Boiler Sizing

Mike T., Swampeast MO

Will try to keep this short...

HVAC-Calc loss is 77 mbh; my "adjusted" loss is 68 mbh. "Adjusted" loss is good enough to predict the surface temp of my standing TRVd iron rads to a degree during maintenance conditions. As best as I can determine, the actual gas used at outside design temp agrees with adjusted loss--that is if the nameplate, my estimate of transmission losses and cycle losses are reasonably accurate.

Neither of these losses include distribution loss in the basement, so I'm considering IB+R rating for gross sizing. Gravity iron mains newly and reasonably insulated--intentionally uninsulated in the basement garage however.

Condensing/modulating boiler I'm looking at has IB+R rating of 70 mbh. This would seem to be a perfect fit with my "adjusted" heat loss calculation of 68 mhb. Low-end modulation is 22 mbh--which is right at my adjusted heat loss at 50° outside and VERY near my current WWSD point of about 53°.

No additions planned. Do not plan to use an indirect for DHW.

Do I trust my calculations for what would seem to be a "perfect" boiler that is going to be firing (and modulating) nearly 100% of the time?

Or do I jump up to the next size with 97 mbh IB+R and low-end modulation of 33 mbh? Possible reasons why: 1) enhance recovery from setback 2) my calculations may not be correct 3) extreme, protracted cold snaps that are fairly common in this area.

Alan(CaliforniaRadiant)Forbes

That's good

Mike "Try To Keep It Short" Swampeast

Short answer:
Trust your numbers

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Boilerpro_3

Mike

I see your WWSD point is 53F. I assume this is your building balance point. Have you adjusted your heat loss for this balance point? It sounds like you haven't, if your load is 22 mbh at 50F. If this is the case I certainly don't think you want to go to a bigger boiler.

Boilerpro

Mike T., Swampeast MO

Building Balance Point

Not certain of that term. If you mean the point at which the inside temperature stays at perfect room temp indefinitely, no. But then again, when does outdoor temp stay stable?

If you mean the point at which I can shut down the system without objectionable low temperature inside, yes. It's been at this level for years. BUT, I use currently electric resistance heat until outside temps start regularly dipping into the mid-20s at night. My poor old oversized boiler is just woefully inefficient until it really starts to get cold...

Boilerpro_3

Basically yes

Its the outdoor temp where heat losses perfectly balance internal gains, in other words the outdoor temp at which you can shut off the heat. For most older homes this temp is about 65F, hence utility bills usually report the degree days at this temp. By lowering the building balance point you save twice...you use less energy when you need supplemental heat, and you shorten the heating season. This is why thermal upgrades to a structure are so much more cost effective than the purchase of high efficiency equipment. High efficiency equipment only allows you to use less fuel when you need supplemental heating, it doesn't reduce how long the equipment needs to run.

Boilerpro

Mike T., Swampeast MO

I guess you realize that this is putting most everything I've ever done/thought of/written about in the way of heat to the acid test...

Steamhead

That makes perfect sense, BP

thanks for the insight!

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