Water heater radiant problem

[MiPaul]

I think I have a water heater stacking issue. DIY system, I'm using a dual rated water with side taps for domestic and radiant floor heat in my barn and I'm getting an occasional T&P blowoff on temperature that I think is related an abrupt warmup in weather. It appears the lower side tap return is well aligned with the gas valve thermostat casusing the WH to fire on a heat call. I use ODR to run a variable speed pump on the HX input to control loop temp, thermostat setpoint is room air, not slab temp. The system has worked well when it stays cold but seems to create a stacking the issue when the HX pump runs slower & shorter on warmups. My thought is to swap the connections and pull water from the lower tap and return to the upper tap, which is the exact oppposite of the instructions from AO Smith. Is this a plausable cause & solution, or will this proposal create a different set of problems?

 

[Fitter30]

Is there a expansion tank on dhw and have you checked water pressure to air charge set at 2lbs lower ? By reversing piping you have colder water cooling off your hotter water depending on the dhw use. When the load drops off the pump is also slowing down water temp to floor is going to be higher. What is the temp difference pump at full speed? What water temp is the dhw? Now many feet and size of pex loop lengths and number of loops?

 

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[John Gayewski]

I think I have a water heater stacking issue. DIY system, I'm using a dual rated water with side taps for domestic and radiant floor heat in my barn and I'm getting an occasional T&P blowoff on temperature that I think is related an abrupt warmup in weather. It appears the lower side tap return is well aligned with the gas valve thermostat casusing the WH to fire on a heat call. I use ODR to run a variable speed pump on the HX input to control loop temp, thermostat setpoint is room air, not slab temp. The system has worked well when it stays cold but seems to create a stacking the issue when the HX pump runs slower & shorter on warmups. My thought is to swap the connections and pull water from the lower tap and return to the upper tap, which is the exact oppposite of the instructions from AO Smith. Is this a plausable cause & solution, or will this proposal create a different set of problems?

The t&p shouldn't blow off under any circumstances related to the weather or the pumps or anything.

There's an issue with your piping or expansion tank.