Are Seven Heads Better Than Three?
A GBA reader wonders how many ductless minisplit units he should install in his new house
John Bell, building a 3300-sq. ft. house in eastern Pennsylvania, is weighing his options for heating and cooling, and it comes down to a conventionally ducted air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. or a multi-head ductless minisplit system made by Fujitsu.
Bell sought several quotes from contractors. The first proposal, parts of which gave Bell pause for thought, was for a 5-ton conventional heat pump. A second HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor thought that a 2.7-ton system would be enough. So Bell went for a third opinion and so arrived at the ductless minisplit option.
His real dilemma is deciding how to configure a ductless minisplit system in his extremely well insulated house. One bid includes seven minisplit heads.
Each of the heads would be connected to its own compressor outside, and Bell says both he and his wife have no problems with that. Nor will they mind looking at all those fan units on interior walls. He does wonder, however, whether this plan makes any sense.
“It might be overkill with that number of units,” he writes in a post at GreenBuildingAdvisor’s Q&A forum. “I will be ventilating the house with an UltimateAir ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.. I would like to go with the minisplits for the whole house, but I don’t know anyone who has used them.”
That’s the subject of this month’s Q&A Spotlight.
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Why so many indoor units?
Bell plans on installing one minisplit head in each of the house’s four bedrooms, one in the living room, one in the kitchen and another in the den.
He thinks that number of fan units would make for a more flexible, versatile heating and cooling system. When his children aren’t at home, for example, their rooms can essentially be shut off.
Plus, he adds, by going with a one-compressor/one-head system, he increases the overall efficiency from 17 SEERSeasonal Energy Efficiency Ratio (SEER) is the total cooling output (in BTU) of an air conditioner or heat pump during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period. The units of SEER are Btu/W·h. SEER measures how efficiently a residential central cooling system operates over an entire cooling season. The relationship between SEER and EER depends on location, because equipment performance varies with climate factors like air temperature and humidity. to 26 SEER.
Bell thinks the system will come in at about $19,000, not including roughly $4,300 for an energy recovery ventilator. He compares that to a single 22 SEER, 3-ton heat pump plus ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. for an estimated $31,700.
What’s not to like?
Seven heads are way too many
GBA senior editor Martin Holladay is one of several people who doubt Bell’s house needs that many minisplit units to stay comfortable.
Holladay doesn’t see any glaring downsides to having that many indoor units, but he did note three issues: the proposed system would cost more than a simpler system; there was always a possibility that the fans might keep a light sleeper awake at night; and the seven-unit system would ultimately require higher maintenance costs than a simpler system.
Holladay advised Bell that his house probably didn't need seven minisplits. He pointed to the experience of Carter Scott, a Massachusetts builder “who has built not one but at least 18 net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. homes in the 1,800 to 2,000 square foot range — all heated and cooled with just 2 ductless minisplits (one in the downstairs living room and one in the upstairs hall).”
“As long as your bedrooms don't have unshaded east-facing or west-facing windows (that is, subject to overheating from solar gain), I don't think they need their own minisplit units -- assuming, of course, that you have an excellent envelope and really good windows,” he says.
Keith Gustafson shares that point of view. “You do not need 7 minisplits,” he writes. “I have lived with minisplits for AC in 2 houses for the last 6 years or so,” Gustafson says. “My current house is 2,800 sq. ft. and has half the insulation of yours and is perfectly comfortable when it is 95°F outside in every room with 3 minisplits. Big one upstairs, main living space. Two small 9K units, one in the downstairs family room and one in the master suite.”
There are problem with having more minisplits than you really need, he adds. “Minisplits 'think' a lot, and they never shut off,” Gustafson says. “They want to run at a very low level to be efficient. This means that if they are oversized for the space as any 9K unit is for a normal bedroom they are blowing cold air on you when you no longer want it. So you turn it off. Then it is stuffy in the morning. Having a more central unit and making it work for a living is better for comfort, especially if you have some means of air exchange room to room.”
Even the UltimateAir rep Bell has spoken with, who’s also a PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. consultant, is “completely comfortable” in his own house with a two-head multisplit system.
Heat calculations can’t be right
One of the HVAC contractors Bell has met with ran heat-load calculations and came up with a requirement of roughly 36,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. per hour, but Dana Dorsett isn’t buying it.
“The 36K heat load number is bigger than the measured heat load number @ +5°F of my 1923 bungalow with 2x4 framing, less than R-19 roof insulation, and antique double-hung windows with storms,” Dorsett says. “It's unlikely your heat load is more than 25K unless you have a ridiculous amount of window area and you cheaped out on U-factors.”
With the insulation levels Bell is building into the house, Dorsett says there’s no way he would need one head per room. Floor plan and head location are keys to keeping the house comfortable.
“But I'd be surprised if a 3 or 4 head 3-ton multi-split couldn't be set up to heat and cool a 3,300 sq. ft. house pretty comfortably at less than half the cost of 7 or 8 3/4-ton units, each grotesquely oversized for their 1-room loads,” he adds.
Our expert’s opinion
Our expert opinion comes from Marc Rosenbaum, an engineer and energy expert at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. Rosenbaum has increasingly been using ductless minisplit heat pump systems for his low-energy high-performance projects. Here's what he has to say:
I would not put seven single-zone systems in, despite the rated efficiency benefit, because I think they will likely be too large. How successful the one-cassette-per-floor strategy will be is dependent not only on how well the house is built and the amount of glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. in each closable room, but also on how the residents operate the house. Whether they keep their bedroom doors open or closed is a big factor, and so is how they define comfort.
For example, we have heated our "pretty good house" on Martha's Vineyard for two winters with a single minisplit heat pump with the cassette in the kitchen/dining/living area of the main floor. The upper floor, with doors open, runs about 2°F cooler on typical winter days (say, 30°F) and about 4°F cooler when it's 10°F outdoors. That's completely acceptable to us, but might not be to other occupants. If we kept the doors closed, the temperatures might not be acceptable.
Another example is the eight superinsulated homes we've built and monitored in West Tisbury. They each have a single-zone system with a wall cassette in the main living space, and electric radiant panels in the bedrooms. There is a large variation in how much people use the radiant panels (we have metered data), which to me says they use the houses differently and they have different definitions of comfort.
We have put up to seven individual cassettes in a superinsulated house without issues, albeit on only two condensers. What we tend to do on low-load homes with more closable rooms is use a ducted system or two, because then the capacity of the system is much closer to the actual load of the house.
- Scott Gibson
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