Influence of DHW Setpoint on ODR Setpoint

CapeCod

My question is about the effect of the domestic hot water setpoint on the space heating target setpoint. I have posted here previously about my oversized boiler and short cycling woes. I have just recently had a buffer tank installed so I have been observing my boiler’s performance more closely now.

The set up (pic 1) is a triangle tube prestige excellence combi boiler connected to panel rads fitted with TRV’s, constant circulation with a Wilo ECO pump simply plugged into the wall outlet. The Wilo is set at the lowest possible setting which I believe is 4 ft of head. My most recent addition is a 30 gallon boiler buddy buffer tank seen in the picture.




The weather is very mild now (50’s) so my ODR setpoint on the low end of the curve is around 100F. The setpoint for the DHW is 194F. When there is a call for hot water the boiler works its way up to 194 and then slowly (very slowly) works its way down to the 100F space heating requirement. Because of this the radiators are fed with much hotter water than desired and the house gets very warm (read too warm). I included pics from the boiler that graph the desired setpoint (pic 2)


and the actual boiler supply temperature (pic 3). You can see it looks like a mountain range. It takes a very long time for those DHW temps to bleed down.




I decided it was pointless to have my boiler try to fire for space heating requirements in this mild weather so I disabled heating (decreased the warm weather shutdown setting) but left the constant circulation pump plugged in. This still resulted in an overheated condition in the house. Then I unplugged the constant circulation pump….still not much of a difference. I finally resorted to closing the shut off valves in the basement that feed the individual radiators but I’m thinking there has to be more elegant solution. Is this normal?

Ironman

The problem is happening because your pumping hot boiler water into the buffer tank during a domestic call. The internal diverter valve may not be fully closing off the space heating.

CapeCod

Thanks for the quick answer...that sounds like a logical explanation. Any way you can easily diagnosis that it is sticking or something or is it easiest just to replace it?