System and Components
Gravity systems operate by air convection. Simply, heated air expands, becomes lighter and rises. Cooler air is more dense and falls. There is no mechanical force moving the air. The difference in air temperature alone creates the 'convection' or motivation for air movement.
The return grille or duct of a gravity system must be unrestricted. Even a filter is too restrictive. This is necessary to develop positive convection and more efficient distribution.
The furnace consists of a burner compartment (firebox) and a heat exchanger. The heat exchanger is the medium used to transfer heat from the flame to the air, which moves through the house. Besides being the medium of heat transfer, the heat exchanger keeps the burned fuels separate from the air.Evaluating the heat exchanger in any warm air heating system is difficult, evaluation is even more difficult in a gravity system.
Inspect the cabinet or jacket for burned paint, distortions and/or scorch marks. Scorch marks may indicate that the firebox is not performing its designed function and heat is escaping. Such problems indicate that the flame is getting beyond the furnace firebox and may be creating fire safety, economic and functional problems.
Forced Air Systems
This type of system is also known as a Heat Exchange. As mentioned with gravity warm air heating systems, the heat exchanger is the medium of heat transfer and separates the burned fuel from the air that moves through the house.
When a heat exchanger cracks or fails, there are two points to consider involving air pressure. When cracks and rust, etc. are located at air pressure points, such as at turns and/or at the top of the heat exchanger (where the direction of the exhaust is not directed toward the flue), you may get combustion gases forced into the flow of the air in the plenum and ductwork.
The following evidence indicate positive pressure on a heat exchanger:
- In a gas-fired, forced air system, positive pressure may be visible when the fan comes on, because it may deflect the flame pattern.
- Look at the flame before the fan comes on to note the flame pattern.
- Look again, after the fan comes on, for changes in the pattern.
- To establish a benchmark, you may want to gently blow air across the burners before the fan comes on to get a sense as to the amount of deflection caused by small amounts of air. Generally, the amount of air needed to blow out a candle will turn the burner flame bright orange. This will help keep small or questionable flame patterns in perspective.
- A metal frame mirror with an extension handle and a good flashlight will improve visibility in the vertical heat exchanger tubes. It will not help at depressions, baffles, and at most welds, or with horizontal heat exchangers. Basing firm conclusions on observations with a mirror is cautioned. A careful visible inspection of the 10 to 40 per cent of the heat exchanger you can see should allow you to make a reasonable determination of the condition of the heat exchanger. If the plenum has an opening from a removed humidifier, you may want to consider removing it.
Most of the gas-fired information noted above also applies to oil-fired heat exchangers, except for the flame pattern information. Air pressure on a unit with an oil burner will seldom, if ever, have a visible effect on the flame.
There is evidence which may be noticeable at times on these units. Lift or swing the burner view door open when the flame is on, but before the fan comes on. Assuming the chimney is drafting properly, note the draw from the view door and a relatively small amount of residual heat. After the fan comes on, open the view door again. If there is positive air pressure being exerted through a crack or failure in the heat exchanger, you may feel more than a small amount of residual heat as the air pressure is pushed into the heat exchanger and out of the view door.
Negative pressure in an oil or gas-fired heat exchanger poses a health and safety hazard. Air is drawn into the plenum and ductwork. Look for soot at the registers that are closest to the unit. Also try to evaluate whether the registers and the areas adjacent to the registers have been cleaned recently. This may suggest that someone is trying to hide a problem.
Smoke bombs, combustible gas detectors and sodium bicarbonate solutions used to determine the condition of a heat exchanger are suspect as best. When using a smoke bomb, the heat exchanger must be brought up to temperature to allow for expansion of cracks. Access to the plenum or a portion of the ductwork is needed, and the flue should be blocked. If you are concerned about the heat exchanger after evaluating it, contact the local utility company or a responsible HVAC contractor.
Blocked Chimneys Or Vent Connector
Blocked or partially blocked vents or chimney flues pose health and safety problems, and are relatively simple to recognize.
With the heater on, a blocked or partially blocked flue will create excessive heat and moisture in the basement air, especially at the heater. When combustible gas exhaust backs into the basement from the flue, the humidity of the basement air is dramatically increased because the cooler basement air has less ability to hold moisture.
If you cannot operate the heater, you may still recognize present or previously blocked chimney problems. In a gas-fired unit, the area around the bonnet or draft hood will have evidence of rust. This is a result of hot (greater than 400 degrees) exhaust gases contacting the cooler metal around the bonnet or draft hood. You will not see the moisture because it evaporates quickly on the warm metal, however, the rust will be evident.
In an oil-fired unit, there may be excessive soot around the burner view door and around the barometric draft control. This is a major health, safety and operational defect.
Caused by late or faulty ignition, such as a fouled nozzle or weak ignition arcing. Intermittent puffs of unburned fuel and combustion gases from the burner and combustion area back through the burner and any other open areas. Unburned fuel oil that is deposited in the combustion chamber is likely to ignite when the burner finally comes on. Adjustments and service usually resolves this problem.
Caused when the flow of the combustion products and air in the chimney are reversed due to reduced air pressure. Causes could be whole house fans, exhaust fans, or other combustion appliances. This is similar to a partially blocked chimney, however, it is typically not as bad and it is usually intermittent.
The length of the vent pipe is critical. The longer the vent pipe is from the heater to the chimney, the more likely it is for heat to dissipate and develop condensation, which will cause corrosion and vent pipe failure. Ten feet is long; longer than 15 feet is not allowed; and the shorter the pipe is, the better it is. Long, horizontal vent pipes also cause condensation in the chimney. This may explain water coming down the chimney in many situations.
