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Utilizing central forced air in fan only mode for ductless heat pump whole house air circulation?

Nicholas Laskovski | Posted in General Questions on

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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  1. Expert Member
    Dana Dorsett | | #1

    The heating efficiency of moving 72F air into a 62F room with an air handler is pretty lousy, due to the low temperature difference. It makes sense with a furnace since it's taking in 65F and delivering 115-135F air (a 50F or greater temperature difference), but the amount of electricity used by the air handler might not even make up for the heat load of the air-handler induced infiltration.

    Also, 75F air or even 80F air coming out of a register has a wind-chill effect, nothing like the 110F "warm summer breeze" coming out of the FH18. Even if it keeps the rooms above some predetermined temperature, it's not exactly providing comfort.

    A typical furnace air handler is on the order of 750watts, which if used in a resistance heater such as an electric panel radiator would deliver ~2500 BTU/hr of heat. In most cases you will be better off using right-sized (for the heat load) resistance heaters, operated under occupancy/vacancy sensor control to manage comfort in the cooler rooms, delivering only the heat needed where & when it is needed.

  2. Nicholas Laskovski | | #2

    Yup, currently doing that now with small portable radiator. The furthest bedroom is the guestroom, so no need to heat unless people are spending the night. In a way, we treat it like an attic, keeping the door shut, almost all the time. But... doesn't the idea of whole house air movement by converting an existing old ducted furnace blower to have an auto/manual fan-only mode (currently not an option) with an added bonus of whole house air filtration give me extra points? Otherwise I have all this sweet duct work that seemingly is to the way side. Joining the ranks of the telephone wires - which brings me to my next topic, what the heck do I do with all my old telephone jacks ? Surround sound?

  3. Expert Member
    Dana Dorsett | | #3

    If you want ventilation, ventilate, but don't think of random uncontrolled infiltration as ventilation. In some instances duty-cycling a larger ducted system for purposes of mixing & distributing ventilation air from known sources to other rooms can sometimes be useful, but the duty cycle needed for ventilation air mixing & distribution is quite low- a handful of minutes at most per hour. So, if you installed a pair of Lunos in the rooms you cared about the most, then set up a duty-cycler to run the air handler to run 1-2 minutes every half-hour there would be at least some ventilation air distributed to all rooms with supply registers.

    If you're looking to use more power to soak up excess PV output, buy an electric car (really!). In my area a used Nissan Leaf in good condition isn't very expensive, and it's a pretty decent car- bigger inside than it looks from the outside. Range is limited (particularly in winter in colder climates), but most families already have the range extending technology know as "the other car". In winter when the Leaf's range performance is worst is when the heat pumps are already using a good chunk of the available solar output anyway, but during the shoulder seasons & summer it's pretty good. The net benefit to you (and the planet) would be much better than using using it on inefficient air handlers for moving heat at low temperature difference.

    Telephone wiring is pretty light duty, not designed for moving much power- you won't be running a sub-woofer with anything approaching decent fidelity. In the age of wireless-everything most of the existing home telephony wiring is probably just scrap copper to be recycled.

  4. Nicholas Laskovski | | #4

    Already own a Volt! Funeral services are being planned for our duct system.

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