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Heating Systems

What's to Know about Furnaces

Many homes are heated with a furnace. This energy system transfers heat created by burning gas, oil, coal, o r by heating a resistance element with electricity to air that is circulated thought out the house. The furnace itself is designed to be trouble free and requires very little care but the gas or oil burner that creates the heat does require maintenance to run safely and efficiently. Here are some maintenance tips to keep a furnace operating efficiently:

Annual Inspection. Service a furnace in the fall before the heating season begins. It is best to use a heating professional to perform this annual maintenance. This professional will probably follow these following inspection and maintenance steps:

The outside of the furnace will be inspected with careful attention given to the flue pipe leading from the furnace to the chimney. He will check for loose connections wherever two pipes join, at all elbows, and where the pipe joins the chimney. If there are loose sections, they may be reattached. Also, the furnace will be checked for large rust spots especially on the bottom of the pipes. Condensation may cause rusting and this is a sure sign of a maladjusted burner. If there is loose or missing cement surrounding the pipe, it will be replaced or repaired.

Air Filter. Most furnaces have an air filter located in the return air duct system, usually at the bottom of the furnace where the large duct enters the furnace. This filter requires regular replacement if it is not a permanent foam type filter.

Blowers. A furnace's blower that forces the air through the heating system. Some blowers have a V belt drive that should be serviced every year. Some newer furnaces have direct drive blowers and are not belt driven. Both systems require cleaning and lubrication

Humidifier. If the furnace is equipped with a humidifier, it will require at least yearly maintenance.

Safety Concerns with Heat Exchangers

How unsafe is a failed heat exchanger in your forced-air system? The main safety concern with warm air furnaces, sometimes called "hot air heat," is keeping the products of combustion from mixing with the air in the home and negatively affecting the health of the occupants.

When fuel is burned, three products are produced: (1) heat, (2) carbon dioxide (C02), and water (H20). This example assumes complete combustion. If there is incomplete combustion, other products will also be present. These may include the compounds such as carbon monoxide (C0), formaldehyde (HCH0)and numerous other aldahydes, nitrogen dioxide (N02), and sulfur dioxide (S02). The technicians who set up furnaces try to keep the C0 to less than 100 parts per million (ppm) in the exhaust.

Problems develop when there is a blocked or partially blocked chimney and/or a failed heat exchanger. A blocked chimney can fill the area where the heater is located and the first floor with toxic C0 gases in a few hours, depending on how much air flow there is in the house. In most situations, a blocked chimney is relatively easy to clear. 

A failed heat exchanger is much more difficult to determine, but, in almost all cases, is much less dangerous than a blocked chimney. In fact, when the furnace's fan is running, the heat exchanger is pressurized from the house air side. In almost all cases, this pressure will not allow dangerous gases to accumulate in the house air. The path of least resistance for these exhaust gases is up the chimney. This may not be the opinion of most gas utilities in the country, which is somewhat understandable based on the liability exposure.

The pressure on the heat exchanger has a significant effect on the tendency of flue gases to pass from one side of the heat exchanger to the other. If the fan is off, the pressure from the burner will cause the burner side to be positive and the C0 or C02 gas can pass to the house side. The amount of gas passing from one side to the other is based on the size and location of the failure in the heat exchanger. However, it is rare this amount would exceed the amount of C0 or C02 gases emitted from a kitchen as range flame.

When the fan comes on, the house air side of the heat exchanger, in almost every case, is positive. The positive pressure from the house air or fan side would cause the house air to be pushed into the exhaust side, not vice versa. The only exception may be some power burners which would maintain positive pressure on the burner side while the fan was on or a heat exchanger failure which was large enough to get your fist into. The main thing to remember is that high pressure will always move to a low pressure. There are a few other factors which must be added to be totally accurate. These would include the location of the failure and the design of the heat exchanger.

This is not to say that failed heat exchangers are safe, but you should know they are rarely as much of a concern as we hear from most information sources.

One last item: According to the American National Standards, it is almost impossible to construct a heat exchanger that is entirely air tight. Therefore, any test method developed to detect flue gas leakage needs to have quantitative aspects. It would not be desirable to identify as unacceptable any heat exchanger leakage that meets the requirements/standards for heat exchanger joints. This standard says the leak should not be more than 2% of the flue gases with the internal pressure raised to .1 water column (WC) static pressure.

