Multi split ?
Hi, I have a 1500 sq ft house in zone 4 ( close to DC). I currently have a gas furnace and AC which is a downdraft and my ductwork is under slab and under sized. I am going to either get a heatpump with gas furnace- ductwork will be in attic or a 42000 Btu multi split or a 3 zone and a 2 zone mini splits.
The 3 zone would have 3 – 6000 Btu heads and the 2 zone would have a 15000 and a 9000 Btu head on it. I thought about doing the 3 6000 Btu units on individual compressors but that really raises the cost. Any advice would be appreciated. I think both the duel head and 3 head units have 12600 minimum and the 42000 one is 14000 btu. My load calculation came out at 39000 btu. The multi and duel compressors are a lot cheaper than ductwork but when I tried to split into more compressors the quote came back more expensive than ductwork.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
" My load calculation came out at 39000 btu"
Was this a manual J or a fuel based calc (https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler)?
That's seems like a huge heat load: I'm north of you and have about half that heat loss in larger, old house.
In general, a ducted system is the highest comfort option and often the most efficient too. You can explore mixing the two though: duct what is easiest and use a ductless for the rest.
Are you comparing the cost of a ducted heat pump vs. the multi-split? Or ducted + furnace vs. multi-split?
I used hvac.betterbuiltnw.com for heat load. My house was built in 1969. .The Mitsubishi multi split was thousands cheaper than Carrier green speed plus furnace and ductwork. Using 2 compressors we’re about the same price as the multi but when I asked about individual compressors for the bed rooms the price was a couple thousand more than the ducted. I really hate to have ductwork in the attic. I know I could put r 30 insulation over it but after installing ducts ,I may not even be able to access the attic.
Deleted
That’s apples to oranges. Can you get a ducted Mitsubishi price?
3 ton came in $1000 below Greenspeed
The most accurate way to size your equipment in an existing building is based on past fuel usage.
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Note oversized multi split equals unhappiness!
Walta
I also did heat loss calc that way which came out at 20000 but wouldn’t be totally accurate because of a couple space heated we usually use a night. ( under sized ductwork to back bedrooms). You should have heard the pushback when I told HVAC guy that I thought maybe I could use a 40000 Btu furnace. 😀 If I end up going with ductwork, I’ll probably get the 3 ton cold climate Green Speed with a 2 stage 60000/40000 backup furnace. I like the idea of mini splits but I’ve read that multi splits are really inefficient and one to ones seem really expensive for 5.
In zone 4 any cold climate unit will easily carry the place without backup, adding the complexity of a 2nd system makes no sense. The Greenspeed unit is also a good choice if the price is right but skip the furnace backup.
The cost of a multi position air handler plus hyper heat outdoor unit is less than a 3 zone multi split, so not sure where this is all coming from. The Mitsu multi position air handlers can be run in a downflow configuration so it should be a very simple retrofit.
Fixing the distribution issue might be a challenge though.
20k fuel based calc + couple of plug in heaters is nowhere near 39k, somewhere between the two is your actual load. I would say closer to 20k. If you are really worried about sizing, add a strip heater, the smallest one is 5kW which would add 17000BTU which is almost your heat load.
If you fix up your house model, I would not be surprised this can carry your whole place. The backup heat and the heat pump can run at the same time, so in your climate, this could deliver almost 40000BTU of heat.
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_SUBMITTAL_SVZ-KP24NA_SUZ-KA24NAHZ_en.pdf
The Mitsubishi ducted was also replacing ductwork and a 10kw toaster coil.
I wonder just how much electric a multi split would use compared to a ducted unit. If I went with the multi I could probably mostly run the 15000 btu head - the multi it puts out 14000 btu minimum.
10kW toaster is 34000BTU, which will carry your whole place plus way more, there is not point for anything that big. I guess you can add it if you have the room in the panel, my $0.02 is add enough backup heat to keep your place from freezing if your heat pump fails plus which will be more than enough for a boost during those polar vortex days. I'm in colder climate and the heat pumps I've sized don't have backup heaters.
A bunch of wall mounts on a multi split is never the answer. Definitely not the case if you have ducting already. They are hard to get right, they are more maintaince and improperly sized can create comfort and efficiency issues. The min modulation spec on most multi splits has a lot of asterixis attached, it is near impossible to get an answer of how well they will run at low load. There is no way to answer how much extra energy a multi split will use but anecdotally, it will be more (my oversized multi split for cooling is about 1.5x the operating cost of a comparable ducted unit).
I had 3 different hvac guys out. I think they all worry about the 20 year cold snap. One was pushing me to get an 80000 Btu furnace- way oversized although I don’t know if it matters if a furnace is oversized as the price difference doesn’t seem to be much. ( furnace for duel fuel) 50% more electric for a multi split is a lot but still overall might not -money wise be to bad - depending on total cost. In my case $9000 buys a lot of electric. Although even at that difference I’d probably still either run ductwork or do separate compressors. Seems like I’m paying either direction. Thanks for info, I was wondering how bad multi splits were.Do you like your multi otherwise?
Another option: keep the existing ductwork, add electric baseboard where needed. Won’t be the most efficient but might not matter.
+1 on this.
Or run extra ducting to these two rooms only. If these are home run ducts without any joints or splices, even through the attic won't be all that bad as long as the boots are properly sealed. Probably less cost than the resistance heat.
My ductwork under the slab really needs replaced. That’s the only reason I was looking at mini splits to began with. The multi was 9k cheaper than the Green speed. I’ll probably call the HVAC company tomorrow and get the ductwork installed. I still have some odd jobs to do in the Attic so I hope tomorrow is a little cooler. Thanks guys😊
If the plan is to move the duct work into the vented attic. it is a very poor idea and you should make a new plan.
As bad as oversized multi splits are it is much better than ductwork in the vented attic.
Just how bad is the sub slab ductwork?
Sure the ductwork will be too small if the HP is 3X oversized.
Walta
The houses in my subdivision were all built by the same builder and some have asbestos ducts, I am getting away from the buried ductwork. That is why I’m having problems figuring what to install. On top of that my heater is on one end of my house having to push air the whole length of my L shaped house and will have 4 ninety’s at the start. I was going to have the ductwork installed in the attic but HVAC guy wants to talk to their estimate guy to recheck pricing on separate mini split condensers and heads. My wife would rather have one multi split but if they really use 50% more electric I’m not sure I’d want it.