Ductless minisplit heat pump in a poorly insulated home?
I’m trying to find some information on the pros and cons of Ductless mini split heat pumps. (Mostly if one could work in my situation.)
My home is a 1930’s Cape Cod style home (Climate zone 4) with a full shed dormer on the front. The home is roughly 1100 sq feet. The first floor approx. 780 sq feet has an open floorplan that connects living/dining and kitchen with a sectioned off bathroom and guest bedroom. The 2nd floor approx. 275 sq feet serves as a master suite that has a sectioned off bedroom and bath.
My existing heating and cooling system is a forced air heat pump w/ supplemental oil furnace for cold days. Currently there is no duct work traveling to the 2nd floor. The heat rising from the first floor barely keeps it tolerable through the winter, while a mobile AC unit is installed in the 2nd floor during the summer. Keep in mind this is an older home with little to no insulation in the walls and minimal insulation in the roof.
My current heating system works hard to heat and cool, but it does the job. I typically keep the thermostat at 65-68 during the winter months and a consistent 75 in the summer. My thinking is that in order for a mini split it to be effective, I would have to go into the walls and roof and insulate properly.
The instillation of a Ductless system seems like it would be less intrusive that running ducts to the 2nd floor and it would provide significant space saving in the basement that could be finished later, (which then in turn would also need to be conditioned).
How critical is good insulation for the effectiveness of these systems?
Are there instances where you can use a combination of a mini split system with a supplemental
heat source such as a pellet or wood stove?
Any thoughts/ info would be appreciated.
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The short answer is: Of course you can heat and cool your home with ductless minisplits. In a small house like yours, the usual installation would require one minisplit downstairs (to provide most of the space heating) and one minisplit upstairs (to provide most of the cooling).
The first step to designing any heating or cooling system is to perform a load calculation. Here are links to articles on load calculations:
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
Who Can Perform My Load Calculations?
Thank you for the response.
So, if I understand this correctly no matter how leaky or poorly insulated your home is (assuming there aren’t any gaping holes to the outside) it can be heated and cooled with any type of system as long as the BTU rating of the system exceeds that of the heat loss calculation.
However, by having a well insulated and air tight home, this would lessens the amount of heat loss, therefor allowing you to install a system with a smaller BTU rating? I’m assuming this in turn is what leads to the energy savings.
Am I off track here or is this a general sense the concept?
Q. "If I understand this correctly, no matter how leaky or poorly insulated your home is (assuming there aren’t any gaping holes to the outside) it can be heated and cooled with any type of system as long as the BTU/h rating of the system exceeds that of the heat loss calculation."
A. That's basically correct -- although for some systems (and especially for very leaky homes), the design of the distribution system (ducts or tubing, depending on the system) is as important for comfort as the BTU/h rating of the heating appliance.
Q. "By having a well insulated and air tight home, this would lessen the amount of heat loss, therefore allowing you to install a system with a smaller BTU/h rating?"
A. Correct. A well insulated and relatively airtight thermal envelope also improves occupant comfort.
Q. "I’m assuming this in turn is what leads to the energy savings."
A. You've got it. Energy savings and comfort improvements.
If it's an open stairwell you can sometimes cool the whole house with a mini-split head located at the top of the stairs, but a room-by-room load calculation would be in order, and it may not be the best investment.
Spending the money on air sealing and insulation would improve your wintertime upstairs comfort quite a bit, and you might be able to cool the whole house with portable AC upstairs. (This is the case with my 1920s 1.5 story bungalow.)
A full shed dormer with little to no insulation in the roof is going to add quite a bit to the cooling load. If it's 2x4 or 2x6 rafters it's probably worth filling the cavities with fluff then re-roofing at least the shed dormer, putting 2-4" of reclaimed roofing foam above the structural roof deck held in place with a nailer-deck through-screwed to the structural deck. Blowing insulation into the walls is also going to lower the air leakage rate and make it MUCH warmer in winter.
Air sealing and insulating behind knee walls can be a real PITA in this type of house, but not impossible. Fixing the big leaks first is important There's often multiple vented mini-attics behind the knee walls with open floor joists bays running under the floor of the conditioned space. Short of closing off the vents to the attic spaces and insulating at/above the roof deck, installing even heavy cardboard air dams stapled caulked to the framing where the joists cross under the knee walls is a good start.