HVAC efficiency changes furnace natural gas consumption

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!

Cmerski

asked 2019-03-19 17:48:47 -0500

11.8k

shorowit

updated 2019-03-20 10:02:25 -0500

edit flag offensive 0 remove flag close merge delete

Comments

add a comment see more comments

2 Answers

Sort by

oldest

newest

most voted

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.

11.8k

shorowit

answered 2019-03-20 10:06:11 -0500

edit flag offensive 0 remove flag delete link

Comments

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 -0500) edit

add a comment see more comments

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?