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# Heat Pump & Supplemental Coil Sizing and Control in ResStock

I am digging into the heat pump modeling and ResStock and was surprised at the amount of supplemental heating energy used by ASHPs in the simulations. For instance, in climate zone 5A, the supplemental heating coil consumes almost as much electricity as the heat pump over the course of the year for one particular simulation I looked into. One thing that stuck out to me is that the heat pump heating coils are sized smaller than I would think they are in the field. For instance, a 1700 sqft single family home gets a 2 ton heat pump. Would think that most contractors would put a 3 ton system since unfortunately rules of thumb seem to be widely used for existing home installations. This smaller sizing would result in the heat pump satisfying less load than it could if it were sized larger and greater use of the backup electric resistance heat.

Since these values are hard coded into the IDF files, can anyone from NREL (tagging you guys @shorowit and @Eric_Wilson) explain the sizing or point me towards some documentation on it?

In addition, how does the simulation call the DX heat pump and supplemental coils? If the DX coil capacity is able to satisfy the load, then does the supplemental coil stay off? Once both are required, does it call them simultaneously and each coil is activated at their rated capacity at given temp/conditions? So for instance, if 100 BTUs are required in an hour and the heat pump can only supply 78 BTU/h, does the backup coil get activated and let's assume it's capacity is 140 BTU/h so that the heat pump only ends up supplying 100*(78/(78+140)) = 35 BTUs for that hour.

Or is it a topping off method, where the heat pump supplies 78 BTU for the hour and the supplemental heat supplies 22 BTU. This would obviously be preferable energy consumption wise but would require a lot more complicated control algorithm to modulate the supplemental heat pump heating coil contribution which probably isn't industry standard for heat pumps in the field right now.

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@shorowit tagging Eric failed, how do you tag a user with a space in their name?

( 2023-05-10 07:48:28 -0500 )edit

@Eric Wilson should work to tag them, @dpud12

Never mind, that doesn't work because "Eric" is already taken as a profile name. You can try reaching out via email, which is listed in Eric Wilson's profile.

( 2023-05-10 10:35:01 -0500 )edit

I think @eric-wilson works. Edit: nevermind, it doesn't. But you can use the resstock tag to notify me and others.

( 2023-05-11 07:36:17 -0500 )edit

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I'm assuming you're using IDF files made from the HPXML files from the ResStock dataset EUSS-Res 2022.1 because that has heat pump upgrades in it. The EULP 2021.1 Release is baseline (no upgrades) and assumes all baseline heat pumps are lower efficiency and single-stage.

The EUSS-Res 2022.1 release was intentionally conservative by sizing heat pumps that re-use existing ducts with cooling load priority following ACCA Manual S (up to 1.3x the cooling load for variable-speed heat pumps). This was chosen to acknowledge the fact that, unless they are modified, existing duct systems may be too small for the airflows required by larger heat pumps. There are also potential concerns about the ability of heat pumps that are oversized for cooling to provide adequate dehumidification (which is the reason ACCA Manual S limits to 1.3x cooling load; I believe ACCA is currently revising Manual S to allow more flexibility).

For EUSS-Res 2022.2 the team is planning to include some upgrades that size ducted heat pumps for the larger of the heating and cooling loads, to include a more optimistic heat pump savings profile.

EUSS-Res 2022.1 docs describing the sizing are here: https://resstock.nrel.gov/datasets

See HPXML docs for the different ASHP sizing options available in OpenStudio-HPXML: https://openstudio-hpxml.readthedocs....

Re: control of supplemental heat, I think it is the latter (top up method). Descriptions of the various options available in OS-HPXML are here: https://openstudio-hpxml.readthedocs....

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Thanks for the detailed answer Eric! A follow on to this, when is the release of the EUSS-Res 2022.2 expected?

( 2023-05-31 08:46:52 -0500 )edit

@dpud12 I'm not sure. You could ask load.profiles@nrel.gov.

( 2023-05-31 09:30:28 -0500 )edit

( 2023-06-01 11:57:14 -0500 )edit

One of the odd things that we've found when digging into the ResStock simulations is that the system sizing is done off of the current home heating/cooling loads. Therefore, there is a bit of a disconnect between what the system size would've been back in the day (and therefore what the duct size would've been), and what are now the loads. So you end up with a 1960s home that is 1700 sqft but has a 1.3 ton AC replaced with a 1.5 ton heat pump unit, even though the heating load is 5+ tons. Back in the 1960s this home would've gotten sized with a 3 ton AC unit (or larger) easily.

( 2023-06-13 12:11:25 -0500 )edit

So I feel like there is a bit of an idealization where even though the design loads according to the ACCA would specify a 1.3 ton AC unit, the house would've likely had something like a 3 ton AC unit originally, and I'm hesitant to think that most contractors would be brave enough to downsize the HVAC system that significantly.

Does this seem totally off base? Anything I'm forgetting? I realize that dehumidification issues could be present if you're talking single speed systems where the upgraded efficiency of the home lowers the design load.

( 2023-06-13 12:26:25 -0500 )edit

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