Advice for sizing a Cooling-Dominant Heat Pump to prevent short cycle in heating season
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Does anyone have advice for heat pump sizing in a scenario where the Cooling load is around 50% higher than the Heating load? If I size the condenser per energy code to cover the cooling+latent load, the capacity becomes so high that I worry even with significant turndown (i’ve been looking at Mitsubishi heat pumps), the heating load is low enough that the equipment will short cycle for large portions of the heating season. Climate Zone 4a, 2400sf apartment in lower 3 levels of a Townhouse (cellar, ground, 2nd floor, no roof in unit, hence the low heat loads and high cooling/internal loads), well-sealed (aiming for 1 ACH), code-required insulation, ERV, heat pump intended to be primary heat. I did a Manual J calc that gave around 18.8k btu/h cooling (15.8k sensible/3k latent), 11.9k btu/h heating load. To save on maintenance, my preference is to have potentially 1 or 2 ducted, low or medium static pressure air handlers in the conditioned cellar rather than multiple heads in each space. I intend to install an 80-gallon heat pump hot water heater that will actually add slightly to the cellar heating load and slightly reduce the latent load – neither is accounted for in Manual J. I know Allison Bailes is a proponent of undersizing heat pumps (say in this case maybe sizing the system closer to 15k btu rather than 18k), as are the Norwegians apparently (though they are heat-dominant and tend to have supplemental heat source). Am struggling to find appropriate heat pump equipment combination in the Mitsubishi catalog and wondering if “undersizing” relative to the Manual J is a viable approach here – I worry about the latent load capacity with undersizing – or if I should consider another heating/cooling strategy that will work well while maintaining historic character of the property. Is radiant floor heating plus ducted A/C recommended in a scenario like this? Sounds expensive – but so is replacing a heat pump that burns out from short cycling. Any wisdom the community can share is appreciated!
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