Choosing a Multisplit Heat Pump System
Hi all,
I’m seeking some advice on whether to go with the hyper-heat or not. I’m replacing baseboard heating with Mitsubishi multi-splits and I will have no back-up heating. Located in Vancouver, BC, Canada.
The three companies I’ve contacted have said the regular non-hyper-heat models will be sufficient. The coldest ever temperature in Vancouver was -18C in 1950 and 1968, and generally -8 to -10C is the coldest day each year (although we hit -16 for one day in 2021).
Any thoughts? The hyper-heat is approximately $10,000 CAD more for the home (~2400 sqft). I’m thinking they are probably right, and I could always get portable electric heating units if it ever really does get cold enough that the heat pumps stop heating. The companies all claim the non hyper models will keep heating down to -20C (at reduced capacity, of course). Any personal experiences with this?
Thanks,
Brian
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Replies
Taking a step back here might be appropriate:
1. Did any installer do a heat loss? If so, what kind? What was the result? Was a fuel-usage heat loss considered?
2. Is this a multi-split installation (one outdoor unit serving multiple indoor units)?
3. Was a ducted heat pump considered for any portion of the home?
These questions matter a lot more than hyper heat vs. not. If a non-hyper heat system is sized for the heat loss, due to the loss of capacity at lower temperatures, it'll be oversized at mild temperatures. If this fact is combined with a multi-split ductless system (usually involving a lot of ductless heads, sometimes 1/bedroom), you'll have extreme oversizing and short cycling, which leads to garbage efficiency, which frequently comes up in the Q&A. It's really important to match loads to equipment for comfort and efficiency's sake.
Yeah, great questions.
I've asked them all if they have done a proper calculation, as I think they are just doing a rule-of-thumb calculation. They have all assured me it will not short cycle and is appropriate for the house and they have done hundreds of similar houses, etc. I pushed a bit more and they said they will come double check the sizing if I commit.
No ducts in the house, so it will all be ductless. Sizing wise they are all recommending two outdoor multi-split units for a combined 72,000 BTUs. Indoor units in 4 bedrooms, 1 large open main floor living space, and 2 kitchens. They recommended the same size units whether it is hyper heat or not. When I looked up the general rule-of-thumb sizing it pretty much exactly matched my square footage which is why I suspected that's what they all have done.
The house is not well insulated, 30 years old with 2x4 walls. I am upgrading to good new windows later this summer, though.
Yup that's a recipe for short cycling for sure. If you have some historical fuel data, give this a shot: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler.
Each outdoor MXZ-SM36NAMHZ-U1 (Hyper heat) has a max output of 42k Btu at 17 F and a MIN output of 22.5k Btu at 47 F.
Each outdoor MXZ-SM36NAM-U1 (non-hyper heat) has a max output of 29,400 btu at 17 F and the same min output of 22.5k Btu at 47 F.
Since they haven't done a heat loss and they've offered both options, they think the non-hyper heat one can carry the load. Assuming the load/outdoor unit is right about 29kBtu at 17F and about 0 at 65 F, that implies a load of <11,000 btu/hr at 47 F, which means both are 2x as large as needed. I suspect the true heat loss based on fuel usage will be much less than 58,000 (with potential new windows driving it down further), so the oversizing will be even worse. It's impossible for the system to not short cycle in this situation, so they're not being honest about that.
Mitsubishi themselves advises installers against this exact situation: https://www.greenbuildingadvisor.com/article/multi-zone-heat-pump-issue
So in terms of solutions after nailing down the heat loss:
1. Single zone systems modulate better than multi-zone (the MSZ-FS06NA modulates to 1600btu at 47F and still outputs 12,840 btu at 17 F).
2. Ducted systems can fit the load better because the minimum ductless head will be too large for most bedrooms, while a duct can provide lower CFM. A centralized system would be great, but any consolidation of rooms would be better than multi-splits with 1 head/room.
I think it's also worth mentioning that the installers have no way of knowing whether the systems they have installed in the past are short cycling. There just aren't any logs available from the systems to examine. Similarly they have no idea what the efficiency of the systems they have installed is ... they could install energy monitoring to at least know what power was used but since they don't know the BTU requirements of the house they can't know what output the power usage created.
At least put your house details into a simple calculator like coolcalc to get some idea.
Have you considered leaving some of the baseboard heat in place, as a booster for really cold periods? That might remove the question of hyperheat or not.
What's the plan for ventilation and filtration of the air with no ducts at all?
I need to get rid of the baseboard heating (hydronic) entirely to qualify for the local government grants that are available.
I only moved in in January, so the Feb gas bill was the largest one I have so far. I did a quick calculation based on that linked article, based on HDD 15.5C it came out to:
19355 BTU/C-hr
27097 1.4 oversizing
32903.5 1.7 oversizing
Does this mean that the heat pumps total BTU rating should not be more than ~33000 BTU? It was colder than usual this February, but certainly it was a bit colder overall in December and January.
