New construction ducted minisplit sizing with inexperienced HVAC subcontractor
Reading over this forum has been really helpful for realizing I’m not alone. This is my first post. Questions are at the end. If I should post something shorter or break it up, let me know.
I’m building an energy efficient house outside Boston. We started out with one architecture firm specialized in efficient buildings, but unfortunately had a falling out around communication and other aspects of home design. That led to switching architects and making some modifications to the design. We’ve also gone with a different contractor, who we like a lot, but who is not as experienced at efficient building as we could wish. And his usual HVAC subcontractor is even less experienced at it, and their initial proposal was laughably oversized for our heating and cooling load.
Three months ago, at the start of this pandemic stuff, I realized the HVAC sub was not the right sub, and tried to fire them and find a different one, or find a consultant to help with evaluation or design. Unfortunately, we have not been successful at getting a new subcontractor, and it is delaying our project. My GC is pushing to have his usual sub do the work, and do it according to our specifications. That means we need to come up with specifications.
The original specification for the house was super energy efficient. 4,200 sqft. Air sealed to 1ACH50. Walls to R-46, Attic to R-67. Foundation R-26, Windows U 0.15, SHGC 0.27.
The modified house adds 250sqft bedroom over the garage. It’s being built to 2 or 3ACH50. Walls R-30. Roof R45. Foundation R-39. Windows U 0.30, SHGC 0.18 south / 0.40 north.
We’re expecting to stick with a Mitsubishi heat pump system, with a separately ducted Zehnder ERV. The ERV originally specified was a Zehnder 350 operating at 100cfm for 0.17 air changes per hour, boosting to 200cfm (.34 ACH) when bathroom exhaust is called for.
We have four (!) analyses of the building to consider:
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The original architect proposed a system with a 45k heat/36k cool outdoor unit (MXZ-4C36NAHZ) and 3 zones of 11.4/9k, 13.5/12k, 13.5/12k for the basement, first, second floor. This is before we added an additional 250sqft of space on the second floor, and assuming better windows (U=0.15 vs. U=0.30).
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My green building consultant (who I trust the most of all these people) did an HERS rating and found the whole house load to be 40kBTU for heating and 26kBTU cooling.
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An engineering consultant did a Manual J calling for a total of 50kBTU heat, 49kBTU cooling, and designed a system with 2 outside units totaling 67k heating, and four inside units 15k, 21k, 15k, 15k heating. We think we prefer four zones for comfort, splitting the second floor in half. I think he made some errors, and it took him over a month to come back with a system, so I’m reluctant to try to have him make adjustments.
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Our original HVAC subcontractor, after being told to come back with something less ridiculous, did a Manual J that sized the whole system as 77k heating / 42k cooling.
I trust the original architect mechanical specification, but obviously we’ve changed the insulation, and it’s tricky to estimate the impact. Unfortunately the green building consultant does not want to specify or design a system.
I’m finding myself having to do the work, and have now proposed the following system:
Single exterior unit: MXZ-4C36NAHZ2 This is below the rated total of the indoor units but will work fine.
Basement: SEZ-KD12NA4R1.TH or PEAD-A12AA7
First Floor: SVZ-KP12NA
Second Floor Rear: SEZ-KD09NA4R1.TH or PEAD-A09AA7 (Master Suite and Office)
Second Floor Front: SEZ-KD09NA4R1.TH or PEAD-A09AA7 (Bed2, Family Bath, Laundry, Bed3)
Choosing between SEZ or PEAD based on whether or not they have enough static pressure for the duct runs. I prefer the SEZ unit and I think it should be possible, especially upstairs.
I’m interested in the forums thoughts on a few questions:
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If my green building consultant has a whole house heat load of 40k, how much more capacity is likely to be required for a room-by-room system?
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If two people do a Manual J sizing and are off by a factor of 2 in some rooms and 50% overall, both using Wrightsoft, is that to be expected? Or is one or both of them doing it wrong?
