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Ductwork Air Flow Tips

Static Pressure - Not Electricity

If you have an existing house that has cooling problems, there may be numerous things that are contributing to the problem. We have already discussed the sizing of the air conditioner and importance of proper return air. But there are other problems that can cause poor cooling. One of these is static pressure.

If you are healthy, did you know that your blood pressure is very nearly the same at your heart as it is in the last digit of your index finger? Why doesn't the pressure drop as the blood goes way out to your finger? That's easy. Your blood vessels get smaller and smaller the farther away from your heart. At each bifurcation (a "fork" - where a single vessel splits into two vessels), the two new vessels are smaller than the original. This is because the energy to push the blood has now been split. If you kept one vessel the same size as the original, there would be less energy to push the same amount of remaining blood!

This is the same thing that must be considered with the air in your ducts. The air which is flowing from a duct near the furnace should have very nearly the same force as one which is far away. Take a look at your ducting system if it is visible. You will notice a large trunk line which comes off the furnace. Sometimes it will split and go two directions. These should be smaller than the original. Then, notice how every so many feet, a pipe is branched off to go to a room(s). Does the trunk line get smaller? If it doesn't, especially after two branches, you could have a static pressure problem.

Gentle Turns

While you are looking at your ductwork, pay attention to how the branch lines come off the main trunk line. Are they abrupt 90 degree bends? Or, do they resemble an exit ramp from an expressway? You are looking for exit ramps or wye fittings. These fittings allow the air to change direction in a gentle fashion. Abrupt fittings absorb energy and cut down on the air flow.

Are there lots of turns or 90 degree turns in a branch? Remember, each bend absorbs energy. Turns or bends in ductwork should be minimal.

The A Coil - Dirt Catcher

We discussed earlier that you really have two coils as a part of a central AC system. One is outside you house. It dissipates the heat into the atmosphere. The other coil is inside your extended plenum at your furnace or air handler. This is the coil that gets cold. It cools down the recirculating air inside of your house. Well, guess what? Most people can't see this coil. It is hidden behind the plenum sheet metal. What happens if you still use those worthless fiberglass filters? That's right, this hidden coil can get covered with dirt. If it does, its ability to cool the air is significantly diminished. Buy, use, and regularly clean either high quality paper air filters or a rinsable polyester filter like I have in my furnace. Electronic air filters work very well too.

Helping Your Air Conditioner

The next time hot weather hits, I want you to touch the ceiling of your second floor rooms at about 5 o'clock in the afternoon. Pretty warm aren't they! In fact, these ceilings can radiate vast amounts of heat into your house. Why not reduce the amount of heat in your attic? This will really help your AC.

Install one or more thermostatically controlled attic fans. These devices turn on and off automatically. They exhaust hot air from your attic space. If you install some excellent soffit vents so that cooler (90 degree vs. 150 degree or more) air is being introduced into your attic, you will go a long way in helping your air conditioner.

Reflective Foil Sheathing

Have you seen the foil faced foam sheathing that we builders use on new homes or room additions? Did you ever think how that stuff could work to bounce the radiant energy back towards your roof? Trust me, it works well. You can buy this stuff and cut it into 2 foot by 8 foot sheets that possibly can be inserted through your attic access panel. Staple or nail these to the bottom of your attic roof rafters. If you have little gaps here and there, it is not important. The more aluminum that faces towards the roof, the better.

Continuous Fan Movement

Several readers have contacted me about success they have had by allowing their furnace motor to run continuously. They indicate that this seems to balance the room temperature. No doubt this is true. The air movement also helps to evaporate your own perspiration. Evaporation is a cooling process in and of itself. That is why you feel more comfortable when a breeze blows. Look at your thermostat. There is a little switch that says "On, Off, Auto". Switch it to "On" for continuous fan blowing.

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2 Responses to Ductwork Air Flow Tips

  1. Hello, I have two rooms that there is very little air flow at the end of the run. Here is what I have found. The unit seem to be working well. I have 16 runs total. This is what seems to be wrong to me. The trunk line is about 50 feet long and is insulated steel duct that measures 10x12 rectangle and has 8 runs on it. There is a secondary trunk line that comes off the primary trunk line. This line rises five feet with two 90 degree turns and extends for 26 feet. This line measures 13x14 steel with 8 runs with 2 of these runs on the end. Is this not backward from what it should be? All help would be appreciated!

  2. Does the installation of 'Reflective Foil Sheathing' to the attic rafters with the reflective side up impact roof (covering) life-expectancy ?
    Thanks

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