Sunday, September 28, 2014

Mini split heat pumps

My regional electric utility has been promoting air-source heat pumps, and  my friend Dan who works installing these types of units tells me they have become quite popular over the last few years.  I discussed the efficiency of geothermal heat pumps in my heating costs comparison post a few years ago, so I figured it's time I did a similar analysis of air-source heat pumps.

As with any type of heat pump, the bigger the temperature difference (called lift), the lower the efficiency of the heat pump.  The efficiency rating for air-source heat pumps is usually given as a heating seasonal performance factors (HSPF).  This is a seasonal average of BTUs of heat provided per watt of energy consumed.  To convert HSPF to COP that is the usual performance rating for geothermal heat pumps, divide the HSPF by 3.4 - the number of BTUs per Watt.

HSPF by itself is not a useful performance measure, since it depends on the heating season outside temperature.  If the unit does not specify the temperature for the HSPF, it is likely 8.3C (47F for those who don't think in metric).  This might be a useful measure for someone living in Vancouver, BC, but not so much for someone living in Halifax, NS where the average January temperature is about -5C.

NrCan states:
At 10°C, the coefficient of performance (COP) of air-source heat pumps is typically about 3.3. This means that 3.3 kilowatt hours (kWh) of heat are transferred for every kWh of electricity supplied to the heat pump. At –8.3°C, the COP is typically 2.3.

A COP of 3.3 equals a HSPF of 11.2, and a COP of 2.3 equals a HSPF of 7.8.  So if your heat pump has a HSPF rating of >7 at -8.3C (17F), it will produce heat for less than half the cost of an electric resistance heater.  The hard part is finding out that efficiency rating.

Mitsubishi Mr. Slim is a popular mini split system, so I tried to find the full specifications for it's efficiency.  I couldn't find them on Mitsubishi's web site, but I was able to find them on a Mitsubishi reseller's web site.  Page 13 has a chart with the efficiency of several of the heat pump models, but the HSPF is only given for 17F.  There are performance numbers given at other temperatures, and since HSPF is BTUs per Watt times 3.4, the HSPF can be calculated at different temperatures.

I started with the GE24NA, a nominal 2 ton unit with a HSPF of 10 at 8.3C.  At -8.3C, it outputs 16,000BTU and consumes 3290W, for a HSPF of only 4.9.  This equates to a COP of 1.4, far short of the typical 2.3 COP stated by NrCan.  Compare that to the D30NA, a nominal 2.5 ton unit with a lower HSPF of 8.2 at 8.3C.  At -8.3C, it outputs 19,500BTU and consumes 2400W, for a HSPF of 8.1 (2.4 COP).  For heating a home in Nova Scotia, the D30NA is a much more efficient unit.

Another reason the D30NA is a much better choice is because at 19,500BTU it will be able to provide more of your heating needs than the GE24NA at 16,000.  Both units will probably need supplemental heat on the coldest winter days.  If you heat your house with electricity, you can look at your electric bill to figure out your heat load, remembering that 1 Watt is 3.4 BTUs.  If you can't find your bill details, but remember your electricity costs, you can figure it out from that.  For example a $450 electricity bill in January when electricity costs 15c/kWh means your consumption was 3000kWh, or 10.2 million BTUs of energy.  Dividing by the number of hours in the month gives an average of 14,000 BTUs per hour.  During a January winter storm with high winds and temperatures of -20C, you'll likely need more heat than the GE24NA can put out.

To analyze the economics, it helps to look back at my heating costs comparison post.  Electricity and oil are slightly more expensive than they were two years ago, but not by much; heating oil is selling for $1.07 per litre.  Wood pellets can still be found for $5/bag.  That puts the cost of a BTU of heat from electricity at about 1.4 times oil and 2.5 times wood pellets.  The conclusion is that a decent air-source heat pump is cheaper than heating with oil, and about as cheap as heating with wood pellets.  If you can get the time-of-day tariff and use a smart thermostat to avoid using electricity during peak time, the heat pump will be even cheaper than wood pellets.

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