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Uponor zone short cycle

geller Member Posts: 3

Our Weil-McLain boiler was designed for one circuit to the radiators in our 1928 house. We added an addition heated by uponor underfloor heating. We now have two zones controlled by two thermostats, each with a pump, with the boiler relay being activated by either zone. The controller turns on the boiler and the appropriate pump for whichever zone is calling for heat. The uponor zone is fed by a mixer valve set to 100 deg. F.

The problem is that if the uponor zone calls for heat, the boiler turns on and then short cycles because of reaching the temperature limit. My diagnosis is that this is because that zone cannot distribute the output of the boiler, which cannot regulate its output level. When this happens, if I activate the other zone, the water temp immediately drops and the boiler turns on again. If the only zone calling for heat is the radiators, there is no issue.

I'm assuming it's piped correctly because it was done by a reputable contractor. Unfortunately, they retired.

Any help would be appreciated.


    EBEBRATT-Ed Member Posts: 15,517
    edited November 2021
    I would recommend you have a buffer tank installed to increase the boiler run time. Looks like he did a nice job other than that issue
  • hot_rod
    hot_rod Member Posts: 22,121
    Great components also😃 You nailed the challenge exactly. With a non modulating boiler, micro zones  create some issues. That boiler really needs to get above 140f or so on every cycle to prevent condensation inside it and the flue passage. At least a 10 minute run cycle would be the goal.
    If the low temperature zone is more then 10-15% of the boilers output, a return protection valve should be used

    A properly piped and pumped buffer could solve a few issues
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream