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

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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

PeterBerr

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

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__AmirRoth__

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

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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.

craigch99

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

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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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rraustad

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

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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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Eric Wilson

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

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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.