Bruce Harley’s Minisplit Tips
A Vermont energy consultant provides expert advice on using air-source heat pumps to heat and cool a home.
Bruce Harley is a Vermont energy consultant and heat-pump specialist. To our readers, he is probably best known as the author of the Taunton book, Insulate and Weatherize — one of the best available books on residential energy retrofit work.
As the recent Better Buildings By Design conference in Burlington, Vermont, Harley shared a wealth of information on ductless and ducted minisplits. The title of his February 8 presentation was “Real-Life Air Source Heat Pump Performance Testing: Results and Reasons.”
Since Harley left his longtime job at Conservation Services Group, he’s been working as an independent consultant. Harley told the audience, “Now 90 percent of my work is related to heat pumps.”
Asian manufacturers have developed models for cold climates
Until recently, North American homeowners’ opinions of heat pumps were strongly influenced by the unimpressive performance of U.S.-made forced-air units designed for warm climates.
Harley explained, “Air-source heat pumps got a lot of bad press in the 1980s. Occupants noticed they cost a lot to run because of the electric-resistance heaters. Back in 2012, when I wanted to install a ductless minisplit at my house in Vermont, a contractor told me, ‘A heat pump won’t heat your house in Vermont. Maybe if you were in Texas in Oklahoma.’”
Japanese engineers have revolutionized the market for heat pumps, however, by improving their units’ cold-climate performance. “Ductless split-system heat pumps have been in mass production for 40 years,” Harley noted. “The technology has made steady advances. Now they are optimized for cold weather. It’s a rapidly growing market. Most of these heat pumps do not have an electric-resistance heater. Lots of people are installing one in an existing home with oil heat to offset the high energy bills.”
At his Burlington presentation, Harley summarized recent research into the performance of residential minisplits. Harley said, “The state of Vermont assumes that minisplits have an average seasonal COP of 2.4. Multi-zone systems have a lower efficiency than single-zone systems.”
He continued, “According to a Building Science Corporation study from 2014, there’s a big issue with on/off use. The researchers found that deep setbacks don’t work. The takeaway is: don’t turn them off.”
Later in the session, Harley reinforced this point. “With air-source heat pumps, temperature setbacks don’t save much,” he said. “After the setback, the system will run at high speed as it recovers. It’s better to set it and forget it. Only use setback if you will be gone for several days.”
Harley also cited a 2015 study of ductless minisplits conducted by Robb Aldrich of Steven Winter Associates: “The researchers found that if you direct the vanes [of an indoor unit] to a certain direction, you reduce the air flow and hurt the efficiency. They also found that in rooms with stratification, the warmest air in the room is sucked back into the unit. The efficiency would be higher if you could return cool air to the unit.” That’s why it makes sense, if a ductless minisplit is used primarily for heating, to choose a floor-mounted indoor unit.
Lessons learned from monitoring exercises
Harley shared information he learned from monitoring the electricity use of a Fujitsu ductless minisplit installed at his Vermont house. “I monitored it for the whole winter,” Harley reported. “I estimate my COP was about 2.8.”
He said, “I noticed that it didn’t perform very well at low-load conditions. It wasn’t ramping down; it was cycling on and off at a high rate. Why?
“It turns out that efficiency in mild weather is highly dependent on user settings. Lots of users choose the low fan speed, because the low speed is quieter. But the low speed isn’t good in really cold weather. Setting the fan to ‘auto’ boosts the capacity of the heat pump, which is important in colder weather.
“Don’t use ‘auto-changeover’ that switches between heating and cooling, or you’ll end up operating the air conditioner on a sunny winter day.
“Fujitsu now offers a setting called the ‘high insulation’ setting. You have to select that setting to avoid cycling. My unit was cycling on and off at 1000 to 2000 watts. Then I installed new firmware from Fujitsu and selected the ‘high insulation’ setting. With the new firmware, it was running longer cycles at a much lower wattage — about 400 watts, the minimum for this 1.5-ton unit, though you can see it ramping up to 1,200 watts at times.” (See Image #2, below.)
Harley continued, “Fujitsu still hides this in their option ‘settings,’ and they call this the ‘high insulation’ setting for ‘efficient houses.’ But any house turns into a ‘high insulation’ house when it’s over 40 degrees outside.
“I learned that you need to override the setting that reverts to factory default settings after a power outage. You have to flip the dipswitch to the ‘remember the settings’ setting. Unfortunately, the default mode is still the mode that allows cycling. You have to follow a complicated menu thing to get it into the ‘high insulation’ mode. You have to shut off the power at the breaker, and then turn the power back on, or it won’t remember the setting. If you have one of the early RLS models from Fujitsu, they will send you the new firmware. But it seems to sometimes forget the setting. Look for #95 on the menu, and set it to 01. After that, make any adjustments to temperature offset. (Selecting 01 wipes out any previous offsets.)”
Solving the cycling mystery
After the conference, I emailed Harley with some follow-up questions about his cycling problem. Harley responded, “There were two issues: the firmware/controls, and the way I operated it. The first showed up both as cycling and also as some slight overheating in mild weather, when I first got the unit.
