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# Decision Logic for Supplementary Heating of ASHP

Using ResStock 2.2.4, OpenStudio 2.9.0, EnergyPlus 9.2

For the heat pump measures provided in the ResStock project, what is the decision logic for determining when to use the supplementary heater? Is it dependent on heat load? Is it dependent on temperature?

I ask this because I'm trying to model a dual-fuel ASHP/MSHP, but with a propane furnace as the supplementary heater versus an electric resistance. Because ResStock does not currently have that as a measure, a workaround for me is to just take the heating output from the heating & supplementary coils and convert it to an equivalent amount of propane needed to burn to meet that output. (i.e. HEATING COIL HEATING ENERGY)

However, I'd like to use the propane furnace only when the ASHP/MSHP falls below a certain efficiency, say for example, an instantaneous COP of 1. I determine the instantaneous COP by dividing the HEATING COIL HEATING ENERGY (RES ASHP HTG COIL) [J] by HEATING COIL ELECTRIC ENERGY (RES ASHP HTG COIL) [J]. So this begs the question: how does the ASHP/MSHP measure in ResStock choose when to start using the supplementary heater?

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Currently in ResStock, a heat pump supplemental heater will operate only when the heating load exceeds the heat pump capacity, such that supplemental heating is needed to meet the remaining load and maintain the setpoint. So the controls are not directly linked to outdoor temperature, though outdoor temperature influences both the heating load and the heat pump capacity.

(Note that ResStock is being transitioned to use the OpenStudio-HPXML workflow behind the scenes for its modeling. This workflow includes additional modeling capabilities, including dual-fuel heat pumps. So at some point in the not too distant future, ResStock will natively support DFHPs.)

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Single speed heat pumps also have a min_tempargument that specifies the minimum outdoor temperature for heat pump compressor operation: e.g., https://github.com/NREL/OpenStudio-Bu...

( 2020-06-11 09:40:27 -0600 )edit

So for ASHP's in particular, supplementary heating operation is decided by min_temp instead of heating load? Furthermore, are the ASHP's in ResStock split systems or packaged system? And lastly, is the only way to model a heat pump in ResStock without a supplemental electric resistance heater is to set the supplemental capacity equal to zero?

( 2020-06-12 10:41:34 -0600 )edit