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January 13th, 2023
We use the term HVAC quite a bit, and frequently we need to explain the acronym: Heating, Ventilation, and Air Conditioning.
The ventilation part of this contains several pieces: vents, registers, dampers, air filters, and blowers. Air conditioners, heat pumps, and furnaces supply the conditioned air. A blower located in the furnace pushes the air through the ducts, which go throughout the home. There is a supply side and a return side; they are equally important.
When we consider gas furnaces, we need to consider the combustion chamber, the flame sensor, the gas valve, burners, the heat exchanger, and the draft inducer motor. Once combustion takes place - which produces the heat - the blower sends the heat through the ducts. Vital to this process is the air ventilation system that sends the byproduct out, away from the house.
In a perfect system, every dollar you put into your energy bill would translate into heat in the winter, and cold in the summer. Alas, it is not so.
There’s simply no way your gas furnace can be 100% efficient unless your furnace is electric. The best furnace from Carrier, a high-efficiency 59MN7, is rated with an Annual Fuel Utilization Efficiency (AFUE) of 98.5 A secondary heat exchanger ensures that 98.5% of your dollar goes toward heat.
But there’s a caveat: It doesn’t account for how much heat could be lost in your vents.
According to the U.S. Department of Energy, a rough estimate is that the average duct efficiency is about two-thirds. So one-third of the energy supplied to the ducts is lost before it gets to the living space. And you wonder why your gas and electric bills are so high.
Let’s talk about the two sides of ventilation: When we enter a home to give an estimate, we look at both sides of the ventilation system. One side is comfort; the other side is safety.
Proper Ventilation Is Essential for Your Home’s Safety
There are two forms of ventilation. A gas furnace is a combustion appliance. The air that you breathe is on one side of the heat exchanger, and on the other side are the flue gases. They can’t mix. That’s why if there’s a crack in the heat exchanger, there’s a danger because the byproducts of combustion can seep into the ducts and spread into your home.
Byproducts from a gas furnace include carbon monoxide, polycyclic aromatic hydrocarbons, aldehydes, hydrocarbons, sulfur oxides, nitrogen oxides, and particulates (e.g. soot). There’s also water vapor.
How Dangerous Is Carbon Monoxide?
The byproduct of carbon monoxide is the one that HVAC technicians and homeowners need to be especially aware of. It is odorless and colorless. If your gas furnace isn’t vented properly, that gas could drift through your home, and it can build up.
Carbon monoxide is also released if you burn fuel in automobiles, gas lanterns, grills, fireplaces, and ranges. And it doesn’t matter if you’re burning oil, propane, or natural gas. We’ve been in homes where a propane heater was supplying additional warmth, except the heater was in the middle of a room. That can make people sick, or kill them if they breathe enough of it. A carbon monoxide detector is a wise investment.
How Are Combustion Byproducts Removed Via Gravity Draft?
In earlier times, the way to get rid of the byproducts of a gas furnace was through gravity draft, aka natural draft. The chimney would create a draft because it’s warmer, and warm air rises. It’s a natural draft, and it pulled those byproducts of combustion up and out of the chimney.
A draft hood was placed above the uppermost part of the gas furnace to draw air into the chimney, which makes it possible to draw as much air through the chimney as necessary to create a constant flow.
You’re not supposed to vent that out through a masonry chimney without a liner, but for years they did. You need a chimney liner. Some HVAC companies don’t tell homeowners that.
A chimney liner has several benefits. It prevents overheating of the chimney, which lengthens its lifespan. A liner promotes better energy efficiency because it helps the fire to burn hotter while minimizing loss of heat. It helps in maintaining optimal airflow within the chimney.
Most important, a chimney liner provides a safe outlet for combustion gases.
Without a liner, there’s less heat in that chimney. The flue gas contains moisture. If it starts condensing in that chimney, we get moisture in there. I go to people’s houses where the wall near the fireplace is all soggy from the condensation. That is going to deteriorate the chimney (not to mention the wet wall), and it’s not going to draft properly. That creates an unsafe condition because the byproducts won’t easily leave the home.
