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Second Radiator on Home Run Circuit?

OaklandNSOaklandNS Posts: 45Member
After using my system for a couple of days, I think I've undersized the emitters in one of my rooms. All the circuits are getting plenty of flow, and after running some additional heat loss calculations I think the room just loses more heat than the emitter can provide.

I have two home runs in the room, each feeding a radiator that is rated to put out 3,480 btus at 140* water temp. I would like to add a second radiator to one of the home runs that is rated for 3,120 btus at the same temp.

The current piping is 1/2" fostapex to each home run. The 1/2" should have no problem carrying the btus at a 20* delta t (each radiator needs only about 1/3 a gallon/minute of flow to meet the btu demand). My question is the best way to pipe in the new radiator:
  • Pipe both radiators in series with a single TRV controlling flow to both radiators based on the temperature of the room where they are located.
  • Pipe both radiators in reverse return configuration with 1/2" legs from the 1/2" mains supplying and returning to each radiator, with each radiator controlled by a dedicated TRV.
  • Pipe both radiators in direct return configuration with 1/2" legs from the 1/2" mains supplying and returning to each radiator, with each radiator controlled by a dedicated TRV.
  • Add a new home run.
Option 1 is the cheapest and easiest, but I read that a single TRV piped in series can cause problems because the temperature will drop too much in the first radiator and under power the second. Running parallel piping will be easy for me, and the cost of the additional TRV isn't so bad. Running a new home run would be a pain, and I would like to avoid doing it if any of Options 1--3 will work.

Thanks for your thoughts in advance.


  • Jamie HallJamie Hall Posts: 10,453Member
    I'd pick your parallel reverse return -- your option 2.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • OaklandNSOaklandNS Posts: 45Member
    Thanks Jamie---if that will work, that's a preferred option.
  • GordyGordy Posts: 9,264Member
    edited January 4
    Pipe in series with a single trv, or add new home run if manifold has the space. You didn’t say how short you were in emitter, only how much you were adding. However can’t be much, and the additional headloss of series piping won’t diminish returns that much. 1/2” will carry 15k. The trv only knows proportional demand, and both rads will get the flow.

    Cheapest, and will be effective.
  • kcoppkcopp Posts: 3,324Member
    Why not just replace one of the radiators w/ one that has a higher output?
  • OaklandNSOaklandNS Posts: 45Member
    Thanks Gordy---the current radiator is rated for 3,5000 btus at my target temp of 140*. The one I want to add is 3,130 btus. So the 1/2" line will be carrying 6,630 total. Well under 15,000. My calculation is not as short as it sounds because there's another radiator in the same large room on a different home run that supplies another 3,500 btus.

    Kcopp---I thought about that but there are already holes drilled into the floor for the existing radiator (and I can't use one of the same length due to wall space constraints), not sure if I can return the already-installed radiator, and adding another will put heat on 3/4 walls in the room which I hope will increase overall comfort.

    I can't decide if it's a bummer I need to add this radiator because all the other rooms are super comfortable, or if I should count myself lucky that this is the only problem (so far) on my first go around on something like this.

    Thanks for all your responses!
  • GordyGordy Posts: 9,264Member
    Have you tried raising your water temps first? If everything has trvs then doing so shouldn’t effect the other rooms.

    If you are using a mod/con you could try bumping up in 10degree increments see if it helps.
  • OaklandNSOaklandNS Posts: 45Member
    I did raise water temp to 160 and it's still losing heat overnight.

    There's still a bit of guessing because we intend to insulate the attic to at least R-30 (currently uninsulated) and the floor will be insulated as well. But redoing my heat calculations suggest that even after we insulate, I will still need the extra btus. Since the floor joists are open, it makes sense to me to add the additional radiator to ensure everything works well and I can get the emitters sized for a 140* design temperature once we have all the insulation installed.
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