Mitsubishi PUZ-ZM Series Freezing Up

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dazdaz

21 posts

Geek

#318804 23-Feb-2025 10:32

I’m experiencing outdoor coil freezing issues in heating mode with two different Mitsubishi PUZ-ZM outdoor units, and I wanted to see if anyone else has come across this problem.

First Issue – PUZ-ZM71 Freezing Up

7 months ago, I had a Mitsubishi PUZ-ZM71 installed with a PEAD-M71 indoor unit. From the start, the system kept freezing up in heating mode, even when outdoor temperatures were well above freezing (10-15°C).

One thing I noticed early on was that the outdoor fan speed was very low—only 200 RPM unless the outdoor temperature dropped below 10°C. This meant that in mild conditions, the fan wasn’t moving enough air over the coil, allowing it to drop below freezing and ice up.

I also found that pouring cold water over the outdoor temperature probe would trick the system into thinking it was colder outside, which made the fan ramp up and quickly remove any ice forming on the coil. However, once the probe warmed up again, the fan slowed back down, and ice would start building up on the coil again. This confirmed that the system was controlling fan speed purely based on outdoor air temperature rather than actual heating demand or coil temperature.

After extensive troubleshooting, the entire system was replaced, including:

• Outdoor unit (PUZ-ZM71)

• Indoor unit (PEAD-M71)

• Copper lineset

• Control panel and wiring

• Refrigerant charge (checked twice)

Despite all this, the exact same issue persisted—the system would freeze up in heating mode and enter defrost even in mild outdoor temperatures.

Mitsubishi tested the unit in their own workshop and confirmed that there was an issue, but they never specified the exact cause. They eventually replaced the PUZ-ZM71 with a PUZ-M100, and that completely resolved the issue. One major difference was that the M100’s outdoor fan runs at 450 RPM when needed, preventing freezing and allowing proper operation.

New Issue – PUZ-ZM140 Freezing Up

At the same time Mitsubishi replaced the ZM71 with an M100, I also installed a PUZ-ZM140 (twin-fan unit) with a PEAD-M140 for my living room.

Out of curiosity, I ran some tests on this unit today at 15°C outdoor temperature, and it’s freezing up just like the ZM71 did.

Here’s what I observed:

• At 9°C outdoor temp, the fans run at a higher speed.

• At 13-15°C outdoor temp, the fans slow to 240 RPM, and the coil eventually drops below freezing, leading to ice buildup and defrost activation.

• The PUZ-M100 does not have this issue—it ramps its fan properly and does not freeze.

It seems like the PUZ-ZM series may have a fan control logic issue, where fan speed is based purely on outdoor air temperature rather than system demand or coil temperature. This means that when the outdoor temperature is mild, the fan does not ramp up even when the coil is dropping below 0°C, causing unnecessary freezing.

In contrast, the PUZ-M100 properly adjusts fan speed based on demand, preventing freezing.

Has anyone else had freezing issues with a PUZ-ZM series heat pump in heating mode?

Its so frustrating as ive wasted so much time on this already.

Cheers.

tweake

2391 posts

Uber Geek

#3345860 23-Feb-2025 11:00

i wonder if the factory got temp sensor 6 and 7 swapped around. they are on the same plug. th6 is on the coil and th7 is the ambient air.

dazdaz

21 posts

Geek

#3345862 23-Feb-2025 11:05

Here is a video of the ZM140 icing up (10 min after recording this, it went into defrost mode) https://youtube.com/shorts/AiWEoTG8eHc?si=R3z8vku-RVe-bUcX

And a older video of the ZM71 icing up https://youtube.com/shorts/KRxClBhewVs?si=L2GHEM_ie0UWjX1G

dazdaz

21 posts

Geek

#3345863 23-Feb-2025 11:08

tweake:

i wonder if the factory got temp sensor 6 and 7 swapped around. they are on the same plug. th6 is on the coil and th7 is the ambient air.

I did wonder that at first, but the request code for the outdoor air temp and for the coil and all thermistors is reporting correctly.

tweake

2391 posts

Uber Geek

#3345906 23-Feb-2025 13:00

well at least mitsi confirm the issue so its warranty issue. even if it means getting a refund and going to a different brand.

the other thing is the install isn't helping things. its pretty tight, tho its not the cause.

it would be interesting to measure the airflow, maybe the factory fitted the wrong fan.

dazdaz

21 posts

Geek

#3345908 23-Feb-2025 13:31

tweake:

well at least mitsi confirm the issue so its warranty issue. even if it means getting a refund and going to a different brand.

the other thing is the install isn't helping things. its pretty tight, tho its not the cause.

it would be interesting to measure the airflow, maybe the factory fitted the wrong fan.

It’s a tight fit, but everything is within spec.
I actually have 5 mitsi heatpumps at home, two ap80s in my shed and an ap71 next to the m100, the ap71 is hardly ever on. The 3 APs operate fine, their fans run a lot faster than the ZMs.

Interestingly, the fan on my older PUZ-ZM71 was 480 or 490mm in diameter, whereas the fans on both my newer PUZ-M100 and PUZ-ZM140 are 550mm in diameter with much wider blades. Despite this, the M100’s fan runs at 450 RPM, while the ZM140’s twin fans only run at 240 RPM.

I really don’t understand why Mitsubishi’s ZM-series heat pumps aren’t programmed to adjust fan speed based on the outdoor coil temperature. Surely, it would be far more efficient to slightly increase fan speed to prevent the coil from getting too cold, rather than letting it freeze up and then having to enter a defrost cycle. Defrosting wastes energy, disrupts heating, and seems like an unnecessary consequence of a fan control strategy that relies only on outdoor air temperature rather than actual coil conditions.

Cheers

tweake

2391 posts

Uber Geek

#3345913 23-Feb-2025 13:51

dazdaz:

It’s a tight fit, but everything is within spec.
I actually have 5 mitsi heatpumps at home, two ap80s in my shed and an ap71 next to the m100, the ap71 is hardly ever on. The 3 APs operate fine, their fans run a lot faster than the ZMs.

Interestingly, the fan on my older PUZ-ZM71 was 480 or 490mm in diameter, whereas the fans on both my newer PUZ-M100 and PUZ-ZM140 are 550mm in diameter with much wider blades. Despite this, the M100’s fan runs at 450 RPM, while the ZM140’s twin fans only run at 240 RPM.

I really don’t understand why Mitsubishi’s ZM-series heat pumps aren’t programmed to adjust fan speed based on the outdoor coil temperature. Surely, it would be far more efficient to slightly increase fan speed to prevent the coil from getting too cold, rather than letting it freeze up and then having to enter a defrost cycle. Defrosting wastes energy, disrupts heating, and seems like an unnecessary consequence of a fan control strategy that relies only on outdoor air temperature rather than actual coil conditions.

Cheers

the zm is the hypercore models for low temp areas afaik. i would expect that they calculated that there is enough airflow at low fan speed in those conditions. the question is why isn't it getting enough airflow.

hard to say if its a design problem as it doesn't appear for others to have the same problem. tho that may be they havn't noticed the problem. it may simply be a manufacturing issue, especially as covid screwed up a lot of manufacturing in china etc.

dazdaz

21 posts

Geek

#3345921 23-Feb-2025 14:18

Ok, I didn’t realise they were for low temp areas. To be fair, once the outside temp drops below 10c the fans speed up and it operated fine, I was getting significantly more heat out of the ZM71 under 10c than above. One solution would be to attach the outside thermistor directly to the coil, that would fool the system and cause the fans to ramp up. But its not ideal for a brand new system, I shouldn’t be having these problems.