Replies

1. Expert Member
   Akos | Oct 05, 2021 05:38pm | #1

   Looks like Mitsubishi improved the cold weather performance on their wall mounts this year. When in doubt, get the most up to date data sheet from Mitsubishi's website:

   http://meus1.mylinkdrive.com/USA/M_Series/R410A_Systems-1/Outdoor_Equipment/R410A_Outdoor

   Most manufacturers have similar rated and max differences. Fujitsu's 9k wall mount delivers almost 16000BTU at 5F. Since the units will be running at a reduced modulation at their "nominal" rating, they provide much higher efficiency and better SEER rating but still be able to deliver a fair bit of extra heat when needed on those cold days.

1. 
   kieran973 | Oct 06, 2021 12:03am | #4

   Thanks, Akos. That’s a great resource. And those specs match what I was able to find from the submittals for the 6,9, and 12k deluxe wall units on some other websites (gotductless, etc).

   I provided some more info on my overall hvac situation in my reply to Kyle below. My minisplit project stalled this spring and I’m just now returning to it. One problem was the IRS took almost six months to issue my 2020 tax refund, and we need that money for the minisplit project. The other issue was I had an extremely difficult time finding any contractors who are willing to do actual whole house air sealing, with pre and post blower door tests, projected ACH levels, etc (you recommended doing this - thanks again for educating me about the importance of this). But I finally found someone willing to do this and the work should be completed soon (I’ll have a better idea later this month). In the meantime though, I’d like to get going on the minisplit install since we have at least 3,000 annual solar kwh that we’re not using and with net metering here it’s better to use them for something than to sell them to the utility at 4 cents per kwh….

2. 
   kyle_r | Oct 05, 2021 05:58pm | #2

   The difference is the H in the outdoor unit. The indoor units you reference are the same, the outdoor units are different. The H denotes the hyper heat model which has improved cold weather heating performance.

   The maximum capacity is without defrost cycles, so the longer term average would be less than the maximum output.

1. 
   kieran973 | Oct 05, 2021 11:51pm | #3

   OK, interesting. So how do I account for the reduced btuh of defrost cycles?

   My 99% design temp is 11F, which is exactly halfway between 5F and 17F. So I’ve just been averaging the specs in the submittals at 5F and 17F to get the presumed max output at 11F. So for example, the max output at 11F for the deluxe 12,000 btuh should be (17,410 + 14,690)/2 = 16,050 btuh.

   Based on my last blower door test in the spring, my current whole house heating load is about 38,000 btuh. But since then I have replaced 8 first floor windows and 5 basement windows. I’m also about to put glass doors on the firebox, and most importantly, I have an upcoming whole house air sealing project in the works. My best estimate for where my whole house heating load will end up after the air sealing project is around 31,500 btuh - this is assuming a reduction from the springtime ACH of 20 to an ACH of 10. Interestingly, I could achieve that 31,500 btuh heat load with just two deluxe 12,000 btuh units. I could also do it with three deluxe 6,000 btuh units. So I’m thinking the best options right now are either two 12,000 btuh deluxe units downstairs (1,000 sq ft) and none upstairs (500 sq ft), or two 6,000 btuh deluxe units downstairs and one upstairs. Two 12k units would probably cost less; three 6k units might be more comfortable in terms of air flow. My staircase is pretty open with two entrances to it (one from the living room, one from the kitchen) - neither one has a door. You and @Akos both gave me a lot of good advice last spring, and I know you were advising a ducted solution, but I just don’t think that’s going to work on either the first or second level, for different reasons. The problem with the first level is that I’d have to make a bigger hole in the foundation between the basement and crawl space in order to get some of the ducts to some of the first floor rooms; plus, ducts leak, and we have some historical and some recurring asbestos/mold/radon issues that I am dealing with, but I don’t want to risk blowing our nasty basement and crawl space air onto the first floor; also, the prices quoted for ducting just the first floor are around $20K while two to three wall units are $10-15K. The problem with the second floor is that our attic is also currently unfinished/unconditioned
   and I don’t want to stick ducts and hvac equipment up there for efficiency reasons; I also don’t want to do this right now because we’re planning to finish the attic soon and I don’t want to compromise the usability of the space with all that ductwork and equipment. So I’m back the ductless wall unit solution. I don’t mind the look of them. We have hydronic radiators on both floors that could provide auxiliary heat after the ductless units go in. Currently, the radiators are supplied by a 10 year old gas boiler, but I’d like to fully decarbonise the house one day and replace this boiler with an electric one (or maybe even one day an air to water heat pump if the service/maintenance network for these improve in the US). Domestic hot water is a 65 gallon Rheem heat pump water heater. Stove is a GE induction range. We have an 8 kw LG solar array that produces around 10,000 kWh per year. Cars: one Nissan Leaf Plus (12,000 miles per year), one Honda CRV (1,500 miles per year). So really, the last fossil appliance to replace is the heat source (well, also the CRV, but we barely drive it).

