A dirty little tale of a Mitsubishi mini-split heat pump

I’ve worked from my home office in Houston since May of 2001. Technically, it’s the apartment that’s part of our detached garage. When we bought this place it was equipped with a hefty through-the-wall heat pump that roared like a locomotive. It wasn’t long before that noisy old beast failed.

Since then we’ve had three different ductless mini-split systems installed. And learned a bit about them along the way. Some lessons take some time to really sink in. That makes them worth sharing.

History

Initially, I had a 12K BTU Soleus air conditioner. At the time mini-split systems were not at all common in the US. Mitsubishi was selling under the “Mr. Slim” name. I bought Soleus on E-Bay because it was cheap. It was in essence, an experiment. I paid a local AC tech install it. Back then, most local tech’s had zero experience with them.

The 12K BTU system was a little undersized for the space and climate here in Houston. So, while it worked well enough most of the year, there were times when it just could not keep up. Nonetheless, it proved the point that a mini-split was well suited to the task.

A few years later, when cash flow allowed, I had the Soleus system replaced with a larger Fujitsu Halcyon system. Still only for cooling. That system was costly, but it lasted a long time. The contractor I bought it from was very familiar with them. They had staff that was recently returned from military service in Iraq, where they had been involved in running mini-splits to cool mobile data centers. Basically, tractor trailers full of racks of computers, with multiple mini-splits mounted at each end to cool the gear.

I needed a 24k BTU system, but the vendor could not get one. We waited many weeks for one to become available. Eventually, they offered me a 30k BTU system for the same price. That was actually too big for the space, but it was a variable speed model, so I thought it would be ok given the amount of equipment I had running in my office at that time.

The Third Time is the Charm

In the spring of 2021 the Fujitsu unit developed a leak somewhere in the drain pan such that condensate tricked down the wall in the office. Not all the time. Only on the warmest days.

I had several techs out to try and repair it. Nothing they did lasted. Working on the inside of the things is really difficult.

In November that year, I had the Fujitsu unit replaced with an 18K BTU Mitsubishi heat pump. This third system was right-sized for the space given the dramatic reduction of servers I had to run after I changed jobs. Also, it provided a heating function that was long overdue.

What they didn’t say

In all that time, I cannot recall any HVAC vendor providing any specific insight about cleaning mini-splits. And they most certainly do need to be cleaned!

When this Mitsubishi was nearing a year old I had the same contractor who installed it out to perform routine service. The young tech offered to clean it, but all he really did was use his tank of compressed refrigerant to blast the accumulated gunk from the blower.

I’ve come to realize that’s a lazy approach. The blower and the evaporator coil really do need to be thoroughly cleaned now and then. At least once a year. Perhaps twice a year in some places.

How do I come to know this? Toward the end of its second year the heat pump failed. It was on the occasion of our first significant cold front of the fall. I set the system into heating mode…but no heat came out.

Thinking it was nearly time for its annual servicing, I had the very same HVAC tech to come check it out. He reported it was basically fine…no refrigerant leak…but it was experiencing unduly high temperature differential. That resulted in an error code that stopped it dead.

Diagnostics: Blinkenlights

The error code was expressed as a blinking LED on the front panel. I had found the error code in the user guide. While I had power cycled the unit on my own, I did not know how to clear clear the error. The tech knew that it was important to remove power for at least 10 minutes.

Together we went on to discover that it would not heat (or cool) if the fan was left in Auto mode. However, if we manually set the fan speed to a faster setting, it worked normally.

We surmised that not enough air was blowing across the evaporator coil. This was very possibly because of a dirty blower. The blower was easy to visually inspect. Yup. It was obviously dirty.

Potentially also because of a dirty evaporator coil. The back of coil is not so easy to inspect without taking the unit apart.

The tech departed, charging me just as diagnostic fee for the trip. He suggested that I should explore YouTube for a guide to cleaning a mini-split heat pump. It was something that I could DIY. Moreover, he wasn’t really very interested in doing it for me.

DIY

Since manual control of the fan allowed me to continue to use the system, I bid him farewell, thinking that I’d clean the unit myself when I could find a couple of hours free time.

In the mean time, I searched YouTube for some guidance on how it should be done. I found several videos that were quite informative:

• HVAC School: Ductless System Install & Maintenance
• Home Performance: Clean Your Ductless Mini-Split! NOT Maintenance-Free!
• Home Performance: Ductless Mini-Split: How to Take Apart & Clean DIY (Mitsubishi FH High Wall Series)

Since Mitsubishi is the most popular make, these videos address the very same kind of system that I have here. This made it quite simple to see how it should be cleaned.

Having done the requisite research, I was able to remove the blower and clean it using just water. Also, the cavity where the blower lives and the drain pan. All three were rather nasty. I should have taken pictures in support of this effort.

Thereafter, the system worked normally, even with the fan in auto mode. Auto mode is the quietest, which is important to me.

I was not able to clean the evaporator coil right away. There was clearly gunk on the inside of the coil. It should be cleaned with water or a cleaner specific to the task. To not create a massive mess I needed either a bib kit or some coil cleaning foam. I opted for the foam, which I will use without once again taking the unit apart.

The Saga Continued in 2024

All the above was written near the end of 2023. It was left in draft form for a year. Having solved the problem of the moment, I set the matter aside, only to be reminded of it a year later when the very same trouble occurred!

This time, on the first occasion that I needed heat, the system worked as expected. However, when that cold front has passed and I once again needed cooling, the system would not work reliably. The green LED on the inside unit blinked 14 flashes. The internet reminded me that means “unknown error.”

Again, I had a tech pay me a visit. He reported the refrigerant pressure was high, reminding me of the lesson learned a year earlier. That also jogged my memory enough to know that, as a temporary measure, removing power for 10-15 minutes would clear the error, allowing the unit to run for a time, until I had the opportunity to clean it properly.

In order to clean it quickly I acquired a pistol-like blow gun nozzle for my air compressor. This cheap little tool allowed me to do what the first tech had done, using high-pressure air to quickly blow the worst of the crud off the blower.

Where he used high handy tank of refrigerant, I used my air compressor. I probably used less pressure, but it worked reasonably well. The blower actually sounds different when it’s not completely coated in fuzzy stuff. And of course, there’s a lot more air flow.

A New Strategy For Deep Cleaning

It costs $150 to have the tech out for a diagnostic, even if they don’t do anything. I don’t mind, if I learn something significant from the visit. I have now learned this lesson very well indeed. The mini-split needs to be cleaned. If it doesn’t get cleaned, it acts up.

It takes an hour or two to clean it. I know from my first effort that I need to be careful when doing this. There are some small plastic parts that would be best not broken. The blower has to be removed, cleaned and dried before it can be reinstalled. Also, the back of the coil should be sprayed with cleaner.

In order to make the routine a little easier (and shorter) I have purchased a spare blower for the unit. My Filter Company has spare parts for Mitsubishi. Going forward, I will always have a clean blower readily at hand. Just swap them out each time it’s required. The spare part cost less than a tech visit, and should make the process a lot faster.

In Closing

I’ve had three different mini-split systems in my home office over the span of 20 years. In general, they’re great. They do the job. They’re quiet and energy efficient, but they do need to be regularly cleaned. And the vendors rarely mention this.

I appreciate that these cleanings are not fun. Certainly not a much of a source of revenue for a contractor. In fact, one of our local HVAC companies has decided that they will not service mini-splits beyond the one year of warranty on the installation. I guess there’s just not enough money in it for the amount of trouble.

You can DIY these cleanings, as I have learned to do.