Best defrost strategy amongst 2 ton minisplits

archibald tuttle

There is a constant drone in the background of discourse on small air to air heat pumps, i.e. 'minisplits', regarding how cold the outdoor temperature can be for them to work. Somewhat overlooked in looking for a number is what demand they can satisfy at the low temperature but, even more overlooked in terms of measuring efficiency is the defrost problem in the focus climate zones for the wall (4 and 5). This can create low efficiencies even at the more modest temps just above freezing when higher relative humidity and low 'condensor' temps can cause plenty of icing (funny how they still call it a condensor just because it is the outside piece even though it is functioning as an evaporator, but i digress).

I'm looking for the best defrost management in like 1.5 to 2 ton units if anyone has studied this or found a brand that is implementing more advanced strategies which, heretofore, have been reserved for larger units.

The study that got me going on this
(Francisco, P. W., Palmiter, L., & Baylon, D. (2004). Understanding heating seasonal performance factors for heat pumps. proceedings of the 2004 ACEEE Summer Study on Energy Efficiency in Buildings) has this illuminating chart showing efficiencies of nameplate 7 and 9 hspf units in different climate zones which starts to look a lot to me like comparing nameplate efficiencies of condensing boilers to that actually achieved depending on return temps.

Ca

I hope maybe better programming has improved the declines shown here since this was published in 2004, but, as you can see our northeast and midatlantic climate zones have the worst hspf and, in fact, the coldest climate zone represented actually demonstrates increased efficiencies. One also notes that the slope of the decline of the more efficient unit is more precipitous than the decline of the less efficient unit. The graphs are poorly drawn (they should have left the units the same on the Y axis so you could make easier comparison) but if placed on the same graph, the lines wouldn't cross. the more efficient is always more efficient, but less so as climate zones increase through zone 5 and then anamolously rising in zone 6.

I have got to believe that this represents the defrost problem and the amount of time spent defrosting in moderate temperature operation from a few degrees below freezing to approx. 10 degrees above freezing. And the apparent anamoly of better performance in the colder zone is likely explained by colder air over the continent being much dryer.

I've got to retrofit 2 ton units in multiple similar sized apartments in zone 5 that have existing heating system that will become the backup and be used during cold conditions with cutout relays to the heat pumps and cut in for the backup likely around 18 to 20 degrees (and perhaps after this study triggered under 40 if a humidstat is registering high humidity.)

All of this is making me way more concerned about the defrost strategy than the minimum operating temp or even the nameplate hspf although there would be some allowance for the efficiency of defrost algorithm reflected in the nameplate. LG Rep sent me a really nice bunch of specs for their intelligent heat approach that responds to a humidstat to reduce refrigerant flows/pressures to keep the 'condensor', i.e. outdoor unit, from experiencing icing conditions to cut down on defrost.

Meanwhile the defrost algorithm itself is held off until it is detected that the inside unit is not keeping up with demand (the literature does not specify the indicia by which this determination is made, but I imagine it is implemented by monitoring pipe temperature of the 'evaporator', i.e. indoor unit piping and the temp of return air. These seem like sensible surrogates for minimizing unnecessary defrosts although I kind of wonder if they have a humidistat why they don't potentially incorporate that as a time to defrost regulator as a check on the meeting inside demand parameter). In any event, this was really cool until I found out it is only implemented on 6 ton and up units. The smaller units are managed just by setpoints relative to outdoor air temp and outdoor piping, i.e. refrigerant temp. The temperature parameters don't seem to be particularly fine in a way that actually employs the refrigerant temp to infer icing, but maybe i'm being too retentitve. The LG explanation is that emperical testing suggests complete defrosting is better so they seem to err on the side of more defrosting, except that they say that colder temps are less likely to result in defrosting so those setpoints would appear to pick up icing.

