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Conventional heat system with a Split/ductless AC system for cooling

BobDog42 Member Posts: 2
We have an oil fired hot water system about 10 years old, the house is 35 years old, well insulated with new windows. The House is warm on the coldest days. With the somewhat drastic changes in the climate here …we have decided to install AC throughout the house using a split/ductless system with a Heat Pump. I’ve heard a number of stories…good and bad relating to heat Pumps as a single source of heat, initial plans would be to use the conventional system as a back-up for heat. We live in the Boston area, I would appreciate hearing opinions and experiences from folks with a similar set-up. The House is of open construction with 4 heating zones. As we have not spoken to any HVAC folks yet…so I don’t have any idea how the AC system would be set-up. I’ll be anxious to hear from folks.


  • Jamie Hall
    Jamie Hall Member Posts: 23,554
    From the heating and cooling standpoint no reason why it won't work just fine. There are a number of such installations out there. The usual strategy is to use the oil or gas hot water or steam when it's colder out (the definition of "colder" is a somewhat movable number) and the heat pump for heating and cooling otherwise. In fact, Cedric's home also has a 4 head mini-split in an apartment which is part of the building, but which needs to be kept warmer than the rest, and it works just fine.

    An open floor plan, however, may present some challenges in terms of placement of the indoor units and in terms of control. I would suggest finding a really could and well qualified local installer and spend a good deal of time with them on options for that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,899
    This is a very typical installation. However, there’s an avoidable, common mistake: you’ll want to make sure that you size these ductless heads correctly. In particular, if you go with a multi-split (that’s one outdoor unit connected to multiple indoor units) you need to be extra careful with the smallest load spaces, usually the bedrooms. Often a ductless unit in every bedroom will be massively oversized and inefficient when part of a multi-split. Solutions include some combination of the following; combining small loads if possible using tiny ducted indoor units, using 1 outdoor to 1 indoor unit setups for the smallest loads and/or using resistance heat in the smallest load spaces. If you have a basement and/or attic, running ductwork for most/all of the house avoids many of the pitfalls and adds other benefits, like better filtration, distribution, etc. 
  • pedmec
    pedmec Member Posts: 994
    My recommendation would be to get stage thermostats and use that combined ductless/baseboard. You see, although they claim it produces heat with the outdoor temperature down to subzero or close to sub zero temperatures its not going to be blowing 120 degree air out of the heads. As with any type of heat pump you get diminishing returns the colder it gets outside. You will most likely be getting 85-95 degree supply air on less than 10 degree ambient outdoor temps. And with a low indoor humidity it feels more like a cool breeze. if your on the younger side it might not bother you but when your older you feel it. we get complaints from customers because they don't like the cooler air. they were not told about it. If you stage it so the boiler comes on at some point you can have the best of both worlds. Mitsubishi sells staging thermostats to work with the mini splits and boiler.