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Ducted Heat Pumps - Terminology Help

Hi all,

I've got a single story on-slab 1550sqft home with tons of attic space that is mostly going to waste. My house is shaped like a "U", with all of the current mechanicals on one side of the house in the conditioned space (nothing in attic). Across most of the house I've got 5' clearance or better with a scissor truss setup in the main areas, and normal truss roof over garage and 2nd/3rd bedrooms. It's unconditioned attic, and my house is currently heated via hydronic heat fed from an old commercial hot water heater.
You can see the post discussing those issues here.

I'm trying to learn more about ducted heat pumps, but the terminology isn't particularly clear to me.

Components of the system:
- Heat Pump or Condenser (sits outside)
- Air Handler (fan, radiators, sits in closet or attic)
- Ducts
Right? What do we call the pipes and stuff that connect the outside unit with the indoor air handler? Refrigerant lines?

If the condenser units can operate outside...why can't it operate in my attic?

I see people who "condition" their attic spaces for better efficiency of the ducts in the attic, but it also looks like a cheaper option is just spray foam the ductwork, right?

Mr. Cool has a wide variety of products, and I generally hear good things about them. Any reason to avoid them?

Lastly, what do I need to know about gas/electric heat pump systems? I like the idea of having the gas as supplemental heat for the coldest days.


  • Jamie Hall
    Jamie Hall Member Posts: 23,452
    Refrigerant lines.

    And the reason for the condensor/evaporator and compressor being outside is two fold. First place, most of them are noisy. Perhaps more important, it requires a lot of air to either get rid of the heat from the house in air conditioning mode, or to provide the heat in the heat pump mode. A LOT of air. It varies with the unit, but somewhere in the vicinity of 10 times the air being moved by the inside units would be a place to start.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,633
    The outside unit is the "Condensing Unit" available as a "Heat Pump" or as a straight "AC Condensing Unit"

    Refrigerant lines are just that sometimes referred to as a "line set"

    They don't make condensing units for indoor use any more although they did in the old days

    Noise and heat transfer as @Jamie Hall said are the issues with putting them indoors

    The "Air Handler" is the indoor half of the unit it can be a "Heat Pump" air handler or a "AC Air handler"

    Air handlers are sometimes be equipped with hot water or steam coils for heating in addition to the cooling coils. They can be had with chilled water coils as well (mostly for commercial work).

    An air handler is basically a box with a fan, air filters, condensate drain pan, controls and heating and or ac coils.

    If you want gas with a heat pump you would install a gas warm air furnace with a cooling or heat pump coil attached to it. The gas furnace becomes your air handler.
  • Pollymath
    Pollymath Member Posts: 9
    Thanks all,

    What's the status of "compact" gas fired furnaces? Is there anything on the market that allows for more DIY install? I'm accustomed to seeing these huge closet-sized furnaces, and my last house, even having a smaller furnace, that unit was still pretty big. A quick glance around the net shows a "Chinkook Compact" as being pretty small.

    I assume heat pump coils come in various shapes and sizes and can be added to just about any NG Furnace setup?

    My buddy runs a home energy efficiency company and we were talking about my options (including the idea that more insulation on the skirt of my house may yield big improvements). Anyway, he specializes in Mini-Split installs. He is not a traditional HVAC contractor, so no experience with gas fired furnaces or even radiant systems. His suggestion was "just run some mini-splits!" And while that may work in some areas of the house, it might not in others.

    My primary reason for wanting to avoid a traditional mini-split is because the air handler is a very visually intrusive thing, and in my kids bedroom, as well guest bedroom, they'd stick out like a sore thumb and look kinda "hacky".

    In my master bedroom, I could actually kinda hide a minisplit air handler with a walk-in closet area.

    This is what has me interested in ducted heat pumps. Vents are more acceptable, more discreet.

    My buddy also suggested that the air handlers be as close as possible to the heat pump, to shorten the "lineset" runs, so he didn't like the idea of 20' of lines running across the attic space to feed a flush-mount square air handler in my hallway. Do the units lose efficiency rapidly with longer linesets?
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,886
    If you have attic access, ducted will be easier. Just make sure to insulate the ducts heavily. Minisplit vs. "conventional" outdoor half is mainly a question of shape as the function/performance is very similar. Getting a variable speed compressor for either is quieter and more efficient. Refrigerant lines can be very long, but depends on the system. 20 feet is nothing to worry about. Fitting a furnace in the attic will be very easy too. Should you? Depends on the design temperature/load, but likely it's more a peace of mind thing than an economic/performance decision.
    EBEBRATT-Ed Member Posts: 15,633
    Todays modern gas fired furnaces even with an AC or HP coil are pretty small. You could fit one in a closet.

    First thing you need is a heatloss/heat gain calculation then knowing the load you can look at options on equipment selection