Shut off mini-split fan when setpoint is reached once a wired controller is installed?
I got a lot of help from this forum early in the summer in dealing with the poor performance of my Mitsubishi mini-split (1.5-ton, 18 SEER, single indoor air handler). Much of this stemmed from its measuring room temp inside the unit close to the ceiling. My last step was to install a wired simple MA controller so that it now knows the actual room temp.
Having done this, the mini split locks onto the setpoint temp when the outdoor temp goes beyond 100 F and regulates the room temp successfully as long as it stays hot. However, once the outdoor temp fall below 100 F (late afternoon, evening, overnight), the room temp falls. The intake fan runs continuously (if on low) and the room temp will drop as much as 3 F by the next morning, which is uncomfortably cold to me. Adjusting the setpoint does nothing at all.
My conclusion is that the unit can’t turn coolant flow down low enough to track the setpoint except on the hottest parts of hot days and over-cools the room most of the time that it is in cooling mode. This unit is oversized as the room’s design cooling load is in the range of 10-12,000 btu/hr. (I am having a Manual J study done to confirm that.) The unit’s minimum capacity is 5,800 btu/hr, which is one-half or more of the design cooling load!
I can stop the air circulation fan once the setpoint is reached. This is done by cutting a specific jumper on the control board. The unit would continue to circulate unneeded coolant, but that shouldn’t cool the room if the fan is off. However, I cannot find any information on when the fan will turn off and back on. If that is close to the setpoint (within +/- 0.5 F), that would be fine. If those points are +/- 1 or 2 F from the setpoint, then stopping the fan won’t improve the comfort level. Does anyone know what to expect? I don’t want to discover that cutting the jumper was a mistake.
Thank you for any help you can offer!
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In case this helps someone else, I did cut the indicated jumper to stop the circulation fan in cooling mode. It seems to help the unit regulate very, very close to the setpoint when it is very near the setpoint. However, it does not help my problem -- which is that the unit is forced to run and overcool the room once it cools off outside because of its high minimum run capacity. The fan continues to run while this is happening, perhaps because it must in order to protect the hardware.
My next step is to replace this 4 year old unit with one that is properly sized.
I'm not sure oversized has much to do with it. I think it's just how minisplits work, with the constant fan, which I hate, even when not cooling (or heating).
Have you tried simply turning the temp up at night to compensate? Often you can program them to do that automatically.
If your minisplit is programmable, couldn't you just set a program which shuts it off for a while and turns it back on?
Or if you set it to Auto mode, it should cool or heat to maintain a temp. My Mr Cool DIY has both.
But I would much prefer if it would turn off the fan after temp is reached, and then turn on every 10 mins to check air temp.
Also, why don't you simply turn the temp up at night?
Once the room temp starts to drop, turning the thermostat up 1 or 2 deg has no effect. The unit must dump 5,800 btus/hr (it minimum run capacity) of cooling into a room with a design cooling load of 9-10,000 btus/hr. Turning the thermo up 3 or more degrees sends it into what I call "give up" mode where coolant flow stops except for periodic bursts of flow (for some reason). The fan continues to run and the room gets colder. Today, our outdoor high was 101F. The room started out at 76.5 F from overnight and never reached the setpoint. It insists on dumping 5,800 btus/hr of cooling as long as the room temp is not more than 2 F below the setpoint.
I have a 3-zone 2 1/2 ton (27K BTU) mini-split (MR Cool DIY), with 3 air handlers (12K, 12k, 9k). 12K in my finished attic (without mini-split running, it easily reaches 85F up there on hot days, no higher than 67F on cold days...not the best insulation...heat radiates in from hot roof, or cold enters. We have Central A/C as well, but not enough ducts in the finished attic to cool or heat effectively).
What I've noticed is that you really have to turn the setpoint at least 2 degrees higher than what you really want. But when it stops cooling, within a 10 minutes of so, the air coming from the air handler is room temp. I don't notice any bursts of flow. I've checked and the condenser fan is stopped outside, even though the indoor unit fan is spinning.
What your unit is doing is very strange, honestly, once you reach the setpoint, the outdoor condenser should shut down completely, not keep cycling refrigerant. It's not like it has to "get rid of the cold" in the refrigerant. Honestly, it sounds defective, and I'd have someone look at it if it's under warranty.
If it's not defective, and you are able to program it, it may be effective to program it to turn off at say 10pm for an hour, turn back on at 11pm, run for an hour, and keep it cycling off an on. You may need to adjust the cycle times and off times.
If you end up replacing your mini-split, I'd recommend either the MR Cool DIY, or a MR Cool Advantage (similar to DIY, but needs installed by HVAC person).
You may also want to consider getting a multi-zone unit, with say 2 air handlers. Each maybe 9K, instead of the 12k you have now. So, in the hot part of the day, you run both. At night, you run only one, as cooling need is much less.
To be clear having oversized equipment will not make an HVAC unit overshoot its set point.
Your single headed unit should be able to operate down to 15 % of its max capacity or 2700 BTU that is almost nothing and if that tiny amount of cooling is still too much it should turn off. There is no logical reason to overshoot the target temp.
You have some sort of control issue. I recall some posts about Mitsubishi having updated firmware for similar issues but I don’t seem to be able to find it.
Just to be 100% clear the unit is NOT set to its “DRY” mode?
Walta
Walta, my unit is NOT set to "DRY" mode. It CANNOT turn down to 15% of max capacity. It is 18,000 btu/hr rated capacity with minimum capacity of 5,800 btus/hr per the Mitsubishi spec sheet. I estimate the room has a design cooling load of 9-10,000 btus/hr and am starting the process of having a Manual J done.
If it is 104 F outside, the unit locks onto my 79F setpoint and follows it. Once temps fall much below that, the room temp starts to fall with the fan still running. This time of year it bottoms out below 77F overnight. Your previous comment taught me about the minimum run capacity. I have to believe the unit is stuck putting 5,800 btu of cooling into a room that doesn't need it. My unit is limited in its ability to turn down (to only 32% of max capacity) and then it is from at least 50% up to 100% oversized for the room ... a double whammy.
Thank you for the comment about firmware update. I will find that information before I go any further with the replacement. This forum is a great source of information and help, which I simply cannot get from any of the local HVAC companies (even Diamond Elite).
Later: I've done a quick check on firmware updates and have found this issue in several places on the web. https://www.greenbuildingadvisor.com/question/mitsubishi-hyper-heat-minisplit-noise-defrost-cycle-issues. I do NOT have a hyper heat model and my problem is in the cooling cycle. I will keep looking for something else.
Oversized should make it cycle on and off not make it overshoot.
The small range of modulation makes me wounder if the outdoor unit was intended for mutable head applications.
If you are desperate, you could bypass the factory controls with “Thermostat Adapter Interface” and a standard thermostat. You would give up the variable speed features and are reduced to off, low and high for control.
PAC-US445CN-1
Walta