down the heat when it’s above a certain temperature outside.

Even with the thermostatic valve in the 0 position, some radiators are still emitting significant heat.

Can I turn off the valve to those radiators without affecting the entire system? Is there some way I should be able to turn off the hot water to the whole system without affecting domestic hot water?

There are two main feeds coming from the furnace, each with an on/off valve and some kind of check valve/pump. I assume one is for domestic hot water, but they are unlabeled.

I have someone coming to walk through the system with my next week, but in the meantime it’s 80 degrees. What’s my best step?

Thank you!


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    Post some pictures of the boiler and piping. I suspect there is something wrong with the control system in this case.—NBC
  • kcopp
    kcopp Member Posts: 4,392
    Turn the TRV all the way down to 0.. That should help. If not turn off the ball valve on the supply going out to the heating part of the system.
  • dtmaine
    dtmaine Member Posts: 15
    @nicholas bonham-carter I will take some pictures the next time I'm there. What components are you referring to when you say the 'control system'?

    @kcopp the TRVs on the warm radiators are turned all the way down. As to turning off the supply to the heating part of the system, I have two questions:

    1.) Being new to a forced hot water radiator system, is it OK to turn the supply on and off like that? Do I have to worry about that creating unwanted air in the system? Likewise, is it ok to turn off the supply to individual radiators? (Would that affect all the downstream radiators? Unfortunately I don't know how the system is plumbed.)

    2.) Do you have any suggestions on how I might distinguish (at the boiler) which supply is going to the heating part of the system and which is going to domestic hot water? It looks like one of the supplies feeds to a water tank -- would that be the one for domestic hot water?

    Thank you both!
    HVACNUT Member Posts: 5,744
    edited July 2018
    Seems like there are faulty TRV's and the rads are warming from a gravity heat rise when there's a domestic hot call. Which, in an apartment building is probably very often.
  • Zman
    Zman Member Posts: 7,542
    Your system should be setup with outdoor reset and warm weather shutoff. Your problem could be in the control settings or the way the boiler is piped.
    Pictures of the boiler piping would help.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • AMservices
    AMservices Member Posts: 610
    Maybe a bad flow check on the heating circuit.
    When domestic hot water pump circulates, water could be pulled backwards through the system heating those radiators.
    If you close the valve on the heat loop (either on supply or return) that will stop water from circulating.
  • kcopp
    kcopp Member Posts: 4,392
    really need pix to tell what valve
  • Ed_32
    Ed_32 Member Posts: 33
    Most likely induced flow during DHW production. Best way to insure no flow is to zone the space heating line with a zone valve on the supply, and a check valve on the return.
  • jason2018
    jason2018 Member Posts: 3
    Add a check valve on the return for your space heating. What’s most likely happening based on the information you gave is that when your domestic hot water tank is calling for heat, you are getting gravity or “ghost flow” through your open return. Hot water is is less dense than cool water and will rise if given a path.