Horizontal vent pipes should always slope up from the heater to the chimney. Negative slopes may cause drafting problems, which could endanger the health of the occupants.
Check the draft by carefully touching the metal vent pipe. This pipe should be hot when the unit is operating properly. If the pipe is cool three feet to six feet from the heating appliance, there may be problems with drafting. Potential causes may include:
- Chimney not high enough
- Draft obstructions at or adjacent to the chimney flue
- Blocked or partially blocked chimney
- Vent pipe pushed too far into chimney flue
- Vent pipe too long
- Restricted air movement—two or more 90 degree turns
- Gas-fired systems have draft diverters to balance the exhaust draft.
- Oil-fired systems have barometric draft controls to balance the exhaust draft
Lack of a barometric draft control on the vent of an oil-fired heating system may affect the operation of the burner because of possible restricted flow to the chimney. This restricted air, which causes some amount of backpressure to the burner compartment, may alter the flow of air through the burner that is needed for proper combustion of the fuel oil. The repercussion of this condition is usually an inefficient burner and fouled nozzles about one month after proper servicing of the burner.
If more than one heating appliance is using one flue, there are some general conditions that apply. Check local codes and safety requirements in your area.
- If two vent pipes are joined, they should be joined with a ‘wye’ and not a ‘tee’ coupling.
- If gas and oil-fired appliances are vented into the same flue, the oil-fired, or hotter appliance should be lower than the cooler gas appliance.
- The length of the vent pipes should be equal. There is a possibility that the longer pipe will have some spillage.
- The diameter of the collar at the appliance dictates the diameter of the vent pipe. As long as the combined areas of the appliance vents do not exceed the area of the flue(s) being used, the drafting should be acceptable.
- Check the draft at each appliance when both appliances are operating and have had time to stabilize.
Limit Controls -- Warm Air Heating Systems
Limit controls typically have three settings—two lower settings turn the fan on and off and the high setting turns the burner off. A typical cycle would be as follows:
- Thermostat calls for heat and causes the burner to come on.
- The fan does not come on until the temperature in the heat exchanger reaches the high fan setting. This is typically150 degrees Fahrenheit.
- If the fan does not come on and the temperature in the plenum reaches the highest setting (usually about 200 degrees Fahrenheit), the burner will turn off. This situation would reflect a problem and should be reported.
- Assuming the thermostat was satisfied, the burner will turn off, but the fan will continue to run until the temperature in the heat exchanger drops to the lowest setting,which is usually about 100 degrees Fahrenheit.
- The cycle starts over on command from the thermostat.
The thermostat and limit controls generally operate at 24 volts.This is a low voltage system. A typical 120-volt circuit is run to a transformer and the voltage is changed from 120 to 24 volts.These transformers can be found on the outside of the electric service panel or adjacent to the heater
One cubic foot of gas requires 10 cubic feet of air to burn.Technicians typically set up gas heating appliances at 13.5 to 15 cubic feet of air to ensure that there is enough air to provide complete combustion, or as complete as the appliance can provide.
Natural gas burns very complete; the typical amount of carbon monoxide in exhaust gases is less than 1/4 of 1%. Consider a typical gas range in a kitchen. If CO was a problem, ranges would be vented, just like water heaters and heating systems.
The reasons that gas-heating appliances are vented are:
- Approximately 94% of the exhaust or waste is moisture, and heat is necessary to carry the water, in a vapor state, up to the chimney and out to the atmosphere.
- The size of the heating appliances, coupled with the amount of time a heater may be on in very cold weather, may allow the small amounts of CO and aldehydes to accumulate and possibly affect the health of the occupants.
- The waste consists of water in a vapor state, carbon dioxide (CO2), carbon monoxide (CO), and traces of aldehydes, such as formaldehyde.
Check for sources of combustion air—a louver or vent into the utility room; open ceiling that can draw air from the floor joist system, which may extend throughout the basement; or a louvered or permanently opened window to the exterior. If there are not adequate interior sources, outside air will be necessary. Logic is your best guide. The source must be permanent.
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Heat Pump Inspection
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 refrigerant charge. Consult with a licensed or certified HVAC technician for evaluation and cost estimate.
If the temperature differential is too much, possible causes may be:
- Fan too slow
- Restricted air
- Ductwork design
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:
- The outside thermometer control will not allow the supplemental heat to come on if the outside temperature is not below the balance point.
- Incorrect wiring to the supplemental coils.
- Incorrect or failed coils.
- In the air conditioning mode, measure the temperature difference across a supply and return. This difference should be in the range of 14 to 20 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
Refrigerant gas is compressed. Anything compressed will develop heat. A hot, high-pressure gas comes out of the compressor and into the outside coil. 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.
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.
Burners, vents, and flues are the same for steam, hot water and furnaces. 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.
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.
- One pipe system
- Two pipe system
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.
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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.
Quantity of Heat Given Off By Radiators -- Hot Water And Steam
Width in inches x height in inches x 2.5 divided by 144 = SF per radiator section.
SF per section x number of sections = total SF of heat distribution surface.
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.
Hot water systems operate at lower temperatures than steam systems. 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.
§ 8'-wide radiator
§ 30" high
§ 20 sections
§ Hot water system
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).
83.2 SF x 150 BTUs = 12,480 BTU’s per hour from radiator
Note: The 2.5 factor may change, based on the radiator configuration, however, the range should be between 2.2 and 2.8.