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I was hoping to get some advice on the proper setup for this system. I live in Long Island, NY and just moved into a large house with an extremely old oil burner and 3 a/c condensers. I'm about to start an oil to gas conversion ... when I was introduced to this brand new Bosch inverter ducted heat pump, which is amazingly efficient.

https://www.bosch-climate.us/products-bosch-thermotechnology/geothermal-heat-pumps/geothermal-residential-product-offering/air-to-air-heat-pumps/inverter-ducted-split-system.html

I thought to myself ... since I'm putting up solar panels anyway, maybe this would be the best way to go about it. But winters might be cold, so I was thinking of using a tankless Navien combi to run a hydronic coil in the air handler to supplement. But it's hard to find people talking about heat pump + hydronic coils, as it's not very common ... although I'm not sure why not since so many people are putting up solar panels nowadays.

But I was wondering if anyone could provide their thoughts on this setup and what I should be careful of. Yes, I know that natural gas is really cheap right now, but 2 5-ton heat pumps should be able to replace the heating and cooling ... I think.

Or am I completely wrong and should just install a conventional gas boiler to heat my large home? And leave my a/c system as is since it's still working.

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Heat pump + hydronics is a fantastic combo, provided you can get an air handler that supports it

While you are correct that the combination of a heat pump with hydronic backup heat is uncommon, it is not bad by any stretch of the imagination -- when an efficient boiler is used, it will beat a forced-air "dual fuel" setup any day of the week when it comes to overall fuel efficiency as the hydronic setup can run the hydronic coil at part load to make up what the heat pump can't provide, something a forced-air furnace can't do, and is far easier on your house's infrastructure (and the electric grid!) than all-electric setups are, especially in cold-climate areas.

However, due to this rarity, not all heat pump manufacturers provide air handlers that have hydronic coils factory fitted in place of the normal electric standby heat. I am not certain if Bosch provides that option; if not, you will need a skilled HVAC shop's assistance to field-fabricate or field-modify an air handler for your setup.

You probably want a water tank somewhere in your setup though

However, on the hot-water side of the picture, you probably do not want the boiler to try to serve hot water loads directly -- this is inefficient as it forces the boiler to try to balance between competing needs. Instead, the superior approach is to use an indirect tank hot water heater that runs as a hydronic zone off the boiler, with the water in the tank is heated from the boiler water via an internal heat exchanger.

A high-efficiency boiler and indirect tank is as efficient as a good gas water heater, while providing excellent longevity with the availability of stainless steel or cement-lined tanks (vs the normal glass-lined steel tank) and superior capacity due to the extra heat exchanger area compared to most gas water heater designs. It also avoids the comfort issues that are possible in tankless setups (such as the "cold water sandwich" problem), and requires less peak capacity from the gas service.

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