Summary: HVAC ductwork sizing is critical for proper heating and cooling in every room. Return air ducting is necessary in almost every room. Have to get the air back to the furnace. These ductwork tips will help balance your air conditioning and heating system.
HVAC Return Air Ducting
Air Conditioning Sizing - Tips on Ductwork
There is nothing more uncomfortable than lying in bed on a hot summer night sweating. Tossing and turning does no good. What’s worse, you know that the first floor or basement of your house is much more comfortable. Does it have to be this way? Absolutely, positively NOT!
My house and lots of others that I have built and remodeled are very comfortable, even in the hottest weather. The reason is simple. My HVAC contractor, Richard Anderson, knows the importance of return air ducting AND sizing correctly both supply and return air ducts. This is the key to comfort.
Existing Hot Houses
I’ll bet that 3 out of 5 people who read this bulletin have an existing home that has no or inadequate return air ducts. Don’t worry, we are going to solve those problems. It isn’t always going to be easy or pretty, but it can be done.
Return air ducts are necessary in just about every room. However, there are two rooms that I never place return air ducts - kitchens and baths. Let’s try to keep the garlic odors and hair spray in these rooms if you don’t mind! The reason for return air is simple. Your furnace is simply a recirculating pump. Instead of pumping water, it is pumping air. We need to get the air back to the pump. Furthermore, without a return air duct in each major room, the supply duct air has to “push” all of the air in the room out of the way as it makes its way towards you. The return air ducts help pull the air as it is being pushed.
Where Can You Put Them?
If you have an existing ranch style home, installing return air ducts in each room can be done fairly easily. You do what we do in new homes. You use the wall cavities as the duct. Think of it. Drywall that is nailed to 2×4s is really a duct - a box with 4 sides. All you have to do is cut out the floor immediately below the wall cavity as well as the bottom wall plate. Once you have created the opening, sheet metal will allow you to connect this to your existing return air system.
Two story houses can be more of a challenge. You have to figure out a way to get return air from the ceilings of the second floor to the basement or furnace room.
Do you have a laundry chute that you don’t use that often? Can it be sacrificed for the cooling cause? We have used these with tremendous success.
Sometimes you can use a kitchen pantry closet or a hall closet. We have often been able to put a duct in one corner of the pantry or closet. This duct extends up to the second floor where it often lines up with an interior hallway wall.
Often you can collect the ceiling air through a network of flexible pipes in an attic. These pipes join together in one central location that permits you to run a duct down to the basement or a second floor closet.
As a last resort, you may have to put a duct in the corner of a room(s). These can be drywalled easily. To effectively disguise it, you may build another one several feet down the wall. Shelves can be put between these two “stacks”. If they are in a child’s room, they can often be painted in a decorative manner as if it was intended for them to be there.
It was common in many older homes to have projections in rooms such as this. These bump outs often were fireplace chimneys as they rose through to the roof.
Balancing Return Air Flow
If you want to really have a way to get the most bang for your return air buck, think of installing grill covers on the wall that have operating louvers. This will allow you to choke down rooms possibly on the first floor that are drawing too much air. This in turn creates a greater suction in the rooms that really need the cool air. This principle is no different than dampers which are used on supply lines to regulate air flow.
The grills with louvers are inexpensive and effective. On an older existing home that is being retrofitted for central air-conditioning, they are a must.
Reversed Air Flow On Old Homes
Do you have a house that is approximately 80 years old? If so, your supply ducts may be on inside walls. You may have a single giant return air grill in the floor or at the bottom of a staircase. This is all wrong.
Supply ducts should be located on outside walls, preferably under windows and near doors. The intent is to wash the exterior walls with heated or cooled air. You are trying to combat the heat or cold at its source.
The single giant return air grill in the floor balances the load at the furnace motor, but does nothing to promote cross ventilation in each room.
What, you don’t think you can match the hardwood flooring? Rubbish! Go to a first floor closet and get the flooring from there. A good hardwood installer can make the repair and no one will be the wiser. Who cares if the flooring inside the closet doesn’t match.
I wish you luck in your effort to stay cool. Don’t hesitate to contact me if I can help.
Air Conditioning Sizing Considerations
Residential air conditioners, both central and window units, are like shoes. They come in various, different sizes. Buy the wrong shoe and you will be uncomfortable. It is no different with air conditioning.
Bigger Is NOT Better
So you really want your house cold. What are you going to do? Buy a monster 5 ton central unit? That could be a mistake. If you put in a unit that is too powerful (produces too much cooling), it will short cycle. It will turn on, blast out vast quantities of cool air, and then shut off. The result will be improper dehumidification. You will be cool and clammy. The air conditioner needs to run for a sufficient time to squeeze out the humidity that has seeped into your house. You achieve the highest comfort when the air conditioner removes as much humidity as possible from your interior air.
How Are They Sized?
To properly size an air conditioner, one has to perform a heat gain calculation. In other words, you must determine how fast heat is getting into your house and what things inside the house are contributing to heat gain. To perform the calculation, you need to know how much insulation is in your exterior walls, how much is in your attic, the size and type of glazing for each window, the type of construction of each window, extra special lights or cooking appliances that generate heat, compass direction that each wall of your house faces, etc. In other words, you have to do some work to get the equipment sized right. Tables and charts have been around for years that convert these measurements into BTUS. Once you know how many BTUs your house is gaining, you can purchase the correct sized air conditioner.
When purchasing a unit, ask for these calculations! Don’t just buy the same size unit you now have. Maybe it was not sized right. Maybe you have upgraded your windows or insulation. Maybe you added a small or mid sized room addition or remodeled an attic space.
Replacing An Existing Unit - BEWARE!!
If you have an existing house which needs a new central AC unit, be careful! For you to achieve the published efficiency and performance that is sold to you by the salesperson, the interior coil housed in the furnace must be compatible and matched for size! If it is not, the outdoor unit will struggle to keep your house cool. It is a question you must ask the installer. Make him/her prove to you that the interior coil will handle the outside unit. Remember, the inside coil could have been wrong from the beginning! The original furnace contractor could have downsized one size to save money. The AC will work, but not as well as it should have!
Ductwork Tips - Air Flow Problems - 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 2 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 either “On Off Auto”. Switch it to On for continuous fan blowing.
