Duty Cycle and CoP
I have 2x 12K Fujitsu’s hooked up to a 24k Multi, non hyper heat. In my attic office, it is way oversized so I run it on low fan speed for heat.
When just using the single attic 12k, the outdoor unit spins for 2.6 minutes, then is off for 2 minutes. Inside, the fan is in the sensing temp speed extra low, and then will go to low for 10 seconds. Obviously short cycling. Outdoor temps in the low 40s.
If I add in the first floor 12k split which I have on high speed (it runs almost continuously), the attic head now runs for 3 minutes on low, and is off for 17 minutes.
How do these scenarios differ for either kWh usage or CoP regarding attic usage? Or is it pretty marginal at these outdoor temps?
Kevin
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Replies
The best way to determine how the short cycling is affecting your electricity usage would be to measure it. Use any commonly available home energy monitor.
In a general sense, the compressors in the outdoor unit of the minisplits has a large spike in energy every time it starts up. So short cycling on and off every 2 minutes will definitely use more energy and be hard on your components.
I guess I'm confused why the run times are so different. A solo split on a multi will run the fan for :10s inside every 4.5 minutes. Turn on another head and now it runs for 3 minutes every 20 minutes. Shouldn't the goal be to maintain the set point?
The problem is that your outside unit can only modulate down it's capacity to some minimum point. Just for fun, lets say that it is a 24,000 btu/h unit and it can modulate down to 10,000 btu/h. When only one indoor unit is running, that unit has to output the full 10,000 btu/h to the room, satisfying the thermostat very quickly.
Now, if you activate another indoor unit, and that unit needs 7500 btu/h, now the 10,000 gets split between the two. The first indoor unit can now modulate down to 2500 btu/h, making it able to run longer to stay close to the setpoint.
Note this is very simplified, and there is also a minimum for the indoor units, so they can't keep going down forever.
Hope that clarifies things!
The spike when a motor starts is of such short duration that it isn’t a significant contributor to overall energy consumption. It is the operating conditions of the compressor that will make the difference here, not the number of starts (within reason).
Actually measuring the energy use of the unit under different operating conditions is the best way to really understand what the most efficient settings are going to be though.
Bill
The spike itself is not the only increase in energy consumption caused by starts, it also takes a few minutes for the system to reach steady state operating efficiency. It's been hotly debated how much extra energy this actually uses if the oversizing is only 50-100% but the industry consensus for now is right-sizing is important for energy efficient operation. Most of the research has only been conducted on air conditioner cycling though, not heat pumps. There are additional parasitic losses like crankcase heaters to be accounted for with heat pumps.
I definitely think the short cycling he is describing would be bad for energy use and for equipment longevity.
There different reasons for short cycling. Figure out which one you're experiencing.
1. Poor feedback due to using the built in return air sensor (most likely)
2. Oversizing (likely but made worse by #1)
3. Low load ( think running low fan speed on one zone of a multi split)
While over sizing is a real problem, even a properly sized unit can easily short cycle if the controlling sensor prematurely senses that the space is satisfied. Using a remote sensor (basically an extension harness), or a wall thermostat placed appropriately can do wonders.
Turning up the indoor unit fan speed may help keep the outdoor unit running. If a single indoor unit is running on low fan speed. It may be below the minimum capacity of the outdoor unit. It will be a compromise though, since turning up the fan speed will make over-sizing problems more evident (more CFM on a multi split = more BTU/hr).
This problem is specific to multi splits (1:1 one will vary the capacity even with a fixed fan speed, multi splits often have a pretty constant capacity at each fan speed and tend to cycle more like traditional furnaces).
If you have an accurate and responsive thermometer. Place it somewhere else in the attic space, if the unit is cycling before it changes much than your problem may be more of a feedback issue than a sizing issue.
My 1:1 9000 btu unit is probably slightly undersized, but it would still hunt until I switched to a remote sensor/controller. The return air was affected way before the room was satisfied. Probably because I have low ceilings. Will the remote sensor the room temp is much more stable and the unit never shuts off until it's probably 60degF outside (1:1 with a wide modulation range).
While the inrush doesn't really add to your kwhr since it's so short. There is a minute or two of runtime at startup where the compressor is running but you aren't really getting any heat. This startup cycle is wasteful.
Buy an electricity monitor and you can see exactly how its modulating. Seeing the live modulation data has been an enormous help in running both my 12k units efficiently. Other members on here took my suggestion for one and found major issues with their units or how they were running them. You will find that fan speeds have a big effect on efficiency and how they operate.
I use an Efergy Engage that cost only $130 and can monitor up to 5 circuit's. Here is a screenshot.