Figured out the problem with my 18K Mr. Cool mini-split install

[jmarkwolf]

I bought my Mr. Cool 18K mini-split air conditioner for my 28ft x 30ft shop with 10ft ceilings from Costco back in the summer of 2019. I used the template provided for locating the hole in the wall for the hose/cable set relative to the steel mounting bracket. We had a hell of a time getting the inside unit to hang on the wall properly. It didn't want to snap into the supplied bracket properly. After a lot of pushing and pulling and gnashing of teeth we got it hung, with the cable set somewhat centered in the hole in the wall, but I wasn't impressed with their published install procedure.

It wasn't working all that well either. I was afraid maybe I selected the wrong capacity for my shop space.

Two weeks later, the unit stopped working entirely and showed a blower motor failure error code. I called the Costco store, they referred me to the main Mr. Cool distributor if I remember correctly. I described the scenario to them, after I sent them a pic of my receipt and the manufacturers data plate they offered to send me a whole new inside unit or blower motor. I opted for a whole new inside unit. It arrived quickly.

By this time it was late in the summer and I never got around to removing the old one and installing the new one. Nor did I the following summer (2020).

In the mean time, I always wondered if the trouble we had hanging the inside unit might've stressed the plastic chassis enough to bind either the blower motor or squirrel cage, causing the early failure.

Today, I decided to install the new inside unit which was still in the box, and taking up valuable floor space in my shop. I took some careful measurements and discovered that the dimensions on the supplied template were off significantly, relative to the locations of the bracket dogs and the hole through the wall! The template could be used for two options of hose set routing through the wall, and/or two different models of inside unit, neither of which were correct for my unit! This indeed had been preventing the original inside unit from snapping into the dogs on the wall hanging bracket properly. Consequently, I very well may have inadvertently stressed the plastic chassis trying to force it onto the bracket dogs with the hose set centered in the hole in the wall, causing the blower failure.

This time, I used my own dimensions for locating the steel hanging bracket relative to the existing hole in the wall, and it went much much better. The unit snapped into place properly with no grunting expended!

The unit is humming along beautifully, reducing the temperature and humidity nicely.

Stay tuned campers!

 

[Cougar67]

You complete projects as fast as me. I was wondering when I watched a YouTube of the installation. How close to the ceiling can you mount the inside unit? It looks like you have to raise it and drop slide it into the bracket? Keep us updated on how well it's cooling.

 

[jmarkwolf]

Update from OP here:

A little over a week and the mini-split is kicking booty, at least temperature wise. It'll make my shop an ice box. But it's inconclusive so far as to humidity control.

The best humidity number I've gotten so far is 60%, at least on a cheap plastic thermometer/hygrometer combo instrument. And it does feel humid in there. Disappointing since much of the reason for installing the mini-split was to prevent my machine tools from rusting.

Will keep experimenting.

Also, how can I contact the moderators? I should've posted this thread in the Heating and A/C forum, and would like to have it moved.

 

[Worsedog]

While I never would claim to be an A/C expert, as I understand it, the effectiveness of humidity control is a function of the cycle time. If it doesn't run long enough the moisture doesn't get drawn from the air. Other than huge air leaks the most common moisture control problem is oversized equipment. Having said that, my A/C guy has said that the inverter based units are more forgiving of that, but you can still go too big. YMMV.

My best example of the oversize issue is hotel rooms that have package unit A/C in each room. You can make the room into a fridge, but it's muggy. I think they get oversized so guests don't complain it is not cooling down fast enough.

 

[CombatNinja]

Rule of thumb for HVAC is that if your space is cold yet damp, the unit is likely oversized for the application. It comes on and short cycles because it is so large that it will cool the space too quickly.

 

[NUTTSGT]

Also, how can I contact the moderators? I should've posted this thread in the Heating and A/C forum, and would like to have it moved.

I saw this and was going to move it anyway, no big deal.

To find a Mod or Admin, click on the 3 lines in the upper left... pancakes.

On the drop down, click Members. Scroll down to Staff members. This is your forum Moderators and Admin.

