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Fri, Oct. 18

Heat Pumps and Dual Fuel Heating and Cooling Systems - the Best of Both Worlds

The largest energy used in a home is for heating and cooling; it's about 46%.

For 90% of us the choice is natural gas or electricity, but for a few it is heating oil, propane, coal or wood. There are also renewables, wind and solar; however, renewables are a small part of the residential energy supply and in most cases can't provide stand alone power sufficient to support a home 24 hours a day without grid or battery backup.

The bottom line for any decision is how much energy I need to condition my home for comfort and how much will it cost; this is where an analysis comes in. Ideally you should know how much energy your home loses through conduction, convection, radiation and air leakage through its outer skin. This is the amount of energy your heating and cooling system has to replace to maintain in home comfort. A comprehensive energy audit can supply this information, but for many it is an unknown.

When comparing heating and cooling systems the common energy measure is the British Thermal Units (BTU). The gas company bills a homeowner in Therms, which is 100,000BTU and the electric company bills its customers in Kilo Watt hours (KWh) where a KWh is equal to 3412BTU; now with the conversion in place it is easy to compare the cost of electricity with that of natural gas or any other fossil fuel.

Now look at the cost of these fuels. Based on numbers from the Energy Information Administration (EIA) the average price of natural gas is $1.137 per Therm and electricity 11.84c per KWh, or $3.47 an equivalent Therm.

It is important that you use costs based on your own energy bills as they tend to be higher than the average. All energy bills are based on the fuel you use and a large fixed cost due to distribution and taxes.

From above the same Therm of electric energy supplied to heat or cool your home costs approximately 300% more than natural gas - why, because over 50% is lost in the transmission to your home from the generation station. This is why in most cases electricity isn't used for home heating. That is unless you consider a heat pump.

A heat pump is a device identical to the air conditioner in your house or car, the only difference is it can reverse direction and heat in winter and cool in summer. There are two configurations, one uses air as its energy medium and the other called geothermal uses the ground and or water as its medium. Heat pumps are a science unto themselves and beyond the scope of this discussion.

The important point to note is that heat pumps extract energy from the air or ground and generate more energy than they use; that is, for a KWh of source power the unit generates 3KWh (+300%) of output power. All other heating systems absorb energy during the input to output conversion called efficiency factor, the heat pump is the only device that supplies more energy than it consumes because it steals energy from the air or ground.

However, it has limitations; its efficiency or Coefficient of Performance, the ratio of input to output energy level varies with air or ground temperature. The air based system is more susceptible to temperature variations than geothermal as ground temperature is reasonably stable below thirty feet at around 55°F. However, for this discussion we will concentrate on the more popular, lower cost and readily available air based systems.

We know that natural gas costs $1.137 per Therm and electricity costs $3.47 per equivalent Therm, but if we now consider the heat pumps energy efficiency characteristic below; it shows that as the outdoor temperature rises the heat pump efficiency increases and at 60°F costs only 71c a Therm; but as the temperature drops below 40°F the cost increases above natural gas reaching $1.40 at 28°F and $2.10 at 8°F.

This is why a stand alone air based heat pump is not economically viable in very cold climates; however, new dual fuel systems that use natural gas as backup to the heat pump can provide the best of both worlds. Be careful with the electric only systems as the electric back-up unit is costing 300% more at $3.47 per Therm and not the $1.137 of natural gas.

Another advantage of a natural gas dual fuel system is that as fuel prices change the temperature at which the system switches fuels can be adjusted for the best cost model. And remember your heat pump is also your air conditioner and with a device called a desuperheater your heat pump can provide effectively free hot water to your home when the heat pump is running.

Dual fuel systems cost a little more than a stand alone natural gas furnace and air conditioning unit, but as a cost effective system dual fuel makes a lot of sense. It provides heating, cooling and hot water from one configuration and with a low operational cost.

All backup data and calculations referenced in this article can be found at

For more information contact Paul Scrivens

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