Mitsubishi Heat Pump Setup
We in climate zone 4c and are about to have a Mitsubishi heat pump installed in a very small house with one floor and a basement. We are unsure about one of the options presented. We’ve decided to have a multi-position air handler installed in the attic with ductwork to each room and no immediate indoor unit in the basement.
The first option is Mitsubishi SVZ-KP24NA variable speed air handler paired with Hyper-Heat SUZ-KA24NAHZ2.
The second option is the SVZ-KP24NA air handler in the attic paired with MXZ-3C30NAHZ4 to allow for a possible future addition of one 6k btu indoor unit for the basement.
If we never connect an indoor unit for the basement, is the second option okay or will it be less efficient, less effective, etc?
Thanks for any input!
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I think the second option with the potential basement unit is a flexible choice. If you don't add one, it should still work well for the main floor, but may be slightly less energy-efficient compared to the first option with a dedicated indoor unit.
The last thing you want to do is install equipment in the attic. With a basement, install there and run the ducting to floor registers. You can also now easily heat the basement by adding a takeoff or two to the supply.
Before looking at equipment, it is worth while to do a bit of math based on previous energy use and figure out what size heat pump you need:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
P.S. You can't have just one head on a multi zone. Most (all?) require at least two connected and will operate with just one running.
I would look closely at the minimum capacity of the outdoor unit in option 2. Others have advised on here that the multi split units can be challenging and certainly less efficient if the minimum capacity by the outdoor unit is much larger than the indoor head is calling for.
After living with a single zone Mitsubishi wall mount unit for a few years now, I will double down on my suggestion to use multiple Single zone units (just my opinion. Not an expert or builder.)
I don't see the efficiency gains with multi head units. Any system leaks or parts failures take down the whole system vs leaks in individual single zone units. This redundancy in and of itself is worth it to me in exchange for additional one or two outdoor units (which can be powered with smaller gauge and a single could be powered with a smaller 7000w inverter generator in an emergency)
However I am considering hiring a duct/system designer to size duct
Work for my shop project so I can utilize one of their air handlers. For shop use I'm concerned with not having proper filtration in front of the indoor coil if I go with a ductless wall mount.
While I'm an electrician and went and grabbed my 608 cert and the tools so I can install ductless systems for myself, I don't have the knowledge or time to learn duct design at the moment.
I appreciate the info shared here. A detail I should have included is that we are removing an old oil furnace from the basement. We want to remove the ductwork to give us more headroom for a finished basement. Otherwise, we would use the ductwork and existing registers with a new heat pump. I wondered about the 1:1 pairing with the multizone. It's disconcerting that the salespeople spec equipment that won't work with the system they've designed.
We were also quoted options for ductless wall units:
One 18k in the main living area, which would heat and cool living/dining/kitchen, and two 6k units in the bedrooms. I'm worried that bedroom wall units will be overkill given how tiny the rooms are.
The other option quoted is one 18k wall unit in the living room with the assumption that we'll add little radiant heaters in the bedrooms and bath.
The bedrooms are each only 132 sq. ft. and the entire floor is only about 800 sq. ft with 8 ft. ceilings. The house was built in 1939 and is insulated but leaky with big single-pane windows (with storm windows). The salesperson calculated using Manual J.
We liked the attic option because it would provide heating and cooling to each room without visible units. We are really not sure what to do at this point.
There is no point in talking about equipment untill you know what your heat loss is. Do the math in the link I sent earlier, pretty easy if you have oil fill data from last winter.
Once you have the heat loss in hand, we can look at options. I know you want the space in the basement but I think taking the ducting out is a mistake. There are ways tor rework the ducting to get a bit more headroom that would cost you less the new ducting in the attic or an oversized multi split setup.
Like Akos says I’d avoid an attic install.
The high head one-to-one units have significantly better efficiency than the multi heads, which have larger compressors and can’t turn down the way single heads do.
You are right that separate heads in the bedroom will be significantly oversize.
Oversizing an outdoor unit in order to have additional options later is almost always a bad idea because of the efficiency/durability hit you’ll take with an oversized compressor.
To me, it sounds like the one to one 18k with electric backup is likely to be optimal. If the manual j was in the neighborhood if 24k for heating, I’d guess the single head will probably handle ~95% of your yearly load.
A basement airhandler (svz kp18 with optional electric backup) is a good option for upstsirs comfort but as you say at the expense of basement space.
You didn't say what your insulation was like. If you haven't beefed up all 6 sides of your envelope yet, maybe get a 1 zone unit and duct it to the first floor and maybe part of the basement. Then spend the left over money on beefing up the insulation. If you look at my threads, you will see I installed 2 to 3" of foam on the exterior of my 1930 house. Then I have a 30K mitsubishi air handler in the attic which has no problem delivering air to both the 1st and 2nd floors of my 2000 sq. ft. house. I have the same technology, I have the SVZ-KP30NA air handler.
You may need a small supplemental heat pump unit in the basement if you want fine control over the temperature. Of course if you are confident that your insulation in the basement is sufficient you could go with the 2 zone system right off the bat.
You're in a tough spot if your contractor is wedded to their one-size-fits-all approach. The different equipment options here are wonderful, but not if you can't find an imaginative installer. Their attic proposal, in theory, should be easy to shift to the basement with smaller ducts than that old oil burner had.