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Feedback on Cold Climate Air Source Heat Pumps as Replacement for Oil Boiler Hydronic Heating System

user-1145135083 | Posted in General Questions on


I’m looking for feedback about plans to overhaul the heating system in my single-family house, replacing an aging oil boiler with hydronic basement heaters with a cold climate air source heat pump.

Some general background: I live in Hanover, NH, US heating climate zone 6A, heating 99% dry bulb = -3 degrees F. I am currently looking for an HVAC engineer to perform heating and cooling load calculations for the house. But I have some reasonably accurate data from past heating oil usage during the coldest months of the year (calculated using the method described here:, which indicate an implied heating load (65 degrees F) of approx. 40,000-42,000 Btu/hr for the house. The true heating load is likely slightly lower because the oil boiler is also used to supply hot water for the house. 

The house was built in 1964, wood frame construction, with aluminum siding and a standing-seam metal roof. The exterior wall insulation is poor by today’s standards, but the attic was recently air sealed and insulated to R-60. The basement blockers and runnings were also insulated with spray foam to R-22. All windows are double-pane vinyl windows, and new patio door has a U-factor of 0.29. There are two old doors that are somewhat leaky: the front door with single pane glass sections (combined with a storm door), and a basement bulkhead door with a single pane glass section. Another source of heat loss is a fireplace that is not used. We have three bathroom fans to address humidity, which run at 30 CFM at all times and at 110 CFM when switched on. We will be getting an energy audit with a blower door test, but I do not currently have data on the house’s air leakage. The house does not have ducts, and it would be very challenging to retrofit the house with ducts, because there is very little space to work with between first and second floors of the house, and attic is unconditioned. The first floor is 1,056 sq ft, the second floor is 800 sq ft, and the basement (mostly unfinished) is 1,056 sq ft. Adjacent to the living spaces are a 240 sq ft covered porch and a 288 sq ft garage (both unheated). The current oil boiler has a heat output of 83,000 to 104,000 Btu/hr (depending on the nozzle size, which I have not yet been able to determine), which seems significantly oversized in light of recent improvements to the insulation of the house. 

I am thinking of having a Mitsubishi H2i cold climate heat pump with ductless mini-splits installed. They have a good reputation in my region, and there are numerous contractors who are certified and have experience with these heat pumps. As mentioned above, ducts are not really an option for the house. So I have been thinking about the best sizing for a heat pump, how many indoor air handlers to install, and where to place the indoor units.

The second floor of the house has three bedrooms, a large primary bedroom, a medium size bedroom, and a small bedroom. The two smaller rooms are the warmest because they face to the southwest. The first floor consists of a large kitchen/dining room/living room area, and a separate office space (roughly the same size as the large primary bedroom).

I am thinking of placing the air handlers as follows: 
1st floor: Kitchen/dining room/living room: 12k Btu/hr 
1st floor: Office space: 9k Btu/hr
2nd floor: Primary bedroom: 9k Btu/hr
2nd floor: Medium bedroom: 6k Btu/hr
2nd floor: Small bedroom: 6k Btu/hr (well positioned to heat/cool a hallway landing space as well)
Basement: 6k Btu/hr

The total capacity of the six air handlers is 48,000 Btu/hr, with 21k on the 1st floor, 21k on the 2nd floor, and 6k in the basement. This exceeds the 40,000-42,000 Btu/hr estimated heating load of the house, but since the small ductless air handlers have a minimum capacity of 6k Btu/hr, it did not seem possible to distribute heat to all areas of the house with less overall heating capacity. Even with this plan, the entrance area to the house will only be heated indirectly from the adjacent office space.

For the outdoor units, I am looking at two different possibilities:
1) Two 24k Btu/hr heat pumps, one on each side of the house (specifically: MXZ 3C24NAHZ2); or
2) A single 48k Btu/hr heat pump (MXZ 8C48NAHZ).

I am leaning towards the two heat pump solution for a few reasons:
– They would be well placed to run lines to the relevant rooms in the house. One unit (on the southwest side) would serve the two smaller bedrooms and kitchen/dining room/living room (6+6+12=24k Btu/hr). The second unit (on the northeast side) would serve the primary bedroom, office space, and basement (9+9+6=24k Btu/hr). 
– By contrast, a single unit would require running lines from one side of the house to the other, likely through the attic. 
– Having two heat pumps would provide a backup if one unit should fail or need repairs.
– The 48k heat pump would require a branch box, and it is perhaps somewhat oversized. The 24k heat pumps have heating outputs of 25k Btu/hr each, whereas the 48k heat pump has a heating output of 54k Btu/hr. 

I am also giving some thought to a backup or supplementary heating system. The Mitsubishi H2i heat pumps are supposed to have 100% capacity down to 5 degrees F, but with a 99% dry bulb = -3 degrees F for my area, we commonly get even colder temperatures. The cold climate heat pumps are supposed to work down to -13 degrees F, but not at full capacity, and we do routinely get temperatures that low in the winter.

In the short term, the oil boiler can be kept as for backup/supplementary heat, but in the medium-term, I would like to go without fossil fuels entirely. One idea for a backup would be replace the oil boiler with a small capacity electric boiler that could use the existing hydronic basement system. Another related question concerns the hot water system, whether to replace the oil boiler with an electric water heater or with a heat pump water heater. 

I should also add, finally, that in recent years our cooling needs in the summer have increased. We are seeing higher levels of humidity and higher nighttime temperatures in our region. We currently use a window A/C units (one per floor) to cool the house, with fans to distribute the cool air. This is an imperfect system and it does not cool and dehumidify the house as much as would be ideal.