The house is currently naturally ventilated (i.e. exhaust fans only). Eventually I'm hoping to get an HRV or ERV installed, but not anytime soon.
Thanks,
Brian
2x4 construction is closer to 18C balance point. This means your load is even less, so I would not go for much above a 2 ton unit overall.
When it comes to fuel based sizing, it doesn't matter which winter month you use, using a colder one is just a bit more accurate.
The 1.4 and 1.7 oversizing is for fuel burners. When it comes to heat pump I would look for the smallest unit that can cover you heat loss at design temperature, no need to oversize especially if you have any backup baseboard heat.
Generally the best bang for your buck is right sized two zone setup with one wall mount in the middle of the main floor and one upstairs in the hallway. Add electric baseboards/ panel / floor heat for each bedroom and bath upstairs to supplement when doors are closed.
The next step up is a single wall mount in the main floor and a slim ducted unit upstairs for all the bedrooms. This means finding space for a ducted unit and ducts. Usually not a problem as most can be mounted in the ceiling of a hallway or closet and run the ducts through bulkheads.
If you main floor is chopped up, you can go with a ducted unit for the main floor and basement.
You can also replace your gas boiler with an small electric one (say sized for 50% of your heat needs) and use it to even out the temperature plus supplement in case of those polar vortex days. This would save you having to install extra electric heat if you don't go for a fully ducted setup.
P.S. Installing 6 tons of heat in a 2400sqf for house with any amount of insulation in your climate is beyond silly. Never mind the equipment costs, you would be lucky to see much above a COP of 1.5 (that is slightly better than resistance heat) because of the crazy oversizing and resulting short cycling.
HVAC guys don't like to do ducts, they prefer the easy money of a bunch of wall mounts and out. It is the homeowner that gets stuck with the power bill and maintaince costs down the road.
If they don't want to do a proper manual J calculation, you can hire an independent energy rater to do an unbiased room-by-room manual J. This removes the conflict of interest that the contractors have in cutting corners on the calculation and oversizing equipment.
> The companies all claim the non hyper models will keep heating down to -20C (at reduced capacity, of course)
They're rated down to -15C so that does sound fairly believable.
Google "Mitsubishi (model number) submittal" to find specs yourself. Here they are for the GL18NA: https://www.mitsubishitechinfo.ca/sites/default/files/SB_MSZ-GL18NA-U1_MUZ-GL18NAH-U1_202005.pdf
Another oversizing story in the making: "two outdoor multi-split units for a combined 72,000 BTUs".
I've got over 2,100 sq ft in Montana (design temperature 1 F, -17C) running on a single 28,600 BTU/h Mitsubhishi hyper-heat multi-split, which does fine even at -10F, -23 C. ACH50 is around 2 or so (measured at 2.6 prior to some tweaking) and total heat load was about 28,600 BTU/h, hence, no oversizing whatsoever on the heat pump.
Design temp in Vancouver BC is -4.5 C (+24 F). Best to get a Manual J according to your 99% design temp: http://cms.ashrae.biz/weatherdata/STATIONS/718920_s.pdf
Aside from sizing and all the expert opinions above my first consideration is:
The difference in price between the 2 units hardware wise would be less than $1,000 USD (based this off a 36k btu outdoor)
The installation would be the same.
Why are they charging you 10K difference?
Everything tends to be more expensive in Canada (~30-40% more CAD than USD), and especially Vancouver generally things cost more typically even than the rest of Canada, but I'm guessing it is the increased price for the two outdoor units plus the increased cost for the 7 indoor heads going from the lower end GL to the higher end FH.
The GL heads work on hyper heat outdoor units. (see link below).
The number one mistake people make with multi-splits is selecting outdoor units based on the indoor unit count, rather than load matching resulting in massive oversizing.
Installers know it's a lot easier to slap a wall mount unit in than run ductwork in a finished house, and they can charge a premium for "zoning each room". So sales are biased towards a head per room.
There also seems be a wide spread wrong assumption that oversizing doesn't matter because "the unit is modulating". But, often the oversizing is so bad that even the min modulation is oversized for the peak load.
A better approach is to shoot for 100% sizing (no oversize), with a provision to make up any difference such as heated floors in the bathrooms. The multisplits often don't have as large of turn down ratio (as the 1:1 splits) so you want to make sure that the range aligns with your actual load profile (ie. how warm can it be outside before you start to get cycling).
https://mylinkdrive.com/viewPdf?srcUrl=http://enter.mehvac.com.s3.amazonaws.com/DAMRoot/Original/10006\M_MXZ_Series_Multi-zone_Indoor_Outdoor_Combination_Table-en.pdf
This fuel use calculation is likely to be much more accurate than any manual J done at zero cost by a bidding HVAC contractor.
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Is this an apples to apples bid in that the non-hyper heat unit is 30% larger so it would produce the same number of BTUs at the design temp?
Walta
I'm building a passive solar home in Buena Vista, CO and when I had an independent analyst do the numbers and design the system, Hyper Heat was strongly recommended. I could chance it and probably would not need it all that often but I also wanted to "future proof" my home for more severe weather being caused by climate change. So I spent up for hyper heat. It probably cost me $5k USD total across the three compressors.
