There is a “new” type of heating system being marketing called a ‘hybrid’ heating system. Actually, this is NOT a new system at all, but a new name for an older system that used to be called an ‘Add-On Heat Pump.’ An air-source heat pump is added to a natural gas, propane or oil-fired furnace. The air-source heat pump runs when the outdoor temperatures are above the ‘application’ or ‘economic’ set-point temperature, and the fuel-fired furnace takes over when it is colder.
For information about how heat pumps work, see Heat Pumps
Application Set-Point – the capacity of an air-source heat pump varies with the outdoor temperature; the colder it is, the lower their capacity and their efficiency. As it gets colder outside, the heating demand of the house goes up. At some temperature, the falling capacity of the heat pump and the increasing demand of the house meet at a point called the ‘Application Set-Point.’ At this point – which is an outside temperature – which has generally been estimated based on calculations – the fuel-fired furnace must be turned on or the house will continue to get colder, even if the heat pump continues to run.
Economic Set-Point – depends on the price of electricity versus the price of the fossil fuel. It may be different than the Application Set-Point. For example, an Application Set-Point may be 40F; that is, when it is colder than 40F, the heat pump no longer has the capacity to supply the house heating demand. However, due to relative low natural gas prices and relatively high electric prices for this specific house, the Economic Set-Point may be 50F; that is, the heat pump still has the capacity to supply the load, but because of its lower efficiency at the lower temperature, it’s cheaper to run the fuel furnace.
The problem with the Economic Set-Point is that it must be constantly re-evaluated as the fuel prices change. Therefore, it’s rarely used, and typical installations are set-up with the Application Set-Point in control.
Hybrid heating systems can be advantageous to most any home owner who also wants central air conditioning. The additional cost for the air-source heat pump has become more affordable as fossil fuel prices have substantially increased in price. Air-source heat pumps are far cheaper than geothermal heat pumps and have none of the installation issues (water loops, wells, etc.) that exist with geothermal heating systems.
The speed of the payback for the price premium for the heat pump over a standard A/C system will depend on the local energy prices, the climate and the heating demand of the house (run hours).
Hybrid Heating from Bryant www.bryant.com/learnmore/guides/hybrid_heat.shtml
See Heat Pumps for more information about heat pumps and some vendor links.
Source: Text Bob Fegan 12/2008; images from Bryant web site at http://www.bryant.com/interactive/hybridheat.html 5/2008;