Mitsubishi Multisplit Hyperheat Excessive Defrost Cycling and Long Heat Up Time
Hi All,
I have issues with a recently installed 3C30NAHZ4 with three heads.
Summer went well in cool mode.
I am in my first winter heating with the unit.
Below -15c (5F), the unit cycles very often into defrost. At -18C the other day, I was getting 15 minutes of “heating” (actually 10 minutes of cold / ambient air and 5 minutes of starting to heat) followed by 10 minutes of defrost; rinse and repeat …
When it defrosts, the outside coils are not frosted at all, compared to expected defrosts at milder temps where there is visible frost buildup
The house temp goes down drastically and raising set point does nothing for this.
Installer came over, does not suspect refrigerant charge as when it DOES heat, temps are very good. I asked if there could be an issue with thermistors falsely flagging a temperature or temp delta that calls on a defrost, they didn’t’ think so.
Instead, they mentioned that Apparently before Christmas they were made aware of a required electric control board replacement (I guess for firmware / software update). Some other clients had it replaced as they were getting 20 minutes of heads producing cold air before heating up!!
Should have it replaced within the next month…
I was wondering if anybody fell upon the same or similar problem and if the board fix is effectively the right one?
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Replies
I’ve read some other complaints of very same issues. This is not normal and does indicate possible firmware issue. I’ll make some inquires but it does sound like firmware/board issue.
Interesting I have similar defrost frequency and slow heat recovery issues with my ducted MXZ-SM48NAMHZ that has behaved like this for a year since install. I have been getting the run around since for a year being told this is working correctly and no resolution. Just had a small leak repair last week and recharged to spec so very unlikely low refrigerant is the issue. At 27f with high humidity I got 10 defrosts in 6 hrs. Nearly an hour of blower off completely, 2hrs at very low output and half the time blowing heat. Added temp monitors a month ago to try and figure out what was going on. When colder, below 20 cycling is similar and recovery to heating is worse so my house starts losing temp.
Curious if your unit is cycling similarly.
Hi Dave, from what I can see in the charts yes it does look similar.
You barely hit 110 F duct temp and it goes back into defrost.
I run a non-ducted system but the behaviour seems similar.
Defrost frequency varies, your 10/6 = 1.66 defrosts per hour, in worst cases I’ve been getting three defrosts per hour.
I am waiting on my installer to receive the new control board and install, I will keep you posted. We have some very cold days coming up (getting close to the unit’s -13F capacity), so it would be good testing if they manage to change the card in time ..
Cheers
And yes … colder temps worsen heat recovery…
I am wondering if this is normal.
That is, what should be the expected time for a head (or duct) to start blowing warm/hot air.
@Mitsuman??
Wow, three defrosts and hour is dreadful. I really hope you get your unit fixed and working correctly before the very cold temps. Thankfully my defrost frequency is not that high and it can keep up at 20 even with the slow recovery.
That doesn't look like defrost to me.
Typical defrost is a short pause, run at full power, short pause back to old power level. That power trace looks more like the unit cycling. A bit strange in this cold of a weather.
Appreciate the comment and it is definitely defrosting as the blower turns off (which it doesn't do when cycling) but agree the power usage is very strange (also didn't show power above above but near perfect alignment with supply temp or btu output). I only have 5 min resolution on power but I have a 25 min period per defrost cycle with a decrease in power usage. 15 minutes at the bottom and a 5 min period in between the high and low on both sides. Getting it looked at next week but so far no insights into issue.
I am experiencing very similar behavior from a recently installed MXZ-3C30NAHZ4. Defrost cycles are occurring at intervals that range between a minimum of 25 minutes and maximum of two hours when temperatures are less than ~10F (temps have not gotten less than -10F). Two connected indoor units (MSZ-FS12NA and MSZ-FS18NA) are both capable of producing 115F degree air but their outputs are inconsistent; sometimes after a defrost cycle they produce lukewarm air or stop blowing air altogether such that the room temperature drops 20 or more degrees depending on the duration of the incident. Please do share your updates as you get them. I have a feeling that the solution to your issues will be applicable to mine.