Ironically though, with the seasonal average hspf of nominal 9 hspf units reduced to between 6 and 8 that suggests that at these high defrost times the hspf could be much lower-approaching a COP of like 1.25 or 1.5 which aren't particularly convincing ways to save energy although the backup for these apartments is steam that runs around 75% efficient so depending on propane costs, etc. it might still be rational to run the heat pumps in humid conditions on the shoulder of freezing temps, but its a way closer thing than it looks when people are spouting about these things being 3 times more efficient than resistance heat.

what do ya got?

brian

archibald tuttle

and, for this morning's scintillating 6AM reading, I saved this on advanced defrost strategies, which, for the most part are yet to be implemented in smaller household units (although it doesn't speak much about the strategies I am seeing which are modestly more sophisticated attempts to relate outdoor temps and refrigerant operating temps as surrogate for icing).

But the most obvious favored strategy in terms of technological readiness (TRL, y-axis in the graph–the x-axis is effort to implement as in cost and complexity vs. technological readiness) and size of potential savings is a humidstat. Not sure why this lags in implementation. It is sideways implemented in the larger LG units I mentioned (and probably larger units from other manufacturers, i just happened to be most easily get LG reps on the phone) which use a humidstat to regulate compressor load and 'condensor' temp to limit icing in the first place rather than to time defrost cycles.

fentonc

Fascinating! I was wondering about something like this: https://www.gwultrasonics.com/other-technologies/ultrasonic-de-icing/

archibald tuttle

fentonc said:

Fascinating! I was wondering about something like this: https://www.gwultrasonics.com/other-technologies/ultrasonic-de-icing/

Glad to see others are considering these questions, both in present and future implementation. I imagine that method might be subsumed under the "M2-Use of vibration methods to shed Ice" bubble in the chart above. Sorry it is tough to read, wish there was an enlargement viewer for laptops using this interface @Erin Holohan Haskell , but if you squint, the M2 is mid-sized yellow bubble midway up the left hand side of the graph charted as moderately ready technologically and low difficulty to implement; so even though I'm not going to find units this year or by waiting for next year's models that would probably be out in the fall, this could be a promising approach. I'm assuming in aircraft, the biggest issue is to insure that the method doesn't have the unintended consequences of doing any minor but accumulating structural damage. A little less risk of crashing a condensor although depending on transmission of the vibrations it might challenge connections as a downside, but i'm all ears for someone to get this right.

Also, after this study, I'm really toning done the aesthetic placement decisions for condensors compared to sunny protected location (balanced with line length considerations and the creative use of faux railings.

If anyone has had any side by side experience running the available (that might be a euphemism in todays never just in time supply chain) units where you have had occasion to note comparative energy use in shoulder conditions or observe icing, I'm trying to find the best currently available combination of controls and algorithm. I did find conflicting LG literature on a few of the 2 ton units that suggest they use head pressure to track defrosting requirements. I'm still trying to track that down.

brian

Hot_water_fan

I have a 2 ton Mitsubishi hyper heat and its seasonal COP is about 2.5 with plenty of hours below freezing so perhaps defrosting is not that big of an energy penalty? It  

Erin Holohan Haskell

archibald tuttle said:

fentonc said:

Fascinating! I was wondering about something like this: https://www.gwultrasonics.com/other-technologies/ultrasonic-de-icing/

Sorry it is tough to read, wish there was an enlargement viewer for laptops using this interface @Erin Holohan Haskell

Depending on the browser you're using, you should be able to click View and then click Zoom In for a closer view of a page.

archibald tuttle

Hot_water_fan said:

I have a 2 ton Mitsubishi hyper heat and its seasonal COP is about 2.5 with plenty of hours below freezing so perhaps defrosting is not that big of an energy penalty? It  

sounds like that is your experienced COP and not nominal? Did you replace resistance heat or monitor changes in bill or use a direct wattmeter to estimate? What's your location climate zone and relative to oceans or great lakes humidity?

while the hyper heat is able to get performance out at low temps I would think the general problems physics of icing wouldn't be better or worse although they may have invested more heavily in sensors and control for defrost cycle given that the hyper-heat stuff is the high end flagship of their line.


thanks

brian

archibald tuttle

Erin Holohan Haskell said:

archibald tuttle said:

fentonc said:

