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The easiest way to recognize a heat pump is at the thermostat. When you remove the cover from the thermostat, you will have dual bubble mercury tubes controlling the heat. These bubbles or mercury tubes are mercury switches, similar to a single pole light switch. The top bubble controls the compressor; the bottom bubble controls the back-up or supplemental heat. The bubbles operate in tandem with the top bubble being engaged about 2 degrees before the bottom bubble. The air conditioning is usually controlled by one mercury tube on the right side of the thermostat. If you see a second pair of bubbles, the air conditioning system has dual compressors, which operate on demand and can operate at lower demand level, which could reduce operating costs. Dual compressors are usually found in commercial units. When inspecting a heat pump, turn the system off. Then move the thermostat up slowly so only the top mercury tube is engaged. You must have the cover off to see this. If you allow both switches to become engaged, and the unit is on, you may have to wait for a time delay to release the supplemental heat or check the supplemental heat first. Once you have the thermostat set properly, turn the system on. Assuming the outside temperature is above the balance point (32 to 40 degrees), with only the compressor engaged (top bubble), and after about 5 to 8 minutes, measure the difference in temperature between a supply and return. The temperature difference should be 18 to 30 degrees Fahrenheit in the heating mode. The outside temperature will have some impact on this temperature. If the temperature difference is not high enough, the probable causes are a laboring compressor or low freon charge. If it is a laboring compressor, figure about $500 to $600 per ton, plus $250 to $350 if the condensing cabinet fan and coil are replaced. If it is a freon charge, figure $125 to $175 for a charge and service. If the temperature differential is too much, possible causes may be:
Engage the lower mercury switch bubble to check the supplemental heat. Assuming there is electrical resistance supplemental heat, you should find an additional 10 to 25 degrees Fahrenheit at the supplies. If you get more than this, there is probably restricted air in the system. If the air is slowed, it will pick up more heat off of the coils. The first place to look is the filter. The second place to look is at the supply registers. If more than 30 percent of the registers are closed, the airflow will be significantly restricted. Other possibilities would be the way the coils were wired, slow fan speed, the size of the supplemental coils or improper duct design, especially the returns. Some reasons for inadequate temperature rise when the supplemental heat is engaged at the thermostat are:
In the air conditioning mode, measure the temperature difference across a supply and return. This difference should be 14 to 19 degrees Fahrenheit. The reasons for low or high measurements will be the same as outlined for the heat mode above. How a Heat Pump Works Air Conditioning Mode
As this gas moves through the coil, the fan cooling the coil reduces the temperature of the gas. Before the gas gets through the coil, it is warm but cool enough to condense. This is the point where the gas turns to liquid. The warm liquid flows out of the exterior coil and into the inside coil, usually in the heating system plenum. As this liquid reaches the coil, it is evaporated at an expansion valve and the cooler coil. The cold, evaporated gas moves from the indoor coil back to the compressor for the start of another cycle. Heat Mode The heat mode is exactly the same as the air conditioning mode. The difference is a reversing valve, which redirects the gases. In the air conditioning mode, the outside coil is warm and the inside is cold. In the heat mode, the inside coil is warm and the outside coil is cold. Steam Heat
Steam heat operates like a teapot. No steam is developed until the water boils. Once the water is boiling, steam rises up supply pipes, which lead to radiators. Steam is constantly being cooled in the pipes and radiators, changes back to water and flows back to the boiler for reheating. The site glass on the front or side of the heating plant indicates the level of the water in the boiler. There is no water in the pipes or radiators like a hot water system. Steam Valves When the steam moves up the supply piping toward the radiators, it must displace the air in the pipes and radiators. This is done by steam valves, which are usually located on the radiators, but can be located on the supply lines. There are a couple types of steam valves; most utilize a bi-metal material. When the air is pushed out of the radiators and the steam approaches the steam valve, the heat of the steam will cause the bi-metal valve to close. This allows the air to escape while the valve is cool, and closes it when it gets warm from the steam. When the thermostat is satisfied and the radiators start to cool, the steam valve cools and allows air to re-enter the radiators. If there is corrosion (white stain) around the steam valve, it indicates failure. The stains are the residues from steam, which has escaped. Low Water Cutoff A low water cutoff senses the level of water in the system and is designed to turn the burner off if the water level is low. If there is not adequate water in the boiler, the excessive expansion may cause the boiler to rupture. Low water cutoffs must be drained regularly (weekly for large units, monthly for small or residential units). This draining is needed to flush the minerals from the water that was converted to steam, away from the cutoff mechanism, and assure its functionality. The pressure gauge only senses pressure when the pipes and radiators are filled with steam. The pressure gauge reads zero until this happens. Steam Limit Control Steam limit controls measure pressure in pounds, unlike a hot water system which measures pressure in PSI or altitude pressure. The limit control is usually a small, gray fixture mounted at the top and in most cases adjacent to the round, steam pressure gauge. Residential limit controls typically operate between .5 and 5 pounds of pressure. The function of the steam limit control is to turn off the burner when the designated pressure is reached. Automatic Water Feed Water feeds for steam systems act on the level of the water in the boiler instead of pressure, because there is no water pressure in steam systems. They are located adjacent to the low water cutoff because the level of the water is important to both of these fixtures. Pressure Relief Valve The steam pressure relief valve is set at 15 pounds, unlike the valves on hot water heating systems which are set at 30 PSI or valves on water heaters set at 150 PSI. Distribution
A one-pipe system is simply that. All steam and water flow in the same pipe. One pipe comes out of the heater and supplies all radiators. When the steam condenses back to water, it flows down the same pipe in which it came up as steam. The critical thing to remember is that this pipe must always slope toward the boiler. Two pipe systems have a supply and return. Some steam may condense and return to the boiler via the supply pipe, however, a return pipe receives the condensed steam and carries it back to the boiler. Return pipes are always located lower than the supply pipes. Hot water system returns go into the bottom of the boiler. Steam returns used to go into the bottom, however, when the return rusted and failed, it would drain the boiler. This would cause the low water cutoff to turn the burner off, and if the lower water cutoff did not work properly, the boiler could rupture. The Hartford Insurance Company paid numerous claims for this situation until a trap was designed to allow the water from the return to enter the boiler above the boiler water level. This trap has been named a “Hartford Loop” after the Insurance Company.
Width in inches x height in inches x 2.5 divided by
144 = SF per radiator section. The amount of heat delivered by a steam heating system is different than a hot water heating system. Steam radiators are hotter than hot water radiators. Steam radiators deliver 240 BTUs per hour per square foot of radiator surface. SF of heat distribution surface x 240 BTU’s = total amount of heat delivered. NOTE: This assumes 215 degrees Fahrenheit steam with 70 degrees Fahrenheit air temperature.
SF of heat distribution surface x 150 BTUs = total amount of heat delivered. NOTE: This assumes 170 degrees Fahrenheit water temperature with 70 degree Fahrenheit air temperature. Example:
8 x 30 x 2.5 divided by 144 = 4.16 SF per section. 4.16 x 20 = 83.2 total SF of radiator surface (heat distribution surface).
should be between 2.2 and 2.8. Heating Main Warm Air Heating Systems Hot Water Systems Fuel Cost Comparison |
Freon
gas is compressed. Anything compressed
will develop heat. A hot, high-pressure
gas comes out of the compressor and into the outside coil.
Burners, vents, and flues are the same for steam, hot water
and furnaces.