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BEopt Question regarding Peak Loads

asked 2022-02-10 13:21:11 -0500

edquinlan's avatar

updated 2022-02-10 15:59:10 -0500

I am running an analysis on a house for a NZE webinar. I have created a building envelope which is common to all the iterations (modelling runs). All that I am varying is the different variety of heat pump systems (ducted, ductless, Geo, etc.). The peak heating loads are all the same (52 MBH) regardless of system type. The cooling loads are all over the place (lowest @25 MBH, highest at 46 MBH). I realize that there will be a little variation on peak cooling load due to fan energy differences, but these differences are way beyond that. Same ERV in all models. I am selecting the stock equipment in the library without alterations. Anyone have an idea what what be causing these differences?

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Can you provide your BEopt project file? You can share it using, e.g., Dropbox or Google Drive.

shorowit's avatar shorowit  ( 2022-02-10 15:27:25 -0500 )edit

Here's the file link

edquinlan's avatar edquinlan  ( 2022-02-11 18:53:02 -0500 )edit

@edquinlan it looks like you need to change the "Share" settings on Google Drive so that anyone with the link can access the files.

Aaron Boranian's avatar Aaron Boranian  ( 2022-02-11 20:43:45 -0500 )edit

Sorry.........lets try this

edquinlan's avatar edquinlan  ( 2022-02-11 20:48:10 -0500 )edit

Any update on this question/answer?

muhl's avatar muhl  ( 2022-09-14 14:49:59 -0500 )edit

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answered 2022-09-15 17:27:34 -0500

I think some of the outputs are being misinterpreted. Here are a few comments to better ground you:

  1. You called these "peak loads", but they are actually HVAC capacities. For heat pumps, note that "cooling capacity" is the nominal/rated capacity of the heat pump and "heating capacity" is the capacity of the backup heating. See the help file for this information. So for the heat pumps, it's the backup heating equipment capacities, not the heat pump capacities, that are being sized at 52 kBtuh and equal the furnace capacities. (In BEopt v3, this will all be clearer because, for a heat pump, we will report three capacities -- heat pump heating capacity, heat pump cooling capacity, and backup heating capacity.)
  2. You've specified that the air source and mini-split heat pumps should be "sized for max load", which in a heating dominated climate means that the heat pump's nominal/rated capacity (at 47F) will be increased relative to the ACCA sizing methodology so that the heat pump can still meet the heating load at colder temperatures. So for example if a heat pump only puts out 25% of its nominal/rated capacity at that colder temperature, it will be sized 4x the furnace capacity.
  3. BEopt unfortunately does not output the design loads (what you are calling "peak loads"), which is what the equipment capacities are based on. It's on the wishlist to add to BEopt.
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To further elaborate: If you bought a 3-ton heat pump, it would put out less than 3 tons of heat at a colder temperature. Or said another way, if you need a heat pump to put out 3-tons of heat at a colder temperature, you would need a heat pump that is larger than 3 tons. BEopt is telling you the size of the heat pump, not how much heat is delivered at the colder temperature.

shorowit's avatar shorowit  ( 2022-09-15 17:45:51 -0500 )edit

Scott, If Im reading you right, in the autosize option, my "traditional" gas furnace / DX AC system capacity is 25 KBTUh cooling, while the ASHP cool capacity is reported to be 80 KBTUh because the heat pump "nominal" capacity must be increased due to heat capacity derate at colder ambient conditions. The Geo HP system's cool cap is only 39 KBTUh because it doesn't suffer as much of a derate penalty due to higher ground temps. Also, it appears the heating Cap's do not include DHW loads. When is V3 of BEopt coming out? Are their local training seminars available for BEopt

edquinlan's avatar edquinlan  ( 2022-09-16 14:48:36 -0500 )edit

Yes, you've got it exactly right. I don't understand your question about heating capacities vis-a-vis DHW loads, what is the connection? A beta release of BEopt v3 is targeted for the end of the year.

shorowit's avatar shorowit  ( 2022-09-16 18:08:32 -0500 )edit

Scott, Regarding the DHW question, I assumed the HVAC capacities reported on the Output screens was just SPACE heating, but I couldn't find a definition. DHW loads in the morning can be significant, so I wanted to make sure what the HVAC CAPACITIES included. So I re-ran the model without a DHW system and the heating capacity remained the same (which means the Heating capacity is only space heat). So question answered. Are their local training session for BEopt (im in the Boston area). If so, where can I find info. Thanks again for your help.

edquinlan's avatar edquinlan  ( 2022-09-17 10:02:39 -0500 )edit

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Asked: 2022-02-10 13:21:11 -0500

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