HVAC efficiency changes furnace natural gas consumption

asked 2019-03-19 17:48:47 -0600

Cmerski
51 ●2

updated 2019-03-20 10:02:25 -0600

I'm running multiple model runs with all parameters held constant except for the central AC. I've noticed that when I use the same furnace with SEER 14 and SEER 15 Central AC I get identical therms / year output for the furnace. However, the same furnace will increase about 15% in therms / year when I use a SEER 17 or SEER 24.5 AC. What is changing in the model that causes the same furnace to use more gas when the AC system changes from SEER 15 to SEER 17? Is it the two stage compressor?

The attached picture shows this happen twice, once with an 80% AFUE furnace cycle through SEER 14,15,17,24.5 AC's and the 4 bars on the right show the same progression with a 90% AFUE furnace

C:\fakepath\Furnace.PNG

Thanks!

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answered 2019-03-20 10:06:11 -0600

shorowit

11891 ●201 ●9 https://www.nrel.gov/b...

It looks like this is a BEopt question, so I re-tagged your question with "beopt".

If you look at the properties for the air conditioner options, you'll see that the supply fan power values are different for lower SEER units vs higher SEER units. If the supply fan power is reduced for a higher SEER unit, then it will provide less fan heat and the furnace will have to provide more heating. If you wanted to verify that this is the cause of your increased furnace heating, you could create custom options where the supply fan power does not change across air conditioner SEER levels.

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After running the model more times with different supply fan power values I believe this is exactly what's happening. Thanks for the reply!

Cmerski ( 2019-03-21 12:11:52 -0600 )edit

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answered 2019-03-20 06:42:17 -0600

Jim Dirkes
3554 ●27 ●16

Essentially, this cannot happen ... unless ... the gas heating coil efficiency is different for the higher SEER models OR the part load curve is different. Has you checked those?

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