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Radiant Heating loop tweaking

Ringo311 Member Posts: 22
I have a weil mclain ultra boiler that has supplied a HWH and domestic baseboard heating loop for a couple years. I just recently expanded to include a radiant heating loop which is controlled by a taco relay station. I had it up and running successfully for a few hours yesterday. When the boiler is running at 180 degrees the radiant loop is pumping and running at 130-140 degrees and heating well.

Here is the issue: when the primary heating loop thermostat is satisfied,, the boiler modulates and the boiler water temperature drops to as low as 130 degrees. This is causing the temperature in the radiant loop to drop to below 100, which isn't really heating well. The radiant loop wont satisfy the temperature for it's own thermostat and it just keeps pumping luke warm water through the loop until the primary loop calls for the boiler to fire again.

When the primary loop is not telling the boiler to heat and the radiant loop isn't satisfied, should the radiant loop be telling the boiler to continue heating at a normal range? Is it possible the wire between the taco relay station and the boiler motherboard isn't working correctly? For anyone that knows Weil Mclain I used P-11 (1 and 2). These are the only things I can think of getting in the way, since it's otherwise working well.

Any help would be really appreciated.


  • Ironman
    Ironman Member Posts: 7,376
    I'm not sure, but I believe the Ultra can have two reset curves programmed into it. Which means it would drop to the lower one when only the radiant zone is calling.
    What type of radiant do you have? In slab or "staple up"?
    The water temp for a properly designed radiant floor should never be in excess of 130* and the surface temp should never exceed 86*.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    Also, What kind of boiler control are you using. And how is the radiant mixing being done? Specifically, what control is your 'Taco Relay Station'
  • Ringo311
    Ringo311 Member Posts: 22
    I'm using a SR502-4 relay switch. I wired it exactly like the diagram shows, which is pretty easy. I am using a mechanical mixing valve for the mixing.

    So I may have to program the reset curve in the boiler when just the radiant loop is calling? Would I have to program it to recognize the new pump and set my own temperature controls? (It's staple up - i'll adjust the mixing valve to not exceed 130)
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I have an Ultra-3. It has three thermostat inputs. Each input gets its own reset curve (though the indirect ignores outside temperature). #1 is connected to an indirect hot water heater and it is set to operate at 175F. Default is 190F. It is highest priority.

    #2 is the thermostat in my radiant slab zone. That goes from 76F to 128F depending on outdoor temperature. It is middle priority and has the greatest space heating demand.

    #3 is the thermostat in my my baseboard zone. It hord gtom 110F to 140F depending on outdoor temperature. It has the lowest priority, and the least heat demand.

    When #1 controls the boiler, the boiler circulator is off and the circulator from the boiler to the indirect is on.

    When #2 controls the boiler, the boiler circulator is on, and the radiant slab circulator is also on.

    When #3 controls the boiler, the boiler circulator is on.
    Also, if #3 is asking for heat, the baseboard circulator is on. In the case where that is the only heat demand, that is fine. If the #1 has control, the baseboard circulator runs, but does not do much. If #2 has control, the baseboard circulator also runs, but the water delivered to the baseboards is cooler: it gets the same temperature as the radiant slab zone. Better than nothing, but would never satisfy the thermostat.

    The reason this setup works is that in the Ultra-3, there are timers that keep any one heat demand from hogging the boiler. So even if the indirect asks for heat, after a while, the radiant slab zone can seize control for a while if it needs it. Likewise the baseboard zone. But I need no external controller, no temperature control valve, etc. I do have an aquastat on the supply to the slab and if it gets too hot, it will temporarily shut off the fire (it is in series with the other safety devices), but it has never operated. The only time it might is if I was heating hot water and it got satisfied, and the radiant zone immediately took over. But the capacity of the boiler is only about 3 quarts, so it cools down fast.