As I am relatively new to learning and reading about this topic, I would appreciate feedback, ideas, and suggestions for further reading.

Thank you,

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  1. walta100 | | #1

    Have you done this calculation it is based on fuel purchased and local weather data.

    In my opinion almost every unhappy mini split owner who posts to this site has two things in common.
    1 Every problematic system seems to have more than one head per compressor.
    2 The systems seem to be oversized.


  2. user-1145135083 | | #2

    Yes, that's the exact calculation I did based on heating oil purchased and heating degree days data from a local weather station. I did the same calculation for several 30 day periods in winter months (Jan. and Feb.) during the past two years, and each returned a similar implied heating load in the range of 40,000 - 42,000 Btu/hr.

    What would you recommend as far as keeping a 1:1 head to compressor ratio in a house whose construction makes ductwork very difficult (if not impossible) to implement?

    Given the layout of rooms in the house over three levels, I'm not certain that I can combine rooms into a smaller number of zones in a sensible way. Bedroom doors, for example, are typically closed at night. And the two main rooms on the 1st floor are separated by a hallway, with an entrance area and guest bathroom in between. 6 distinct zones, as described in the original post, would seem to be the minimum.

    Would it be worth considering multiple single-zone cold climate air source heat pumps such as the Mitsubishi MUZ-FH series? They are available in 6k, 9k, and 12k Btu/hr capacities. But would it make sense to have six individual heat pumps, one for each zone? Is there an advantage in efficiency to doing so?

    I agree that I should avoid oversizing the system. With an estimated 40-42k Btu/hr heating load, a 48k Btu/hr heat pump system would be slightly oversized. But is there a sensible way to design a smaller system with 6 zones given the unique characteristics of the house? Would smaller heating outputs for the larger rooms be called for?

    Thanks for your thoughts and ideas,

  3. user-1145135083 | | #3

    Looking for more insight into the potential problems with multi-zone ductless heat pump systems, I came across this very informative podcast on "The Multi-Zone Heat Pump Issue":

    So I see the potential problem with an oversized multi-zone ductless system: in non-peak heating times, it's unable to scale down to a heating capacity that matches the house's needs, resulting in short-cycling, poor efficiency, and uneven performance and comfort in the different zones.

    That being said, the discussion here focuses on high-performance homes in cold climates with very low heating loads (such as 24,000 Btu/h). Dana Fischer does note that for homes with heat loads equivalent to 700–1,000 gallons of oil per year, multi-zone ductless units can be well-suited.

    Question: would my house fit into this category? For the 2022-23 heating season, total consumption of 790 gallons of heating oil, and as noted above, an estimated heating load of 40,000-42,000 Btu/h during the coldest months (based on local weather data and oil consumption). Some of this oil is being used for hot water, but it still seems that the house has fairly significant heating needs, so perhaps a multi-zone unit could work well?

    At the same time, the podcast notes that you really want to size your unit at 80-100% of the heating load, and include a supplemental heat sources for the very coldest days (wood stove, electric radiators, etc.). So the proposal of a 48k Btu/h multi-zone unit (with max. capacity of 54k Btu/h) or two (2) 24k Btu/h multi-zone units (with total max. capacity of 50 k Btu/h) does appear to be too big for the house.

    What about this idea:
    Basement: Single-zone 6k Btu/h
    1st floor: Living/dining/kitchen: Single-zone 12k Btu/h
    1st floor: Office: Single-zone 6k Btu/h
    2nd floor: Primary large bedroom: Single-zone 6k Btu/h
    2nd floor: 2nd and 3rd smaller bedrooms: Single-zone 6k Btu/h, with a small attic duct set-up to distribute the 6k between the two bedrooms, which are adjacent to one another.

    This is a total of 36k Btu/h, which is about 85-90% of the house's heating load. It would involve 5 single-zone heat pumps, one of which would be shared between two rooms using ducts, while the others would be ductless.

    Does this seem like a better system design?

    Thanks for thoughts and feedback!

  4. charlie_sullivan | | #4

    I do think that's a better system design, for the reasons you say.

    An option to consider if the layout works for this is to put the 6 k for the two smaller bedrooms in a hallway between them. The bedrooms will be colder than the hallway, but that might be OK. You have good attic insulation (kudos for doing the air sealing along with that), and so even if your hallway has to be a lot warmer than the bedrooms, you aren't losing much of that heat through the ceiling--most of it will still be going to heat the bedrooms. The small-duct approach will be more efficient, but the difference might not be that much.

    The backup heat is a tricky question. One thing you might want to consider is whether you want to have a non-electric backup heat option to keep at least reasonably warm in the event of a power outage in the middle of winter--which could be a backup source of electricity powering your heat pumps or at least some of them, or it could be a combustion heat source, which could be a wood or pellet stove to avoid fossil fuels. Our neighborhood can have air quality problems in the winter from the many wood stoves, but if you got a clean burning one and operated it well your contribution to that problem would be pretty minimal compared to the ones that are burned long hours through the winter choked down to a smoky fire.

    But aside from the power outage scenario, supplemental heat could be effectively supplied by your present hydronic system with an electric boiler. If you want 10 kBTU/h from that, that's 3 kW electric, so not likely a problem for your electric service , and from an efficiency perspective, you'd hope to use it very little, so the COP = 1 isn't really a problem.

    I would recommend a heat pump water heater. In the summer it will help combat basement humidity problems for free, and in the winter, the net efficiency even including the extra work that the mini-splits may need to do, is still better than an electric water heater.

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