Minisplit cooling mode at night
Hello Masters of the Cooling/Heating world. This is the first ‘cooling’ season for us with our Mits split system. We only plan to run it when it’s crazy hot like it was/is yesterday, today, and tomorrow.
As many of you may already know, the day temps will run 90ish, but at nights, even times like this, the temps end up below the set temp.
During the day, we are quite satisfied with a set temp of about 73/74 (actual room temp on Acurite thermometer shows 75ish). At night, the outdoor temp (last night/tonight) drops to about 69ish. I have not been able to get substantive answers from searching with Google, other than anecdotal practice/theory.
Potentially further compounding the issue is that the bedrooms being at the far side of the house (the Mits is a single head unit located in the living room, and it has worked out great for this Winter), doesn’t get quite cool enough for ‘comfortable sleeping,’ though it’s plenty fine otherwise. Without bogging in details, it is related to the design of the house than capacity of the Mits.
We have window a/c units from our pre-Mits days, so I was wondering what is the ‘best practice’ way to handle the situation during the night – especially where many times it will be hot enough to use the Mits during the day, but outdoor temps drop fairly lowish during the nights.
I was thinking:
Run the Mits during the day to cool the whole house generally, keeping bedroom doors open to also cool the rooms adequately, and prevent the rooms from getting too hot (making it easier later for the window a/c’s to take over at night when we want it cooler). Shut the Mits down at night once outdoor temps drop, and open windows. Close bedroom doors and run window a/c at night. Reverse steps back in the morning as temps rise.
这是基于理解(我可能是错的), that mini splits are more efficient at maintaining temp versus having to drop the temp down. For example, it’s better to start up the Mits in the morning before indoor temps rise, and shut it down when the load from outdoor temps drops (using window a/c just for the bedrooms), rather than come home to an indoor temp throughout the house of 85ish and starting it all up.
This is also based on a theory that if I let the house “heat up,” it will take more energy to cool it down due to the residual heat throughout the house. Is my thinking wrong?
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Replies
Wayne,
The main disadvantage to opening your windows at night is that you allow lots of exterior humidity to enter your house. The next day, your air conditioner has to spend a lot of energy removing the moisture you introduced at night.
Your energy bills will be lower if you keep the windows closed at night. The only seasons when it makes sense to keep your windows open at night is when the weather is mild enough that you can get through the day without any air conditioner use at all.
Thanks, Martin. So is it better to keep the split running at night, or to actually shut it off? I don't know if the split is actually running/cycling/doing anything - what usually happens with a split in situations like this?
Wayne,
You can leave the minisplit on or turn it off -- it's your choice.
You can turn on the window-mounted air conditioner or leave it off -- also your choice.
The rooms served by these cooling appliances will obviously be cooler if you operate the equipment than if you don't. The only downsides to operating the equipment are (a) higher energy costs and (b) the environmental effects of electricity use.
So whether or not you need the air conditioner depends on your budget, your environmental concerns, and whether or not you are comfortable sleeping.
Thanks, Martin. You are such a wealth of knowledge/information. So I am thinking this would be the best trade off between sleeping comfort / energy use:
During day:
1. Keep bedroom doors open.
2. Run split set at 73ish 74ish. Would setting it a little higher be better if no one is home (workday for example) - especially considering I would have to ramp the temp down upon occupation?
3. Keep bedroom ac off - not needed since it's comfortable enough for daytime activities with just the split running.
At night:
1. Keep windows closed.
2. Keep split running (keeping set at 73ish) to assure indoor temp doesn't rise too far @ night, and keep humidity out.
3. Close bedroom doors and run window ac to cool bedrooms further for comfortable sleeping. Also, because the bedrooms were being kept reasonable cool during the day, this would present an easier load on the window ac to cool the room further.
Wayne,
Your plan sounds sensible to me.
Depends, what's the dew point at night? If it's low open the windows, if it's high close it. You need to calculate the total energy in the air (temp + humidity), then compare it with the total energy of the air inside and you also need to compensate for the fact the outside temperature at night changes.
Running the total energy math is a bit cumbersome- there's an easier way to deal with it:
The dew point of human & house-healthy 50% RH 75F air is 55F. If there is a reliable high quality local weather station (airport weather stations are usually pretty well maintained & calibrated), check the local dew point to verify that it's ~55F or lower before throwing the windows open.
Night time ventilation strategies work well in locations with summertime dew points averaging in the mid-50sF or lower (which includes most of the Rocky Mountain states & further west in the US), and not very well at all in locations where summertime dew points average in the mid 60sF or higher (most of the right half of the US map.)
Turning the mini-split on in the AM and letting it run all day will usually use less energy than turning it on the afternoon when the house is already hot, since part load efficiency can be many times higher than running full-out playing catch-up.
Thanks all for the great insight. Yeah, don't know if I want to go down the spreadsheet road for what equates to a few days per month - at least for now. Maybe as heat waves become more prevalent in the not too distant future, I'll actually hook up some electricity monitors and compare different scenarios (esp since monitor systems are getting quite reasonable).
For now, I think Dana's "rules" will likely suffice for my use. After tomorrow, everything drops back down (including dew points). I think I'll try my CAM (cheap *ss monitor) in the meantime - check the electric meter before the hot weather starts, then look at the monitor daily until it ends, then compare to "normal" usage, and try difference scenarios.