“The overheating was corrected when I put in a wall-mounted control and adjusted it [to sense] the actual room temperature. With the built-in control using the return-air sensing and the hand-held remote, I had to turn the set point up higher the colder it got outside to get consistent room temperatures. This is likely because it was a single wall unit heating the entire 900-square-foot first floor and incidentally heating much of the second floor via an open stairway. The unit was in a corner of the first floor and was sensing ‘its own’ heat to some extent.
“I believe that the way I operated it that first year, I simply neglected to keep turning the setpoint down as it got very mild outside, especially on mild sunny days. The second autumn (before I changed the controls) I was more diligent about turning the setpoint down lower or even off on mild days, and the efficiency went way up.
“After I changed to the wall control mounted in the center of the first floor, this issue basically went away, and the temperature target has been maintained well even as the outdoor conditions vary widely. So I guess the overheating and temperature control in general was some combination of user operation (before I got the wall control, I had figured out how to manage it well) and controls (after, I no longer needed to ‘intervene’ manually).
“The cycling issue is purely a firmware issue — the unit came with an early 2012 version that had no option to control the cycling issue. I replaced the indoor unit PCB (all models of this manufacturer have the new firmware since sometime in 2013 or early 2014), but you still need to go into the installer settings menu to choose the ‘efficient house’ setting. They don’t tell you that virtually all homes are ‘efficient’ when it’s 40°F to 55°F outside; and this is only a low-load issue. To my awareness, many products from various manufacturers have this issue — that cycling occurs at higher than minimum capacity — and it’s not always predictable. Only Fujitsu has the ‘high insulation’ setting as an option that I am aware of. Ducted systems (at least 1-to-1 single zone) seem to avoid it better than wall cassettes. Multi-zone systems seem to have more issues with this; and wall-mounted controls (with more thermal lag?) seem to help, but I have not found any consistent reliable way to eliminate it. Even the Fujitsu ‘high insulation’ setting has seemed to disappear twice in the last three years on my system, and I have had to re-set it (even though the settings appeared to be correct) to change the behavior to the preferred mode.”
Tips for installers and homeowners
At the Burlington conference, Harley provided a list of system design tips. “If you have a two-story house, the first-floor unit will be the primary heating unit,” Harley said. “Remember that multi-zone systems are a lot more expensive, they aren’t as efficient, and they have more cycling problems.
“If you’re adding minisplits to an older house, the first unit that you put in will save you the most on your heating bill. You’ll need one upstairs if you want air conditioning upstairs, or you can put in a little air handler to serve each bedroom. The air handler can be installed in the attic, covered with insulation. If you have several bedrooms upstairs, ducts help.
“For a retrofit installation, a unit that is undersized will generate a little more heat. As long as the house has an existing heating system that will stay in place for backup, put in a slightly small heat pump. It will run more efficiently.
“For the downstairs unit that will provide most of the heating, put the unit low on the wall. A floor-mounted unit from Mitsubishi is a good unit for the first floor. Or put a small air handler in the basement.
“Avoid oversizing for small zones. The bedroom in almost any ordinary home won’t have a design heating load that’s more than 2,000 or 3,000 Btu/h, so a 6,000 to 9,000 Btu/h head will be way oversized for the space.
“Your outdoor unit’s size will be based on the sum of all the indoor units.
“Remember, multi-zone systems can’t turn down very much. They have a bad turn-down ratio. You are making a compromise with a multi-zone system. So don’t try to put a zone in every room. You want a small air handler to handle several rooms as one zone.
Harley continued, “Use the wall-mounted controls. Adding a wall-mounted thermostat helps reduce cycling. The option is not cheap, but they make a big difference. These controls sense the temperature in the room, not the return-air stream. The controls are hardwired for Fujitsu, but they are wireless for Mitsubishi. Be sure to set the control to sense the temperature at the thermostat — the default is to sense the temperature at the indoor unit, so you have to reset it.”
If you’re a nerd, buy monitoring equipment
Finally, Harley advised, “It’s a good idea to monitor your system. Even if you monitor, you really can’t tell the output, just the input. The only downside to monitoring is that it adds $300 to $1500 to the installation cost.
“I like the eGauge because there is no subscription fee. The eMonitor is more consumer-friendly, but you have to spend $20 per month subscription fee. Another option is the Sense monitor. It costs $300, so it’s cheap. I put it on the heat pump circuit.”
Harley’s advice: “Don’t use the HSPF rating to estimate or compare performance. Instead, consult the cold climate heat pump listings on the NEEP web site. There are 60 columns of data, including capacity and COP ratings at 47, 17, and 5 degrees F. The NEEP guide is really helpful.” (NEEP is the Northeast Energy Efficiency Partnerships in Lexington, Massachusetts.)
Harley also recommended NEEP’s heat pump sizing and selection guide.
Finally, said Harley, NEEP has published a guide for installers: the Guide To Installing Air-Source Heat Pumps in Cold Climates. It includes valuable information for installers to share with homeowners.
Originally published on GreenBuildingAdvisor.com. Martin Holladay’s previous blog: “Henri Fennell’s Advice on Cathedral Ceilings.”