Mitsubishi Hyperheat vs Fujitsu Airstage
Hello folks,
I have a fairly large home (8500 sq feet) that is currently heated with oil. I have done a lot of insulation and sealing work and heating system optimization, yet I am going through about 2000 gallons of oil annually (it used to be much much worse). With the oil at $5.50 right now I simply refuse to survive another winter with oil. Gas is not an option as the local company (Eversource in MA) will not run a gas line to my neighborhood. The only option left is cold weather- capable heat pumps.
I have a good grasp of the Mitsu Hyperheats and I like their 1 to 1 setups with heads and air handlers (1 evap connected to 1 condenser). I greatly dislike their multi-split setups as they are terribly inefficient because the smallest ceiling cassette is 9k (no wall warts for me thank you) and that 9k is oversized for anything but larger rooms. Also add the fact that the EV is in the outside unit and the turn down is not that much and I really do not know what they were thinking… Those things cannot be sized properly in most cases and are bona fide energy hogs. Not to mention if you run them all the way turned down for extended periods of time they spike the compressor to return the oil… such is the nature of the beast. Add the somewhat rudimentary drain pan heater algo and the standby losses do add up… but I digress..
I have done manual J calc and my heat load is just about 100k btu on design day which is about 0 degrees F here. Not good, not terrible.
My setup is going to be large, but fairly simple and conservative- on one hand there will be three 2 ton condensers with matching air handlers using existing duct work. The forth zone is where my problem lies. I could do a 4th 2 ton condenser/ air handler, but I do want a bit more temperature resolution as that zone has 4 bedrooms and a living room. Thus I would like individual temp settings in each room. Initially I was thinking about a 3 ton Hyperheat with 5 heads, but turns out that condenser supports max of 4 heads so no go (unless I want to Mickey Mouse and branch one line to two heads)… This limitation would bump me to the 4 ton condenser and .. look at my second paragraph.
The problem is that two of the bedrooms are small and two are large. The large ones prolly can be matched to 9k heads, but the small ones have 1-2k heat demands on design day and they short cycle the condenser to death let alone let it modulate…
Looking around my obvious choice for that multisplit is true VRF like the Mitsubishi PUMY that will turn down and modulate even with smaller loads. But the PUMY is $$$$$ and the project is already expensive. Another option is Fujitsu Airstage – a true VRF with EVs in the heads. I might just do that as they have smaller heads (3k?) and the compressor can turn down 4:1 if needed.
So… thoughts? Ideas? I do not slave to any brand so anything goes.
Thank you
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Replies
"The problem is that two of the bedrooms are small and two are large. The large ones prolly can be matched to 9k heads, but the small ones have 1-2k heat demands on design day and they short cycle the condenser to death let alone let it modulate…"
Sounds like you have it almost all figured out. For the two smaller bedrooms, why not use electric resistance? It'll be 2-3x the energy cost, but with a heat load that small who cares? The equipment savings will buy you about a decade or two of energy. That leaves you only 3 zones for the multi-split. Otherwise, I'd consider combining the two smallest zones if possible. Those zones won't be individually controlled, but we've gotten by for decades without individual zoning.
Seems like a spot that AirZone might help,
https://www.airzonecontrol.com/na/en/control-solutions/easyzone/
But recent threads have not been encouraging:
https://www.greenbuildingadvisor.com/question/a-warning-and-a-question-about-mitsubishi-zuba-zoning
But yes, under-supply the bedrooms from a single 1-to-1, then add individual controlled electric resistance (baseboard or something in the ducts into each room)? Not useful for cooling, though, but that might not be an issue?
Thank you for your replies. I really appreciate the thoughts.
Me being an engineer would like to have a "fluid" and well integrated HVAC solution that is not going to make me second guess myself. Also, I don't like to concede technological "defeat" so a resistive heat albeit probably being the right solution is a no go "patch" for me.
What I have decided is the following:
1. I will run a full VRF Fujitsu 3 ton Airstage multi with 2x 4k 4-way slim cassettes and 3x 8k 4-way cassettes. The turndown on the Airstages is awesome and I can step down the EV in the 4k cassettes prolly to 1-2k realistically
2. I will run 3x 2 ton Fujitsu Halcyon 1:1 minis paired to matching air handlers through the existing ducts of the house.
There is also something else - on the supply side of things the Hyperheats are tentatively scheduled to be available to distributors in October at the earliest, while the Fujitsus have a 2 week lead time. Just an FYI for folks looking at options...
Wall mounts or ceiling mounts are not the best idea in a bedroom. Even if sized right, they can be the source of noise that will make sleeping challenging. Speaking from experience as I have a wall mount in the bedroom.
