mitsubishi heat pump seems to not be able to ramp down compressor fully when fan is on high or super high setting

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aaron1

Member
Oct 9, 2012
188
Poughkeepsie, NY
Hi, I just got heat pumps last spring and am now trying to figure out how to most efficiently use them . I have 2 units on my first floor, an 18k in the living room and 12k in the kitchen. I have been using the 18k mainly for now, as it's not that cold out yet. I have been monitoring the current draw of the heat pump at various times to get an idea of how much the compressor is ramping up or down. I have seen anywhere from 2.2 amps or so to maybe 6.6 when running full out and then it has gone up to like 9-10 when coming out of a defrost cycle one time.

One thing that really confused me the other day is that it appeared that it was short cycling (it was in the high 50s outside two days in a row and the house probably had built up a large amount of heat. I left in the morning with it set on high (3 bars) fan speed. It is set to 76F, which results in a 70-72F temperature in my room most of the time. It was getting warm out and I noticed it kept climbing past 78F (a temperature sensor is mounted up on the unit outside of where the unit's sensor is located so I can gauge what it is seeing as the room temp.) It will usually level off at 78F when it is colder outside, presumably by ramping down the compressor some. However, since it was so warm out and the house had been so warm for a few days, the compressor output it was using was making it go past 78 to 79F. Once it hits 79F up at the ceiling sensor, it will short cycle. I went down to test the current and it was pulling maybe 4.2 amps (~1000 watts.) Theoretically, it should be able to put out only 2.1-2.3 amps (500ish watts) or so on low compressor mode. I am wondering why it was refusing to ramp down further than 4.2 amps. Is it because I have the fan on manual mode instead of AUTO mode? Is it trying to make sure the temperature of the air coming out of the unit is a certain minimum temperature? I've rarely seen the current less than like 4.2 amps. I did see it that low one time when it was on AUTO mode with a very low fan speed, so that made me think that it should be able to go below 4.2 amps. I assume it is trying to keep the air leaving the unit at a certain temp and will gladly short cycle over and over to do that rather than drop the temp coming out down (by ramping down to a lower compressor setting), which would allow it to keep running on high fan speed and not short cycle? I think I would prefer to avoid short cycling and have a lower compressor output, as short cycling drives me crazy and is a waste of energy)
 
You may be “over operating” these things. You bought a premium product with more brains than you might think. Let it do it’s thing. Use auto.
 
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Remember its heat output depends on the outside temp. The unit probably only cares what the inside temp is and knows what output to hit the set point is.

With the fan on high they probably measured the temp across the whole coil and realized that at any compressor output lower than this 4.2 amp setting some of the coil may not be getting hot waistline energy to blow cold air through a cold coil. Just a thought.
 
You may be “over operating” these things. You bought a premium product with more brains than you might think. Let it do it’s thing. Use auto.
The problem with auto is that the unit is scared to death to increase the fan unless it absolutely has to. This means that the compressor runs pretty hard but the fan speed is anemic, which results in poor heating.
 
The problem with auto is that the unit is scared to death to increase the fan unless it absolutely has to. This means that the compressor runs pretty hard but the fan speed is anemic, which results in poor heating.
In heating mode the lowest air flow feels the warmest. Many people complain when switching to heat pumps that they feel colder even though the temperature is the same because the supply temp is lower than let’s say a gas furnace. It’s all about balancing comfort and efficiency. They done the mast and know the most efficient way to operate it but it may not be the most comfortable way.
 
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In heating mode the lowest air flow feels the warmest. Many people complain when switching to heat pumps that they feel colder even though the temperature is the same because the supply temp is lower than let’s say a gas furnace. It’s all about balancing comfort and efficiency. They done the mast and know the most efficient way to operate it but it may not be the most comfortable way.
Yeah, I think that is a big part of it. They got so much complaining that they decided to just make sure the output air is always a minimum temp. even though it results in wasted heat production on a warm day. I think I need to integrate my heat pumps into a IFTTT setup with Kumo Cloud (Mitsubishi online split control software) so that I can force them to just shut off when it gets warm outside.
 
