Mismatched Heat Pump Units
In Oct of 2020 we’d had 3 minisplits installed in bedrooms that the central ducted furnace never quite kept warm, and we’d been very pleased. That set included Mitsubishi‘s MXZ-3C24NAHZ2-U1 outdoors, and one MSZ-FH06NA and two MSZ-FH09NA indoors.
Then, in early Feb of this year, the heat exchanger on our gas furnace broke. It was cold (we live near Boston), but our bedrooms were warm, and we’d been planning to replace the furnace with a ducted heat pump. We bought a couple space heaters and solicited bids.
First quote we received was from the same company that installed our original minisplits and it was for a second outdoor unit, SUZ-KA36NAH, and the 3 ton air handler to match: SVZ-KP36NA. Unfortunately, then we were told their supplier couldn’t get the outdoor unit quickly due to supply chain problems, so they proposed replacing our two ton outdoor unit with a four ton unit (MXZ-8C48NAHZ2-U1). They gave us credit for the two ton unit they hauled away, and since we wanted heat ASAP, and we trust them, we accepted the proposal. Our new equipment was installed and fully operational by the end of February.
Since we had the space heaters, we did not have them install any kind of resistance heat in the air handler.
My question is, is there a problem with having the outdoor unit be 4 tons, but all the indoor units add up to 5 tons? If so, any advice?
I tried to do the “15 minute calculation”described in Dana Dorsett’s “Replacing a Furnace or Boiler” using our natural gas bill for Dec – Jan of this year (although our 3 minisplits were contributing to some of the load, too), and I got about 3 and a half ton. (The natural gas furnace that was replaced was 7 tons!)
Also, I want to thank Jon Herrod for the excellent 5 part series on Planning a Furnace to Ducted Heat Pump Retrofit – I learned a lot!
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