Every time a unit cycles up and down, you will hear the refrigerant rushing through the pipes. If the outdoor unit needs to go into defrost, which might not happen in your area very often, you'll get a pretty loud whoosh through the pipes. This will most likely happen in the middle of the night, if you are a light sleeper it will wake you up.
The fans and coils also needs periodic cleaning which is labor intensive and if not done properly it will cause the unit to rattle.
Also avoid any condensate pump in a bedroom. They are quiet but not silent.
A ducted unit with an Airzone system is your best bet. With a bit of work, you can get a Honeywell zoning to work if you use a thermostat interface module. This won't be as well integrated but installers will be more familiar with it. If you are concerned about sound, an in-line duct silencers of an S bend in flex does a pretty good job of limiting sound transfer.
If you enjoy complexity and a challenge, why not look at air-to-water heat pumps? Much easier to zone and a buffer tank solves the short cycling.
Thank you again for all the replies.
The air to water heat pumps are "not there yet" in terms of water temps I need. On design day I need 157 deg water to maintain 68 degrees indoor and while there are some water-water heat pumps that can get to 140-150 deg I am not sold on them. I have a friend with a hydronic water-water pump and while it does maintain indoor temps ok, its reliability has been horrible. I can imagine an air-water heat pump would be even worse... but who knows. Not to mention drilling alone cost him upwards of $75k in my region...
Zoning is definitely an option, but I have had really bad experience with zoning dampers and I am a bit shy. Also, I am not sure how well the step-down and up will work on the VRFs with air handlers with changing pressure when dampers open/ close. .. I would not like to be the test subject - I did mention to Fujitsu about zoning with a single air handler and their answer was - this was not tested you are on your own. Oh well...
Anyway- I have decided to go with Fujitsu Airstages all around with a mix of air handlers (x3) and 4-way ceiling cassettes (x5). I will probably sound proof a bit the sump pumps on the cassettes and call it a day. Hopefully they will run at the lowest speed most of the time which when they do is barely perceptible to the ear lol...
I will report back once I have the setup up and running. The Airstages are few weeks out (but are stateside at least) and the project is slated to be complete some time in early September.
Bumping an old thread but curious if you ran the Fujitsu airstage VRF and if it worked out as you expected?
Yes, I have now 2 winters under the Airstages “belts”. I ended up with x3 2 ton air handlers with 3 outdoor units and one 5 head 3 ton multi split.
They work like a charm. The Airstages can be configured at 100, 80,60 and 40% consumption so I was able to match them pretty close to the loads.
The multi split does short cycle- typical cycles are 10-15 minutes which while not optimal does not seem to be terribly inefficient.
About modulation I have watched them closely for all this time so I know what they do under different conditions and loads. They ones connected to the air handlers in 1:1 config will turn down to about 1/3 and the multi split will go down to 1/4 with a single head calling (6k).
On cold weather performance I know for a fact that they will go into turtle mode at -21f. We had that bout once and I had to turn on the oil for couple hours. Mind you design day here is 0f.
On COP I am seeing anywhere from over 5-6 around 45-50f outside temp down to 2.5 or so around zero.
That’s all. Overall I am very happy with the Airstages.
Greenright,
Thank you for your reply and I glad to hear that your system has worked well for you. While the size of the house I am working on is not close to the size of yours, I have the same problem with trying to find a solution for a second floor that has three bedrooms, none of which are large , but one has a heating load that is just under 2K on design day, (something that doesn't happen much in Toronto anymore, usually much warmer these winters). You mentioned in your 2022 post that you for the multi split unit, you were going with 5x 4-way ceiling cassettes, but in your reply today, you mentioned "heads".....are you using the 4 way cassettes or did you switch to mini split heads? I am thinking of using 5x 7K heads with a 3 ton cold climate outdoor unit but have been afraid to pull the trigger in fear that the 7K heads will be oversized.
Thanks!
Heads = 4 way cassettes- yes. The smallest Airstage cassette is 6k- oversized for two of the bedrooms (smaller), but I configured the outdoor to limit its power to 40% and along with the “quiet” fan setting on the cassettes they do get 10-15 mins run times at 40f outdoor. At zero f outdoor they run for 20-30 mins when called (70f indoor set temp). When the smaller heads are calling the outdoor runs at 1400 watts which is about as low as it will turn down to. At that outdoor temp COP is about 3 and daily usage is about 30kwh which is very acceptable in my book for a multi.
I am heating about 8500 sqf and At $3.25 for a gallon of oil I am seeing a 33-50% reduction of heating bill across the board for all units. I am paying 30 cents per kWh of electricity. Rough calculation shows at about $2.25 for oil they break even. The math is further skewed in my favor as I have a 15kw solar array on my roof.