| ELECTRIC HEAT: Heat is produced by resistance heating (like a toaster). It may be either forced air or convection. Often individual units in each room with individual thermostats Advantages: Individual room temperature control, very reliable (few or no moving parts) clean, quiet, low initial cost and very long service life Disadvantages: Cost of operation GAS FURNACE: Heat is produced by burning gas in a chamber called a "heat exchanger". Air is forced over the outside of the heat exchanger by a fan and into the various rooms of the house through ducts. Ignition of the gas is provided by a spark, standing pilot, or resistance heated metal strip. Exhaust of the combustion products is either by a blower or via convection up a chimney. Advantages: Relatively low equipment cost, moderate maintenance cost, supply air temperature typically 120 F to 140 F. Life ~ 15-20 years (shorter on LP). High efficency (95%) available. Disadvantages: Must be on natural gas line or have LP tank. Possibility of gas leak or carbon monoxide problems. This is a central system and normally has only one thermostat so there may be room to room temperature variations without special "zoned" duct systems. OIL FURNACE: Heat is produced by burning vaporized oil in a chamber similar to a gas furnace. Air is forced over the outside of the heat exchanger by a fan and into the various rooms of the house through ducts. Exhaust of the combustion products is normally by convection up a chimney. The oil is vaporized as it is pumped through a special nozzle and ignited by a high voltage spark. Advantages: Air temperature typically 120 F to 140 F. Life ~ 15-50 years. Disadvantages: Needs to be serviced regularly. Possibility of carbon monoxide problems. Good equipment tends to be expensive. Central system - High operating cost, relatively low efficiency(~82% maximum). HEAT PUMP: Freon (typically) gas is pressurized by a compressor and circulated through an indoor coil. Air is circulated over the coil removing heat and changing the high pressure gas into a high pressure liquid. This high pressure liquid is circulated to an outside coil where it passes through an expansion device changing it into a low pressure liquid. Outside air is passed over the outside coil with another fan and the liquid absorbs heat from the air changing it into a low pressure gas. The low pressure gas is then compressed again and the cycle repeats itself. Note that heat is added to the freon both by the compressor (when a gas is compressed it gets hot and also by the outside air when it changes state from a liquid to a gas). Since only part of the heat is added by the compressor and the rest of the heat is taken from the cold outside air the heat pump has an electrical efficiency of greater than one (up to about 4 ). When the unit is used as an air-conditioner, the flow of the freon is reversed by a "reversing valve" and the freon changes from a low pressure liquid to low pressure gas in the inside coil removing heat from the house and transferring it to the outside air. Advantages: One piece of equipment provides both heating and air- conditioning. Very efficient above ~50F. Moderate maintenance. Life 12-15 years. No possible carbon monoxide problem. No need for gas or oil tank. Very high efficency. Geothermal systems are available where water of earth replace the outside air to Further improve efficency and allow its use in cold climates. Disadvantages: Heating efficiency drops with decreasing temperature. Air temperature from supply vents is 90-100 F in heat mode. Requires back-up heat (normally electric) during defrost and in very cold conditions. Requires the availability of more electric power than fossil fuel system if electric back-up is used. Central system |
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