Or quicker, under members, first thing you come to is "Most Messages" or something like that. I'm at the top of the list (yeah, spend too much time here) and you can simply PM me, no big deal. Feel free to PM any other Mod if you choose to.

Just include a link to the thread in question.

 

[metlmunchr]

Your unit should have "dry mode" on the remote. This will make the unit act like a dehumidifier without cooling the space excessively. It actually functions much more efficiently than a normal dehumidifier because it pumps all the latent heat extracted from the condensing moisture to the outside via the outdoor unit where a standard dehumidifier pumps all that heat right back into the space. A dehumidifier will actually warm the space while a mini in dry mode continues to act as an air conditioner but with the balance of latent vs sensible heat removal shifted toward latent heat (dehumidification).

 

[jmarkwolf]

OP here with an update:

Well, I'm disappointed to report that while my 18K Mr. Cool mini-split can make my 785sq-ft shop (27ft x 29ft) an ice box, it will not reduce the humidity below 60% regardless of the mode, nor time spent "at soak". And the only way to get to the 60% mark is to make the room so uncomfortably cold that any tool I pick up is too cold to hang on to.

The room is insulated with R19 everywhere, with an insulated big door, drywall on the ceiling and T1-11 on the walls. There is no water source in the room to contribute to the humidity, and the condensate tube is flowing freely outside.

I double checked the capacity against numerous on-line calculators and 18K seems to be the correct size for the room with 10ft ceilings.

I'll call Mr. Cool next, but it looks as though there may be a dehumidifier in my future as a supplement to my Mr. Cool mini-split. At least it's good to know that the mini-split will be able to compensate for the heat exhausted from a de-humidifier.

Ideas gratefully received.

 

[HoosierBuddy]

What are you using to measure your humidity?

Any chance the gauge is bad?

 

[yeldogt]

What in the min output of the unit? ..... I figure over 1k sf per 12k of BTU ....

I have an 18k for 1800 sf in NYC .... In PA have two units .... for my 1700sf space. (9k and 12k) The 12 is the one running most of the time

 

[Yankeefarmer]

Discussions about humidity that don’t discuss outside humidity levels and air exchange can be very frustrating. We are at the end of June, and the natural temperature of my shop with lots of tree-provided shade and a well-insulated ceiling is below 70 degrees. Even on a 90 degree day, if I don’t open the big doors, the temperature will not rise above 75. Couple that with days with 70 degree dew points, and an air conditioner can not be expected to dehumidify unless set to bone-chilling cold. Dehumidifiers exist for a reason.

 

[Bert_]

OP here with an update:

Well, I'm disappointed to report that while my 18K Mr. Cool mini-split can make my 785sq-ft shop (27ft x 29ft) an ice box, it will not reduce the humidity below 60% regardless of the mode, nor time spent "at soak". And the only way to get to the 60% mark is to make the room so uncomfortably cold that any tool I pick up is too cold to hang on to.

The room is insulated with R19 everywhere, with an insulated big door, drywall on the ceiling and T1-11 on the walls. There is no water source in the room to contribute to the humidity, and the condensate tube is flowing freely outside.

I double checked the capacity against numerous on-line calculators and 18K seems to be the correct size for the room with 10ft ceilings.

I'll call Mr. Cool next, but it looks as though there may be a dehumidifier in my future as a supplement to my Mr. Cool mini-split. At least it's good to know that the mini-split will be able to compensate for the heat exhausted from a de-humidifier.

Ideas gratefully received.

How often does it run on a hot day?

Some calculators are more accurate than others. This can be a hotly discussed topic.

 

[glend123]

I would get a separate dehumidifier.

 

[Crowbarman55]

Is your floor coated, tiled or just the plain slab?

 

[metlmunchr]

Going back to your original post, you mentioned changing out the indoor unit. Did you pump the outdoor unit down prior to changing out? Or just close the service valves? Perhaps the Mr Cool units come with the tubing attached to the indoor section and precharged? I don't know as I'm not familiar with their units.