Hey,
Your unit is the exact same as mine.
However in heating I run a 12 head and a 15 head instead of your 12+18 (a third 12 head is off since heat rises)
They are GL model instead of FH
Sure does sound like same problem… seems to be a trend!!
Can’t wait to have my firmware changed and confirm to you guys whether this is any help.
This night was terrible …
-17.5C ambient (approx 0F)
10 min defrost, 17 minutes (!!!) of cold / lukewarm air, 7 minutes of warm (not super warm air), defrost.
So cycling practically twice an hour with 25% “heating” cycle. House temp dropped 4 degrees C.
Can’t wait till they replace the stupid firmware card.
Confirming this is firmware issue indeed. Headed out tomorrow with Mitsubishi to reprogram a unit doing the same exact thing. It’s funny, it is NOT a typical defrost cycle. Almost fooled me the first time, the compressor did NOT ramp up to typical level during defrost it was spinning much slower….however, reversing valve actuated and indoor vanes pointes straight out, outdoor fan stopped. About 4-5 minutes then back to heating. I observed and documented about 5 cycles, all about 15 minutes apart. Yes the new outdoor board will fix it if they do not have access to program tool. I think there are less than 10 in the US so most likely board is fastest option to resolve. They’ll get you fixed up then you should be good to go. Very rare to have an issue like this but nothing and no company is perfect. It is an unfortunate inconvenience for sure.
Ok so this night we hit a low point of -18c (0F), temps at which the system was problematic.
(I honestly haven’t had time to check the system’s functioning now in detail, but I definitely saw differences last evening when it was warmer (Around 23f). System no longer shoots cold ish air for 10 minutes after defrost but seems to wait till coils are hot enough ..)
Anyways, normally at -18c ambient (0F) I would have to program set point to 22 or 24c (71 to 75f) and hope for a house temp at 20c (68f) in the house, but would usually wake up to 18-19c (64-66f) and a defrosting mini split.
Well, I decided to go all in and leave the setpoint at 20c (68f).
Go downstairs and …. Ambient -18c (0F) and house temp… 21c!!! (70f). TOASTY
I don’t want to celebrate too early but…. The fix seems to have worked!!!!!
ANYBODY DEALING WITH EXCESSIVE DEFROST CYCLING AND UNDERPERFORMING MITS MULTI HEAD HYPER HEAT UNITS IN HEATING : GET YOUR FIRMWARE CHECKED!!!!!!
Just wanted to thank you for this update! We just had two boards replaced on our two MXZ-2c20NAHZ4 units. I showed this thread to our installer, who first had to verify the lines weren't kinked and even replaced the charge in case it was contaminated. After four visits they contacted the distributor who confirmed the firmware issue and sent out new boards. Mitsubishi has been taking in old boards, reflashing them, and putting them back out into the field pretty regularly from what he heard.
After the first board was replaced, the defrost cycles stopped their every-15-minute airplane-toilet flush cacophony and the heads are working like a charm.
My situation was precisely as described by several. At below -8C twice per hour "defrosts" (although almost zero frost on condenser) with extended periods of cool air from the heads, result: 10-15 minutes of real heating per hour. The contractor replaced the main circuit board last March (after 4 months of head scratching, we have an electric furnace too and a Napoleon FP insert). Still same poor performance this past December and they replaced the board again with result that it will now sometimes run 2 hours hot without pause. Not burning any wood but mild winter. At -12-15C the main floor is a bit cool but the unit is working OK though not as continously.
I think the system was undersized by the contractor: 30K BTU condenser with 15K head for 1200 sq ft log house main floor + equivalent of another 800 sq ft of cathedral ceiling and loft. The 9K heads in top floor bedroom and the mostly above ground finished basement have no problem maintaining temps. Problem is, can only go to 18K head on main floor without also increasing the diameter of feeder pipe. Would that be worth a try? Have not a serious conversation with contractor due to lack of prolonged cold periods.
Any wisdom would be appreciated. Thank you Jay