Heat Pump Cycling in Shoulder Seasons
We are about to start our first spring with our heat pump system in CZ6. We have two Fujitsu LZAS/LZAH15 single head systems, one for 1275 sq ft bungalow upper floor, one for basement suite. House is 1994 vintage, blower door test gave 4.82 ACH @ 50 pascals last summer, before some air sealing work.
I’ve read that heat pumps can run at lower efficiency when temps warm to the point that heating load becomes minimal, below the heat pump’s minimum output. This causes the heat pump to cycle on and off. We use and Ambi Climate wifi thermostat, which allows for comfort-based or temperature hold control, so the thermostat does the heat pump management.
Common recommendation is to avoid using the auto mode on the pump to avoid cooling air you just paid to heat, and visa-versa. But when the temperature and heat load are around that minimum heat output level, does alternating between cooling and heating avoid the reduced efficiency of cycling on and off entirely? Does auto mode make sense in the spring and fall?
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Replies
Cycling between heating and cooling is way worse than short cycling. You should have heating and cooling setpoints far enuf apart so you shouldn't be going back and forth between the 2. I never use auto mode, it can only result in more energy use. Do what you can to minimize short cycling, but I wouldn't worry about it. There may be some settings in the thermostat to help reduce the short cycling.
I agree you do not need to use the auto change mode on your thermostat. I think it was written for people that live in high desert locations for days when below freezing at night and uncomfortably hot by afternoon for weeks at a time.
I am sure you like your WIFI thermostat. I am not sure it is the most efficient way to control your mini splits. Most third-party thermostats will only command the mini with 3 options OFF, LOW, HIGH. Seems you me you are giving up much of the minis potentially highest efficiency operating speeds when you select the third-party thermostats and forcing the mini to cycle on and off much more than the factory controls would. The third-party thermostats low will be about 50% of the high speed and the factory controls often will run the unit down to 10%.
I think the best way to operate a heat pump is to set the thermostat and forget it for weeks if not months at a time. How useful can the WIFI functions be when you so rarely change a setting.
Walta
The Ambi is pretty sophisticated. It controls the mini's set point in .5c increments. It uses AI to consider indoor and outdoor temps, sun exposure, humidity and user feedback to decide how high to turn up the mini to maintain a comfortable temp. It supports the mini's auto mode as well.
It's still very new tech - I've had some issues with it losing control of the mini after turning the mini off in low-demand situations. But when it's working, which is most of the time, it is set and forget.
Can the Ambi turn down the output to 10 or 20%? If not all the fancy features save you no energy, and don't reduce cycling
My understanding is that the Ambi is just a wifi-connected RFID controller. Like the Sensibo. It has no more controls or capabilities w/r/t to the actual functioning of the unit than the remote that is supplied with the unit does. The benefit is that you can control the unit from your smartphone wherever you are and maybe there's some additional timer/scheduling abilities and some gimmicky maintenance reminders - which are basically multi-week or -month timers that remind you to clean or replace the filter.
I'm a Sensibo user myself...
@brad The Ambi has its own thermometer it uses to determine the target room temp you want, instead of relying on the split's head thermometer. You place it in a part of the room you use most and at a sensible height in the room. The heat pump may be set higher or lower to help achieve or maintain that Ambi temp. In my experience, when the heat pump's set point gets turned up high, the pump responds by putting out more heat with a faster fan speed, and the opposite when the set point is at or below the current Ambi/room temp - so it can run at minimum output. The heat pump never fully turns off (fan runs low but steady, some heat produced) so I'm not seeing much evidence of cycling outside of deftost cycles.
Ian, feel free to be dismissive but I shopped the Sensibo before I made this choice. The Ambi does what it says - it keeps the room comfortable all day even if I only give it feedback a couple of times a week. Am I eliminated the need to constantly tweak the head's set point because changing conditions (weather, day/night, humidity, etc) affecting our comfort level. If Ambi can work out its reliability issues (rare but frustrating loss of control) it's the best tool available right now for making a heat pump an easy option for most homeowners.