Why does a more efficient residential AC result in higher heating energy?

beopt
reheat

asked 2018-11-16 08:12:58 -0500

PeterBerr
31 ●1

updated 2018-11-16 13:49:05 -0500

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

I have made a simple model in BEopt of a single story 2000-sqft house with no basement which I am testing for three locations in Texas. In one design case I increase efficiency of central AC from SEER13 to SEER 18. Sure enough, my site energy use for cooling reduces from 13.6 to 10.1 MMBtu, but my site energy use for heating has increased from 27.2 to 31.4 MMBtu. This is for Austin Texas. I made no other changes to the design other than the AC efficiency. Is the extra heating energy due to reheat of conditioned air? Or could there be some other dynamics in BEopt which could produce this result.

Thanks in advance.

Peter

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3 Answers

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answered 2018-11-16 10:45:20 -0500

craigch99
21 ●1

Duct losses can be higher with more efficient HVAC systems with variable-speed fans, because conditioned air is in the ducts for more hours as the fan operates more continuously with modulation.

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answered 2018-11-16 13:22:43 -0500

rraustad

13599 ●72 ●12 http://www.fsec.ucf.edu/

Check the coil energy transfer to see if it is the same. I would expect less cooling and heating electricity with higher COPs while energy transfer should remain the same.

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answered 2018-11-16 10:10:17 -0500

Eric Wilson

1853 ●12 ●8 https://resstock.nrel.gov/

The SEER 18 likely has a more efficient fan, which produces less waste heat during heating mode, resulting in the higher heating energy use.

Reheat is not common in residential buildings.

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Good idea, thanks. The electricity use for the fan is much lower for the SEER 18 system. Another bit of context is that heating (and cooling) duct losses are higher with the SEER 18, not sure why, but it seems this might contribute to the higher heating requirements.