Letters From Our Readers: July 7, 2014
Editor’s Note: The following remarks were made in response to the “Heat Pumps Riding a Wave of Popularity” article, by Joanna Turpin, published May 26 in The NEWS.
Calculating COP in Heat Pumps
Mrs. Turpin, congratulations on your front page article in the May 26 issue, only problem is the tech is using analogue gauges, which aren’t accurate enough to properly charge anything, let alone a heat pump or a/c. Improper charge using analogue gauges can result in significant efficiency reduction from either over or under charging.
Another area I didn’t see covered is the heat sources resulting in a heat pump’s higher output measured by its COP (coefficient of performance). A COP of 1 is equal to the heat of 1 watt of electricity or 3.41 Btu. As you can see from manufacturers’ equipment specifications, a significant heat pump COP is a result of three inputs. One, the heat in outside air being absorbed through the outdoor unit into the refrigerant; two, the heat of compression caused by the compressor raising the pressure from low to high; and three, heat given off cooling the compressor motor, all contributing to the final capacity.
As the outside air temperature drops, the heating capacity of a heat pump drops. As the outdoor temperature drops, the heat required to maintain structure thermostat set point increases. If you plot the two capacities on the same graph, you’ll see the lines cross, here in California, usually in the mid-30s. This point is called the balance point. It is the point at which one needs supplemental heat to maintain indoor temperature usually energized by a second-stage thermostat. It can be accessory supplemental electric strips or a fossil fuel furnace, actually it can be any secondary heat source.
It seems manufacturers have drifted away from supplemental heat, trying to exaggerate efficiencies by equipment oversizing, trying to match calculated Manual J heat requirements with heat pump heat capacity at outdoor design temperature. In California, this grossly over sizes the unit in the cooling mode.
In the old days, we broke the heater’s second stage with an outdoor thermostat set at the balance point plus the thermostat had a bypass sometimes called “emergency heat,” allowing it to energize both stages for rapid pickup, say, for entering the house after an unoccupied period with no regard for operating cost, trumped by the need for comfort. The second stage was set anywhere from 2-5°F behind first stage. Setbacks greater than 5° weren’t recommended. Airflow is also more critical with a heat pump, since resulting lower discharge temperatures increase the chance of draft discomfort. I’d like to see an article from manufacturers on how they recommend addressing heat pumps versus conditioned space characteristics at mid-to-low outdoor design temperatures.
Troy R. Helton Jr.
Santa Paula, California
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Publication date: 7/7/2014
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