   Anyway, long story short: given all of this, what sounds like a better solution? Two 12k btuh units downstairs and see how much heat moves upstairs from the two access points? Or two 6k btuh units downstairs and one upstairs?

3. 
   kyle_r | Oct 06, 2021 09:02am | #5

   Can you remind me where in the country you are? I imagine that with an 11 F design temperature Ac is a need for the first floor?

   Also on that original floor plan you drew, can you tell me where the bedroom closet is?

1. 
   kieran973 | Oct 06, 2021 11:03pm | #7

   You reminded me of how bad that photo was, so I attempted to redraw it here. The guest room closet is on the wall which that room shares with the living room.

   I’m in southern CT, close to NYC, just barely climate zone 5. AC is a need for the first floor - we’ve been getting by with a few window units, which is fine. Thanks.

   
4. Expert Member
   Akos | Oct 06, 2021 08:41pm | #6

   Whether you have ducts or no ducts, that nasty crawlspace/basement air is already going through the house. The only way to prevent that is to deal with the radon/mold/asbestos issues. Once that is done, you'll have a great, clean and fresh smelling basement/crawl and there would be absolutely zero issues with running ducts through it.

   Otherwise, the in-between option is a single larger wall mount in the main floor and a slightly smaller one upstairs in the hallway. A 12k+9k hyper heat is probably about the right size if you are looking at 30000BTU. This would mean using the existing radian to help with remote/closed rooms. Since both heads would be operating at a reasonable load most of the time, a 2C20/3C24/3C30 would not be too far off, but a one-to-one would still be more efficient (HSPF 11 VS 13.5). Since the equipment cost of a 2 zone multi split is about the same as 2 single zone units, there is not much benefit to go for the multi split.

5. 
   kieran973 | Oct 06, 2021 11:15pm | #8

   Ha, good point about the nasty air regardless - you make it hard to argue with you, Akos.

   Asbestos has been professionally abated but there is some question about whether they got all of it. I’m looking into that. Mold has been professionally remediate too, but I found some more in the hatch for the sewer clean out pipe a few weeks after a sewer backup into our basement this spring. Radon has spiked to 6.4 after putting a vapor barrier in the crawl space, but the radon company is widening the suction point for the radon fan, plus there’s a little 2’ x 8’ food storage type room in the basement with a dirt and gravel floor, so we’re concreting that next week.

   I agree with you about the in between option and the importance of one to one mini splits. One 12k downstairs and one 9k upstairs would be about 29,000 btuh total at design temp. If I did two 12k units downstairs, that would be 32,000 btuh but no direct mini split heat upstairs (only the hydronic radiators). If I did three 6k units (two down, one up) it would be 35,000 btuh, and three 9k units would be 38,600 btuh.

6. 
   kyle_r | Oct 07, 2021 08:38am | #9

   If Ac is important, I would be concerned about the downstairs bedroom being comfortable with ductless units in the kitchen and living room. If it was my house I would install a ducted unit for the first floor. Get bids from Mitsubishi, Fujitsu, and Carrier if you can. Install that and use your baseboard as needed for upstairs. When you remodel the upstairs/attic you can put a ducted unit up there. When the gas boiler fails, hopefully by then you will have convinced yourself you don’t need backup heat and don’t have to worry about an electric boiler.