Your first question may well be, "Well, what are ASHPs?!?" For that, we've got you covered. ASHP is short-hand for Air Source Heat Pump. It's the technology we use and love, because it's such an efficient way to heat and cool a home (particularly those great homes that have well-built exterior shells and thoughtful, pre-filtered air exchange!).

Air Source Heat Pump Technology is so efficient because it is, in fact, the moving of heat from one place to another, rather than the combustion of fuel to generate heat. This means not only that it's a system that can use energy harnessed from the sun, but also that it is significantly more efficient than your average everyday boiler.

As these systems are relatively new to most homeowners, though, there can be some head-scratching as to how best to use them... This is where our local expert, Dana Fischer comes in. As the Area Manager for Mitsubishi Electric's HVAC division, he's the perfect expert to chime in on those questions we get from homeowners - so we threw a few zingers his way...

Q: What is the best rule of thumb for setting temperatures on mini-splits?

The best suggestion is to set the temperature to whatever setting makes you most comfortable in the most space as possible and avoid making big setbacks at night or when away from the house for the day. Depending on your home and setup and what is going on outside, this may mean setting the temperature much higher than you would expect, and in some situations, higher than what actually results. This is normal strategy for our climate and results in the most effective use of the heat pump to provide comfort and displace the maximum amount of conventional fuels. As the outdoor temperature drops, crank the heat pump up as needed to meet the need. The cost of operation will not change much, but everyone will be happier.  

Most heat pump installations use a handheld remote to control the temperature setpoint, but measure room temperature inside the wall mounted cassette which works great for cooling, but may not accurately reflect or adjust to the room temperature 20 feet away. Remotes do not sense temperature, but are one-way communicators that blast out a full set of settings every time a button is pushed. The indoor unit may only “hear” the change in setting if the remote is pointed at the indoor unit and in some reasonable proximity. If consumers would prefer to set a temperature and not have to adjust the unit as much over the season, a wall mounted thermostat placed 20 to 30 feet away from the unit and measuring temperature at human height can make a big difference and limit difficulty finding the right comfort setting.

Also – for at least New England, I would recommend using “heat” or “cool” mode and avoid “auto” which theoretically would switch between heating and cooling as needed to maintain temperature setpoint in a space. For example, keeping the unit in heating in the winter prevents the system from going into cooling mode in the middle of a sunny January day when that wonderful sunshine is pouring in through the southern facing window. And vice versa for avoiding turning on the heat during delightfully cool nights in the summer.

Q: When there are multiple heads running off of a single compressor, what is the best way to set up all units in order to optimize efficiency?

Multi-zone systems are amazing devices for what they can accomplish, but have some limitations that users should be aware of related to their size relative to the size of each of the indoor units to which they are attached. In most circumstances, best comfort and efficiency will result from keeping most or all of the units on with each set for maintaining a comfortable room temperature. Only operating one indoor cassette can create a mismatch between the capacity of the indoor and outdoor units which can lead to short cycling and lower efficiency.

Q: Is there any problem with taking one or more heads offline, if they are in spaces that are occupied less frequently?

No damage to the system will occur from running with less than all the connected indoor units. Multi-split systems are programed to reach targeted set points at any and all attached indoor units, but are also designed to take whatever steps and maintenance cycles are required to ensure their longevity for whatever conditions they face. Having fewer active heads can result in more frequent cycling of the system as the outdoor unit has limits as to how low an output level it can operate at, under which it will turn off the compressor. When there is a call for heating or cooling again, the unit will undergo its standard startup procedure, which consumes some electricity without providing heating or cooling. Heat pumps provide maximum savings when they are able to operate at level rates over long periods like a car in cruise control on the highway. High frequencies of starts and stops lowers overall efficiency, just as it would with a vehicle in traffic.

Q: How often should filters be cleaned or changed on ductless units? On ducted?

The joke I always use in presentations on this topic is “it depends on how many cats you have.” It is a good idea to check the dust screens on ductless units every month or so while it is in daily use. It takes two seconds to pop the cover and see if they need to be sprayed off in a sink improving both efficiency and output. Ducted units with conventional furnace style filters can be changed less frequently, but again it depends on the house and occupancy. Indoor units should be cleaned professionally every few years or perhaps every year depending on usage, animal friends, and occupancy to ensure good indoor air quality and performance.  

Q: What care and maintenance do the outside compressors require, and how should these units be cared for/maintained in winter months?

Outdoor units mostly want us to give them a glance from time to time to make sure they are free and clear of yard debris, ice, snow, or anything that would impact airflow through the units. These systems need to be relatively unobstructed to operate at their optimum. When you shovel your front steps after a storm, take a minute to shovel some of the snow from below and around your heat pump. During scheduled professional maintenance, technicians will spray out the coil to remove grass clippings, leaves, spiders and anything else that flies around your backyard and would lower the rate of heat transfer from outdoor air through the unit.

Q: Is it possible to run one mini split in cooling mode while another is in heating mode, when they both run off the same compressor?

Large commercial heat pump systems designed for hospitals, hotels and skyscrapers are capable of providing simultaneous heating and cooling to different parts of the same building. Affordable residential mini-splits do not incorporate the same degree of expensive complexity and components to be able to accomplish this feat with a single outdoor compressor. If it is imperative that both heating and cooling are provided in a residential, multi-family or small commercial application, the best strategy is to utilize multiple outdoor compressors where one could be providing cooling and one could be providing heating to different parts of the building at the same time. Caution in design, layout and controls would be want to be excised to prevent the systems from trying to heat and cool the same space at the same time. There are no winners with dueling HVAC systems!

Q: Are there important things to know about fan settings versus temperature settings? (i.e. if you are cold, and you turn the fan up, will the fan ramp back down automatically after a given time? Or should you just turn the temperature up?)

Fan setting strategies are similar to temperature set point in that the correct answer may depend on the space and occupant preferences. For many homes, setting the fan to “Auto” may be the simplest and most effective option. With the fan set on “auto”, the system will ramp up and down to match the need for heat in the space. If homeowners are trying to provide some amount of heating or cooling to more than just the room where the indoor cassette is located, a medium high setting may work better to increase distribution range. When the fan is set at a single speed, it will use that blower speed anytime there is a call for heating or cooling, but ramp down to a low and quiet flow rate the rest of the time.

If it is a matter of temperature and not distribution, treat the temperature set point like a “sleep number” or extra logs in the wood stove. Set it to whatever makes you comfortable and gets the heat to travel to as much living area as you prefer. I’d encourage folks to experiment and take their heat pump on a joyride to see what it can do. Crank it all the way up to 80 and see how your house responds over the course of several hours. Good times!

Q: Is it advisable to apply the “yankee frugality” principal of cycling mini-split from cooler at night to warmer during the day?

For many years it has been common practice for Mainers to turn down the heat when going to bed and turn it back up in the morning to save on heating costs. With heat pumps that operate most efficiency when idling and maintaining a set temperature, the ramp up period in the morning would take much longer and negate much of the energy savings that homeowners might be seeking. On the other hand, if homeowners would prefer to have the house a few degrees cooler at night, there is no big penalty to turning the system down a couple of degrees for comfort, as long as there is understanding that there isn’t much, if any energy savings, and that it may take a while to get back up to warmer temperatures in the morning.

Q: Other tips and tricks for optimizing comfort and operations of your mini-split system?

The number one tip is to keep on using them all winter long for maximum savings. The highest performance heat pumps function well even when it is silly frigid outside, and are designed and proven to run hard for many years. Crank it up and be comfortable year round!

A special thank you to Dana Fischer for providing expert advice on Air-source heat pumps!

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