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News Room: Industry News

7 Tips to Get More from Mini-Split Heat Pumps in Colder Climates

Thursday, April 4, 2013   (0 Comments)
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PostedApril 04, 2013 11:40 AM by Peter Talmage 


Air-to-air heat pumps are getting more popular as a primary heat source in colder climates. Here’s how to get the most from your system.

[Editor's Note: This guest post comes to us courtesy of Peter Talmage, P.E., an energy and design consultant and an instructor in theRenewable Energy & Energy Efficiency program at Greenfield Community College.]

I have heated my various homes with wood since 1975. It was always a love/hate relationship. The wood fuel was "free” off my land, but burning it was a very dirty business in many ways.

This Fujitsu 3/4-ton model 9RLS is in its third season as the primary heater for our 1,500 ft2 home in Northfield, Massachusetts. The interior unit is 18" off the floor, and certain creatures like that very much.
Photo Credit: Peter Talmage

Mini-splits in cold climates? Yes we can!

Three years ago, I installed a ¾-ton Fujitsu modelair-source mini-splitheat pumpto heat my historic 1790 cape home here in Northfield, Massachusetts. It has been a great success.

During the winter of 2010–2011, the heater for my 1,500 ft2home consumed 1,757kWhfrom October 2010 to June 2011. For the warmer winter of 2011–2012, the usage was only 1,247kWhfrom September 2011 to April 2012.

So far this winter, from October 2012, to March 23, 2013, the usage has been 1,501kWh. I have a 5.4 kW PV array that supplies about 200% of my electrical consumption, including that of theheat pump, so the heating system is very "green.” I have since installed mini-splits in two other houses.

Below are my suggestions for successful house-heating with a mini-split—even in a cold, Northern New England climate like mine.

1. Reduce load first

Improve the thermal envelope of the structure to minimize the size you’ll need and to reduce overall energy use.

2. Size it right fortypicallow temperatures

Heat-pump output drops as the outdoor temperature drops. I recommend sizing theheat pumpto meet heating load at, say, 10°F. During periods of lower temperatures, use simple electric resistance heating or another source to make up the difference.

The compressor in the Massachusetts house is located on the east side of the house and has a shed roof installed over it. The big pile of snow on the left had just slid off the roof cover.
Photo Credit: Peter Talmage

Also, remember that aheat pumpdoesn’t have the capacity to quickly bring a cold house up to temperature. I set the temperature to 60°F whenever the house is unoccupied temporarily or at night and down to 50°F for extended periods of no occupancy. (At the 50° setting, the interior units typically keep air circulating constantly to prevent overly cold spots from developing.)

3. Prepare for a little noise

The interior unit makes noise—not a lot, but a varying level ofwhoosh. Make sure you can live with it before you install one. Find an installation and listen. If you like a dead-silent house, a mini-split isn’t for you.

4. Let it snow—but not on your outdoor unit!

The outdoor compressor unit needs to be mounted at least two feet above the ground here in snow country. It also needs to be well protected by a roof or cover that does not restrict airflow butdoeskeep snow off and away from the unit.

In normal operation, the evaporator will freeze moisture from the air, which takes some extra energy. This ice is melted off during the defrost cycle. The melt-water drains out under the unit and sometimes forms a small glacier. The energy balance of this evaporator freeze/thaw cycle isn’t all that bad becausethe ice releases heat as it changes phase.

What can drasticallyreducethe performance of aheat pump, though, is when the evaporator gets plugged with snow. There is no gain of latent heat here, only energy consumption to melt the snow out. If the evaporator is located so snow can easily be sucked into it, the compressor will spend a great deal of its time melting snow and not heating the house.

The compressor for this Kennebunkport, Maine, home is set up high on a stand on the south side of the house. It draws air from a three-season porch that has glass panels installed in the winter, pulling air up through the gaps in the floorboards. A protective roof will be installed as well.
Photo Credit: Peter Talmage

My latest mini-split installation has the evaporator drawing air from an enclosed porch space. Air is pulled into the porch at low velocity through the spaces between the floorboards. Snow drops out of the air before it enters the porch, so it can’t plug up the evaporator. A second benefit is that the porch warms up in sunny weather, improving efficiency.

5. Get the low-down on indoor mounting

For heating, the interior unit should be mounted about 18 inches off the floor and should have a good, clear shot into the living space. Mounting the unit low has many benefits for heating:

  • First, it operates more efficiently because it is pulling in cooler air to warm up.
  • Second, the warmed air is blown out across the floor and mixes with the cold air at floor level.
  • Third, the air isn’t blowing directly on occupants, which can cause discomfort in the winter unless the moving air is very warm.
  • Fourth, it is very easy to access the filters for cleaning.

6. Right-size the pipes too

The interior and exterior units need at least 15 feet of piping to ensure no noise transfer from the compressor to the inside unit. Greater lengths of tubing are allowed, depending on the manufacturer, but will lower efficiency.

7. In warmer climes, get maximum efficiency

In colder climates, heat pumps need to strike a balance between efficiency (measured as heating seasonal performance factor, or HSPF) and lower operating temperatures. The warmer your climate overall, the more weight you should put on the efficiency side of the equation.

In central New England and south, go for units that have higher HSPF rating over lower operating temperatures.Mostof the time, the compressor will be seeing temperatures of 20°F or higher. Rarely will it be running at –10°F. The latest Fujitsu 9RLS2 has an HSPF of 12.5Btu/Wh.


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