Multizone heat pump design
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Hi all,
I am in the process of working through the HVAC design on a new home that I am building in Massachusetts. We are utilizing a Zehnder ComfoAir 350 erv and Fujitsu heat pumps. Currently my hvac contractor has 2 36k outdoor units with 8 heads inside of various sizes. After reading the recent article here on GBA I dug a bit deeper into this and suggested to my hvac contractor that we utilize multiple outdoor condenser units and decrease the size of many of the indoor heads (smallest available are 7k btu units)
I think the pro’s would be:
– in event of unit failure other units are still available
-smaller indoor units may remove humidity more effectively via the need for them to run more often
-avoid over-cooling and unwanted condensation in rooms where no cooling is called for but refrigerant still moves through heads.
-easier to individually control zones
What are the cons to designing the system with four or five smaller outdoor condensers as opposed to two large multi-zone units?
I’ve attached a layout of what I am going to suggest to my HVAC contractor. Many of the details shown on this drawing have changed but the floor plan remains the same.
We are utilizing what I hope to be a “pretty good” method with continuous zip r6, liquid flashed penetrations and rough openings, and rockwool batts in exterior walls with r49 cellulose in the attic. Acoustical sealant on sill plate mating surfaces and anywhere else it made sense.
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Replies
I would strongly suggest you run a room-by-room heat load calculation before you go too far down the path of selecting equipment. My guess is you’ll find that the loads are well below the turn-down ratios of multi zone units and perhaps even below that of 1 to 1 units.
Given this is a new build, it may be a good candidate for an air-to-water heat pump system. Spacepak, Nordic, and Arctic all make some nice equipment that can be paired with indoor wall fan coils, baseboard hydronics, air handlers, or radiant heat.
Also, Massachusetts’ Mass Save program may offer some incentives in this space too. I’m not sure how it applies to new construction, but it’s worth asking them about.
I tried to find the information on turndown ratio for the smaller 7k units but wasn’t able to find that posted, even in the product specifications manual online.
The MassSave program has some incredible incentives and my HVAC contractor has put in for all that we can, if we can get our blower door down to a 1.5 Ach50 or less the incentives come in at $15,000 and even more.
My wife is still skeptical on using heat pump technology for our hot water and as the sole source of heat in our home and the ability to manage temperatures in individual spaces is a trade off I think I am going to have to make to make use of heat pumps as opposed to some other traditional method of heating
Unit you have lived in it, you can't appreciate how comfortable a well sealed and insulated house is. There are no drafts, no cold spots, the temperature is even everywhere.
A well designed and installed modulating heat pump just makes this even better.
A properly sized unit will will be running and supplying some amount of heat all the time, which means you never have the blast of hot air followed by long waits until the thermostat calls for heat again you get with a typical oversized gas furnace. A cold climate hyper heat unit will easily heat your place the coldest of days well bellow 0F.
Make sure to figure out what you actual heat loss is, size the unit accordingly and make sure someone competent designs your ducting layout.
72k of heat pump in a 2400sqft PGH house with basement is probably about 3x oversized.
I think the 7k unit is only for multi splits.
We did a remodel and used 4 Fujitsu one to ones. 3 are slim ducted and 1 is wall mount ductless. For the ducted, one 12k supplies 3 bedrooms, 2 baths, foyer, and living room. One 9k supplies 2 bedrooms, one bath, and living room all in the walk out basement. An 18k supplies primary bedroom, bath, and loft office (lots of glass and outside walls). Finally, we have a 9k ductless that does the kitchen/dining area which is above the garage. We actually ran part of the winter without the ductless due to backordered parts. Kitchen was a little cool but everyone was happy in their bedrooms. Overall system works great and is fairly balanced. It was a challenge figuring out where to mount some of the outside units to keep the freon line runs under 66'.
That is another consideration for multiple smaller outdoor units for us. We can place them opposite ends of house to keep our refrigerant runs shorter. The house is a single story ranch 70'x36'. We have a large wall of window/door in the great room with a cathedral ceiling and currently is spec'd for one 12k floor mount adjacent to the windows to blow across where there would be any draft from them. I should attach a floor plan with my proposed HVAC plan for reference.
I would definitely consider using multiple 1:1 single zone units vs 2 multi zone units. It will give you such a higher degree of control and margin for error during shoulder seasons.
