Too-frequent defrost cycle? What’s the normal ratio of heat:defrost when it’s this cold?
Like everyone heating with minisplits in the upper Midwest these few days, I’m marveling at their capabilities. BUT, I’ve got one that’s not keeping up, and I’m hoping to hear others’ experiences. This unit runs for 30-40 minutes, then defrosts for 15-20 inutes. Given the long ramp-ups every cycle, I feel like I’m getting actual heat less than 50% of the time. With 0-10 degree outdoor temps, the unit can’t reach setpoint.
Mitsubishi SUZ 18K hyperheat outdoor unit paired with PEAD 18K concealed ducted handler. It was aggressively sized based on a study of the previous actual heating bills.
Even if it turns out it’s undersized by a bit, why is it spending SO MUCH time not putting out any heat at all? We spent up on the hyperheat model specifically to give us peace of mind on these cold nights. Any suggestions/compairsons would be most appreciated!
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Is the outdoor unit actually frosting up? Defrost depends a lot on local conditions, generally you'll see much more defrost during snow storms as the air is more humid.
There is no such thing as normal for defrost, but generally happens the most just bellow freezing. In deep cold, you should see less provided outdoor humidity is not too high like when snowing.
If the outdoor unit is not actually frosting up and you are seeing a lot of defrost cycles then something is off.
Thanks, Akos. My impression is that "something is off" since I'm watching several other units that are not defrosting so often. Ambient here today: 10F, 50% humidity with dew point -5F.
I've been through this other thread on the topic which suggests low charge and/or bad firmware. It is so frustrating to lean on Diamond techs who say the best fix would be to pull out my 18K unit and put in a 60K.
https://www.greenbuildingadvisor.com/question/mitsubishi-hyper-heat-minisplit-noise-defrost-cycle-issues#:~:text=It's%20because%20your%209k%20unit,it%20is%20a%20firmware%20mismatch.
The FW issue was with multi splits.
Your unit could be down on refrigerant. Quick check is to crank the thermostat to max and measure the temperature rise across the indoor unit. If you are delta is low, the unit is down on refrigerant.
Hmm, I have no baseline of what it's supposed to be, but should I go with a calculation, given my unit's nominal max output? (The formula has probably crossed this forum more than a few times...)
delta T x 1.08 x CFM. You can use the CFM rating from the spec sheet. delta T should be around 35F.
Like Akos said there is no set amount of time between defrost cycles but my Rheem’s lifetime average 13,000 hours / 3200 defrosts = 4 hours or so each cycle is about 3 minutes.
Did you see last week’s thread about Mitsubishi defrost software update and parts shortage?
Walta
For what it's worth, here in CO it has been 0 to 10F the last few days, dew point 0F to -10F, and my MXZ-SM42NAMHZ (Smart Multi, 42kbtu, hyperheat) units have been cranking, running 19-23h per day.
For all that, my defrost and preheat time is adding up to 5.3% and 5.6% of the total runtime, respectively. The last few weeks including more moderate temps is 5.0%.
If you are at 25% defrost time, something is wrong. My guess would be low refrigerant, but firmware or some other setting could be the culprit too.
Update: I see your chart below on how you calculates time in defrost/preheat - slightly different but still looks a good bit better than what I see.
I am really interested in how you calculate the defrost and preheat time. I probably should start a new thread but have a MXZ-SM48NAMHZ ducted and I calculate that over the past week looking at 0-8am temps down to -10 and low humidity I was averaging 25% of time in defrost or recovery (I set recovery to be low fan and delta T under about half of steady state max (using under 20F)). This 20-25% is the best I have seen. On higher humidity days I see this combined defrost recovery time go over 40% of runtime. Also means I got more heat at -5 to -10F with low defrost frequency then I get at 15 or 30F with high frequency of 40-60minutes. (My unit runs at 48Hz or 1.5-2kW for 10 minutes after every defrost cycle and that accounts for the slow recovery).
Thanks
I control my indoor units using an ESP microcontroller running ESPHome, and this project: https://github.com/echavet/MitsubishiCN105ESPHome
It is all a bit esoteric, but ESPHome is a project designed to interface low-cost ESP32 microcontrollers with HomeAssistant, a home automation/data collection software. It takes a little time to get deep into the project, but the amount of things you can control and monitor is impressive and addictive for data nerds like us. This particular ESPHome project reverse engineers the Mitsubishi serial protocol allowing control of the indoor unit as well as a lot of datapoints. It is still a work in progress, so we can't read all data points yet, and some give odd/inconsistent output.
So in this case, Defrost and Preheat are the unit reporting those states directly, and using their definitions. Defrost is an active process sending hot refrigerant to the outdoor coils, and preheat sends extra current (out of phase) to the compressor to heat up the compressor windings.
I'm not processing further to concepts like recovery, and I'm not measuring indoor delta T yet, though I keep meaning to add some sensors to one of my ESPs so I can measure inlet and outlet air temps.
I calculate the BTU/hr rate at any given time, but that is a guesstimate from the power readings and indoor/outdoor temps.
Your numbers definitely seem high. Here is another snip of my west heatpump's operation. It is 19F and 86% humidity today (currently snowing), and here is the behavior of the three indoor units, and my calculation of current heat energy. It is pretty consistent, with only occasional bumps up and down in power, and defrosts at 2.5 hr spacings right now.
That is great and much appreciate the write up. Mitsubishi should be making this information available so that it is easier to assess performance.
Do you know if your unit defrosts when the temperature is above freezing? Mine doesn't and it will freeze up if there is enough humidity with temps in the mid 30s- though it will quite effectively defrost once temps get below 32. It's an 11 year old non hyper heat Mitsubishi (admittedly a somewhat apples to oranges comparison to yours) but does act similarly to your unit with temps close to single digits, and I've always assumed something was off.
I didn't own the house when the unit was installed so I have no comparison from new and this is the only mini split I have experience with.
Yes, it will run a defrost cycle whenever it needs to, though above freezing they tend to be shorter. Today is a good example, the arctic river is moving on from CO so my temps shot up from -5F last night to 43F right now. When it was 4F and 80% humidity this morning, it ran a 10 minute defrost/preheat cycle. Midday, the unit is modulating down closer to its operating min (1700W or so), and it did run a defrost/preheat cycle at 40F and 20% humidity, though it lasted less than 5 minutes.
So, from Post #5, I'm calculating delta-T * 1.08 * cfm. That's 20 * 1.08 * 600 = 13,000 BTU/hr.
My outdoor unit is SUZ-KA18NAHZ, which is supposed to put out 21,600 BTU/hr at 5F. It was 4F when I was taking measurements.
So plainly I'm not getting the heat I expect from this unit. But more critically, that delta-T of 20F only happens for a few minutes per hour. The rest of the time, it's slowly ramping up from a delta T of zero. The total runtime never exceeds 40 minutes before it enters defrost mode again for 20 minutes. (There is zero frost anywhere on the coils when defrost begins.)
I'm going to have to A) convince and pay my Diamond dealer to weigh out the refrigerant and reinstall; and B) look into this firmware issue.
You are down on refrigerant, means you have a leak somewhere. If it ran for a while, it will be a small leak which is harder to find.
If you can get at the fittings of the indoor and outdoor unit, you can try to snoop test them. If you are lucky, it might just be a loose/bad flare connection. Also check the service port caps, I've had techs forget to tighten that before.