Boiler high temperature rise

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NickelCity

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My 10 year old hot water boiler has a 36F temperature rise across the heat exchanger at 100% fire. This seems high. Not certain if it was this high when installed, but I doubt it. The system otherwise is working OK. I think systems are usually set up for <30F temperature rise.

My question is what is the upside and downside of running the boiler like this, and whether I should change anything, in particular switch out for a different primary pump to increase the flow.

Setup: 110K Peerless Purefire condensing boiler, primary/secondary piping with a Taco 007 on the primary. Install manual calls for 008 or 007, and I think 008 would give me more flow and would be an easy switch. I'm heating pump-zoned cast iron radiators and an indirect DHW.

Thanks.
 

Dana

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The temperatures at which this high delta-T is occuring matters. It could be normal if the output temp is high, but not if the output temp is low.

With a condensing boiler under outdoor reset control the delta-T on the boiler changes with output temperature. When it's only putting out 100F water the radiation doesn't emit much heat and the return water might only be 95F, a 5F delta-T. When it's putting out 180F water the radiation emits quite a bit of heat, and it might be returning 140F water.

With the high thermal mass of high volume rads the delta-T is always pretty high for the first several minutes (even 10+ minutes) of a call for heat if the radiation is full of tepid <85F water and the outdoor reset is calling for 130F+ output, and that beginning of a call for heat would be the only time a PF-110 would be firing at anywhere near it's maximum firing rate. If the outdoor reset curve is dialed in to perfection and no overnight setbacks are being used there will never be an extra-large delta-T, but the delta will rise and fall with system temperaure.

Most mod-cons can handle 50F delta-Ts at a high duty cycle without a problem, some can only handle 40F- I'm not sure what that spec is on the PF series.

The PF-110 ridiculously oversized for the design heat load for most houses. The PF-80 would be more than adequate for most houses under 4500 square feet, the PF-50 a better fit for most <2000' houses. That series only has a 4:1 turn-down ratio, so the min-fire input of the PF-110 is 27,500 BTU/hr, which at condensing efficiency would be ~25000 BTU/hr out. The heat load of my house (2400' of 2x4 framed 1920 antique 1.5 story bunglow + 1600' of insulated but not actively heated basement) doesn't rise to 25,000 BTU/hr until it's cooler than 25F outside. YMMV. It's not worth swapping out if it's still working but even with an indirect water heater zone (zoned priority) it's probably totally overkill.
 
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