Heat pump with mini-splits: best way to choose temperatures/settings?
November 28, 2022 7:36 PM Subscribe
We have a spanking new ductless heat pump in our house, with mini-splits in multiple rooms. But having been used to a gas furnace, it is somewhat mystifying (in particular, seems to runs warmer than set), and I'm not quite sure what settings to choose. I've been told that heat pumps need to be set differently from gas/oil/electric heating, but don't know how to translate this into practice.
We live in Seattle. When we had a gas furnace, we would set up our programmable thermostat to automatically go to 67°F during the day and 60°F at night, and we would close vents in rooms we weren't using.
Now we have a ductless heat pump with Mitsubishi minisplits: there is a minisplit in each of the 4 bedrooms, plus two minisplits covering the large open living/dining/kitchen area. Each minisplit has a separate control.
The first thing we noticed is that the minimum temperature setting allowed for the minisplit controls is 61°F (so setting to 60 or lower is impossible), and the second thing we noticed is that if we leave it set to 61°F in any of the bedrooms, the temperature actually goes up to around 68°F. If we want to keep the temp around 61°F, we have to turn off the minisplit after it hits 65 or so, and wait for the room to cool off. At night, we actually want it to be cooler for comfort, it's not just an energy efficiency thing.
(Currently the outside temperature is 36°F, for reference.)
We're also unsure what it means in a heat pump system to operate the minisplits at different temperatures, or to have some off and some on, and whether it's OK to turn them (some or all of them) off at night. The manuals don't seem to reveal these answers. Any insight would be appreciated!
We live in Seattle. When we had a gas furnace, we would set up our programmable thermostat to automatically go to 67°F during the day and 60°F at night, and we would close vents in rooms we weren't using.
Now we have a ductless heat pump with Mitsubishi minisplits: there is a minisplit in each of the 4 bedrooms, plus two minisplits covering the large open living/dining/kitchen area. Each minisplit has a separate control.
The first thing we noticed is that the minimum temperature setting allowed for the minisplit controls is 61°F (so setting to 60 or lower is impossible), and the second thing we noticed is that if we leave it set to 61°F in any of the bedrooms, the temperature actually goes up to around 68°F. If we want to keep the temp around 61°F, we have to turn off the minisplit after it hits 65 or so, and wait for the room to cool off. At night, we actually want it to be cooler for comfort, it's not just an energy efficiency thing.
(Currently the outside temperature is 36°F, for reference.)
We're also unsure what it means in a heat pump system to operate the minisplits at different temperatures, or to have some off and some on, and whether it's OK to turn them (some or all of them) off at night. The manuals don't seem to reveal these answers. Any insight would be appreciated!
- There is probably a setting for how much above and below the set temperature the heating should run or not run. Setting this as low as possible may help avoid overheating.
- There is probably also a temperature offset feature, so you can adjust the temperature that the thermostat says it is versus the actual temperature (setting this so the thermostat thinks it is warmer than it actually is will help reduce the actual temperature).
- There is probably also a vacation mode that heats only to about 50F or so, though setting this manually every night would be a pain.
- It's fine to turn everything off at night or turn on and off rooms as needed. You do lose some efficiency in heating up a lot of space quickly versus maintaining it at a constant temperature (because the heat pump is more efficient at lower load, within reason), but this depends a lot on the sizing of the unit relative to the load, the outdoor temperature curve over time, how long you have the unit off for, and so on. I imagine there is some kind of eco setting that will allow you make sure you're heating up slowly and efficiently. If you're not talking about heating up the entire house by a large amount daily, you probably don't really need to worry about this. Just turn it off or way down when you don't want it on.
- It's fine to operate different rooms at different temperatures — that's part of the benefit of the system you have (which sounds like it is not technically a minisplit, but just a regular split system).
- Not sure about the details for the Mitsubishi specifically, but if all else fails, you may be install a non-Mitsubishi thermostat than lets you set the temperature lower (this is definitely the case with our Fujitsu).
posted by ssg at 7:56 PM on November 28, 2022
I hope I have got the terminology right, what you are calling a mini-split is just a regular non-ducted split system here in Australia. Temperatures ranges from -3°C to 45°C where I am, which translates to 26°F to 113°F where you are.
My house is set up with a few 1.7kW units in the bedrooms and a larger 9.5kW unit for the living areas, and a 2.5kW unit for the master bedroom + ensuite + walk in wardrobe. I also use Mitsubishi Heavy Industries RCAC units.
It's possible that your units are too powerful for your bedrooms, particularly in heating mode. A 2.0kW unit is suited for a well insulated room 27 sqm in size, while a small bedroom is barely 10 sqm in size. It tends to overshoot the target temperature, making it uncomfortably warm. Tthe 2.5kW unit in my master bedroom is perfect since it covers about 32 sqm, but overshooting temperatures happens in the 1.7kW units in my smaller 10sqm bedrooms. My solution to that has been to simply sleep with the doors slightly open but that might not be an option depending on the nature of your household... we leave the bedroom doors wide open during the day, so it receives heat from the living area, and sometimes don't use the bedroom split system at all.
