Heat pump design and sizing feedback for new build in Maine (with Manual J)
Hi all,
I’m having a forever home built in the woods in the Portland, Maine area. Some stats:
- Climate zone 6
- 2 stories plus a semi-finished walkout basement
- 1500 sqft for each of the main floors
- 10ft ceiling in the first floor, 9 ft in the second
- ICF for basement foundation
- Triple-pane windows
- ZC wood fireplace in the great room for backup heat
Our builder’s HVAC engineer’s proposal (for just the two main floors so far):
- 6 ton, ducted system
- 4 Mitsubishi SUZ units (24, 18, 18, 12)
- 2 units for each floor
Based on all I’ve read, 6 tons seems like a lot, especially since that doesn’t even factor in proposed mini-splits for the basement and garage.
They put together a Manual J (attached), apparently based on a simplified “Tight” house. Given the Manual J numbers and the Mitsubishi stats, it seems potentially oversized though not much. I can’t tell if the Manual J itself is a bit juiced, with a design temp of 0 degrees.
I’m getting a second quote/design to be safe, but I’m brand new to all this and would love some other opinions.
What do you all think about the sizing and the suggested layout of 2 units per floor?
Also, is it typical to size now (during ground breaking) based on a simplified Manual J and circle back for the actual sizes when more of the building is done, or should they be doing a more specific Manual J now?
Thanks!
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Replies
The ManJ is spread out because of all the zones so hard to see what is exactly going on. If you have the summary page showing the loads for each (ie structure, windows, infiltration etc). would help.
Overall, you are overzoned, that is a lot of different units. I do see some rooms have much higher cooling loads, so some control there might be in order, you can usually get that by adding in a zone damper that doesn't close all the way with a local thermostat. The idea is to over-provision the zone to meet cooling airflow and adjust the closed position to meet heating airflow. The local thermostat now just opens/closes that damper to modulate the local airflow without effecting much of the rest of the house.
Mitsubishi units deliver about their nameplate capacity at 0F, so 6 tons works out to 24btu/sqf which is very high for new construction.
Unless a walkout, basement loads tend to be so small that another unit is not worth the cost. Add a couple of registers from the main floor unit to feed the basement.
Also if the ducting is inside conditioned space (as they should be for any new build), there are no duct loses.
Thank you for the thoughtful reply! I don't have a good summary, I'm afraid. What you see is what I have. I should ask for one, though, it makes sense. Bottomline, a 2 ton unit for the first floor, and a decimated 1 ton unit for the dining room (some details below). Then 1.5 tons for the bedrooms on the second floor, and 1.5 tons for everything else there.
I'm also wondering about the number of different zones. I'd asked in vague terms about multiple zones on a single unit using dampers, but the sense I got was that we'd lose some real efficiency. I worry that I didn't/don't know enough to get more specific with my question.
One that stands out to me is the 1 ton unit for the dining room. It's our splurge area, with glazing all around: windows to the south, bifold door to the east, and sliding door to the west. All triple pane, but still. It also has a cathedral ceiling. I get that it will need more heating/cooling than the great room, but there's no door between the two spaces, just a transom. It feels off, but it's where we'll be eating all our meals and I don't want it to be uncomfortable.
As I think about your damper idea for that space at least, the main duct work would be serving the great room first. So a damper would let us reduce the heating/cooling compared to the base amount serving the great room, but not really boost above base. Unless I'm not picturing it correctly.
Yes, it will be a walkout basement downstairs, with a TV room and art space. I still like thinking about the registers from the main floor, especially if we can work the damper controls. The space should always need the same type of conditioning as above, just less of it.
Lastly, yes the ducting will all be inside conditioned space. Does that mean you don't think those "Ducts (R-6.0)" numbers should be included in the calculations? Something for me to ask about, at least.
If you do have an all glass room, a dedicated unit for that space is a good idea even with the extra cost, there is no easy way to zone a central air handler to properly handle such a load.
