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2 zones 2 stats: stat 1 calls both zones, stat 2 calls zone 2

Residence (mine), mod-con boiler, 2 zones

Zone 1 (main zone) is converted gravity, cast iron, heating most of the downstairs and all of the upstairs. Second (smaller) zone is RFH under tile, covers kitchen and adjacent small addition. Open floor plan so these rooms do get some heat from zone 1. Each zone has a stat and a Taco SR501 relay. Three pumps on the system: a return pump upstream of the boiler, one pump for radiator loop, one pump for RFH loop. No outdoor reset (I know ...). Overall happy with this system, in our second winter now.

When the RFH is calling but the rads aren't, the boiler tends to short cycle because the RFH gpm draw is low, so boiler quickly overshoots temp and shuts down. But RFH loop keeps calling, so that soon the temp in the primary loop drops and the boiler cycles again. Most often happens because the RFH is calling for slab temp, not for room temp. To help minimize that short cycling I want to try running the RFH loop whenever the rads are running, to boost the floor temp up in that zone when the boiler is already running (and condensing), but also maintain the ability for the RFH to call and run separately from stat 2. I expect this will boost the RFH temps higher than we have them now but that is OK and I want to try it out.

So, current setup:

Main stat calls ==> boiler called, pumps runs for radiators loop

Addition (RFH) stat calls ==> boiler called, pump runs for RFH loop

desired setup:

Main stat calls ==> boiler called, pumps run on both loops

Addition (RFH) stat calls ==> boiler called, only RFH loop pump runs

Can I do this with existing relays? I considered taking the RFH pump off its relay and running it from the orange wire on the boiler, but then would the RFH pump continue to run when the boiler is called but temp is already up? I'm thinking no, but I'm (obviously) not a pro ...

Sorry for the long post. Current controls diagrammed in images.


  • ErikV
    ErikV Member Posts: 34

    The answer is yes, the current relays can handle this just fine, here is how it is presently wired:
  • geno54
    geno54 Member Posts: 43
    Good call

    What program did you use to create the images?
  • ErikV
    ErikV Member Posts: 34
    nice drawing is from NRT Radiant

    The clunky drawing (of all wiring) I did myself, in Powerpoint.

    The really nice drawing came from NRT Radiant when they designed the RFH. I don't know what software they used. I just modified their drawing in Photoshop when I made the jumper and wiring changes.

    NRT designed the RFH retrofit (swapped out some radiators during a remodel), I installed. I recommend them highly, very fair and helpful. And the design includes great drawings!
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