Bill - How did you find/ choose your analyst? I'm in the process of doing an addition + gut rehab and considering doing this. I have reached out to Energy vanguard which has been recommend on the forums. I'm not sure if I will be able to afford the advice but looking to have more expertise around the system design than just the HVAC company salespeople.
Since your actual fuel-use heat load is 19k BTU, a 24k or 30k non hyper-heat is perfect. You don't need hyperheat and it will have lower efficiency anyways. The real problem you'll be facing is not having enough indoor heads/zones for your typical NA house. Many installers suggest 4 units for your 4 bedrooms plus 2 on opposite sides of the first floor which is 6 branches, requiring a 48k+ system or multiple systems. A single concealed duct to cover all bedrooms would be ideal but ductwork labor is $$$.
What I'm thinking now would be an air handler upstairs to cover the three bedrooms as suggested by several people. I talked to one contractor who thought it would be ballpark the same cost or even slightly cheaper to do that than installing a head in each bedroom anyway, unless I want the controllable airwave dampers which would push it a bit higher. Would also be able to reduce the size of heat pump required as well if I'm not doing a min 6k head in each room. Then a single 18k head on the main floor, and then two single-split 6k heads in the basement bedroom and living area. So would be:
SVZ-KP12NA / SUZ-KA12NAH2 upstairs for 3 bedrooms & 2 bathrooms ~800 sqft total. Min BTU output of 4700.
MSZ-FH18NA / MUZ-FH18NAH2 on main floor, ~1000 sqft. Min BTU 5150.
2x MSZ-FH06NA / MUZ-FH06NAH in basement. Min BTU of 1600 each.
Total min BTU of 13,050. I could do the same setup but on a single 36k multi-split, which is rated at 12,400 min BTU. I'm thinking the redundancy of multiple units plus better individual efficiency would be the way to go? I'm hoping this would make for a reasonably efficient system with my ~19k heat load. FYI I recently had an energy evaluation done, although I don't have the report yet. Guy said the blower door test showed a ballpark ACH50 of 7, which I should be able to cut to between 3-4 after closing up the multiple large vents in the basement as well as some other identified airgaps.
The other positive of having separate units is that you can modulate down further by turning some off - you may find the main floor unit can heat both floors.
Excellent to hear you got an energy evaluation, please post results if possible, you have a good project at hand that is likely to assist others.
As to heat load, if you are at 19k heat load with ACH50 of 7, cutting ACH50 to 3 to 4 will reduce that considerably. https://www.mwalliance.org/sites/default/files/MEEA_Understanding%20Manual%20J.pdf Look at slide 50: Impact of House leakage from 3 ACH50 to 15 ACH50. Going from ACH50 of 10 to 3 dropped heat load from 37k to 26k. Going from ACH50 of 5 to 3, dropped it from 30k to 26k. Perhaps you'll go to 15k or so if you hit ACH50 of 3 to 4.
The units/systems specified appear oversized for 19k heat load, they'll be even more oversized once you close up the low hanging fruit of the leaks. Big ones are easy to find and make a big difference.
Personally, I would not want four outdoor units humming/buzzing/vibrating and making ice piles in winter.
I'll post the results as soon as I get them. I guess at this point I'm struggling with how to lower the minimum capacity while still heating/cooling the whole house on a budget I can afford. Ducting the whole house is not practical for me at the moment.
The 12k air handler is the smallest unit Mitsubishi lists, so I can't shrink that. I could reduce the main floor to MUZ-FH12NAH unit, which lowers the min BTU from 5150 to 3700. I could also got with a single 6k or 9k unit for the basement, and either tell the tenant that they have to keep their bedroom door open all the time, or else look at a backup electric heater for the bedroom. This would reduce the min BTU by another 1600 and would cut the outdoor units from 4 to 3. These two things would lower the min output BTU to 10,000. I don't really see how I could go any lower with mini-splits, although I've only looked at Mitsubishi and Fujitsu specs.
Just as a data point, I have the same size house in essentially the same climate as Vancouver. Early '80s construction, some air sealing work done, and upgraded attic insulation (about R55), but still the original windows and ~R11 walls. And not a lot of solar gain in the winter because there are lots of coniferous trees around the house.
I heat my house almost solely with an 18K Fujitsu ducted mini split, which can output about 20-22K in the coldest temperatures the area normally sees. I have another unit for upstairs (a 12K unit), but I think I only turned it on twice all winter on the absolute coldest days when the main floor's unit was defrosting a lot (I mainly only use the upstairs unit for cooling). So that is to say, 72K is insanely oversized for a house that size in this climate, so I think your 19K estimate is probably fairly accurate.
As for hyperheat, I don't think it's necessary in this climate. My units don't even have basepan heaters and I've never had an issue. That said, I did retrofit one on the main unit this last year because I was going to be away for part of the winter, but it was only powered on during that time.