Quote:
Originally Posted by Slickcraft
The math is straightforward. At minus 15 deg 85% efficient means 100 watts in gets only 85 watts of heat out. Over 0 deg temp the efficiency is over 100% so the heat pump puts out more watts than what is required to run the unit....
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I think the term "efficiency" is being misused here (for a minisplit). The performance of a heat pump in heating mode generally is indicated by "Coefficient of Performance" (COP), that being the ratio of energy delivered inside (as heat) to energy input as electrical power (3412 BTU/hr per KW). A reasonable seasonal averaged COP might be around 3, but this will be substantially higher when the outside temerature is say in the 40s or 50s. The COP also drops as outside temperature drops. Think of it as being like pumping water uphill; for a given amount of pump power, you get a lower flow as you pump farther uphill.
That 85% number for the minisplit likely indicates how much of full rated capacity (BTU/hr) is being delivered from a very cold outside temperature. In general, except for some losses along the path, a heat pump delivers as heat all of the electrical energy input to the compressor plus whatever is absorbed from the source (outside air) by the evaporation of refrigerant on the cold (low pressure) side. Thus the COP will always be above 1.0 and substantially so.
It should be noted that heat pumps installed in heating climates generally are sized according to the expected heating load at some design minimum heat loss rate, which can be calculated sufficiently well for any house. On the other hand, most fired heat systems installed over the years typically are grossly oversized relative to expected demand, often by a factor of 2 to 4. Such a system has a lot of extra capacity, which leads to short cycling in normal use but which does let a cold house recover rapidly after a power outage or other period of deep setback. In contrast, a properly sized heat pump does not have that huge reserve of overcapacity for rapid recovery.