I run my Mitsubishi on auto fan settings and I'm happy with how it does. If the temperature inside is more than 4 degrees or so below the setpoint, the fan will crank up. Otherwise it runs inaudibly quiet.

I think the heat shedding efficiency inside does not need a very high flow rate. I think they crank up the fan mostly when the setpoint is quite a bit higher than the actual temp in order to mix the warmer air it blows out with the colder air in the room.
 
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The problem with auto is that the unit is scared to death to increase the fan unless it absolutely has to. This means that the compressor runs pretty hard but the fan speed is anemic, which results in poor heating.
Maybe the only question is why is your outside unit running high when your inside unit running low and you are experiencing “poor heating”. When my inside unit idles down, so does the outside unit. Of course everything is sort of slow on these, not bang bang on off like traditional splits.
 
Maybe the only question is why is your outside unit running high when your inside unit running low and you are experiencing “poor heating”. When my inside unit idles down, so does the outside unit. Of course everything is sort of slow on these, not bang bang on off like traditional splits.
the unit does seem to ramp down the compressor better at least sometimes when the head fan is on a lower speed, but it won't do it when the fan is in a higher speed. i've read that other people have found that on auto fan mode they will try to run at low head fan speeds until the it drops below the set temp like 5 degrees and then will ramp up fan speed. i want it to run on higher or really high fan speed and then ramp down or up across the full range. another very annoying thing is after a defrost cycle my kitchen one will ramp up to very high compressor to try to "catch up" and in doing so seems to short cycle every time. i really need to install wall thermostats for them to alleviate some of my problems during heating season.
 
I run my Mitsubishi on auto fan settings and I'm happy with how it does. If the temperature inside is more than 4 degrees or so below the setpoint, the fan will crank up. Otherwise it runs inaudibly quiet.

I think the heat shedding efficiency inside does not need a very high flow rate. I think they crank up the fan mostly when the setpoint is quite a bit higher than the actual temp in order to mix the warmer air it blows out with the colder air in the room.
i suppose this depends on how many btus the mini split can put out and how much it needs to put out. it will also depend on how well insulated and air sealed your house is i guess. i don't really want it to have such a large hysteresis. that seems ridiculous to me.
 
Hm. My home has 1977 2x4 walls, excellent windows, excellent sealed and insulated attic. Hysteresis is 1 deg F or less. Both on the unit and on the wall mounted thermometer (and hygrometer) across the room.
 
Hm. My home has 1977 2x4 walls, excellent windows, excellent sealed and insulated attic. Hysteresis is 1 deg F or less. Both on the unit and on the wall mounted thermometer (and hygrometer) across the room.
yeah that sounds a lot better than me lol.
 
I wonder if yours is defective. Check with the installer?
 
I wonder if yours is defective. Check with the installer?
i should be seeing someone from the company next week when i am having an inspection done by someone from the state to make sure they are running efficiently. we can go over all of this. the installer is very responsive so i should be able to make them work as well as can be expected and find out if they are having issues or are just not designed the way i thought they would be.
 
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Good explanation here. The summary is the controller try’s to maintain a specific coil temp so it runs the compressor harder.
Yeah, I was the OP in that thread, lol! It makes sense that is what it is trying to do. It fits with all of my observations. So I think at this point I need to either manually control the fan speed or set up some home automation to control it. Thanks for sharing!
 
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Yeah, I was the OP in that thread, lol! It makes sense that is what it is trying to do. It fits with all of my observations. So I think at this point I need to either manually control the fan speed or set up some home automation to control it. Thanks for sharing!
Have you read through the installer setup manual? They are making user control very difficult/limited so the average Joe can’t mess it up and have a call back.
 
Have you read through the installer setup manual? They are making user control very difficult/limited so the average Joe can’t mess it up and have a call back.
I looked through that when i first got them. fortunately, they have this online control software called kumo cloud that connects up with your household wifi and will allow you to control your splits from anywhere. you can control fan speed, set temp, on/off, lots of things. i don't think it would be too hard to set it up with a smart home hub so that it would ramp the fan up and down depending on the outdoor temp or the temp on a wall sensor.