HEAT PUMP EFFICIENCIES
For the purpose of this website, we will focus primarily on the heating cycle of a heat pump.
There are four key ratings when considering a heat pump.
SEER: Seasonal Energy Efficiency Ratio is a rating for air conditioners. The higher the number, the more efficient it is as an air conditioner. Heat pump SEER ratings typically run from 13 - 26.
HSPF: Heating Season Performance Factor is an efficiency rating for the heat pump in its heating cycle. It is defined as the ratio of BTU heat output over a heating season to the watt-hours of electricity used. For example, a heat pump that generates 120,000 BTUs in a season using 15,000 KWH will have a HSPF rating of 8. The higher the number the better, since that indicates either a high heat output, or a low electricity use. Most heat pumps have an HSPF of 7.7 - 10.
COP: Coefficient of Performance is defined as the useful heat supplied to the system (Q in the chart to the right), divided by the work required by the system, i.e. the electric energy used. (W in the chart). The chart to the right specifically shows the cooling cycle, but the concept can be reversed for the heating cycle. Again, the higher the number the better. Though the math is somewhat involved, a heating unit that produces 50,000 BTUs and uses 7 kw of electricity will have a COP of 2.1. Most heat pumps have COPs in a range of 2 - 4.
BTUs: British Thermal Units are a measure of heat, and for heat pumps they are rated as BTUs produced per hour to heat (or cool) a home. This is also expressed in tons, where a ton of capacity is equivalent to 12,000 BTUs/hour. Thus a 2-ton heating unit has a 24,000 BTU/hour capacity
Thermal balance point
The "thermal balance point" uses some the concepts above to explain the temperature at which a heat pump no longer provides enough heat to heat your home. We're just going to look at whole-home solutions for simplicity's sake, though the same concepts can be used for a floor of a home or even just one room.
Basically, all homes lose heat (unless you live in a home with concrete walls and no windows or doors). This isn't quite true, since if it's hotter outside than indoors, heat will come into your home rather than leave it, and when the temperature outside is equal to the temperature you want inside, let's arbitrarily say 70 degrees F., then by the Second Law of Thermodynamics, there is no heat loss. For heating purposes, the heat pump needs to deliver heat to make up for the lost heat of the home when the outside temperature is below 70 degrees, in this example.
Homes are rated by how much heat they lose per hour at certain temperatures, based on their size, construction (wall material, number of windows and doors) and insulation. As mentioned above, our hypothetical house is designed to lose no heat at 70 degrees. At that point the indoor and outdoor temperatures are in equilibrium. Due to its construction properties, let's say the home will lose 40,000 BTUs per hour at zero degrees F. without any heat source to keep it warm. As we saw above, heat pumps are designed to produce BTUs per hour; and this heat is used to compensate for the heat lost from the building, in order to keep the house at the target temperature of 70 degrees.
In this example, illustrated in the chart to the right, we model the specs of an actual heat pump, a Fujitsu FO314RSJ model, against a home with a 40,000 BTU/hr heat loss. The Fujitsu has two BTU ratings: the heat supplied at 47 degrees and at 17 degrees. These are just two arbitrary industry benchmarks for all heat pumps. In the Fujitsu case, it produces 27,400 BTUs/hr at 47 degrees and 16,600 BTUs/hr at 17 degrees. These points are graphed in the chart to the right with the red line. It shows various points, including these two design points, in a descending line from right to left. That means as the temperature drops, the heat pump produces less and less heat. That's because the ambient heat in the air produces ever cooler refrigerant vapor that the compressor needs to heat.
The blue line in the chart shows the heat loss from the home, assuming an inside desired temperature of 70 degrees, connecting 70 degrees on the right where the home loses zero BTUs, to zero degrees on the left where the home loses 40,000 BTUs/hr.
The thermal balance point is where these two lines intersect. At that point, the heat pump produces exactly the amount of heat lost by the house to keep the inside temperature at 70 degrees. To the right of that point (higher temps), the heat pump produces more heat than is needed to heat the house to 70, and to the left of that point (lower temps), the heat pump dosen't produce enough heat to keep the home warm at 70 degrees.
