Forced Air Systems

Forced air systems are very popular and require the least amount of BTU's to obtain desired temperatures. Unfortunately, they have the greatest amount of distribution loss out of all the other systems. The way a modern forced air systems work is by taking air from the home into a return duct system to the furnace where the air is cleaned first through a filter. Then the air passes over the heat exchanger and then the conditioned air is distributed throughout the home via a supply duct system.

This is considered more or less as a loop. Another way of saying this is one cubic foot of air goes into the system and one cubic foot of conditioned air comes out. This is the reason why the filter should be checked and/or changed monthly. Once the filter gets dirty, it prohibits the amount of air that goes into the system; hence it prohibits the amount of conditioned air that comes out. Your fuel bills can literally be double as a result of a dirty filter.

Other concerns with forced air systems are ducts that are not insulated and leakage in the ducts. Since air does not hold heat well, the point where you actually heat the air to the place where you actually want the heat plays a major factor on your fuel bills. In other words, the difference in temperature of the air at the furnace to the air at the register in your home represents distribution loss.

Let's assume the air temperature at the furnace is 180 degrees F. and the air temperature at the registers is 135 degrees F. There is a 25% loss in distribution. There are a variety of solutions. The question here is which is more cost effective? You could redesign the ductwork thereby optimizing the flow of air through the ductwork. This would cost approximately $3,000. And you would probably reduce the distribution loss by 10 to 15%. You could also insulate the ductwork for approximately $400. And reduce the loss by another 5%. On the other hand, let's say you only insulate the ductwork and not redesign it. Your saving would probably be around 10 to 15%. The reason for this is conductive distribution loss and the law of diminishing return. If you did the redesigning after the insulating, you would probably only save another 5%.

When you consider conductive distribution loss the two main factors are material used in ductwork and surface area. Sheet metal is the most common material used in ductwork and it conduct heat and cold very easily. If you were to take down the ductwork in your basement and unfold it, it would cover your entire basement floor more than once. If only one is done, they both would save approximately the same amount. The difference is the cost of doing either one. It is good to note you only insulate ductwork that is exposed in basements, crawlspaces, garages, etc. and not in the walls. Also, return ducts should be insulated in those areas.

What compounds the conductive distribution loss is duct leakage. This is why the distribution loss in most forced air systems is far greater than 25% to as much as 50%. The solution to this is to duct tape every joint in the ducts or do what is preferred, use mastic. It's cheaper and easier to do than duct tape and does a better job. There are measuring devices such as the duct blaster that can determine how leaky your ducts are. The only time I would recommend the use of a measuring device is when you are having a professional do the duct sealing for you. They will measure the leakage before, during and after to verify the effectiveness of their work. In most home you do not need a measuring device to duct seal, just common sense. It is mandatory that you duct seal before you insulate your ducts. Another consideration is balancing your system, which is probably one of the best things you can do.

However, controlling different areas of the home at different times, depending on use, is accomplished through zoning. The following link gives you an idea on how this is done with a forced air system. Don't Heat/Cool Empty Rooms! and In-Wall Motorized Registers.

Humidifiers on forced air systems primary purpose are to improve the efficiency of the heating unit itself. It will not and should not improve the quality of the air inside the home. Since fanning dehumidifies air, the humidifier replaces some of that moisture. The reason being is that humid air holds heat better than dry air.

The problem here is that if you attempt to improve the quality of the air in the home with a furnace humidifier, you will exceed the recommended levels the manufacturer of the furnace. This will definitely void your warranty and damage the heating unit.