As for turning off some splits during the night... during the colder months, the main split system for my living area consumes about 1kW at steady state to maintain 21°C against an external temperature of about 6°C to 12°C. If I turn it off at night, the temperature in the living area drops to about 18°C by morning in about 8 hours, at which point if I turn it on it requires about two hours at 4kW to bring it back up to 21°C. So turning it off saves about 2kWh per night (saves 8kWh during the time it's off, then consumes an additional 6kWh when we turn it back onto make up for the deficit) which is about 37 cents for me. In % terms, that's 24kWh per day heating if I leave it on all the time, versus 22kWh per day heating if I turn it off overnight, so an 8% increase in electricity bill seems about right comparing the scaling in temperature gradient that results from the cooler house overnight.
I just leave it on 24/7 since the cost difference is so minor, you can treat it like the compressor in the fridge, it runs more or less constantly for 15 years.
posted by xdvesper at 11:15 PM on November 28, 2022
My house is set up with a few 1.7kW units in the bedrooms and a larger 9.5kW unit for the living areas, and a 2.5kW unit for the master bedroom + ensuite + walk in wardrobe. I also use Mitsubishi Heavy Industries RCAC units.
It's possible that your units are too powerful for your bedrooms, particularly in heating mode. A 2.0kW unit is suited for a well insulated room 27 sqm in size, while a small bedroom is barely 10 sqm in size. It tends to overshoot the target temperature, making it uncomfortably warm. Tthe 2.5kW unit in my master bedroom is perfect since it covers about 32 sqm, but overshooting temperatures happens in the 1.7kW units in my smaller 10sqm bedrooms. My solution to that has been to simply sleep with the doors slightly open but that might not be an option depending on the nature of your household... we leave the bedroom doors wide open during the day, so it receives heat from the living area, and sometimes don't use the bedroom split system at all.
As for turning off some splits during the night... during the colder months, the main split system for my living area consumes about 1kW at steady state to maintain 21°C against an external temperature of about 6°C to 12°C. If I turn it off at night, the temperature in the living area drops to about 18°C by morning in about 8 hours, at which point if I turn it on it requires about two hours at 4kW to bring it back up to 21°C. So turning it off saves about 2kWh per night (saves 8kWh during the time it's off, then consumes an additional 6kWh when we turn it back onto make up for the deficit) which is about 37 cents for me. In % terms, that's 24kWh per day heating if I leave it on all the time, versus 22kWh per day heating if I turn it off overnight, so an 8% increase in electricity bill seems about right comparing the scaling in temperature gradient that results from the cooler house overnight.
I just leave it on 24/7 since the cost difference is so minor, you can treat it like the compressor in the fridge, it runs more or less constantly for 15 years.
posted by xdvesper at 11:15 PM on November 28, 2022
We're relatively new mini-split owners, but this is the general idea conveyed by our installers/technicians, and over the couple of months we've had it, it seems to pan out:
Modern mini-splits are incredibly efficient, and work best as "set it and forget it". If you need a lower nighttime temperature for comfort, that's one thing, but try to get the cost savings thing from other heating systems out of your head. Heat pumps are "slow" heat, in that they work best at maintaining a steady comfort level, instead of rapidly heating or cooling on demand. They are so incredibly good at what they do, though, that just setting it to 68F or whatever and leaving it there forever is really their ideal operating mode.
You may need to set a different temperature than what you're used to from a previous system. This may be higher or lower. Ignore the numbers completely; don't look at 72F on the setting and think "that's too high!" or whatever. Fine-tune it until it feels comfortable. Do this over the course of a few days, because it's not instant results. Again, this is "slow heat", and it will take some time to adjust to the new setting. Figure out what works for you, and stick to that.
The temperature may only go down to 61F in heat mode, but it may go slightly less in cooling mode. Whether your system will allow you to have some units in heating mode and some in cooling mode is dependent on how your system is configured. If they each have their own compressor/exchanger unit, then probably so. If units are sharing an outdoor unit, probably not. Our house is very open, and we have two units indoors sharing a single outdoor compressor/exchanger. Both units must be in cooling or heating mode, otherwise they will sort of "fight each other", as one is wanting the compressor/exchanger to run in heating mode, and the other wants it to run reverse in cooling mode.
If you owned this home previously and were the ones who had the new system installed, talk with your installer, and I'm sure they'll answer any questions. Would probably even come out for a visit to go over operation with you.
