Low Air Flow from a Furnace

Low air flow to certain rooms or areas of a building is one of the complaints we hear all the time! Multi level buildings with only one central furnace and air conditioner usually have trouble getting each floor the same temperature for several reasons.

The thermostat is usually on the middle floor. Once the middle floor temperature has reached the set point of the thermostat the furnace or air conditioning system will shut off whether or not the top floor or basement is at that temperature or not. If the home was built with (2) furnaces and air conditioners, one in the attic and one in the basement, most of this problem would go away. To install a second system in the attic will cost thousands of dollars, if it is even possible.

Most of our clients will close registers in the basement or first floor in an effort to force more air to the top floor. This would seem to make sense at first glance. The problem with a homeowner doing this is as follows.

Lets talk about air conditioning first, but the same things apply to heating but to a lesser extent.

For each ton of air conditioning in a home we must move about 400 cubic feet of air per minute. If we don't, we are going to cut the capacity of the cooling system as well as freeze the evaporator coil. A frozen coil will eventually lead to a damaged compressor which will cost hundreds of dollars to replace.

Each register in a home was installed to move a certain amount of air by the designer of the heating and cooling system when it was originally installed using a heating and cooling load analysis. Unfortunately, seldom do any of these systems ever get balanced before the building gets occupied. The photo below shows a Service Technician balancing a homes register. Balancing tries to strike a happy compromise of air flow to different areas of the home without "starving" the system of the 400 cubic feet of air per minute. To illustrate what we are talking about try this-close your bathroom door, turn the heat up to 80 degrees, run the system 15 minutes and see if the bathroom is not much hotter than the other rooms?? If you are like most people once you try this simple test you will close the adjustable lever on the bathroom register. This is where problems begin. If you were to do that on several registers you will be disrupting the 400 cubic feet of air per minute that is needed. One cure for this is to change your original furnace or air handler blower motor to a high speed motor, once this high speed motor is installed you will be getting a MUCH higher airflow from ALL the registers. You can now cut down on some of the registers that put out too much air without causing trouble to the 400 C.F.M we need! click here to see these motors Hi Speed Furnace and Air Handler Motors

Click for larger image

BALANCING A TYPICAL HOME REGISTER. This meter gives a cubic feet of air per minute reading. The service person should go through a home and record what each register is putting out "before" then balance the system and record the "after" readings. This will result in a more even temperature on different levels while keeping the correct cubic feet of air per minute. It is a relatively cheap thing to get done compared to high energy bills, damaged equipment and uncomfortable living. Balancing can't "cure" ductwork problems but will help this situation greatly.

Lets talk about a new 3 story house being built. We are called out to give an estimate to a builder who is just building a "Spec" house. A spec house is a home being built for sale with no home owner involved yet. Lets also say the home has a total living space of 3,000 square feet, has a couple of sky lights and large windows which face south with NO over hangs for shade. This house needs (2) separate heating and air conditioning systems! The builder not being able to communicate with a prospective home owner chooses just (1) large system because it is cheaper to install than (2) smaller systems. The furnace is placed in the basement and all the duct work is installed before the home is sheet rocked. We now have ducts to the top floor with an equivalent length of about 300 feet! Each elbow in a duct system adds the equivalent of at least 10 feet. To get to the top floor there may be 8 elbows! Add to this, the length of the duct itself and we think you can see the problem. This is for EACH duct going to the top floor. We liken this to a garden hose 300 feet long with (8) kinks in it. How much water pressure will you have? What the home owner ends up with here is this. Your thermostat will be on the first floor and this area will always be comfortable. The basement will be cold in the winter and the top floor will be hot in the summer. Heat rises and cold settles and will always be that way! One cure for this is to change your original furnace or air handler blower motor to a High Speed Motor click here Hi Speed Furnace and Air Handler Motors

Read more about how to change your motor here Furnace motor installation photos

Clients want us to over come this some how when they move into the home. We can do certain things such as add an Electronic Zoning system if we can get to the duct work (a trick or impossibility when sheet rock is up ). A zoning system will give you a thermostat on each floor and the problem will go away. Read on to see how they work.

Zoning systems will require separating the ductwork so that we end up with a supply trunk and a return trunk for each zoned area. Obviously if you want (3) zoned areas you will use most of your basement ceiling area up for duct work. If you have a short ceiling height in the basement this is probably not feasible. Even if it is feasible you may not want to finish this much sheet rock off, exactly what builders don't want to do either!! Zoning isn't cheap to install and depending on the duct work involved can run between $1,500 - $4,000.00! If it can be done while a home is being built it has many advantages however.

Instead of having (1) large system running and heating or cooling all areas (that may not need it). You end up with (2) or more smaller zone systems running only when required. Not only do you end up with more comfort but lower utility bills to boot. Makes sense doesn't it?

If you are building a new home especially one with multiple levels, do yourself a BIG favor and ask a qualified H.V.A.C. company about multiple systems or zoning. Just don't expect the builder to pay for the difference. Remember builders are business people and have to have competitive prices which they can't do with multiple systems or zoning. You, on the other hand will be living there, possibly for a LONG time.

Other than zoning there are only a couple of other things that can be done but will never replace multiple systems or zoning. One cure for this is to change your original furnace or air handler blower motor to a High Speed Motor click here Hi Speed Furnace and Air Handler Motors


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