Mini split efficiency
Julie56
| Posted in Energy Efficiency and Durability on
After reading the articles, questions and answers on this site, over the past few months, i am convinced and saddened that my new mini split system, installed last fall, is likely ginormously oversized.
2 Mitsubishi FH 09’s – 1 in each of 2 bedrooms totaling ~ 400 sf. An FH18 for ~ 600sf. House is 1989 rancher with 1000sf, insulated crawl space, zone 4. Attic insulation is 2 layers R19. 8 double pain Pella windows, 1 double pane slider, standard steel front door. Outdoor unit isMXZ3C0NAHZ. Is there any hope that this system can be used efficiently?
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What are your symptoms? Is each bedroom 400 sqft or 200?
Julie,
Although you didn't provide enough details to perform a heat-loss calculation, I think that we probably have enough information to conclude that the capacity of the minisplits installed in your house exceeds your load.
The moral of the story (for other GBA readers) is one that we often repeat: if you are designing a heating and cooling system, the first step is to perform a heating load calculation and a cooling load calculation. In most cases, the easiest way to do this is with Manual J software, although paper-and-pencil methods also exist.
Don't despair, however. Live with your equipment for a few years and assess whether it works. Unless you are uncomfortable, or bothered by the equipment's noise, or burdened by high energy bills, you may be able to accept what you have with grace.
Thank you. I can do that.
What are the symptoms you are seeing?
While I really only use my mini splits for AC, I have found manually running the fan[setting it to low] and moving the vanes so they do not blow on you eliminates comfort issues.
I heating mode you can turn the vanes straight down, and the side to side ones off in one direction [to where you aren't] then turn the fan on low and adjust the thermostat to where you are comfortable[not to a number]
My hydronic baseboard will keep the house within 1 degree[excepting solar overtemp] meaning that the thermostat never changes its reading. Minisplits simply do not do this, apparently part of their efficient design is to allow some temperature fluctuation, and, in my experience, this is worse when they are oversized
I have not had any problem/symptoms. The house temperatures are stable, except during 20 degree nights when the temperature drops 3 degrees from the set point. During the 40 degree days it is on spot or off by 1 degree up or down. The size of the bedrooms are 240sf and 140sf. I have turned off the fh09 in the 140sf room thinking that should help prevent short cycling. Overall, the system is performing well but I constantly wonder, 'am I modulating or short cycling?' Thank you for your interest and help!
There are obviously better experts than I responding but I will throw in my two cents. Mini splits do have a lite learning curve as a previous post said better than I can. The outdoor unit(condenser) on your unit has an inverter compressor and can modulate from 6,000 btu in cooling and ~7,200 in heating all the way to max 30,000 btu. I don't know the Mitsubishi technology like other brands but this should help alleviate some oversizing.
Also, when you get to cooling season the dry setting(if equipped) will help remove more moisture while only slightly widening the temperature setpoint. At least that is how my older, non inverter model works
The heads are clearly oversized, but the more important issue is the total heat load of the house at +47F relative to the minimum modulated output of the MXZ3C0NAHZ at +47F. The total heat load of my 2400' 2x4 framed antique with 1500' of insulated basement is 10-12,000 BTU/hr, so it could probably do OK with that multi-split (even though it's design heat load at +5F is a bit over the 30,000 BTU/hr rating).
For a 2x4 framed ~1000' house with R38 in the attic and better windows than mine you're probably looking at 15,000 BTU/hr @ 0F, and about 4500 BTU/hr @ + 47F, which is well below the 7200 BTU/hr minimum modulated output of the compressor. The compressor is probably only cycling, never really modulating until it's below freezing out no matter how you set up the heads. If that's the case you'll probably get a seasonal average COP of about 2.5 out of the thing, maybe a bit better, but not 3, and certainly not 3.5.
https://www.acwholesalers.com/hvac/pdf/mitsubishi/mitsubishi-multizone/MXZ-3C30NAHZ-sb.pdf
For this house a better choice would probably have been the1.5 ton Fujitsu mini-ducted unit, installed in the insulated-sealed crawl space. It'll deliver 20,000 BTU/hr at zone-4 type outside design temps (probably about 1.5x the actual 99% design heat load, which is fine) and modulates down to 3100 BTU/hr @ +47F. That unit would run almost constantly in a modulating mode, and deliver a seasonal COP north of 3. (It's tested-rated HSPF is 11.3, which is a COP of 3.3, which it should pretty much hit if sized correctly in a zone 4 climate. If 25-50% oversized for the design heat load it might do a bit higher than that, but probably not 3.5.)
http://www.fujitsugeneral.com/PDF_06/Submittals/18RLFCD%20Submittal.pdf
With real heat load numbers and the climate data the guesstimates could be fine-tuned, but I suspect it won't make a huge difference.
Cycling due to insufficient load isn't exactly the same thing as short-cycling. Short cycling is where the head is mounted where the output air is feeding excessively back into the input to the head, making it cycle on and off at fairly short intervals, even when the load is in a range where it could be modulating. That takes a toll on both efficiency and comfort, since room temperature tracking is poor, and it's running at a higher speed than would be necessary to just keep up with the load. With a well-placed head the blower would only run at lowest speed and with a reasonably long minimum on-time when there isn't enough load for it to modulate. But when the compressor has to cycle on/off rather than modulate due to lack of load, it's another layer of inefficiency.
For now, you're probably better off keeping all of the heads on as long as they're not short-cycling (1-2-minutes on, 1-2 minutes off or something). The FH09s can modulate down to 1700 BTU/hr, and having the additional head drawing from the compressor means the others don't have to run at higher speed /lower efficiency just to take up the compressor's output.
Thank you so much for helping me to understand the system and how to use it. Although the situation isn't perfect I can finally put it to rest.
My I ask what your previous setup was and did an hvac company suggest this equipment size?
The previous setup was a traditional York with ducts in the crawl. It wasn't working well and an HVAC company/Mitsubishi diamond dealer, promoted by a local electric coop, suggested and installed the minisplits. The old ducts and air handler were cleared out of the crawl at the same time. The crawl was then encapsulated. It is 3:30 am and since the temps dropped and the units are actually in use they are making intolerable random cracking and creaking noises. All around not a good situation.
The cracking and creaking is probably from quick expansion/contraction of the materials (especially plastics) on the indoor units as the system cycles between regular heating operation and defrost mode. In defrost mode, the refrigerant flow reverses for a short period of time (usually just 2-3 minutes based on past experience with Mitsubishi units) in order to melt any frost build-up on the outside heat exchange coil. The refrigerant temperature moving through the indoor heat exchange coil is relatively cold in defrost mode, which will make the previously warm components on the indoor unit start to contract slightly. Once the system switches back to regular heating mode, everything warms back up and expands.