Sizing an Air-Source Heat Pump
I need help with air-source heat pump sizing.
My house is 1,870 sq ft ranch house with a 900 sq ft finished basement. The house is located in Westchester County, NY. The basement air is already conditioned with a 1-Ton ductless mini-split heat pump. I’m looking to replace the 50-year old central AC for the main level. The house has gas-based hydronic heating with baseboards. The gas boiler is a 120,000 BTU/Hr System2000 EK-1 boiler. It seems highly oversized for the house. The system cycles a lot and the temperature variation is quite high.
To increase the comfort level (keep the temperature steadier), I’m leaning towards using a heat pump to both cool and heat the house. In the winters, I would use the heat pump as the primary source of heat and when the outdoor temperature falls too low for the heat pump to handle, use the hydronic system as the secondary heat source. In the summers, the heat pump would be the only source of cooling.
I had a professional run Manual J and Manual D calculations for me. The Manual J report is attached. It shows total cooling required as about 30,000 Btuh and total heating required at about 47,000 Btuh. I want to go with a variable speed system in order to keep the indoor temperature study. I have narrowed down to the Carrier Infinity Greenspeed 25VNA4 series heat pump coupled with a Carrier FE4ANB005L air handler. My HVAC contractor is recommending the 4-Ton 25VNA448 heat pump (specs can be found at https://ashp.neep.org/#!/product/32814 and https://www.shareddocs.com/hvac/docs/1009/Public/06/25VNA4-02PD.pdf). This is an overkill for summer months but meets the winter heating requirement.
My question for the experts:
1. Should I go with a 3-Ton 25VNA436 or the 4-Ton 25VNA448 heat pump? 3-Ton would be more than adequate for the summers. Given that I have reliable (though oversized) secondary heat source, would I be fine with a 3-Ton heat pump in the winters? One school of thought says to just buy what is needed for heating and let the heat pump adjust speed downwards to meet the lesser summer load. Does the fact that I have a reliable secondary source of heat impact this rationale?
2. Are there published efficiency figures on the variable speed heat pumps running at 25 or 50% of capacity? Without the efficiency ratings at low speeds, I don’t know whether a 4-Ton unit would be as efficient in the summers as a 3-Ton unit.
3. Would I have to wire the heat pump controls to the boiler controller or would it suffice to use thermostat settings to make the heat pump the primary heat source. E.g., I could set the heat pump thermostat to 70 degrees and the boiler thermostat to 65 degrees. Would that work?
I would greatly appreciate any input.
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