BEopt 2.8: Boilers have higher delivered energy than other heating systems

asked 2019-10-17 14:57:46 -0500

Ajey.Pandey's avatar

Hello, I've been working on a building model in BEopt 2.8, and I'm trying to get a consistent output delivered energy for an Excel calculator. For the most part, I'm able to get consistent delivered energy values, but for some reason, boilers deliver way more heating per year, for reasons I can't quite understand. For now, I've been looking at an old-vintage 6-unit multifamily building model. The model includes and unfinished basement, and the envelope and water heater are unchanged between heating systems

For forced-draft gas, oil, and electric boilers (80% AFUE for the fossil boilers), BEopt registers about 120 MMBTU/year of heating delivered for the building. For electric baseboards, 15kBTU mini-split heat pumps, central ASHPs, a gas furnace, and a condensing gas boiler, BEopt registers between 77 and 83 MMBTU/year of heating delivered for the building.

All of these heater systems completely cover heating loads, they all deliver the same amount of cooling (boilers and furnaces use a SEER 10 central A/C, heat pumps do their own cooling, all deliver around 31 MMBTU/year of cooling), and they all have an almost-identical number of unmet cooling hours (4310-4347 hours, they're all undersized). Additionally, all these heater systems have the exact same heating capacity of 118.4 kBTU/hour.

As far as I can tell, this discrepancy between forced-draft boilers and every other type of heater has nothing to do with zoned heating, or with sizing, or with efficiency, or even with distribution, since the condensing boiler delivers only 80 MMBTU/year.

What else could I be missing? Do forced-draft boilers simply deliver more heating, and if so, where does the extra heating even go?

Thank you!

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It's possible that this is the same issue as

shorowit's avatar shorowit  ( 2019-10-17 19:37:49 -0500 )edit

Hmm, good point. I think I already made the fix recommended there, but I'll keep that in mind. Thanks!

Ajey.Pandey's avatar Ajey.Pandey  ( 2019-10-18 08:11:57 -0500 )edit