Mitsibishi Smart Multi and Mitsubishi Intelli-HEAT Dual Fuel
Hey All,
I’m wondering if anyone has direct experience with two newish Mitsubishi products. The first is the new Smart Multi 36-48k H2i multi zone compressors. Really impressive HSPFs and temp-specific COPs, but the same high min-modulation levels. Has anyone used them? Have they corrected some of the issues associated with multi zone units in general like control issues, refrigerant bleeds through unused indoor heads, and poor performance with shoulder season heating? Any real world knowledge of how these are better than their predecessors?
And the other question is their new Intelli-HEAT dual fuel setup. Looks pretty fantastic, or at least so for those of us in cold climates looking to tiptoe into electrified space heating. And the fact you can pair with with a multi zone compressor is really cool. Ducted heat pump-AC swaps don’t solve issues with cooling second floors, especially in old houses or those with poorly designed ducting. Having the ability to add a ductless head for hard to reach spots in addition to the main dual fuel coil seems pretty great.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Hey I’m wondering if you found out much about the intelliheat coils ? I’m going to replace my 28 year old hvac system with a new hybrid system and this sounds good to me…I’ll need a new furnace as well.
It seems I hear only crickets on these dual-fuel / hybrid heating systems. I'm wondering if people either stick with fossil fuel, or gut the fossil furnace and go all-in on heat pumps?
The hybrid is interesting to me because we are out of budget to fully seal and insulate the envelope on our existing home (IECC climate zone 6b/7, 2600 sqf, two-stories, R13 walls, R4 roof, single pane windows).
Using propane to get the temp up when coming home, and then automatically switching over to the heat pump once we're comfortable is interesting and practical. But then with the COP of heat pumps around 3.0, you could almost run the heat pump full-time for the same energy cost. I haven't pencilled it out, because it depends how long you're away / at home. And we will improve our insulation and windows someday.
"The hybrid is interesting to me because we are out of budget to fully seal and insulate the envelope on our existing home (IECC climate zone 6b/7, 2600 sqf, two-stories, R13 walls, R4 roof, single pane windows)... And we will improve our insulation and windows someday."
There's a more to air sealing than replacing windows. A lot more. And a lot of it can be done cheaply and incrementally.
I agree that air sealing can be independent of insulation and windows. Seals around doors, smoke testing to find gaps (lowest hanging fruit). We've done what we can in those areas, but the next level is pulling off the siding (reverse board and batten) and taking the walls up from R13 to R21 or R30, plus adding an air barrier, of course (there's none now). The interior is semi-transparent stained rustic cedar panel with saw marks (no drywall), which we'd like to preserve, so we'll be tackling this from the outside.
We're going after the roof now - that's THE lowest hanging fruit after air sealing. We're going to bump up from R4 to R38 or R49 if we can justify R49 (ROI). We intend to put down some Grace Water & Ice on top of the 4x6 T&G (open cathedral ceiling), then cc spray foam (SIPs or spray directly), then sheathing, underlayment and shingles.
Our current budget is going toward a garage and driveway addition, which includes a family room, and another bed/bath. That will be built using all modern building and energy methods, much of it designed while consulting the experts here on GBA.
We're doing the best we can with our budget, incrementally tackling things in a sequence that is mostly driven by CapEx rather than OpEx, in the hope that that we won't wait too long to keep OpEx down. Covid hammered the CapEx for several years in our area, and we had to wait for everything to calm down to get our project going (we were going to start in 2019/2020).
Dual fuel systems don't get a lot of love here because they rarely make sense.
There's two use cases for dual fuel. Since the COP of a heat pump declines with the outdoor temperature, there will be a temperature below which the heat pump is less economical than a combustion furnace. What that temperature is depends upon your local cost of electricity and gas, the efficiency of the gas burner, and the COP curve of the heat pump. With modern heat pumps, in most of the US the break-even point is so low that you only spend a few hours a year below break-even and you're only saving a few pennies per hour and you'll never recover the cost of the burner over the lifetime of the equipment.
For the use case of having capacity to enable quick warm-up when using a thermostat setback, setback has gone out of fashion. You don't really save that much money. A lot of the heat that is "saved" isn't really saved, the building loses heat as the internal temperature drops, when you warm the building back up again that heat needs to be returned. If the heat pump is more economical than gas, you're using gas to produce heat that could have been produced more cheaply with a heat pump.
For what it’s worth,
I went with an all electric system and now do not have a furnace.
I’m happy I went this route as the furnace would be used rarely so it really didn’t make sense to own and maintain a furnace.
What is your city and what are your utility rates? Do you know your home’s heat load and design temperature?
We're in California, High Sierras. Our utility rate is $0.271 fully loaded (taxes and fees) - painful. I don't know the heat load off the top of my head (need to revisit our CA T24 docs and building simulations), but we are designing for 65F interior, 9000 HDDs (we get between 8800 and 9200 HDDs, and 8600 HDDs in a warm year).
It's hard to put the electricity cost into perspective without also knowing what gas costs where you are.