When we yanked the oil boiler, we replaced it with a wall-mounted minisplit heat pump in the main level open area that includes kitchen, dining, living and our little office area. We closed off the first-floor bedroom and bathroom so those rooms are only heated by conduction and air leakage through the walls, and so they get cold — in the high 40°Fs at the lowest last winter.
The second-floor bedrooms and bathroom are only heated by warm air from the main floor rising up as cooler air from upstairs drops. This is a natural convection cycle, and it works pretty good and for free! Dontcha love physics?
Monitoring indoor temperatures
I’m data-logging temperatures in the basement, main level, upstairs, and outdoors with Hobo data loggers. The data for a week in December 2011 is shown in the graph below.
The outdoor temperature dropped as low as about 16°F and climbed as high as 57°F. The upstairs ran 2 to 4 F° cooler than the main level, except when it was good and sunny and it heated up with more unshaded south glazing per square foot of floor area than the main level.
December 26th was the only good sunny day in the six days shown in the graph, and you can see where the upstairs temperature climbs above the main floor temperature briefly.
We set the heat pump thermostat to 70°F when we’re here and to 66°F when we are asleep or gone. This being Christmas week, we were here a good bit more than usual, since both of us have been off of work since the end of day on the 23rd, which is the first full day of this series shown above.
If one of us were to be working at home upstairs, we’d need some supplemental heat — probably an electric radiant panel, like the ones we put into the Eliakim’s Way homes that I wrote about here. A panel like this heats up quickly and can be located so that it heats a person in a fixed location, like a desk, directly. It’s a good supplemental heat source for a house with a point-source heat pump.
Does our ductless minisplit system need more indoor units?
Of course, we could also install more heat pump cassettes. In the most extensive case, there would be four: the one we have and one in each of the three bedrooms.
I think if we ever use the first-floor bedroom, we might install another Fujitsu single zone unit for that room — albeit the 9,000 BTU/hour model rather than the 12,000 BTU/hour unit we have now, because it’s a smaller space. Then we’d add the radiant panels in the upstairs bedrooms as well, which we may do at some point as I have two of them laying around in the basement!
But for our present use of the house, the point-source system works well, and was inexpensive way to get off of fossil fuels and let the PV system supply the energy we use for heating (albeit on an annual basis…)
A year’s worth of monitoring data
We just passed our first year using the Fujitsu 12RLS for heating, with the exception of a 7 1/2 day period in early February 2012 when I switched back to the oil boiler to get some data, which was equal to about 5% of the heating degree days (HDD) in this one-year period. The heat pump energy consumption at the meter was 1,366 kWh; adding the missing 5% brings the consumption up to about 1,441 kWh/year.
I don’t know if that is the full number, as we have learned that electromechanical meters don’t always pick up the low wattage periods of consumption. The heat pump was off at the breaker for close to 6 months (May to October).
This works out to about 0.28 kWh/HDD. We keep the first-floor bedroom and bath closed off and they only are heated by conduction through the walls. The remaining area (gross square feet) of the house is about 1,300. So space heating energy use is about 0.73 BTU/sf/HDD in terms of input energy.
If the Fujitsu is operating at an overall COP of 3, then heating demand is about 2.2 BTU/sf/HDD, which is believable — a little lower than I’d expect, but we don’t keep it at 70°F unless we’re there and awake (70°F under those conditions, 66°F otherwise).
I’ve got Hobo data loggers set up now, logging temperatures on the main floor where the heat pump is, the bedroom upstairs, the basement, and outdoors, so I’ll have a better idea of performance as we move through the winter. Overall, we’re very pleased with this gizmo. I think we’ve paid about 1/4 the cost of what we would have if we’d used the oil boiler instead.
Marc Rosenbaum is director of engineering at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. He writes a blog called Thriving on Low Carbon. Marc teaches a 10-week online Zero Net Energy Home Design course as part of NESEA’s Building Energy Master Series. You can test drive his class for free.