Carbon Monoxide in the Home

This silent killer claims about 1,500 lives each year in the U.S. 

Carbon monoxide (CO) is one of the toxins that remains from incomplete combustion of fossil fuels. Fossil fuels include oil, gas, and coal. Small amounts of CO, such as those emitted from the kitchen range, will usually be found in the air in the home. These amounts pose no health concerns for the occupants.

However, health problems can develop if one is exposed to CO in large amounts, such as those emitted for many hours from a blocked chimney. In extreme cases, the presence of CO can be lethal. 


Much has been written about heating systems causing dangerous levels of CO gas in homes. The heating furnace itself will not cause CO amounts of any concern to be emitted into the home.

If the heat exchanger fails (the heat exchanger is the part of the furnace that keeps burned fuels separate from the air in the living space) CO is rarely emitted in the air. If CO is emitted, the amount released is not significant. Here's why:

The typical furnace has a fan that circulates the indoor house air to and from the heating system and living space. This fan creates approximately 18 times more pressure on the house side air than the typical pressure created by atmospheric burners. In the event of a failure, this pressure causes the air from the living space to pass to the exhaust side of the unit and up the chimney. 

This is not to say that a failed heat exchanger is acceptable. It is not. However, the likelihood of significant CO gas being delivered to the living space has been grossly overstated. A chimney that is blocked for many hours or days is the only item that would deliver dangerous amounts of CO gas to a dwelling.


Carbon monoxide in homes is difficult to research due to numerous variables, including:

  1. The size and air volume of a home. The more air in the home, the easier gases will dissipate.
  2. The number of air changes per hour in modern homes that have thick insulation, etc.
  3. The type of construction. Various types of frame and masonry construction will have an effect on the air changes and air infiltration.
  4. The type of heating system. Combustion air requirements and efficiencies have some effect on air movement and changes.
  5. Operating fans and exhaust systems. When on, these systems dissipate all the air in the house in minutes. The size of the systems and of the house will determine how effectively this is done.

According to an American National Standard's study on heat exchangers, leakage of waste gases is acceptable as long as the combustion chamber and vent do not leak more than 2% of flue gases. (Testing parameters are .1" water column static pressure on the interior of the heat exchanger.)


CO poisoning kills about 1,500 people a year. CO reduces the ability of the hemoglobin in the blood to carry oxygen to the brain and body. This is akin to not breathing. The blood recovers quickly if the exposure is not continuous. Typical symptoms include headaches, fatigue, insomnia, nausea, and heart palpitations.

The presence of CO in a home can cause physiological effects at any level. However, the following parts per million (ppm) indicate when it is a serious concern:

50 ppm  
Allowable for up to 8 hours of exposure.
500 ppm  
Can be inhaled for one hour without appreciable effect.
700 ppm  
Unpleasant, but not dangerous, effects after one hour of exposure.
2,000 ppm  
Dangerous effects after one hour of exposure.
4,000 ppm  
Fatal in less than one hour

The most desirable condition would be a zero level of CO. To achieve low and safe levels, use a CO monitor. It will detect when levels surpass 10 ppm.

Heating a New Addition

Putting an addition on your home, such as a bedroom or kitchen, is very exciting. It often affects your attitude and comfort level and can truly renew your spirit. One consideration when adding to your home or when creating a new living space from a previously unused area, such as a porch, basement or garage, is the extra heating that will be required. 

Your current heating system is probably sized for your current living situation, whether the unit is original or a replacement. There has probably not been any consideration of a future addition.

The first thing you need to do is assess your current heating situation. A general rule of thumb for heating requirements is that 40 to 50 BTU’s (British Thermal Units) are required for every square foot of living space. So, determine the square footage of your current living space (before the addition) and divide it by the 40 to 50 BTUs. This, of course, will depend on the type of construction and geographical location.

To figure out how much more you would need (assuming what you have is enough) simply add the square footage of the new living space to your current number. When you have this total, you can figure out the amount of BTU’s you’ll need for the addition.