The vent cap (or V Cap) that sits on the roof usually vents the gas furnace and the water heater. There is a bird-proof covering, and the heat coming out of it would melt any snow that could be blocking it.
How Are Combustion Byproducts Removed Via an Outside Wall?
With a 90% high-efficiency furnace, there is a secondary heat exchanger. It takes the byproducts from the first heat exchanger and creates another round of combustion. Those are vented outside via a side wall with PVC pipe. Condensation is sent down a drain. The best setup is a two-pipe system. One vents the air outside, and the other pulls in the fresh air.
That system is all about getting the byproducts of combustion outside as efficiently and safely as possible.
How Does Ductwork Aid Ventilation?
The other side of ventilation concerns the conditioned air in your home: the supply and return air ducts. On the supply side, the furnace is typically in the basement. So you have a supply trunk, which is a large rectangular duct, and there are runouts going off of that to individual registers.
The more turns, the harder it is to get conditioned air to where it needs to go. Every turn adds about 5-15 linear feet of pipe. If you’re pushing air, the rooms closest to the furnace will get the bulk of it. The rooms farther away aren’t going to get as much. It should be balanced as much as possible.
Read more: Is it OK to Close Air Vents in Your House to Redirect Airflow?
What Is a Proper Air Duct System?
The big question is whether the ductwork was designed to provide enough air to each room. The other side is, do we have enough airflow to support the size of equipment we’re installing here? Is the air getting to where it needs to be? Is the customer comfortable?
A lot of times there will be balancing dampers in the runoffs. If one room is getting too much heat, you can close off the damper to help balance things.
Ideally, you have a supply and a return air vent in each room. That allows for proper air exchange. (Air exchange is when conditioned air comes in from the supply vent, mixes with the air in a room, and then goes into the return vent.) That’s how most modern homes are built.
A lot of homes will have two returns; one on the lowest and one on the highest part of the room. The idea is you’re pulling that hot air off of the ceiling, and in winter you’re pulling that cold air off of the floor. Many times, people will have a big refrigerator magnet that they’ll place over one register or the other depending on the season.
Read more: The Art and Science of Designing and Installing Home Ductwork
What If the Air Duct Design Is Inadequate?
Uneven temperatures could be caused by an inadequate air conditioner or furnace, or it could very well be from poor ductwork.
Manual balancing dampers are a good way to try to even things out. They are a much better solution than closing registers, which simply stop the airflow. It doesn’t help the rest of the home.
Running the fan without the furnace making heat is a common way to compensate for uneven temperatures. The fan mixes air as it runs, helping to balance the different rooms that have differing temperatures.
If there aren’t enough return vents, the ducts might be pumping air into the rooms, but all of that air is trying to get back to too few vents.
If the supply and return registers are too close to one another, the conditioned air will head straight to the return without mixing with the room’s air.
What Are Other Issues Affecting Ducts?
Sometimes I see holes cut into the wall, but they are not connected to the vents. Or the builder used the return air registers to sweep sawdust and drywall dust.
That’s why we suggest cleaning your ducts, especially on new builds. Especially if they’ve never cleaned, there might be an obstruction that’s robbing you of efficiency and affecting indoor air quality.
I was at one home that had a furnace, but there was no B vent coming out of the roof. (Type B vents are vents suitable for gas-fired appliances, including most domestic heating and hot water systems.) It turns out the vent ended in the attic, which was now filled with by-products and moisture. It was like a rainforest.
Sometimes the PVC pipes can come apart and separate from the furnace. We’ve been in places where the HVAC unit won’t run and we can’t figure it out till we go outside and see toys stuffed in the intake pipe blocking the outdoor air from entering.
Proper Home Air Duct Design and Ventilation in Columbus, Ohio
We don’t sell water heaters, but I still look at their venting when I’m on a sales call. Because if it’s not vented properly, that’s a health and safety issue. Any salesperson who knows what he’s doing is going to pay attention to the combustion side.
Part of an HVAC contractor’s job is to create a safe, comfortable space. If you’re a homeowner in Columbus or Central Ohio and have concerns about your vents, we’d love to talk with you to help find a solution.
Just fill out the zip code graphic below to see if you’re in our service area.