If you bought the home with this system already installed, see if there's a sticker on the unit for who installed it, or call around to see who services that brand. (We just had a Mitsubishi system installed by Foss Heating in our Anacortes home; not sure if they service Seattle as well, but they were great every step of the way.) Might be a good idea to see if you can schedule an inspection/annual maintenance call to make sure you're ready for the coldest part of the year, and also get any other operational questions answered.
posted by xedrik at 7:56 AM on November 29, 2022 [2 favorites]
Modern mini-splits are incredibly efficient, and work best as "set it and forget it". If you need a lower nighttime temperature for comfort, that's one thing, but try to get the cost savings thing from other heating systems out of your head. Heat pumps are "slow" heat, in that they work best at maintaining a steady comfort level, instead of rapidly heating or cooling on demand. They are so incredibly good at what they do, though, that just setting it to 68F or whatever and leaving it there forever is really their ideal operating mode.
You may need to set a different temperature than what you're used to from a previous system. This may be higher or lower. Ignore the numbers completely; don't look at 72F on the setting and think "that's too high!" or whatever. Fine-tune it until it feels comfortable. Do this over the course of a few days, because it's not instant results. Again, this is "slow heat", and it will take some time to adjust to the new setting. Figure out what works for you, and stick to that.
The temperature may only go down to 61F in heat mode, but it may go slightly less in cooling mode. Whether your system will allow you to have some units in heating mode and some in cooling mode is dependent on how your system is configured. If they each have their own compressor/exchanger unit, then probably so. If units are sharing an outdoor unit, probably not. Our house is very open, and we have two units indoors sharing a single outdoor compressor/exchanger. Both units must be in cooling or heating mode, otherwise they will sort of "fight each other", as one is wanting the compressor/exchanger to run in heating mode, and the other wants it to run reverse in cooling mode.
If you owned this home previously and were the ones who had the new system installed, talk with your installer, and I'm sure they'll answer any questions. Would probably even come out for a visit to go over operation with you.
If you bought the home with this system already installed, see if there's a sticker on the unit for who installed it, or call around to see who services that brand. (We just had a Mitsubishi system installed by Foss Heating in our Anacortes home; not sure if they service Seattle as well, but they were great every step of the way.) Might be a good idea to see if you can schedule an inspection/annual maintenance call to make sure you're ready for the coldest part of the year, and also get any other operational questions answered.
posted by xedrik at 7:56 AM on November 29, 2022 [2 favorites]
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The indoor units have their temperature sensor very close to the heat exchanger coil and fan that is used to heat/cool the room. As a result, it's not particularly accurate - especially, in my experience, when heating. The solution to this - and your minimum temperature issue - is to use an external temperature unit to control the indoor unit. Some heat pump manufacturers will sell them (Daikin does). I use a Sensibo Air to control my heat pump. As an added benefit, I can control the heat pump from anywhere I want. It's particularly convenient to be able to turn my indoor units on just before I drive home. I can also put the indoor units on any arbitrary schedule like your example of alternating between temperatures through the day.
We're also unsure what it means in a heat pump system to operate the minisplits at different temperatures
It means each indoor unit will maintain a different temperature. If the indoor unit doesn't need to heat the room, it won't. The load on the outdoor unit(s) will be the sum of the six indoor units. Think of each indoor unit as a heater/AC unit for that particular room, and the outdoor unit(s) as providing the coolant for all of the indoor units together. As an example, I heat my living room while I use it during the evening to a higher temperature than my bedrooms when I am not sleeping. When I am sleeping, I reverse that (bedrooms higher than living room).
We're also unsure what it means in a heat pump system to [...] have some off and some on
This is, in fact, ideal from an energy usage standpoint if you don't want to heat/cool a particular room. If an indoor unit is off, or is close to the desired temperature, it'll just use less coolant from the outdoor unit(s). As a result, the outdoor unit(s) will throttle back the condensers - and hence, energy usage - accordingly. The ability to turn off indoor units on demand is one of the prominent reasons mini split units can be substantially more energy efficient than central AC units.
We're also unsure [...] whether it's OK to turn them (some or all of them) off at night
Again, this is ideal from an energy usage standpoint. With a modern mini split, your outdoor unit(s) will throttle back its condenser to the exact heating/cooling need from your indoor units. If all indoor units are off, the outdoor unit will turn off. If some of the indoor units are on, the outdoor unit(s) will operate at a lower load, and reduce energy usage. Unlike older central AC units, modern outdoor units are designed to operate at many different load levels, and rapidly change between zero/low heating/cooling loads and higher/full heating/cooling loads.
You'll likely see your outdoor unit(s) referred to as a variable frequency drive unit - the variable frequency enables the outdoor unit to provide a variable amount of heating/cooling capacity at any given time. Older central AC units tended to either be on/off only, or only a few levels of cooling capacity. Modern mini splits can provide exactly the heating/cooling capacity needed at any given time. The energy usage of the outdoor unit(s) will be determined by how much heating/cooling load is needed by the indoor units.
posted by saeculorum at 7:53 PM on November 28, 2022 [2 favorites]