I would still try to reduce the rest of the zones. Maybe one unit for the rest of the main floor/basement and one unit for the 2nd floor.
Summary is helpful to see where your losses are. Sometimes improving air tightness or assembly R value can significantly cut your heat loss.
"don't think those "Ducts (R-6.0)" numbers. Correct. With ducts inside conditioned space, there is no heat loss as all the heat stays within the house envelope. There is even no need to insulate ducting. I'm in the north edge of zone 5 with hot and muggy summers and all interior ducting is bare hard pipe.
That's very interesting about the ducts. I had a follow-up with the contractor and asked him about it, specifically asking if they were taking into account ducts in conditioned spaces. He said they were, but it feels off, especially that 25% load attributed to the bedroom ducts.
He attributed it to the energy used to heat the ducts, comparing it to a ductless unit that can directly condition the target room. Could it be that insulating the ducts does mean that they don't effectively condition the target room as they lose heat, even though the heat is still in the envelope of the house? Or is the guy just wrong?
"Also, is it typical to size now (during ground breaking) based on a simplified Manual J and circle back for the actual sizes when more of the building is done, or should they be doing a more specific Manual J now?"
I'll answer the last part first: you should have the whole house planned out before breaking ground. It's not going to be easier when the house is further along. Once construction starts there will be pressure to keep everyone working or they'll go off to other jobs.
Almost on a daily basis here we get people who are at the stage in construction where it's time to install HVAC and they wish they had planned it all out before framing.
The Mitsubishi heat pumps selected have a nominal size of 2 tons, 1.5 and 1.5, total
sevenfive tons. But if you look at the spec sheets, at -13F they're rated for 20,000 BTU/hr, 17,200 and 17,200. So a total of 54,400. That's at -13F, your design temp is 0F, so you can add a couple kBTU to each.I get 21K on the first floor, 17K on the second and 14.5K for the basement, total of 52.5K. So I don't think you're oversized.
Spec sheets are:
https://ashp.neep.org/#!/product/34581/7/25000/95/7500/0///0
https://ashp.neep.org/#!/product/31992/7/25000/95/7500/0///0
I'm a bit confused on the math there.
There are actually 4 Mitsubishi units suggested: 2 ton for the main first floor, 1 for the dining room. Then upstairs, two 1.5 ton units. So nominally all that would be 6 tons, right?
Here's the spec for the 1 ton model, I believe: https://ashp.neep.org/#!/product/31991/7/25000/95/7500/0///0
When it comes to design temp, the 99% mark for Portland is actually 5F. Now we're a bit inland from the weather station at the jetport, but still. Especially with the wood stove as backup, it feels like 99% is fine to aim for, but open to feedback there.
Given 5F, the heating specs are then:
13800 + 25000 =38,800 for the first floor
21600 + 21600 =43,200 for the second floor
I don't have the details yet, but the engineer was also recommending a dedicated multi-split unit (two heads) for the walkout basement. So these numbers just cover the main two floors.
OK, I missed the other one-ton. And my math is off, I corrected it. Now I do think you're oversized.
On the NEEP.org page you can click on "Advanced Data -- System Sizing" and put in your heating load and zip code and it will show how that heat pump will perform on your house.
I'll take a look. Thanks!
For what it's worth, Portland's 99% heating design temp is 5.2°F. Its 99.6% temp is 0.5°F. It does go down to -10°F occasionally but rarely lower.
How would you size for the area, Michael? Do you aim for the 99.6% to play it safe, or stick with the 99% (or something else?).
The contractor was very much in the play it safe category. I've asked for 5F numbers, too, to understand the impact.
If you take some time and read the posts from people with mini split problems you will find almost universally the don’t have mini split systems, they have multi spit systems with more than one head connected to the same outdoor unit. The other thing they have in common is the system is massively oversized.
In my opinion a manual J calculation is worth what one pays for it! If a contractor does it for free you can bet, they did not but much effort into it. When doing the calculation, they enter a ton of variable numbers. The formula tends to oversize a little by design if the contractor puts his thumb on the scale and rounds up the numbers you can be hugely oversized in the end.
