Utilizing central forced air in fan only mode for ductless heat pump whole house air circulation?
I’ve recently added a Mitsubishi Mini-Split (MSZ-FH18NA2) to our old (1870s) but decently renovated and well insulated 1800 sq ft. house (1100 1st floor, 700 upstairs) Sometime in the late 80s, a forced hot air oil furnace and duct work were installed. All things considered, our house layout is relatively well situated for a heat pump, but as with most retrofits, cold air pockets are going to exist, particularly in the bathrooms and furthest upstair bedrooms. The ductwork from the existing furnace, was actually done well and has good reach throughout the house with intelligent placement of returns, allowing for excellent whole house air circulation. Ductwork was partially rebuilt during the renovation by previous owners 10 years ago. We also have a wood stove which has the ability to heat our whole house, although constant wood loading is required. With the addition of the heatpump, I’m tempted to remove the old furnance, but I will not – at least not until I’ve fully vetted the heatpump – and for multiple other reasons, like one, it still works, and two, redundancy. But, I also have the mindset that I will try to run the oil beast as little as possible, all but for maintenance to keep the thing ready to go for whatever reason (heat pump breaks, selling the house, burning oil somehow ends up being good for the environment?).
I’d like to trick my furnace thermostat into turning on the blower only, no heat. The reason, take advantage of existing duct work to take the warm air from the heat pump and circulate it to the cold pockets, then turning off when temperature stabilizes. My furnace does not have a fan only mode, and my thermostat only has two wires. Currently the blower only turns on when the heat comes on. Technically I can swap some wires around (on both the furnace and the thermostat) but I don’t just want the option to turn the fan on manually, I also want the auto fan kicking on when the temp. drops. I do also like the idea of being able to turn the fan on as needed just for the sake of circulating air during other times of the year, indoor air filtration, etc.
Beyond just the technical ability is it a bad idea? I understand the additional air circulation will increase electrical use, both from the furnace fan and from the heat pump working harder but I have a solar system which is currently generating excess and I need to find decent ways to use up electricity.
Am I complicating matters?
Old/New House Technology Melting Pot Redundant Efficiency Cheapskate
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