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Community and Q&A

Radiant Heating temp for a 1940’s house

layth | Posted in General Questions on

Hi. First time poster here. I have an original gravity boiler that supplies my heat through what I think was called convectors, very heavy cast iron with steel fin radiators. The boiler has developed pin hole leaks that seem to heal themselves!  I want to replace the boiler this summer with a Radiant system.
I have access to the main floor from my basement  and my plan is to use BlueRidge Co. aluminum extruded 4′ x 6″ heat transfer plates in the 13′ long joists ( 6 plates per joist ). So two runs per joist with 1/2 Pex ( 250′ loops ). These plates are 6″ wide so 12″ of plate coverage for each joist cavity. The plates are 1/16″ thick. I am hoping to use 100 degree water for this system so I can use a condensing boiler in the 95% efficiency zone. My upstairs is another problem. No access from beneath. My solution, maybe, is to use the original convectors with 100 degree water to heat the upstairs as a second zone. I think they oversized these convectors because number one they were  gravity feed ( I will be using a pump ) and two the house was fairly leaky.  I have been replacing all my windows with Andersen 400 double glaze and upgrading insulation everywhere so I am hoping 100 degree water will work. We keep the temp at 65 upstairs in the winter so that may help. Any feedback would be welcome. Oh yeah I am working on a Beopt energy calculator to get a better understanding of what I need to upgrade in terms of insulation, air tightness ect. Not done yet.

Tom, Minneapolis Minnesota

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  1. Expert Member
    Dana Dorsett | | #1

    It's unlikely that you'd be able to adequately heat a 1940s house with 100F water in a radiant floor, even if it was an above-the-subfloor solution such as Roth Panel or WarmBoard. A boiler operating under outdoor reset control may be able to get you there with extruded heat spreaders & PEX under the subfloor, but maybe not. It won't run in the condensing zone at the outdoor design temp, but it might be able to run in condensing mode most of the time (TBD.)

    A room by room Manual-J (for the "after window & insulation upgrade" condition of the house) would be a prerequisite to specifying the radiant system that would work.

    A few pictures of the existing radiators might be useful. If the room to room temperature differences are reasonable with the old system, the size of the radiator and water temps/radiator temps could put some brackets around the heat load and the amount of radiant floor it would take to get you there, and at what water temp.

  2. layth | | #2

    Hi Dana. I was hoping the Beopt would get me there instead of a manual J, but maybe not? Anyway I have been thinking about having closed cell foam or high pressure cellulose blown in to my walls, ( have to get some quotes on that ) do you think that would get me to 100 degree water?

  3. Expert Member
    Akos | | #3

    Why get focus so much on 100f, you can get pretty good performance at higher supply temps as well, what matters is the return water temperature.

    Most modcons can handle higher deltaT, you can set it to supply say 140f water but have 100F RWT. This way you can get some BTU out of the old convectors.

    If you want to run everything at one temperature, I would go with something like ultra fin for the floor heat, this would let you run higher temperature water without overheating your flooring. Slightly less efficient than heat plates but quick install.

    Adding a low temp zone for the floor heat is pretty simple as long as you only run outdoor reset on the boiler output. If your floor plan is open and you can set it up so most of your heat can be supplied from your floor heat. This way you'll get pretty low RWT and good efficiency. With the higher supply temp you'll still be able to get some BTU out of the 2nd floor convectors when you close the bedroom doors at night.

    This is pretty close to what I run at home, the only issue I've had is my gravity feed piping is very large and unless I'm careful with the flow adjustments the system flow can reverse between first and 2nd story (all heat goes to the 2nd story rads, then to the 1st story before getting back to the boiler). RWT is around 110f from the cast iron rads and 90f from the floor heat.

  4. tommay | | #4

    Most gravity systems are already piped with a two pipe system for feed and return. Any conventional hot water boiler can be used. You just need to add pumps and controls. The high mass radiators should still work well or you can replace them with baseboards if you choose.

  5. Expert Member
    Dana Dorsett | | #5

    >"The high mass radiators should still work well ..."

    Whether it works as the total solution at condensing temps sort of depends on how oversized they are for the "after upgrades" version of the house, but I agree that's probably a better (and cheaper) option than retrofitting radiant floors.

    If the radiant won't support the design load at a low enough temp, using the high-mass rads in parallel with the floor, or as a second stage can provide both higher comfort and lower return water temps.

    Some pictures of those convecting radiators might be useful for this discussion.

  6. layth | | #6

    To reply to the question of using radiant, one of my concerns was the issue of those large schedule 40 supply and return pipes. They hang down from the ceiling all around the perimeter of the basement which I will be remodeling and hopefully adding two bedrooms . I have already torn out my old water supply pipes which were also below ceiling level and replaced with a pex home run system ( manabloc ). The plan is to remove all those boiler supply and return pipes as well.
    I am not so focused on 100 degree as I am on staying in the condensing mode most of the time. Also I will be able to access the upstairs radiators with the existing metal pipes that run through the walls.
    Just a little context. Last year we had to get our cabin ready for sale which included new plumbing, a new pump, stripping and painting the exterior, replacing the old rail with cable rail 110 feet, installing an electric range and cutting down several large trees, painting the interior. All me. So this no pipe dream.

  7. gusfhb | | #7

    First, I would not worry about an artificial water temp

    The boiler will condense or not condense, it will still be much m ore efficient than the old one

    Second, if the pipes are the issue, remove the pipes.

    With outdoor reset the old style radiators will work well and not needing gravity the will not need giant supply pipes

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