Fascinating! I was wondering about something like this: https://www.gwultrasonics.com/other-technologies/ultrasonic-de-icing/

Sorry it is tough to read, wish there was an enlargement viewer for laptops using this interface @Erin Holohan Haskell

Depending on the browser you're using, you should be able to click View and then click Zoom In for a closer view of a page.

right, i must have the wrong browser that doesn't support the functionality you detail for picture viewing, e.g. .jpg or .png. When viewing a .pdf I do get zoom options but when I right click and view the sort of image in this thread in firefox, I do get the image independent of the page and text, but no zoom buttons and the two finger zooming is also not implemented.

nothing you can probably do about that other than if there were some kind of more native viewer.

thanks, brian

Hot_water_fan

Brian, yup it's experienced: I can compare what the previous furnace used to what the heat pump uses now. It's in central Maryland with 4000 HDD or so. I think the controls must be pretty strong, as this is one of their newest units. Interested to know how they do it!

Erin Holohan Haskell

@archibald tuttle, you can also try saving the image to your computer and using the image preview to zoom. PM me if I can help more. Thanks.

archibald tuttle

@Hot_water_fan

is that an MZ-FS model?

brian

Hot_water_fan

@archibald tuttle nope it's the SUZ-KAxxNAHZ, basically the drop-in furnace replacement.

BobbyB444

@/archibald%20tuttle try the keyboard keys "control +" --or-- "control-" keep clicking until your get the pic size just right

BobbyB444

Hi Folks! I don't understand the science spelled out in detail above, but am searching for an appropriate minisplit unit where southern New England outside ambient temps are generally from -10 degrees F to +100 degrees F max; sunny south side of house; "maybe" place the condenser unit on 4" stone slab, and build an aerated enclosure/solid cap to ward off snow/wind/rain drippings from above (worst snow is 6" drift that side of home). Maybe paint it black to keep its interior warmer?? Insulate drain ext? Dunno

I see some units can operate from -4 to -13 degrees ambient, no wind

Any ideas, or am I completely way off?? My first attempt, yes I'm a "noob"

Thanks for anything...!

Hot_water_fan

@BobbyB444 you’ll have plenty of options from Fujitsu, Mitsubishi, LG, etc. I don’t think the enclosure is necessary, but could help keep ice off the top, and painting it black won’t make a noticeable difference. 

GGross

@BobbyB444

LG RED have ratings down to -13f, meaning they have actually rated the heating output while the unit is in that ambient temperature. They will run far lower than that, and they don't list a "shutdown temp" because the unit will just keep running and running with lower outputs until it can run no more.

With mini splits you want to stick the big names currently, there are a few that are coming up now that are getting a good reputation but these names make great mini splits, with optional models for cold climate.
LG, Mitsubishi, Fujitsu.

These are not cheap units, and they are not comparable to a window shaker.

TAG

The old school single speed split type w/ timed defrost can run into problems .... on many days they are defrosting when there is no need and other days not soon enough or long enough. I have experienced this in SC where many in the family have vacation spots. All my new stuff has on demand defrost.

The modern split systems and the mini-splits being VS w/ big condensers don't seem to pull in the moisture like the smaller single speed splits. I'm in the mid Atlantic and you can see this in the winter with sleet and snow .. even when my carrier is running at top speed the snow mostly just falls right past. The one mini-split I use the most for heat (3x head 30k unit ) is on a wall and they don't seem to have to defrost that often. It's really that above freezing time in the rain when I see them doing it more often.

BobbyB444

Great responses! Thanks!!! tweaking my brain more...some mention optional models for cold climates/timed defrost, but those units I've seen don't mention that stipulation in their descriptions. I also worry about rusting or similar as a result of water intrusion, drain line (which I want to enclose to keep from freezing), water water everywhere, or am I worrying too much?

pecmsg

I like minis for heating but only to a point. Much below 10 -15* they can heat but it’s a cool heat and defrosts are more often. 

Below those temperatures I go to my back up source. In my case NG fired hot water.