My point is whether or not you may have lost some refrigerant in the process. An air conditioner that's somewhat low on refrigerant will maintain most of its capacity to cool but lose some of its capacity to dehumidify due to the fact that the evaporator coil temperature will be higher when some of the charge is lost.

You can get an approximate indication of the unit's charge by putting it in max cool mode and checking the return and supply temps at the indoor unit. If you set the fan on max speed and turn the thermostat down as far as possible, and then let the unit run for 10 minutes, that will put most minis in max cool mode. The temperature drop thru the unit at that point should be between 20*F and 24*F. Any less than 20* is indicative that it may be undercharged. The space temp and outdoor temp should both be at least 72*F or greater when making this check.

For example, the the 9K btu Pioneer mini I installed recently in my den checked at 72° return and 49° supply with outdoor temp about 85°.

The cheap mechanical and electronic hygrometers typically have a stated accuracy of +/-7% Discussions on various HVAC forums indicate the real numbers are closer to twice that in many cases, and that the accuracy is random. I remember one guy saying he bought 10 identical cheap ones and checked them against a known accurate device. He found a couple were within 4% while the remainder were anywhere from 7-8% to as much as 12% off.

Personally, I've used a Bacharach sling psychrometer for 30+ years, and IMO, it is hard to beat when it comes to accuracy because it operates on the basic principles of dry bulb and wet bulb temperatures. You simply measure the two temps and use the built in slide rule calculator to determine RH. No batteries, electronic circuitry, or mechanical gadgetry between the measurements and the answer. Googling will provide demo videos. Available from supplyhouse.com for about $85. Don't waste your time with Amazon as the prices there are ridiculous.

One other thing is to make sure whatever you use to measure RH is located such that the readings aren't affected by the discharge air stream from the unit. Moisture within an enclosed space migrates freely to maintain a uniform RH level within the space, but measurements within the air stream of the unit will distort both WB and DB temp readings which will give inaccurate RH readings regardless of the accuracy of the measuring instrument.

 

[jmarkwolf]

Update from OP here:

Mr. Cool tech support got back with me and confirmed that my 18K unit is basically the correct size for my space, but he offered little in the way of explanation as to why it is not reducing my humidity very much (best case 60%). He said the 18K was perfect for my floorspace with 8ft ceilings, further stated that it was a little undersize for my 10ft ceilings, but that the next size up (24K) would be too large for my space, and would short cycle). I asked him if he could "ball park" the reduction in humidity that I could expect, and he stated that there are "too many variables".

I had been expecting similar performance to my house air conditioning which will significantly reduce ambient humidity in a matter of minutes. Maybe I was simply too optimistic.

So, I'm very disappointed in the humidity reduction performance of my Mr. Cool mini-split, but am confident that the temperature performance (it will easily reduce temps to far too cold) will easily overcome any rise in temperature imposed by a supplemental de-humidifier, should I opt to go that route.

Otherwise I'll just have to get used to the clamminess on the back of my neck while working in my shop, and hope that 60-65% humidity will be adequate to keep my machine tools from rusting.

 

[Yankeefarmer]

You haven’t mentioned what temperature you are trying to maintain, nor the amount of sun exposure your shop has, nor how tight it is. The dew point in Dundee, Michigan( a southeast Michigan location I picked off a map at random) is currently 74 deg F. That means that any air that enters is essentially at 100% humidity if your shop is at 74 deg F. If your cooling load is low, because of a big cold slab, no sunlight shining in through windows, a shady location, etc, an air conditioning-only approach to dehumidification cannot be successful.

Ifyou have WiFi available in your shop, you may want to consider purchasing a temperature/humidity monitor that logs the data for you so you can get a better idea of what’s going on. I’m including a picture of this morning’s data from mine here in Connecticut, where we are at 94 def f and a dewpoint in the low 70’s. While I don’t put a lot of faith in accuracy of the relative humidity numbers, I do believe they are directionally correct. You can see the central air kicked on for the first time this morning at about 11:00 am, and with each cycle the humidity dropped a bit more. Mine is a new building, and is very tight infiltration-wise.