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Tekmar 561, high mass radiant, Tekmar 260. Will it short cycle the boiler?

Good day!

I'm finalizing all the updates to our heating system, and one piece of that is to install a 561 thermostat on the radiant zone.
My understanding is that the 561 has a setting for radiant flooring that uses PWM to modulate the pumps on and off in one minute or so bursts to keep the floor temperature relatively constant once it comes up to temp.
I plan to connect the 561 to a 260 with ODR, which should (I think) work to keep the boiler operating in an efficient range for the radiant needs (supply temps of 100-140 degrees through a mixing valve).

Will the boiler short cycle when the 561 goes into PWM? If it only calls for heat for a minute, with the 260 be smart enough to heat a full differential cycle, or will it shut down once the call for heat goes away?

Thank you!


  • Zman
    Zman Member Posts: 7,447
    I assume you are working on this system? https://forum.heatinghelp.com/discussion/170537/new-boiler-install-g115-and-controls-questions

    A smart mixing valve is the solution, not adding PWM and cycling the pump on and off. Cycling the pump will give you uneven heat because the supply areas will warm every time the pump turns on but it will not even out due to the flow turning off. The tekmar 260 is a simple controller, it will not understand that the t-stat is doing PWM, it will just turn off the boiler. You need constant flow for even heating, you should be varying the water temp and using outdoor reset first. If you want to fine tune from there, indoor feedback and slab temps that are actually communicated to the boiler controller is the way to go.

    In your other post, you were trying to do setbacks and the system was overshooting. Outdoor reset will help with that a bit. The best way to do that is by reading outside temp, inside temp and slab temp and letting the tekmar make adjustments using one of their higher end boiler/mixing controllers. The 260 is not that controller. You will always overshoot if you are doing setbacks and feeding the slab 140 degree water.

    I run the slabs at my house which are not very well insulated (it was the 90's) on a reset curve of 80-125, with no setbacks. Those zones rarely overshoot.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • vilord
    vilord Member Posts: 48
    Hi! Yep, same system.

    I guess I was trying to understand what the PWM in the tekmar radiant controllers is for... I won't use it.

    I do want to put in an ODR-controlled electronic mixing valve, but I'm pretty much out of funds at the moment.
    I already have a 260, which I can use to at least not run the boiler to 180...
  • Zman
    Zman Member Posts: 7,447
    I have successfully used their PWM to get older systems that were poorly designed under control. In a zone valve system you sometimes have one zone that needs hotter water than another to maintain indoor temp. The tekmar will give the one zone full flow and do a gentle PWM on the other (like 20 minutes on and 20 off). The boiler controller is getting outdoor feedback and communicating indoor feedback with all the zones. As soon as the need for hotter water goes away, the boiler lowers the temp and runs at full flow.

    This is a band aid approach for a system where adding a mixing valve to each zone is not feasible.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein