Help figuring out mini-split heating system in central MA house
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
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Replies
The most accurate heat load will come from fuel usage data.
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Have you consider a concealed ducted mini split in a dropped ceiling of the hallway?
Walta
Thank you for the response.
– Heat load from oil:
I touched on this briefly in the post, but I just tried running the numbers again for mid/late winter for three separate fill-ups.
I get an average of 23.5k BTU/hr (!). The highest value is from our first winter in the house, 29.8k BTU/hr.
The two caveats here: we've kept the house cool in winter to reduce oil usage, and the back room was only heated to around 55F. The house heats unevenly: the thermostat is in the office, set at 63F usually, and the living room will usually get up to 66F, the studio a touch warmer. So, it's around 65F on average in the house.
I'm not really sure what to make of these much lower heat load numbers from oil usage before we've even done insulation improvements. Some of the heat lost from the boiler and pipes goes into the basement (combating heat loss through the basement walls?) and probably rises up through the uninsulated leaky floor, so maybe more of the input energy from oil makes its way into the space than the boiler efficiency factor would suggest?
My conclusion from this side was that the air infiltration was maybe not too bad in practice. As well, hyrdonic heating tends to feel more comfortable than forced air, so I'd anticipate needing a slightly higher temperature in the house to be as comfortable.
- Concealed ducted minisplit in hallway dropped ceiling:
It's a small hallway: ceiling is only 7'7", it's 3' wide. There is an attic access hatch there as well, and a ceiling light fixture, so only a 3' x 2' area of free ceiling. Height above doorway molding to ceiling is less than 10".
Would the idea be a ducted minisplit blowing into the hallway, doing enough for all three rooms?
Thanks!
If you have a basement put a ducted unit unit there. Floor vents under the each window generally works quite well.
With such a small space, you can probably home run all the ducts to simple box plenum at the air hander.
The ducted unit would be about 1/2 the BOM cost of the 4 wallmounts. That cost difference buys you a lot ducting although residential retrofit installers don't like to do ducting and pricing tends to be out to lunch.
This is probably in the ballpark, there is also a pretty similar performance Carrier branded Midea ducted unit that would also work:
https://ashp.neep.org/#!/product/160599/7/25000/95/7500/0///0
P.S. You can click on the "advanced sizing tool" on the top right side of the page and enter your design conditions to see how the unit will operate in the shoulder season.
Your favorite on-line retailer will also sell you a mini split stand with a built in roof that might be simpler than any site built option.
Thanks for this suggestion, useful pointers.
(Yes, the NEEP advanced sizing feature is great, very useful.)
Two challenges for basement duct work:
1- The back room is an extension over a crawlspace, not accessible from the basement. This room is a good fit for its own mini-split, so not a problem excluding it from a main ducted system.
2- Two bedrooms ("bedroom" and "office" on the plans) are over a garage. This is one of those garages that's within the basement structure (if that makes sense). The garage space is separated from the main basement by a concrete dividing wall (with a metal door), and the garage has a drywall ceiling.
Running duct work to those bedrooms over the garage is difficult, I'm not even sure how it would be done. There's room to sneak in copper piping and electrical cables through the gaps between the floor joists on top of the dividing concrete wall, but I'm not sure what size duct would fit through. Because of the ceiling in the garage and joist orientation, a fair amount would have to be run through joists.
In the garage itself, we'd need to remove a fair amount of the ceiling to run the ducts. This then means MassSave won't cover planned insulation work for that ceiling as removal would count as a renovation which voids the subsidy. The garage door is also attached to one part of the ceiling, so depending how much access is needed, that might need removing and reinstalling (although, for other reasons, we may have to do this soon anyway).
So, this approach would have to factor in additional cost/time to take out some of the garage ceiling and repair, insulation work, along with running duct work over that wall.
When I was looking at air-to-water heat pumps, this had crossed my mind, remove the ceiling to run wider pipes to allow higher flow rate, reposition pipes to bedrooms for low temp radiators... it's a fair amount of extra costs though, and really relies on having a HVAC contractor you fully trust to know it's worthwhile the extra hassle.
That complicates things but not impossible.
Assuming a vented crawlspace, for the room above the crawl, it is easy to punch through the concrete and run an insulated duct run through the crawl. Make sure to seal the whole where the duct goes through the concrete and air seal and insulated the duct boot and register where it goes up through the floor into the room.
For the two rooms above the garage, you can run a duct up through the interior walls and have a wall register. This avoid having to go through the garage. Now this won't give you good coverage for heating but will cool the rooms fine. For heat you can keep the existing rads and replace the boiler with an electric boiler. These are pretty cheap and since you don't need to fully heat the house with it, it won't require major power. You can also replace the rads with electric baseboard heaters or even better, add resistance floor heat to the rooms.
You can also run the ducts for the rooms through the garage. These would have to be covered in fire rated bulkhead and well air sealed but not impossible.
The issue is with any of the above is that is outside of what an HVAC contractor would do. I would talk to a GC and have them rough in all the ducting so the HVAC guy only has to connect the air handler. This is much closer to their day to day job so you won't get magical pricing from them for the work.
You can also stage the work. Say install the house sized ducted unit but only run ducts to the rooms that area easy, leave the 3 hard rooms on rads or electric baseboard. Connect these up down the road when doing major work in those rooms.
P.S. Those rooms above a garage are always uncomfortable. The simplest solution is to pull the ceiling and insulate with spray foam to seal it up. You can also go with batts and layer of rigid with taped seams under the floor joists (greener, cheaper materials and avoid any potential spray foam issues but more work).
Thanks for these explanations and ideas.
The crawl isn't vented. I was avoiding opening up a hole for duct here: the concrete wall has a crack repair; if I leave it as is, insulation and sealing work will be covered and carried out in the crawl under the MassSave program (I really don't want to do that work!). I'm happy treating that separately and taking a cost hit for a mini-split.
Running duct up through walls is interesting. The interior walls for the office are all over the garage (the bath drain and water supply lines are also over the garage (it's an awful design...)), so access there might be difficult. The bedroom closet is actually partly over the basement, so maybe that could be used (losing closet space might be a problem, it's not that large as it is). Or, as the common interior wall between office and bedroom is only just in the garage, maybe duct work could be fitted over the basement wall and through there with minimal garage ceiling disturbance. Although, if you're opening up just inside the garage, you may as well access the floors.
Given the electricity rates in MA, avoiding straight electric heat seems sensible. If I was going to do floor heat, I'd probably try to have radiant floor off an air-to-water heat pump. Paying for involved duct install and then also paying to use electric resistance heat in MA doesn't seem like it would be more economical than having a mini-split (although I have no idea what costs might be for the duct install side).
Thanks for the pointer on using a GC in combo with HVAC.
The challenge on staging the work is finances. Although I don't know how the kind of duct work above would compare cost-wise, the mini-split option is only possible if we get the MassSave rebate. We do qualify for the option where partial home heating upgrade gives as much of a rebate as whole home, but they have criteria for how this should work. I could maybe reach out and ask what options we'd have there.
The garage ceiling right now would be insulated with dense pack cellulose by MassSave (I think this work is valued at $1k). The plus of taking out some of the ceiling is I could seal the rim joists as well as do insulation (and better organise the electrical cables run to the breaker panel in there... a whole other story), and batts and rigid probably wouldn't quite be $1k. Repairing the ceiling though would be a cost.
Part of the reason I hadn't gone too far into exploring ducted options is that the heat pumps themselves didn't seem as efficient as mini-splits and there's the duct losses to factor in (and I assumed they'd need to be run in the attic). Looking at the NEEP sizing graph, the Mitsubishi M-series unit linked above would start to low-load cycle from the mid-30s (not counting the back room, and I assume this graph wouldn't take into account duct loss). I haven't checked if there are smaller sizes. But I suppose, looking at the input power specs, just powering one unit to condition the house would use less electricity than having four mini-splits running, even if each unit was more efficient in itself.
Thanks again for the help. A lot of it comes down to being able to find good HVAC or GC people!
Yes, a ducted Akos described would be best. I wrongly thought this was a heated slab.
Walta
Thanks, good to know this is a preferred option given the house load and layout.
See my reply to Akos above for the issues with this approach for this house.