Seeking second opinion on HVAC sizing and specs
Hey all. We’re doing a gut renovation of an old victorian in the Bay Area. Climate zone 3. Insulating and air sealing to “pretty good” standards. We have three floors – a basement inlaw, a main living floor, and attic living space. Our square footages and load calcs are:
Basement: 1638 sqft, heating 8,419 btu, cooling 12055 btu
Main floor: 1618 sqft, heating 10,479 btu, cooling 15,985 btu
Attic: 1041 sqft, heating 9,757 btu, cooling 11,795 btu
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Totals: 4297 sqft, heating 28,655 btu, cooling 39,835 btu
Our goals are efficiency and comfort. We’d like to avoid oversizing the system and the resulting short-cycling.
We are required by code to meet or exceed heating loads, but there’s no requirement to meet cooling loads. Our summers are mild and cooling is a “nice to have”. Our decision will be made primarily on heating loads.
Our mechanical engineer spec’d a Samsung VRF 3-zone multi-split:
Compressor: AM060MXMDCH/AA (66k BTU heating)
Basement AH: AM009TNVDCH/AA (10.5k BTU heating)
Main floor AH: AM024TNZDCH/AA (27k BTU heating)
Attic AH: AM024TNZDCH/AA (27k BTU heating)
We shopped the design to a contractor who installs Mitsubishi. He broke the basement off into its own 1-1 system:
Basement compressor: MUZ-GL15NA-U1 (18k BTU heating)
Basement AH: MSZ-GL15NA-U1 (18k BTU heating)
And 2-zone multi-split upstairs:
Compressor: MXZ-SM36NAMHZ-U1 (42k BTU heating)
Main floor AH: SVZ-KP18NA (21.6k BTU heating)
Attic AH: SVZ-KP12NA (15k BTU heating)
My concern with both these systems is size and whether they can modulate down to shoulder season loads. I don’t like that the mechanical engineer spec’d a 60k compressor when the max heating load is 28.6k. Or the contractor’s plan for a 42k compressor for 20.2k load across two zones.
I’ve been on this site long enough to know oversizing happens all the time. I also would prefer to avoid multi-splits. I’m not an expert but I wonder if the following system is more aligned with our needs and goals:
Basement compressor: MUZ-GL15NA-U1 (18k BTU heating)
Basement AH: MSZ-GL15NA-U1 (18k BTU heating)
Main floor compressor: Mitsubishi SUZ-KA12NA2 (13k BTU heating)
Main floor AH: Mitsubishi SVZ-KP12NA (15k BTU heating)
Attic compressor: Mitsubishi SUZ-KA12NA2 (13k BTU heating)
Attic floor AH: Mitsubishi SVZ-KP12NA (15k BTU heating)
When I pushed back on sizing, my mechanical engineer said the nominal tonnage of these units is de-rated in my region. He said because of the dry air in the bay area that I should factor in a de-rate of .6 – .65 output. Is that true? I’ve heard of de-rating for high elevation, but we’re at sea level.
Any thoughts or suggestions are welcome! Thanks to anyone who has read this far.
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Replies
If you read the post on this forum from unhappy mini split owners you will find two things, they all have in common.
1 the equipment is hugely oversized and that they all have multiple heads connected to a compressor.
Since it is a gut job make some room for ductwork and get a ducted mini split.
Look at the concealed ducted systems that could go in a dropped ceiling of a hallway.
Consider paying someone not bidding your job to do the manual J.
Walta
"He said because of the dry air in the bay area that I should factor in a de-rate of .6 – .65 output."
That's wild.
Yup, same though.
The ratings are done at 47F 70%rh, it might get dryer there in the winter time but not that dry.
The SVZ are the ducted units so the OP is on the right path.
For the OP. I think you are on the right track, that 2nd quote is not far off from what you want but with a right sized multi split, no reason to install 60k of capacity with a 30k heat load. The issue the smaller multi splits won't run 2 SVZ air handlers, so the attic air handler would have to be the low static SEZ-KD unit. There is also no need for a hyper heat unit in your climate especially with the cooling load being more than the heating load.
I do think the one to ones is always the better option if you have the space for the outdoor units, it also lets you use the multi position air handlers for both spaces.
I think the mechanical designer is looking to add in a safety factor so they won't be blamed if there isn't enough heat in the place. The one way to get around this hump is to install backup strip heaters. This lets you select the equipment that is right sized but gives the mechanical designer a bit of breathing room.
Thanks for the words of encouragement! I'm still puzzled by my engineer's argument that lower RH% in my area would cut my tonnage to 60% - 65% of the rated value. I haven't seen that argument made here or elsewhere and started doubting myself. I don't fully understand his point either – in my region, on our design heating days, as the temperature drops our RH rises above that 70% rated value. Looking back on the coldest days of the year, our RH% is in the 80s and low 90s.
Are all three floors open plan, or are there rooms? One unit per floor works OK with an open plan. If there are rooms, particularly bedrooms, you're going to want a source in each room. Another vote for ducted minisplit.
Thanks everyone so far for your thoughts!
A couple of you asked about ducting. Yes, the main level and attic will both be ducted.
The basement can't be ducted and so we're going for a single ductless in the main living space. I know it's not ideal, but the rooms aren't big enough to deserve their own heads. We actually live in this area today and we get by with occasionally using space heaters. After envelope improvements I think it'll be a non-issue.
Walta you wrote "Consider paying someone not bidding your job to do the manual J.". That's actually what we've done. The mechanical engineer did the load calcs and spec'd equipment. The contractor I found independently of them.