Doing the math, that thermal equilibrium point is at 31.7 degrees in this example. At this temperature, the home is losing 21,890 BTUs/hr and the heat pump is producing 21,890 BTUs/hr. In warmer temperatures, let's say 50 degrees, the home is losing 11,429 BTUs/hr and the heat pump is producing more than enough heat at 28,400 BTUs/hr.
But at temperatures colder than 31.7 degrees, let's say at 10 degrees, the home is losing 34,285 BTUs/hr but the heat pump is only generating 14,080 BTUs/hr. In other words, at 10 degrees, in this example, you home is losing about 20,000 BTUs of heat every hour.
You may see heat pump manufacturer ads that claim their heat pumps are still running and producing heat at zero, or even at negative temperatures. That may be true, but it's unlikely the heat pump is producing enough heat to keep your house warm at those temperatures. Again, using the Fujitsu example here, even at zero degrees, it's producing 10,480 BTUs/hour and at -10 degrees, it's still shelling out 6,880 BTUs/hr. But as you can guess, our house in this example needs 40,000 BTUs/hr at zero degrees and 45,714 BTUs/hr at -10 degrees; so it's getting so cold in the house that pipes are freezing up, even though the heat pump is running at full blast.
What happens when it gets cold?
It's not unusual for the temperatures in New England to be 30-40 degrees F. during the day, but fall to 10-20 degrees overnight. If you're using a heat pump, it's running fine during the day, but overnight, you'll get cold; and, as is seen in this example, you're could be losing a lot of heat - some 20,000 BTUs every hour when it's 10 degrees outside.
What you need is another source of heat.
Some heat pumps have built in them an electric heat strip. Basically this is a strip of metal that heats up with electricity, like a toaster oven, which generates heat which is blown into your room by the fan in the air handler. Electric heat is expensive in states that have high electricity rates, and electric strip heating as a supplement to the heat pump only adds to the electricity already used by the heat pump (see the next tab, "Cost of Operation")
If you already have another heat source installed in your home, whether heating oil, propane, gas or electric, you can switch off your heat pump when it gets cold and turn on your oil or gas furnace or boiler. That means if you anticipate a cold night you'll need to manually make this switch before you go to bed.
Insulation is critical to making your heat pump work efficiently. You can see this easily from the graph. If your home loses 30,000 BTUs/hr because it's well insulated, rather than the 40,000 used for this example, your balance point will move lower, to 24.8 degrees, which means the heat pump is generating enough heat to keep the house warm at 70 degrees at this lower temperature. If, on the other hand, you have a poorly insulated, leaky house, say at 50,000 BTUs/hr, your balance point will move higher, to 36.8 degrees, which means you'll be losing heat below that temperature.
Cold Climate Heat Pumps
The other alternative is to buy a bigger heat pump. These so-called "cold climate" heat pumps will also push the balance point lower which means they will keep the home warmer at lower temps. They do this with a bigger compressor. Remember, the compressor squeezes (reduces the volume) of the refrigerant vapor, like a piston in a car engine, making it hotter (the "ideal gas law again). Now at colder temperatures, there's less heat to extract from the air to turn the liquid refrigerant into a gas, which means that the resulting gas vapor will be cooler, and even after a normal compression cycle, the resulting heated vapor will only be warm, not hot. That's why you'll need a more powerful compressor to get more heat out of the refrigerant. The tradeoff is that a larger heat pump uses more electricity and is more costly. Note, there is a temperature at which any heat pump won't work at all, and that's when there's not enough ambient heat in the outside air to warm the liquid refrigerant to its boiling point and turn it into a gas vapor. When that happens, the compressor doesn't work and the heat pump fails.
An example of one of these kinds of heat pumps is the Mitsubishi "Hyper-Heating" systems, such as the MXZ-3C24NAHZ. With maximum capacities of 30,600 BTUs/hr at 47 degrees F. and 24,600 BTUs/hr at 17 degrees F., you can see there's less of a fall-off compared to the Fujitsu as temps decline. In fact it's thermal balance point is much lower, at just 24.4 degrees; and it's still putting out 21,300 BTUs/hr even at zero degrees.