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Forced Hot Water Radiator Not Heating

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MPL15654
MPL15654 Member Posts: 45
in THE MAIN WALL
Installed a new boiler on an old gravity converted forced hot water system. When we looked at the job the owner mentioned that 1 radiator on the 2nd floor would not heat. After further investigation we realized the old 11/4 piping that was feeding this radiator must have froze and broke and someone had gone in and repiped it with 3/4 HePex to feed the radiator. I'm assuming once this happened the radiator stopped working. We replaced the boiler and bled the entire system, every radiator is working properly and heating except this one. It has to be a "flow" issue because we were able to bleed the air out of the radiator and we just get cold water out of it. The "supply" pipe to the radiator gets warm but that's about it. The supply and return risers are at the end of the main piping, there isn't any scoop tees in the system to help direct the flow, it's an old gravity converted system so i believe there's a 21/2 x 11/4 tee on the supply and return main that picks up another radiator and then this reduces to 11/4 and now 3/4 and feeds to this second floor radiator. Again all the radiators in this house are heating except this one that was repiped. Any ideas on how to resolve this? I'm thinking we should re route the 3/4 HePex and tie in closer to the boiler, not on the end of the run like it is now.

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  • Gordy
    Gordy Member Posts: 9,546
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    If possible to route the pex that would be a good idea. What size circulator is on the system now?
  • MPL15654
    MPL15654 Member Posts: 45
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    Taco 007, that's what the existing boiler was using, have used it many times on these types of systems and haven't had an issue. Boiler is Piped P/S. System is working perfect in my opinion except for this one radiator that wasn't working even before we replaced the boiler. Owner keeps thinking we need a larger pump, higher head pressure, i keep trying to explain that there is no head, the supply mains are 31/2", i don't think a bigger pump will solve this issue, think it's a flow issue and needs to be re-piped.
  • Gordy
    Gordy Member Posts: 9,546
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    Well here's a good read.

    https://heatinghelp.com/systems-help-center/sizing-circulators-for-old-gravity-hot-water-systems/
  • unclejohn
    unclejohn Member Posts: 1,833
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    The pex may have air trapped in it. I would try a valve and a bleed first.
  • MPL15654
    MPL15654 Member Posts: 45
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    I have valves on it and have bled it, don't think it's air. Do you think the 007 is too small? The house is heating comfortably, no velocity noise, 2nd floor radiators are all working correctly except this one.
  • Gordy
    Gordy Member Posts: 9,546
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    Read that article Frank put a lot of effort into.mif you calculate the EDR of the existing radiation he has pump sizes to EDR tables in that article. Just a reality check.
  • kcopp
    kcopp Member Posts: 4,435
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    Re-pipe the radiator the way it was in 1 1/4".
    You are correct about the Circ.
    You need to mimic the flow.... any higher head pump will really mess up the system balance.
  • Gordy
    Gordy Member Posts: 9,546
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    If the EDR is 810 SF then the 007 is fine., but the 3/4" pex is not mimicking the gravity flow piping.
  • MPL15654
    MPL15654 Member Posts: 45
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    EDR was 750. What would be the best way to get this radiator to heat up. Repiping to 11/4 isn't an option, I'm sure owner does not want to tear open walls and ceilings. Would tieing in closer to the boiler help?
  • Paul48
    Paul48 Member Posts: 4,469
    edited December 2015
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    Try closing the valves(slightly), on the other radiators. I have a gravity conversion in my house. The other radiators won't mind the reduced flow, and you may get that last one running. If so, caution the HO about playing with the valves.
  • kcopp
    kcopp Member Posts: 4,435
    edited December 2015
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    Do you have access to the piping from the basement?
    You could try to put a small circulator on that line to pump up to that rad from the main.
    Treat it as a secondary loop off the primary main.
    Other than that you are end up chasing you tail....
    How bad does he want it to work?
    If the HO wanted it to work he should have replaced what was there w/ the same.

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