I think it is a mistake to go with a system with so many heads. Have a look at the concealed ducted systems that might fit in a dropped ceiling of a hallway and feed all the bedrooms. If you have a basement or crawlspace a full ducted system with the same efficient outdoor unit makes for a much better system with fewer field joints in the refrigeration tubes, better air filters and you can put a duct in every room.
Walta
Hi Walta,
Thanks for the reply!
The proposed design (absent a basement suggestion not listed) is actually all ducted 1:1 heat pump systems, one head to one outside unit. I believe the suggestion for more individual units, vs say one unit for the upstairs and one downstairs, is to address the different heating and cooling needs of the spaces. Otherwise, the engineer was a strong proponent of 1:1 systems and was dissuading me from multi splits. That was a good sign, I thought, though I'm still worried about that thumb on the scale as you say...
Are your ducts in unconditioned space? It looks like you have duct losses in every zone.
It also looks like you have ~200 cfm of ventilation, seems high?
Also looks like you have a fireplace in each zone?
How big is your “play area” to have a ~10,000 btu/hr heat load?
So the ducts will be in conditioned spaces: basement for the 1st floor, insulated attic for the 2nd floor. Talked to the contractor and he said the duct loss was due to just the air passing through the ducts. He compared it to a ductless system where you don't spend any energy hitting up the metal of the ducts themselves. It still seems like a lot to me, especially the bedroom ducts at 25% of the load, so I wonder if they didn't have that calculation right.
Only one fireplace, in the great room. I wonder if that line about fireplaces impacts each zone. That's a good call out that I'd missed.
The "play room" is one of our splurges, both from a cost and efficiency perspective. Above it is an accessible cupola for views of the area and the stars. It's also above the garage. So that combination could totally mess with the load requirements, I can imagine.
He is wrong. Line items that call out “Ducts R6” are calculating heat loss of conditioned air through ductwork with R6 insulation in an unconditioned space. If he was doing a Manual D and calculating flow losses (CFM) I could see his point. This explanation makes me question whether he knows what he is doing.
I would also confirm your ventilation rate, I assume an ERV? Upwards of 200 cfm seems high.
I spot checked a few things as well and it feels a bit oversized, and over zoned. Not sure why the dining room needs a whole dedicated air handler, for instance.
Manual J as a method has a lot of safety factors built into it - it isn't needed or recommended to be extra conservative while running the calculation, because you end up doubling up on all your safety factors. You can get away with it using forced air heating because it will just run less often (but might compromise comfort), but heat pumps are a bit more sensitive to precise sizing.
I'd ask that they re-run using the 5F 99% design temp, move the ducts into the conditioned space (meaning they won't have any heat loss), and take a look at the construction loads to make sure they reflect your better-than-average house design, assuming that if you are doing triple pane glass and ICFs you are probably also doing well on your insulation and air sealing too.
From there, I'd see if you can merge the 1st floor and 2nd floor onto one air handler each.
Thanks for taking a look!
The dining room conditioning plan is changing. The issue is that it's surrounded by glazing: bifold doors to a screened in porch to the east, lots of windows to the south, sliding glass doors to the west. The goal (according to the engineer) was to keep it comfortable even in the winter. That said, they couldn't figure out a good way to set it up efficiently, so are just going to double up the vents to make sure there's plenty of conditioned air getting right to that space.
The contractor straight said he was being conservative with the numbers, to make sure we weren't cold which I didn't love. I did ask him to rerun the numbers taking into account the 99% design temp as well as confirming things like conditioned basement and possibly attic, if they'd missed that. Most importantly, I asked for all the specific inputs this time.
We're also going to look at a single air handler for each floor. I will say, given the challenges of that playroom space (garage below, cupola above), I can see some benefits to having that on its own, correctly sized handler, but we'll see how the new numbers play out.
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