Working with Heat Pumps
by Brendan Kavanagh
Almost 10 years ago Byggmeister started installing air-source heat pumps (also referred to as “mini-splits”) as part of our energy retrofit projects. Since then the technology has improved dramatically and heat pumps have become our go-to solution for heating and cooling. These days the majority of our projects include a heat pump of one form or another, ranging from spot conditioning in a renovated space to whole house conversions.
Along the way, we’ve been monitoring energy use on many of our past heat pump projects and noticed some had higher than anticipated electricity use. About 4 years ago we started working with Bruce Harley, an energy consultant and heat-pump specialist, to help us troubleshoot. Pretty quickly we realized that tweaking systems after installation only gets us so far. To get the best performance we needed to take a more deliberate approach to system design. Bruce now does the design for all of our heat pump installations.
I got together recently with Bruce and Cador Pricejones, our estimator, to discuss common issues that come up with heat pumps and best practices for ensuring optimal performance.
Brendan: Clients often ask us some version of, “I’ve heard heat pumps aren’t efficient when it gets cold out and you need some type of backup heat. Doesn’t that defeat the purpose?”
Bruce: It’s a shame that the reputation of early air-source heat pumps is still so strong out there. We now have a wide selection of equipment designed for cold climates that can operate efficiently down to -5⁰F, and some even colder. All heat pumps lose some heating efficiency and output as the outdoor temperatures get colder, but many of today’s cold-climate heat pumps have plenty of heat output and can provide very high efficiencies overall.
Cador: We’ve done several whole-house heat pump conversions without back-up gas or oil heat, and while there have been some issues, none were due to the systems not performing in cold weather.
Bruce: I looked into a few of those projects. The data shows they perform well in cold weather, but not as well in the spring and fall. The main culprit in those cases is that the outdoor unit is oversized, which causes short-cycling.
Brendan: Could you explain what short-cycling is?
Bruce: Short-cycling is when a system turns on, reaches the target temperature quickly, shuts off again, and repeats this process many times. This can happen when the outdoor temperatures are milder and the capacity of the outdoor unit is more than the indoor space needs to hold the temperature. The outdoor unit can’t slow down enough to deliver heating continuously, so it starts to cycle. The more oversized the outdoor unit is, the more it cycles and the more electricity it uses. We can mitigate this somewhat by adjusting settings and coaching better use habits, but a somewhat smaller system would perform better overall.
Brendan: We’ve known for years that proper sizing is critical to heat pump performance. Yet oversizing remains quite common. Why do you think this is?
Bruce: There’s a bit of a Goldilocks situation with air-source heat pumps that you don’t have with more traditional forms of heat. An oversized fossil fuel based heating system might be a bit more expensive to install, but won’t use much more energy than a right-sized system. Because of that many installers are in the habit of using larger systems to be extra sure the building stays comfortable on the coldest nights of the year. With heat pumps you don’t want to be too big or too small. It’s best to be between 100-115% of the calculated heating load.
Brendan: Beyond getting our load calculations right, avoiding larger multi-zone systems also helps optimize performance. Multi-zone systems are often used when someone needs multiple indoor zones, typically including bedrooms, but they only want one outdoor unit. The problem is, small indoor zones, such as bedrooms, often have loads that are smaller than the lowest capacity indoor unit and so the indoor units end up oversized for these zones.
Bruce: We can avoid multi-zone systems by using ducted systems to serve smaller rooms together in one zone, or using a couple of smaller outdoor units instead of one big one. We generally stick with 1:1s or 2:1s (number of indoor units to outdoor units), with the occasional 3:1 if we have limited options. In some cases two 2:1 systems will cost about the same as one 4:1 system, if we are able to get rid of some equipment that’s no longer necessary like a branch box.
Cador: Clients understandably don’t want any more outdoor units than necessary. It can be challenging to find places to locate them. But a smaller outdoor unit can often fit beneath a window, so it won’t block the view from inside. A larger multi-zone unit on top of 18” stand (to keep it above the snow in winter) can be almost 6’ tall whereas smaller units stand just over 4’ tall.
Brendan: How have our installers responded to our more proactive approach to system design?
Cador: Everyone has their way of doing things and we are asking them to change some of that. So it can be difficult, but Bruce is great at speaking to installers.
Bruce: That’s really nice to hear. My sense is that your installers have been pretty receptive to my involvement. I love how Byggmeister is committed to getting the projects done right. It feels like a win-win for the clients and the environment.
Brendan: That’s the goal. Since I’ve been here I can’t recall a time where we considered a new fossil fuel heating system for a project.
Cador: It’s been at least a couple years since we’ve put in one in. Honestly, I can’t see too many situations where we would consider a fossil fuel system anymore.