Turning down mini splits?
Muddytyres
| Posted in General Questions on
We have a new minisplit system. Is it beneficial to turn it down during the day and for the days we’re not home? We leave many of the rooms turned down all the time but should we be turning down when we’re away?
Thanks!
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Replies
Sarah, this question has seen quite a lot of discussion over the years here. Do a search on the words "minisplit setback"; you'll see articles and Q&A threads that generally propose no meaningful savings result from either manually or automatically incorporating a turndown on a daily basis. I have a different conclusion, though. Without disputing the calculations of whether it's efficient or not, or how it affects the equipment, I argue that ANYTHING that engages a homeowner on the daily and deliberate use of their energy is a good thing. "Set it and forget it" should never be our rallying cry.
An architect I know has what he calls his “two button rule”. His rule is that anything that requires more than two button pushes to accomplish won’t get used.
I would argue that it’s good for the homeowner to know what’s going on, but if it’s overly complex, they won’t use it at all. I think the best option is a configurable thermostat with an easy interface so that the homeowner can properly configure setbacks and the like, and any changes can be made automatically by an occupancy center or a simple button push. I’ve been very happy with my Ecobee thermostats in this regard.
You want “set and forget”, but with a proper setup so that things are handled correctly. Most homeowners won’t bother with much else, unfortunately, so all the fancy stuff needs to be automated as much as possible.
Bill
Sarah,
Turning down the thermostat will save energy, you have lowered the Delta T for that time period. There is a matter of temperature recovery, a right sized system may not be a good fit for deep setbacks. Also, a well insulated home will lose indoor temperature quite slowly so a mid day setback may not save very much energy. If you are away for longer periods go for the setback, with smart technology you can have the house up to temperature when you return.
My system is a multi split that I only use for cooling. What I've found with it, is changing the set point doesn't save much power. Turning a head completely off does.
For each head on the minimum the outdoor unit seems to consume is 500W/head and only modulate up from that with the afternoon sun. The unit only starts to cycle off in the middle of the night once heat load is very small. For me, it is much more efficient to only run the head in the main living space during the day and turn on the one in the bedroom at night time.
I think lot of this is manufacturer specific, without a power meter of some kind it is next to impossible to figure out the best way to run them.
Overall, it is pretty annoying that one has to monkey with these to get good efficiency. These really should be set and forget.
I've found my kwh to be less on days where I setback or turn it (Mits FH single zone) off when not needed. Some have said you'll use more energy to recover, but that hasn't been my experience. I don't use the heating function in deep cold though (woodstove). During shoulder season it easily recovers the space a couple degrees within a reasonable period of time and my total daily kwh are lower when I turn it down/off for the night or when I'm not home, even with the higher energy recovery.
The occupancy sensor (i-see) can do this automatically, but I haven't played with it.
A simple $120 electrical monitor will allow you to see exactly what its doing in real time. I seem to be literally the only one on here that has one connected. You can run these units way more efficiently and completely eliminate short cycling.
I schedule mine using the cielo app to ramp slowly up every hour mid afternoon during peak outside temperatures before I get home from work. The COP is significantly higher compared to turning it up after dark when I get home.
What meter do you use? I have a whole house watt meter, and also my utility gives me pretty good enough But I would love to monitor a couple circuits individually.
How are you calculating COP. You've got the energy in, but how are you calculating the heating/cooling delivered to the space?
A high COP is great, but if your total kilwatt hours are higher it's like take the long way home from work cause you get better MPG but also burn more gallons.
I agree that ramping the setpoint is likely more efficient than large step changes. Another thing to consider if you have a pan heater is that it may shut off if you shut down the unit. I think the heater is around 150watts. The pan heater tanks the COP on smaller units below freezing since it's a higher percentage of the total power consumption.
I have a Efergy energy monitor. I have a couple charts I got from the Midea/Carrier service manual and other sources that list the COP and electrical wattage. I also have a return and supply duct temp probe on both my units.
My Midea/Carrier unit has a heated compressor belt that uses 52 watts and appears to turn on below 30 degrees. The pan heater is only used on the defrost cycle which mine rarely ever goes into. I believe it does use around 15o watts.
I'm curious about this as well. We installed two Mitsubishi FH06's and two FH09's primarily for AC, but sized to be primary heat as well. So far, it's done just fine with the heat in CT, but setting them back overnight and during the working day wasn't wife-friendly. House cooled down enough that the chillier floors / walls weren't as comfortable as our hydronic heat.
After 2 weeks of heating weather, we've settled on 64 degrees downstairs overnight, 67 during the day, and setting the bedroom to 63 overnight/day with a bump to 67 from 5-9 so the separate kids room warms up more to offset the space heater...
I did install an Iotawatt energy monitor when we installed these, and have each mini split on it's own circuit, so I can test and monitor it, but I haven't had time to teach myself the Emon CMS db to export the info...
The extra wrinkle we have is that the COP is the worst at dawn when outdoor temp is the lowest and max heat is called when the setback is turned off.
Is the energy saved by setting it back overnight more or less than the energy used to heat it up (at a lower efficiency).
>"Is the energy saved by setting it back overnight more or less than the energy used to heat it up (at a lower efficiency)."
As a general rule, using overnight setbacks ends up using more total kwh than keeping it at a constant temperature and letting them modulate. This has been shown to be the case both in controlled experiments and with in-situ monitoring studies.
https://www.nrel.gov/docs/fy15osti/63080.pdf
https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/monitoring-mini-split-ductless-heatpumps.pdf
The houses with the worst average COPs were those where the occupants were using set backs.
Has there ever been a study about raising the setpoint at peak daytime heating where COP is much higher? So right now its in the mid 50s here in CT but on a clear day like today the temp will plummet once it gets dark. My unit has a COP of 6.36 at 47 degrees but drops to 2.51 at 17. So wouldn't it make more sense to use my house as giant heat sink and run it at a way higher COP during the day?
Re any temp adjustment plan:
Keep a daily log of energy use and alternate adjustment/no-adjustment on even/odd days and you will eventually know if it helps.
I have the a ton of data being logged its just a matter of sifting through it. The one thing that really stands out is how much the compressor load drops at warmer temps. In the 40s and 50s these things absolutely crank out heat at really low compressor speeds. I can force mine into turbo mode that runs everything at max capacity. At extremely low temps my unit will use around 2kw at full capacity. At temps in the mid 40s and 50s it uses about half that and supply duct temps are 15-20 degrees higher. I regularly get supply duct temps in the 140s in mild weather and at low temps it seems to stay right at 120. All these measurements are recorded at max capacity using turbo mode.