Questioning Heat Pump size in our just completed HVAC Design
Hello and thanks in advance for considering this question. We have just received our HVAC Design plan for our 2900 sq. ft. 1.5 storey home to be built on the shore of Georgian Bay west of Collingwood, Ontario. We are in CZ6. The plan calls for a fully ducted system with two separate heat pumps, a 2 ton on the main floor and a 1.5 ton on the upper floor. There will be a conditioned crawlspace below most of the building (the remainder will be slab on grade). The crawlspace will be heated by the 2 ton main floor unit There will be a conventional wood burning fireplace which could take the chill off on the coldest days, but the heat pumps need to meet code without taking into account that potential auxiliary heat. There will also be an ERV and a Makeup Air unit.
The design was originally spec’d for R22 (nominal) walls (Mineral Wool batts between 2×6 studs on 16″ centres). (Not sure if we can afford better than double-pane windows.) Then the heat loss calc was updated to reflect the addition of R10 ComfortBoard 80 on the exterior and a few other small details. This dropped our total heat loss numbers from 62,536 BTU/H to 55,508 BTU/H.
I was hoping that the addition of the ComfortBoard 80 might allow for smaller heat pumps. If I understand correctly, it is important to undersize rather than oversize heat pumps to avoid the inefficiency and reduced comfort that come with frequent cycling. IOW, in relatively cold weather, the heat pump should be working almost constantly, not intermittently. The HVAC designer has kept the same Mitsubishi units and made this comment relative to the addition of ComfortBoard 80:
“The equipment remained the same, although the heat loss and heat gain values went down (more so the heat loss). The good thing is that the equipment won’t run at full capacity on the coldest day (-25 degrees Celsius) and in turn will use less power on those days.”
Maybe he means the electric resistance component won’t be required as much on the coldest days, so it will be less costly to run. I’ll try to get more info from him on that.
I am attaching a chart showing the recommended heat pump details. Please chime in with your thoughts on his choice of equipment and his comment above. Thanks very much.
Henry Rose
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Replies
Those aren’t 1.5 or 2 ton heat pumps. Together, they output 64,000 Btu at -25C, so they’re slightly oversized but not egregiously. I think you’re fine. You could go smaller if you want, you certainly have enough resistance heat.
Let me start by saying I am not a fan of the half story houses. It seems unlikely you will reconsider at this point but think about going to a full 2 story. Ok so that not happing if you must forge ahead make a detailed plan for your air barrier with an unbroken red line between the conditioned spaces and the unconditioned vented attic spaces. This can get very complicated where the second-floor ends and the first-floor ceiling continues draw the details in advance for the air barriers that will fill each joist space.
You did not say it this will be a ducted system? Will all the ductwork and equipment be inside the conditioned spaces you drew?
I see no need for 2 systems with a well thought out duct plan one system in the crawlspace should do a fine job assuming you are willing to adjust the dampers when you change from heating to cooling.
I don’t often say this but being in Canda I think triple pain windows are a must.
I say do not be afraid of the resistance back up heat. When the HVAC contractor say your heat pump is too small let him add more resistance just be certain the thermostat will allow you to lock out the resistance based on the outdoor temp. My heat pump has the resistance locked out until it is bellow 6°F.
Note in my opinion any spray foam and HVAC equipment located in an attic are red flags in new constriction plans for laziness, poor planning and total disregard for your budget.
Walta
Thanks Walta,
We have to go 1.5 storey because we are rebuilding due to fire and were/will be too close to the water (we’re closer than 15 metres from the 100 year flood line). We can replace what we had, but can’t increase height to 2 storey. I will follow your red line advice.
It is fully ducted. I have specified from the beginning “no equipment or ducting in unconditioned space”. We are looking into triple-pane windows, at least for the north wall.
I know it can feel like the north pole up there but the design temp in Collingwood is not -25C, I would check what the OBC actually calls for. Sometimes it can get colder, that is what the wood stove or the backup strip heater is for. There is no need to oversized equipment.
If you look at the PUZ data book here:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_PUZ-HA-NHA1_NKA1_TECHNICAL_DATA_BOOK_OCD750B.pdf
The PUZ-A42 delivers 42000BTU at -18C and the PUZ-A30 29000BTU, so 71000BTU total, way more than your load is at.
I would also question 58k load for a 2900 sqft new build with exterior insulation. 1/2 story places are hard to air seal, unless you have huge amounts of glazing you should be under 40k load.
You also have 15kW-8kW of backup heat, so 78000BTU. I can't see ever needing something that big even if your actual heat loss is 55000BTU. These units don't shut off when cold, they continue to put out heat at reduced rate. No need to for all that extra electric heat and the service to support it. Size the backup heat to keep your place from freezing in case all the heat pumps fail. This should be more than enough additional heat for those polar vortex days.
I recently got quoted triple pane windows for about the same cost as double pane after rebates, I would double check with your supplier.
Sometimes an option is to try to reduce the number and size of operable windows and keep those a double pane and get all the fixed ones triple. You have to play with coatings here a bit as the triple panes are always darker using the same low-e coatings.
akos, can u share the make/model of triple-pane windows u were quoted that were similar to doubles after rebates?