You may be able to use the same input size heater if you buy a more efficient one. If you replace a typical heater that is 60 percent to 70 percent efficient with a heater that wastes just 5 percent to 10 percent of its heat/fuel, and if it includes an outside air supply for combustion, you could buy a heater sized at approximately 25 percent to 35 percent fewer BTUs for every square foot of living space. More effective energy improvements may allow you to reduce the size even more.

Heater efficiency is based on burner efficiency, transmission of losses to the heater exchanger or boiler and flue or chimney losses. The quantity of heat lost up the chimney is rarely discussed by utility companies or fuel suppliers. However, it is significant. Approximately one-third of all heat generated by a gas-fired unit goes up the chimney. Oil-fired appliances have 5 to 15 percent more waste. However, oil costs less than gas to purchase.

When the distribution of air is from an existing situation, additional ductwork may be needed. The farther you travel from the source, the smaller the ductwork should be to increase/maintain adequate air velocity. Additional fuel and ductwork for equitable distribution.

Before you begin a new addition, consider the changes that will take place. Don’t get discouraged; these calculations are not difficult. The half-hour you may put in will be well worth the yeard of enjoyment you will get from the new living space.

How Your Heat Pump Functions

By learning how the systems in your home operate, you can often troubleshoot problems when they develop. At the very least, you will be able to talk intelligently with a contractor who is brought in to correct a problem. Showing that you "know your stuff" can gain you respect and possibly better service.

A heat pump serves two functions: to heat and cool your home. A central air system simply cools the house. Air conditioning units operate the same way the heat pump's cooling side works. Here's how your heat pump operates:

With your heat pump unit in the air conditioning mode, the compressor compresses Freon gas. (It does this in both the heating and cooling mode.) When you compress gas, as with anything, you develop heat. You get a high pressure gas that is very hot -- typically between 190-240 degrees. The hot gas gets pushed into the outside coil. The outside coil's fan, which is on, is drawing the outside air across the coil. The 90 degree outside air is significantly cooler than the gas inside the coil. This is why you feel warm air coming off an air conditioning unit.

The cooling of the gas causes a change in state. The hot gas converts into a warm liquid, one of the unique properties of Freon. The warm liquid continues through the final portion of the coil and moves inside to the evaporator, which creates space for the liquid to expand. The liquid expands into a low-pressure gas, which is now cold. The cold gas goes through the "A coil," which is inside the plenum of the heating system ductwork. The cold gas makes the coil cold.

The cooling into the house occurs from drawing the warm house air across the coil. The velocity and volume of air across the coil dictates the temperature of the air on the other side of the coil. The technician will set the fan speed up so you get a 15-17 degree differential between the supply and return air.

The cold gas moves through the coil and is pushed back into the suction line to the outside unit, which draws the cold gas back to the compressor where the whole process starts over.

In the heating mode, the compressor does the same thing. However, instead of pushing the hot gas to the outside coil, it pushes it to the inside coil. Once into the inside coil, a similar process happens. Hot gas moves around the coil. The air from the house takes the heat off the coil, which is desirable, but while it does this it drops the temperature of the gas. When it does this enough, the high pressure gas changes into a liquid. This continues through the coil, then through the expansion valve which allows the liquid to change to a low-pressure cold gas. This cold gas is taken to the outside coil -- the opposite of the AC mode -- but simply by moving gas/liquid the other way. Once it goes through the outside coil, it ends up back at the compressor and then starts the process over again. In either mode, the compressor simply compresses the Freon gas.

There is one more important point when the system is in the heating mode and the cold gas comes to the outside coil. If the temperature is low enough, you may start to freeze the outside coil. When this happens, you will develop ice. When you start to form ice, the unit will automatically change to a defrost cycle. An outside thermostat controls this function. The defrost cycle will reverse or change the function back to the AC mode and defrost that outside coil.

Note: While defrosting the outside coil, you will have back-up heat engaged on the inside. The back-up heat could be electric or a furnace operation. This will be dictated by the electric and fossil-fuel costs. 

When the temperature drops too low and too frequently, this situation becomes economically impractical and the compressor will shut down and will heat with the back-up process.

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