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Heat Pump system design in 110 year-old home

shirashira Member Posts: 3
Hi all,
We have a 1908 3-bedroom, 1 1/2 bath, 1800sf. gambrel-roof home with insulated walls and attic. Our oil burner (forced hot air) just conked out and we want to switch to a heat pump system (Electricity is sourced from clean energy RECs, so no more fossil fuel!!). We don't raise the heat beyond 68 in winter and are used to no air-conditioning in the summer, so prefer not to oversize the system, especially to keep the budget down on a new system.

Our existing ductwork runs to floor registers that are located farthest from the exterior wall in each room! Should we then reuse existing ductwork, (with aerosealing), or install new ductwork to better located SA registers? How much more efficient and comfortable will the house be with registers at the window wall?

We've received quotes showing: 2 ton units per floor, (1)3 ton and (1)4 ton unit to cover the 2 floors, all ducted. Some quotes have a basement unit using existing ducts and an attic unit with new ducts dropping into each bedroom via a ceiling register. One quote has a single heat pump option in the basement using all existing ductwork to both floors.

The possibilities are many!! What would be the wisest way to heat/cool this home? Advice is greatly appreciated : )


  • mikeg2015mikeg2015 Member Posts: 1,178
    Omg. A 3 ton should heat and cool that whole house. 2 ton up and down should work. Get full variable speed fan upstairs and a thermostat that can reduce fan speed to dehumidify on demand.

    Downstairs run blower at 700cfm cooling, full 800 heating.

    For reference, a 2 ton heats my 1900sqft downstairs down to 35-38f without heat strips. It stages to my gas steam boiler under 45f if needed. Heat pump disabled at 30f at the economic and comfort balance point.

    Upstairs I did a Unico with mixed results. Should have gone conventional.

    7 tons for a house that size is insane. It will short cycle, be drafty and unbalanced.

    The bigger the system, the more air. 7 tons needs 2800cfm. That’s maybe 300 cfm per room. I would only do that in a small commercial building for cooling loads with 10% outside air.

    All that being said, you should take your previous fuel use anf unit run time and use it to estimate system size.
  • shirashira Member Posts: 3
    This is a huge help, Mike. Getting your take on correct sizing of the units is great. We are getting off a 165,000 btu oil burner that has been doing exactly what you warn against- short cycling, drafty, inefficient. More of the same insanity.

    Any thoughts on location of SA registers? Are they worth relocating to window wall?

    Thanks so much!!
  • JackJack Member Posts: 1,044
    Step one in this process should be a pressure test of the existing duct work. You may find that it is so poorly sealed that it is not worth saving. Duct sealing is difficult and can become quite expensive as access is limited. As well, the duct sizing for an oil furnace may not operate as well on a heat pump.
    I pulled all the ductwork out of my home and made a dump run several years ago. I did the same to my daughters home a couple weeks ago. I installed mini-split heat pumps in her home and the same in mine. I also added a Rinnai Energysaver EX-22 to my home. Our homes are very comfortable with zoned heating, cooling and dehumidification with excellent state-of -the art equipment.
    Given the direction you are going, I would start with a clean sheet of paper and develop all of the options.
  • shirashira Member Posts: 3
    That makes good sense. The ducts are leaky for sure. We are thinking of ductless ceiling cassettes (accessible attic) to supply the second floor- 3 bedrooms that each only have one exposed wall and it are insulated.
    We've been told that mini split ductless throughout is too expensive and typically meant for a single added space.

    Did you use a BDU to have several units make it to one condenser, despite running longer than maximum distance to condenser?

    [excuse incorrect use of terms. I'm not an HVAC person. Architect, new to heat pump technology]
  • Larry WeingartenLarry Weingarten Member Posts: 1,875
    Hello, Here is a link: that discusses both ductless and ducted mini-splits. (see Dave Robinson's presentation) I might back up and do a heat loss calculation so you know the loss at each room, also. B)

    Yours, Larry
  • mikeg2015mikeg2015 Member Posts: 1,178
    If you have a basement and already have registers. I’d stay conventional downstairs and just replace or seal accessible ductwork. Honestly I just used flex. It’s cheap and very quiet. Can’t hear my system ever. Just ran 5 very large registers, one for each room.

    Mini split is good option upstairs. Thats where it’s nice to have room by room zoning. In hindsight it might have stil been a better option for me overall. Unico is a work in progress. Should have just installed it myself. Need a good installer similar to a steam boiler. It’s not hard, but details matter.
  • ratioratio Member Posts: 2,540
    If you're interested in using VRF systems, I believe that many/all major players have software to size the heads and linesets. You can download Mitsubishi's at, you have to register for junk email from them though.

    One thing to keep in mind is that not all systems can operate with heads in different modes, i.e. one heating and one cooling. Usually one mode wins, or sometimes the whole system parks until the offending unit is set back to the same mode. The savings of going without the branch selector box maybe aren't worth the cost of going without!

  • mikeg2015mikeg2015 Member Posts: 1,178
    edited December 2017
    VRFs are crazy pricey. Almost as cheap to go GSHP with multiple indoor units. Not a bad solution done as a hybrid with air to water cooler and boiler or solar to allow undersized loops. Honestly it’s the perfect application for solar. Warming up a 35f ground loop. Air to water heat exchanger can heat the loop to in warm weather.

    Remember, geothermal isn’t really geothermal. It’s thermal mass storage system. Energy isn’t created. It’s stored and extracted because average annual air temps are 50f in most areas so ground gets warm in summer, then cooled in winter. This is widely misunderstood even among engineers. You really need to size based on annual building load, not equipment size. There can be a discrepancy.
  • IronmanIronman Member Posts: 5,739
    Is everyone but Larry missing the MOST IMPORTANT and crucial first step? An ACCURATE load calculation must be done FIRST. If you don't how many btus it takes to heat the house, how do you know what size unit(s) to install?

    To the homeowner: I would immediately dismiss any contractor who won't do a Manual J heat loss calc to determine what size you need. Forget about the size of your old oil furnace: it was grossly over-sized.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
    Larry Weingartenratio
  • JackJack Member Posts: 1,044
    If that is your contractors opinion of mini-splits I'd find someone who specializes in them for comparative info. Line set lengths on multis are quite liberal. My recently installed 24 kbtu w/3 evaporators can handle up to 164' of line set. It varies by manuf. VRF systems are great, but quite costly compared to standard MSHP systems. You may be able to resurrect the ductwork, but before proceeding agree with the contractor on a 6% max leakage and test it before payment. In fact, again, test it first and see if it is worth doing at all. You can throw a ton of dough at unitary (central) equipment but between your comfort and the hi tech equipment you just bought you MAY be strapped over a barrel due to the poor duct work efficiency. Personally, I don't buy the line that "leakage into the house" is okay. If it is okay, why bother designing and sizing a duct system at all
  • mikeg2015mikeg2015 Member Posts: 1,178
    Ironman said:

    Is everyone but Larry missing the MOST IMPORTANT and crucial first step? An ACCURATE load calculation must be done FIRST. If you don't how many btus it takes to heat the house, how do you know what size unit(s) to install?

    To the homeowner: I would immediately dismiss any contractor who won't do a Manual J heat loss calc to determine what size you need. Forget about the size of your old oil furnace: it was grossly over-sized.

    I've seen Manual J still overestimate some homes. It has a lot of trouble with stack effect, windy, solar gain, and thermal mass. Overestimates internal loads at times, especially in larger homes.

    I think it's good to use both load calc and duty cycle/utility bills to estimate real useage.
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