Residential HVAC

Performance of any type of heat engine is described by its thermal efficiency. Air conditioning unit or heat pump is also a form of heat engine and its performance is expressed by the ratio of useful heat to work called COP or Energy Ratio. From thermodynamic relation it is noticed that the COP depends upon the operating conditions of condenser and evaporator. For V.C system it is of the order of 3 for water cooled and 2 for air cooled a.c applications. For domestic refrigerator it will be still less as the evaporator temperature is low. Conventional refrigeration systems work on the vapor compression cycle which is very near to Carnot Vapor Cycle has high COP. Other cycles like air cycle, absorption cycle have less COP.

The Coefficient Of Performance (COP) of an electric heater is 1.00, since it directly converts on Watt of electric energy into one Watt of heat energy. The COP of of a heat pump can be higher than 1.00, since it transfers heat from outdoors to indoors, rather than merely converting heat. In the United States residential heat pumps are rated at high heat (47F (Fahrenheit) outdoor air) and low heat (17F outdoor air). All products are different, but here are some typical ratings: (a) At 47F a COP might be around 3.1, (b) at 17F a COP might be around 2.2, (c) at 5-10F a COP might have dropped to 1.00, at which point it is no more efficient than an electric heater, (d) at abou 0F, many manufacturers would recommend that the heat pump be turned off, since the high compression ration may shorten the life of the compressor.

As was mentioned, the COP depends on what is happening with the condenser and evaporator. Both the entering and exiting fluid temperatures, and of course the delta T (temp change).

What was not mentioned is the distribution systems - which is one of the main factors in determining what is happening on the inside or conditioned side of a system (which could be condenser or evaporator depending on heating or cooling operation). So duct systems or pipes for hydronic heating and cooling will play a role in determining how much usable energy can be extracted from a heat pump with a certain amount of energy input.

So in the example of an air duct system, if the fan or ducts can not move the correct amount of CFM over the water to air heat exchanger, it may not be able to get rid of all the heat, or maybe it gives up too much heat if the CFMs are too high - causing a drop in efficiency. This is a common problem in house retrofits where a heat pump is just dropped into a duct system designed for a fossil fuel burner.

Then there is also any pumps, fans or other electrical draws on the system that should be taken into account.

Bench test rated efficiencies are always optimistically high, for all appliances, in my experience.