There are far to many reports by users here with Multi-split units that suffer both performance and comfort issues as combination of gross over sizing and the multi-split's being significantly less adaptable to temperature swings.
Also I believe you are oversized, I'm running 21k (one 1:1 12k unit one 1:1 9k) of capacity in a 1,100 sqft 1955 ranch with a lot of glass, climate zone 6 NY, r-11 in the wall, r-25-r-30ish in the ceiling and uninsulated basement. Definitely not tight.
I would think your house being new build at 2500sqft, Mass. Climate, a reasonable guess would be around 50k BTU with an oversize margin of error, not the 72k they are proposing.
Are you an doors open or closed doors kind of house?
unfortunately with two girls and my wife I can assume closed door.
I've attached a quick pdf layout of what my suggestion to the HVAC contractor will be.
Is this a slab on grade? I would highly recommend dropping your 9 ft hallway ceiling and running ductwork there to hit all of the bedrooms with a ducted mini split. You could steal some of the coat closet for the air handler or put in the drop ceiling itself. For the great room/dining room/ kitchen you could go with one ductless head or put another ducted air handler in a closet in the corner of the great room to serve these areas. Ductless mini splits are not a good option for the bedrooms.
House is a full 2400 sq/ft walkout basement
Oh then I would for sure go with a ducted heat pump with the duct work in the basement ceiling.
Both options are subpar for a new build, try to use ductwork as much as you can! So much cheaper to duct rooms than put a ~$5k ductless head in each one. Plus then you’re on the hook for replacing each ductless minisplit when the time comes, instead of 1 unit. Also you can put one real filter in a centralized location instead of 8 ineffective ones.
We went through a similar thought process, starting with a ductless mini split in every room, which quickly got panned by the gBA community.
After hiring an HVAC designer, we finally ended up with a zoned ducted air handler for most of the house, with an additional ducted mini-split for guest bedrooms and another for the in-law suite. Our house will be slab on grade with not attic, so ducting will be routed in hallway soffits.
Charly, I'll second evantful's thoughts that the proposal to install 72K worth of heating capacity is oversized, but I'd say grossly so. For a new 2400 sqft house, assuming it does get built to be solidly in the "Pretty Good House" class, I would expect a design heat load of less than 30KBTU/hr. I recommend getting a really good heat loss calc done by someone other than the HVAC contractor, and who will take the time to use ALL the information available (dimensions, sizes and U values of all windows and doors, insulation levels, etc). Short cut calcs using rules of thumb and unfounded assumptions typically will give you an unrealistically high heat loss result.
For comparison, my PGH-class house in central NH, CZ6, is two levels on a 2000 sqft footprint, not counting the attached garage. The heat pump is just a two-ton unit, and it keeps the house at temperature in just first stage at design minimum temperature, which equates to around 19KBTU/HR. The area distributor of the equipment and two other HVAC installers all wanted to put in a five-ton unit, based on quicky calculations that ignored all the information I had provided. I relied on my own spreadsheet calculations and specified the two-ton unit; I was right.
I'll agree too, 72 kBTU is likely way oversized.
I'm in CZ6 with a design temperature of -1F and have an OK house (not a PGH) of 2,100+ sq ft and maintain 68F set points at subzero temperatures with a 28.6 kBTU Mitsubishi hyper-heat with three ducted heads (noting ducted heads sap efficiency). Today/tomorrow we'll have subzero with winds, I don't expect any issues.
I can't imagine a 2,400 sq ft PGH in MA (where I lived for 6 years, evidently all CZ5) having a design load of 72 kBTU. A guesstimate for a true 2400 sq ft PGH in MA would be more like 24 kBTU or less (unless ventilation is cranked up really high).
Oversizing of ASHPs seems to be a common issue on the GBA Q&A forum.
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To assist with sizing of equipment, here's what a Mitsubishi hyper-heat rated at 28.6 kBTU does at -6 F with wind in CZ6, 2,100+ sq ft (not PGH) with three ducted SEZ 15k, 12k and 9k.
The thermostat at 65 F is in a bedroom where the 9k also feeds 2 bathrooms and a utility/laundry room. While the bedroom is at 65 F, the other rooms fed by the 9k are at 70 F or above (I need to tweak the vents slightly).