Thanks Akos,
The Ministry of Municipalities and Housing 2014 chart has Collingwood’s January design temp as -21C 2.5% and -23C 1%.
I like your suggestion to: “Size the backup heat to keep your place from freezing in case all the heat pumps fail. This should be more than enough additional heat for those polar vortex days.“
Going by the -23C, the output at that temperature for the A42 is 39k and the A30 is 27k, so 66k total.
Since you are adding strip heat, you can size for your actual load so you could drop down to the M (SVZ-KP30NA / SUZ-KA30NAHZ and SVZ-KP36NA SUZ-KA36NAHZ). These are each good for close to 30k output at -25 which should just about cover your losses. The M series are cheaper than the semi commercial P series.
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M-Series_Technical%20Data%20Book_01-2021.pdf
Extended temperature data is around P684 for the M series.
I would still question the heat loss estimate from your HVAC designer. If you have the full report, you can post it here and we can see if there is something obviously off on it.
Your engineer does have a valid point about better efficiency from an oversized unit but you have to watch mininum range. Generally the larger units will have higher minium output which can cause a lot of cycling for most of the heating system. If a right sized and and oversized unit have similar min output, the oversized unit will cost less to run. A good option is to try the simple sizing tool here:
https://ashp.neep.org/#!/product/34536/7/25000///0
Click on the "Advanced Data - Sizing for Heating" box and fill in your heat loss and design temperature. It will show how well the system runs through the whole heating and cooling season.
You can search the site for the other units as well and see.
Per OBC SB-1, Table 2 Collingwood January design temperature is -21C because
OBC: Section 9.33.3.2 says that "The Outdoor Conditions to be used in designing heating, ventilating, and air conditioning system shall be the appropriate values for the location as set out in the MMAH supplementary standard SB-1 Climactic and Seismic Data" using the 2.5 percent design temperature criteria."
Having 3 engineers calculate heat loss for me recently (plus helpful people on this site helped me do this using https://hvac.betterbuiltnw.com/), I think calculating it yourself using https://hvac.betterbuiltnw.com/ would help you see where there may be differences/issues.
But if you have a variable-capacity heat pump, oversizing is not as big of a problem as with a fixed capacity, and indeed on Google Scholar there are articles saying efficiency is improved for variable capacity heat pumps that are slightly oversized.
(Yes, I read all I could find on this). I still do not want a hugely oversized heat pump, but approximately right is good enough.
orange_cat,
Thanks for your reply and perspective. I hope to look into the better built site next.
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Akos,
Thanks again and sorry to be so long in replying. I was away on a camping holiday when your last comment was posted and this is my first chance to pick up the thread. I have been comparing Mitsu models on the NEEP site that you linked. I have found replacements for the two spec'd units which have output and COP ratings down to -13F vs. 5F for the original units. My intuition tells me these would be preferable since my design temp is -6F. They have lower minimum outputs which, as you suggest, should limit frequent cycling and they generally have higher COP #'s.
Looking at PUZ-HA36NKA* instead of PUZ-HA42NKA1 and PUZ-HA24NHA* instead of PUZ-HA30NKA. Any thoughts on suitability, initial and running costs and reliability? BTW, the M Series: SUZ-KA30NAHZ you suggested also looks good, but I was unable to find the SUZ-KA36NAHZ model.
While I can filter models by Type, Ducting and Brand, I can't run the Advanced Data - Sizing for Heating function because I don't have a US Zip Code and a Weather Station. Is there a way to find a Zip code and Weather Station that roughly match the Collingwood, ON Weather Station with a design temp of -6F?
Still hoping that we can afford triple-pane windows on the north (Georgian Bay) side of the house which should reduce our heat-loss further. As you requested, I am posting the entire Heat Loss Calc PDF. Thanks again for all your help.
Henry
It is not a standard Wrightsof output so not the easiest to quickly browse through. Without a summary page of where the bulk losses are, hard to nail down if the inputs are correct.
Mostly it looks to be in the ballpark, if I'm reading it correctly, the infiltration numbers are on the high side, looks like they are using 3.5ACH. If you are building something with 2.5" of exterior rigid, I would assume there is enough will and budget to get under 1ACH. Probably knock a ton off the house heat loss.
Even with that you are still close to 16BTU per sqft which feels on the high side but could be correct with a lot of glazing and tall walls.
You can search through the Man J outdoor design database and find a US ZIP that matches your location:
https://farm-energy.extension.org/wp-content/uploads/2019/04/7.-Outdoor_Design_Conditions_508.pdf
Some of the Mitsubishi combinations are not in the NEEP database, you can use the Trane/Mitsu rebaged unit:
https://ashp.neep.org/#!/product/33559/7/25000///0
Mitsubishi provides COP and output numbers for all their product lines including all the PUZ-HAxxNKA models on their my-link drive. They usually list them down to -16F to -20F for any of the hyper heat units:
https://mylinkdrive.com/USA/M_Series/R410A_Systems-1/Outdoor_Equipment/R410A_Outdoor
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