How often should my heat pumps cycle when heating?

allenm

We had two separate Mitsubishi H2i units installed this past June, a MSZ-FS06NA in a bedroom and MSZ-FS18NA in the living room. All seemed fine during cooling season. Now that I've switched them over to heat, they seem to cycle too often. It's not really noticeable during the daytime, but it sure is at night in the bedroom.

According to my energy monitor, both units cycle on/off every 8-10 minutes mainly at night when the temperature drops down into the 40s and 50s. Don't know yet about 20s and 30s or lower.

Is this normal?

pecmsg

Were they sized for heating or cooling?
Where is the house located?

allenm

pecmsg said:

Were they sized for heating or cooling?
Where is the house located?

Good question about sizing for heating or cooling. I guess I don't really know. The dealer entered our room sizes, doors, windows, etc into a program on his computer and came up with 6 for the bedroom and 18 for the living area. Made sense to me at the time.

We are located in southern New Hampshire, about 12 miles north of Massachusetts state line.

Hot_water_fan

Something to look into would be where the temperature is being sensed. I think it can either be at the unit itself or via the controller or a separate thermostat. It could be short cycling because the temp near the head is too warm due to poor air circulation. 

EdTheHeaterMan

That system has a variable output. based on the air temperature sensed at the inlet of the MSZ-FS06NA the outdoor compressor speed (output) will ramp up and down to accommodate a steady room temperature. The thing is that the outdoor unit is at least 24,000 capacity and at the slowest speed, (minimum output) the compressor can only go down to so much. The outdoor unit may have a heating capacity as high as 30,000 BTU (Not sure what outdoor unit you have). At 10% operating speed when the outdoor temperature is in the 40s and 50s the smallest that unit can be is a 3000 BTUh unit. If your room only needs 2000 BTUh to satisfy the load, then the unit will cycle off because the indoor unit is producing 1/3 more heat that the room requires.

As the outdoor temperature drops two things will happen, the amount of heat the compressor will generate will be less and the indoor load requirement will increase. When you get to that "sweet spot" the compressor and indoor fan will not stop. It will modulate as needed based on the room air temperature.

I am guessing that you turn down the Living Room 18 unit at night... If that is true then try this experiment. leave the living room operate at night to use some of the compressor output. By sharing some of that 3000 BTU minimum with the living room, perhaps the bedroom will cycle less or not at all.

Just a guess on my part, not sure it will matter much.

Also not sure what the turndown ration is on your outdoor unit since Im only guessing at what model number you actually have. It may only go as low as 15% or 20%

allenm

Ed - if it has a variable output, that explains why my KWh energy monitor shows these units kind of ramp up to a certain level of current draw and then maybe 8 minutes later the current starts ramping back down. Maybe frequent cycling isn't that big of a deal on these units?

The temperature sensor is inside on a circuit board. I was wondering if a small fan might inside might help. I should have mentioned the bedroom does have a ceiling fan about 8' away. It doesn't seem to make any difference running or not.

My regular heating system (off but still works ok) is hot water baseboards, so no air circulation that way. I'll try running the living room system at night, and see if that helps.

pecmsg

You don’t want a unit starting and stopping but the weather is mild right now so it just can’t slow down and still run. 

Check it again tomorrow night when it’s cooler. 

EdTheHeaterMan

allenm said:

Ed - if it has a variable output, that explains why my KWh energy monitor shows these units kind of ramp up to a certain level of current draw and then maybe 8 minutes later the current starts ramping back down. Maybe frequent cycling isn't that big of a deal on these units?

The temperature sensor is inside on a circuit board. I was wondering if a small fan might inside might help. I should have mentioned the bedroom does have a ceiling fan about 8' away. It doesn't seem to make any difference running or not.

My regular heating system (off but still works ok) is hot water baseboards, so no air circulation that way. I'll try running the living room system at night, and see if that helps.

Actually there several sensors that all connect to the circuit board by wires. The actual sensors are connected to the refrigerant pipes, the coil and and where the sensors can measure air flow temperature before it gets to the refrigerant coil. Operating a ceiling fan should have little effect on the heating operation of your Mitsubishi.

Hot_water_fan

Is this system 1 outdoor unit or 2? 

pecmsg

I’m reading it as 1 outdoor 2 indoor. 

The model # of the outdoor would clear it up. 

allenm

Hot_water_fan said:

Is this system 1 outdoor unit or 2? 

It's actually 2 separate systems, so 2 outdoor units and 2 indoor units. Nothing in common between them.

allenm

A few more things I should have mentioned in the beginning. The bedroom is about 14 x 16 and the door is always left open. It has a ceiling fan, but that makes no difference as far as short cycle times. The hvac room temperature sensor is located inside the wall mounted unit.

It woke me up at 5:05 AM this morning when it came on. I counted the seconds. The fan was ON and blowing warm air for 28 seconds.

Then it came on again at 5:13 for 2 minutes. Again at 5:52 for less than a minute. Again at 6:07 for 1 minute.

The bedroom temp is set for 67 degrees. I have a separate thermometer in the bedroom and it reads right around 68 degrees all the time.

It was 47 degrees outside this morning at 5 AM.

Meanwhile the living room 18K unit covers a much larger open area and was staying on for about 6-8 minutes. Same setting 67 degrees.

pecmsg

Something doesn’t sound right. 

allenm

I fixed the problem, at least to my satisfaction, by installing Mitsubishi's PAC-USWHS003-TH-1, Wireless Temperature and Humidity Sensors for both wall units. These are little battery powered Bluetooth sensors that work with Mitsubishi's Kumo Cloud. I mounted them about 12 feet away from the wall units.

Now the bedroom unit, which was my main concern, is coming on about 3 times an hour for as long as 20 minutes. No more short heat cycles. Hopefully they will work just as well when cooling.