I have an old oil boiler heating system in my house that needs to be retired before winter and I’m looking for feedback on my plans to replace it with mini-splits.

I’ve been looking into this for a few years, but it’s getting to crunch time.

I’ll go over setup then ask numbered questions.

House is a 1950s 1300 sq ft single-floor 3-bed in Worcester MA (climate zone 5A, design temp 6F). Concrete basement mostly below grade with integrated garage, back room extension over crawl space.

I’m attaching a floor plan with room names and heat loads.

House site is distanced from neighbors but with some shelter to from a steep wooded slope and side banks.

We had a MassSave energy audit and have details of work to be done – not completed yet as some other things have had to be fixed before sealing/insulation can be done.

I did the heat load calcs myself using CoolCalc based on planned insulation work, with best guess for infiltration (question on this below).

The house can be considered in three chunks:
– open living/kitchen area -> heat load of 14,000 BTU/hr
– back room -> 8200 BTU/hr
– three bedrooms -> 2700 + 4500 + 3770 BTU/hr

Total heat load = 33.3k BTU/hr
Cooling load = 15k BTU/hr

From our oil usage over three years, I did a rough calc using heating degree days and came up with 28.8k BTU/hr. However, this was with the house at 65F, the back room at 55F (unusued). That said, the boiler did provide hot water during this time from a direct coil (‘provide hot water’ is perhaps generous).

So much for the house profile, on to HVAC companies and the system itself.

When we first moved in a few years ago, I met with three HVAC companies. This year I’ve met with a further two.

Initially I was hoping to do a mix of air-to-air and air-to-water heat pumps and reuse some baseboard, but this doesn’t seem practical with what local HVAC companies are familiar with. It might well cost more too after adapting and upsizing some baseboard and possibly repiping the monoflow setup.

A common ASHP proposal was the Mitsubishi 42k multi with 5 heads. Other proposals involves two multis, always with a head in each room. One company proposed a 48k ducted system. Only one company did a Manual J. No company suggested a compact ducted unit: when asked, one said the low static pressure nature meant it wouldn’t work for three bedrooms.

My impression is system design and explanation is quite poor with HVAC companies, especially given the prices they are quoting. Having done some homework since first looking into this, it seems all these multi systems are oversized to varying degrees and would lead to efficiency and comfort issues – the 42k multi has a minimum output at 47F of 24k BTU/hr!

Looking at my layout specifically, it seems the living/kitchen space and the back room would both be well served by their own properly sized one-to-one. The bedrooms are more of a challenge.

The studio load is quite small so not having a head here seems reasonable. Air mixing will likely be minimal with the doorway orientation, but a space heater might be OK for the coldest weeks, and if needed there might be a way to ventilate air from the bedroom to the studio by putting an inline fan in the closet space that’s between these rooms and installing vents on the walls.

For the bedroom and office, the Mitsubishi 6k has a low enough minimum capacity to make some sense (1.6k at 47F, 1.2k at 17F). The fit for the bedroom is OK. For the office, given the doorway orientation, the head in the living room should be enough for shoulder seasons, then when it gets colder the load will be in the 6k modulation range (this is the case from 38F with current heat load calculation).

So, this is my current intention for mini-split system, all one-to-ones – this is what is on the second attached floor plan with model numbers:

– living room/kitchen -> Fujitsu 12k [rated 16.6k at 5F]
– back room -> Mitsubishi 9k [rated 11.6k at 5F] (the 6k might even work here)
– bedroom -> Mitsu. 6k
– office -> Mitsu. 6k

I have a quote of $26,100 for this system (including snow covers – question on this below – $24,700 without), which is similar to another quote.

OK, question time!

1) System feedback

Does this seem like a reasonable plan? Are there any issues I should anticipate or better setups I should consider?

In a way, it’s a process of elimination approach to what is practical and efficient given local contractors and my current knowledge.

The price seems a little high considering limited design support, but it seems typical for MA HVAC companies from the quotes I’ve had and seen in forums.

In an ideal world, there would be some assistance given in figuring out how efficiently a system would be expected to run and at what sort of operating cost (I’m sure some people have heard of the Heat Geek program in the UK that does something along these lines).

2) Infiltration estimate for heat load calc

I’ve used an ELA of 90 in CoolCalc – think this equates to 10 ACH50, 1600 CFM50 for my house dimensions. This seems close to the “loose” setting in CoolCalc.

Ideally I’ll have insulation/sealing done first and pay for a blower door test.

Failing that, does this seem like an OK value to take for now for my kind of house? I suspect there isn’t a good answer to this, other than do a blower test.

My insulation contractor expected (rough guess) to see around 3000 CFM50 from a blower test after work. He wasn’t sure on the calculation but thought that might be 4-6 ACH50.

Indeed this is a little confusing as I think 3000 CFM50 for my 9600 cu ft house comes out as 18.7 ACH50. Conversely, 6 ACH50 comes out as 960 CFM50. My ELA 90 is somewhere inbetween those.

(I understand the key number is natural ACH for heat load calculations, and getting from a precise ACH50 to this is a bit fuzzy anyway…)

3) Minimum capacity values on NEEP esp. for Fujitsu 9k/12k/15k

The NEEP database is great and has data I don’t find elsewhere – specifically minimum output capacity at 17F, a key bit of info when figuring out how a unit will cover the lower heat loads in the shoulder seasons.

I don’t see this info in AHRI or on manufacturer submittals (minimums only seem to be given for 47F), so not sure where it’s sourced from or how reliable it is.

With the Fujitsu 9k/12k/15k, all three have the same minimum output capacity at 47F (3100 BTU/hr) and also at 17F (5200 BTU/hr).

Typically I’d expect minimum capacity to fall with temperature, here evidently the rise in input power from 0.17kW to 0.57kW accounts for this rise in capacity from 47F to 17F.

Does anyone know if this data is accurate and how the transition in minimum capacity occurs?

The key question is how quickly does capacity rise below 47F, that’s where there might be an impact in dealing with the low loads at milder temps.

Similar Mitsubishi units seem to have a lower and more stable minimum output, and maintain a better turndown ratio over range. The Fujitsu units tend to have higher output maximums but the good modulation range at 47F reduces quite a bit as temperatures fall.

I with manufacturers or AHRI published clearer info on modulation performance over temperature to help size cold climate situations.

4) Snow protection covers for outdoor units

I’ve read snow/ice protection is advisable for a climate like central MA. 

The article “Roof protection for a minisplit outdoor unit” and question thread “Advice for keeping snow off a minisplit” in GBA answer most questions.

I wanted to check on what one company is offering: in-house snow caps for $350 a unit. They seem quite small, I’m not sure it’s worth the $1400 for the four outdoor units. I’ve attached a photo, do these look worthwhile?

We have a hip roof, so some snow sliding off would be expected. As I’ll have the outdoor units together in pairs, a DIY wood-frame roof could end up quite large. Tough to know what the best solution is.

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Thanks for any help, really appreciate the knowledge I’ve learnt from GBA so far.
AG