Is It Safe to Seal off Atmospheric Combustion Appliances in My Historic Sacramento Home’s Closet?

One of the best parts of getting to do our work in Sacramento is seeing some of the historic homes in the area. I don’t even know if I could pick a favorite historic neighborhood. Richmond Grove (south of R and north of W, between 12th and 19th) is up there, but I also love all the beautiful old homes near the redevelopment that’s going on in Oak Park. Then there’s all the 1920s houses in the Yale Avenue Historic District, between Broadway and X. It’s just too hard to pick a favorite.

I appreciate the beauty of these homes—and the complexity they add to my job. Problems come up sometimes when it comes to their compatibility with modern HVAC systems, which weren’t even invented when these houses were first built. You’ve got to work with (and around) the existing architecture—which is a starting point for a question that’s come up several times this fall: how do I know if it’s okay to seal off a closet that has two gas appliances inside?

Homeowners often assume (rightly) that sealing off a closet with a gas furnace and water heater in it would make their house quieter and less prone to the potential danger of a hazardous air leak. But should they do it? Seeing as the vast majority of these appliances are atmospheric combustion appliances, the answer, it unsurprisingly turns out, is more complex than a simple yes or no.

Do You Have Atmospheric Combustion Appliances?

First things first, homeowners need to figure out what sort of gas appliances they have inside the closet they want to seal. In most instances, the appliances are going to be a gas furnace and a gas water heater, because these are the most common, especially in historic homes. The more important question, though, is are these atmospheric combustion appliances?

Atmospheric combustion appliances draw air from nearby and heat it. They pull in air to mix with natural gas, then they burn the air and gas mixture, and exhaust the combustion via a flue. Pulling  the cover off of this sort of appliance will reveal a blue flame. The flame in an atmospheric combustion appliance is open to the air, because combustion is a chemical reaction that needs fuel to mix with oxygen in order to take place. These appliances need consistent access to air, otherwise they will quickly use all the nearby oxygen, and begin to burn more fuel while not functioning as well—which is far from ideal for homeowners who want to keep their houses heated and their costs low.

Know Your Space

While you may be tempted to seal these appliances off to make your home quieter and cut down the chances of hazardous air leaking from them when they wear down, don’t rush off and grab some fire-rated caulk or threshold seals just yet. Look at the first word in atmospheric combustion appliances: atmospheric. These appliances need air from the atmosphere in order to function properly, and if you seal them off without making sure there is still an air supply, you’re in for big trouble.

You can still seal atmospheric combustion appliances off, you just need to make sure air is able to flow freely away from and to them. For this, you need one of two things:

1. A very large closet. And chances are if you have that, you’re probably not going to be sealing it off, because why waste all that storage space? But if you do want to, make sure this large space has enough airflow that for every cubic foot of air that leaves a space, an equal amount of air can come back in. More likely, however, you’ll have:
2. A combustion air supply duct. Pumping the air out of the house through a duct to the chimney is a better way to make sure your atmospheric combustion appliances are working right. This duct is usually a combination of high and low pipes—if you see those in the closet, it is then possible to determine if they’re sufficient for airflow. However, if you’re running atmospheric combustion appliances in a space that is not properly conditioned, you’re also increasing the infiltration of expelled air into your home. That’s not good.

Under Pressure

Pressure is key in making the final determination on whether it’s safe and smart to seal atmospheric combustion appliances off in a closet of your historic home. With the high and low vent or pipe combination as your combustion air supply duct, the idea is that one vent or pipe will supply air to one appliance and one to the other. Just because there is a high and low setup, however, doesn’t mean it’s okay to go ahead and seal. You have to consider air pressure.

I’ve seen a lot of high and low setups where air is actually being carried out of the closet because of a discrepancy in pressure. Hot air from the combustion process will rise in the closet and get carried out of the higher pipe, which then causes negative air pressure in the closet. Negative air pressure then makes it tough for air to reach one or both of the atmospheric combustion appliances, which makes their efficiency dip (and costs you money). Remember, your historic home wasn’t built with this new HVAC technology in mind, so you have to be extra diligent in getting these nuanced technical issues figured out.

The only sure-fire way (that’s a furnace pun, by the way—feel free to laugh) to make sure the pressure difference is at a level that makes the high and low setup effective is to have the gas appliances in question tested by a professional. I’ve talked before about making sure historic homes are energy efficient, but this time we’re talking about making sure they’re quiet and, more importantly, safe. Take good care of your historic Sacramento house, and it can take care of you for decades to come.

Contact the trained professionals at Bell Brothers for all your HVAC needs. Whether your house is modern or historic, we’ve got you covered.