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Do I really want 4 zones? The original idea was to separate the north facing and south facing parts of the second floor, and allow the master suite to have it’s own thermostat. If the ERV is mixing all the air, would I be better off having just two zones? The finished walk out basement is open to the stair column and the first floor. I could put two MVZ/SVZ systems in, one for the bastement/first and one for all of the second. I could even put those in with two 1:1 outdoor units for greater efficiency, I’m told.
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Or should I actually have lots of little zones, like Mitsubishi website implies? I could do an MXZ-8C48NAHZ2 and then have four zones upstairs, two on the first floor, two in the basement, using SEZ air handlers or even ductless units in some rooms.
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Is there someone I should hire who will turn around a new system design in less than a week, or otherwise work with me to get a system designed an implemented quickly?
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Will I be ok working with this inexperienced subcontractor as long as I supervise them closely, or are they going to mess up subtle things?
Thanks for your time. Questions for clarification more than welcome.
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Replies
For unit sizing, you'll have to go through the wrighsoft printout and make sure the person didn't have their finger on the scale. Check for things like correct wall/roof/window R/U values, infiltration numbers, shading and outdoor design temperature. If these are set right, the software will provide good results.
For the actual system layout, I would stay away from multisplits as they don't module well and the indoor units have no turndown. Provided you size them correctly (9k is WAY too big for two rooms), it can work but a 1:1 will always be better.
This is even more important when heating and cooling with one since the cooling load will be significantly less than heating. Comfort, efficiency and moisture removal will suffer with an oversized multi.
I would go for one mid static ducted unit for the basement and main floor and a low/mid static for the upstairs.
If you want some additional zoning, you can do dumb zones for selected areas. What this means is to provide additional flow to a room but install a motorized damper in the duct to partially restrict the flow. The damper should be adjusted to only partially close so the flow is reduced but not shut off. A room thermostat drives the damper but because it only modulates the flow a bit, it doesn't need to tie in with the main thermostat.
Most ducted units can handle some flow difference, as long as you are not going nuts on zones and limiting zone modulation to say 80% of flow, the unit will run just fine.
100cfm for that size of house seems too little.
You also mentioned something about the ERV mixing the air in relation to heating/cooling. The amount of air an ERV moves is not enough to affect that.
I don't have anything in the way of helpful answers to add here (sorry!) - but I did want to say that I'm absolutely interested in the question, and I'll be watching this thread for answers!
I'm still in the design-phase for a new build just outside of Boston, and as California transplants, we're deathly afraid of the weather. Despite all of the great things we hear of mini/multisplits in efficient and tight homes, my wife is a bit reluctant to forgo a gas furnace.
Seeing you get this right will be great!
Architects aren't engineers. Sounds like you know more than any HVAC contractor so why not just do it yourself.
I would just hire someone like Energy Vanguard or someone they recommend to design a system and have it installed per specification.
In a well insulated house, the need for zoning is diminished. The only issue that can occur in a such a home is if a particular room has a significant amount of south or west facing glass. That can be addressed with dumb zoning as Akos mentioned. Also as Akos suggests, 1:1 ducted systems are vastly superior as they modulate keep running while meeting the load.
Do you have a floor plan you can share?
Even 40K design heat load at +8F (or whatever your town's 99% temperature bin is) seems high for the house described. A house that well insulated can usually even undersize the heat pump for the design load without comfort issues, even without strip heat backup. Did the RESNET rater show the math on the U-factor analysis of the walls?
The "head in every room" approach with an 8C48 would REDUCE comfort, since some individual room loads are surely low enough to overheat/overcool even on the bypass refrigerant running through the head when any other zone is calling for heat/coolth, even when the head is nominally "off".
A 12K ducted cassette is probably overkill for the basement, unless it's a walk-out with a lot of west facing glass slider door or window area, or unless you plan to host polka parties in the basement with a couple dozen 200lbs Czechs dancing with a keg on each shoulder (in which case it might even be undersized.)