Venting Gas Appliances in Masonry Chimneys

Gas Log Set
Gas logs are formulated to look like a natural fire.

Gas appliances, depending on the type of appliance, can be directly vented, vented through a masonry chimney, or even vented through certain stainless steel. Some gas log sets are specially made to not need to be vented at all. Natural gas is a relatively cheap way to heat the home, and as time goes on new advances in technology allow less and less gas to be used to produce the same amount of heat. Using gas a fuel for heating your home can be great, but there are some things that you should know regarding situations where a gas appliance is being vented through a masonry chimney.

For starters, even though gas burns clean (or should when conditions are up to code) you still must have your chimney inspected annually. The products of combustion that vent up the chimney when gas is burned are highly acidic and can eat away at your masonry. This could create an unsafe situation in which the extremely hot gases could cause a chimney fire.

White Stain on Chimney
This white stain may be efflorescence, which is another indicator that your appliance and venting system aren’t compatible.

There are so many regulations regarding the way a particular appliance must be vented that there is no way to delve into all of them. The most important thing to remember is to follow manufacturer’s regulations to the letter. A chimney professional can help you evaluate your current system to ensure that things are up to code. When installing a new appliance into an old chimney, it is especially important to have a professional out to check not only that the two are compatible, but also that the chimney is functioning properly. A chimney must be in good working order in order to vent a gas appliance. There can be no cracks or missing mortar joints, and if these problems do exist they must be repaired prior to venting the appliance.

Water vapor is a product of combustion of any fuel. If the flue system and the appliance are compatible, a draft that is strong enough to move these byproducts out of the chimney before the water vapor has time to condense on the walls of the chimney. If the relationship between the appliance and the flue system is not appropriate, you will experience problems with different acidic residues building up in your flue system and that will eat away at the inside of the flue. If you are experiencing peeling wallpaper or bubbling paint, damp areas on the inside or outside walls of the home, or stains on the ceiling, you may have a ventilation system that is not appropriate for use with your appliance. If this is the case you should contact a professional for diagnosis.

Clay Lamb
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4 thoughts on “Venting Gas Appliances in Masonry Chimneys

  1. I have my heating (oil fire burner) and my water heater (gas) both venting up the same flue, is this a problem?

  2. Yes it is! Two dissimilar flues (Oil & Gas) can not be manifolded together. The soot from the oil can accumulate and fall back and block the gas fumes…possible CO ;-( Start off by buying CO detector(s) for downstairs and outside the bedrooms. It will buy you some thinking time.

  3. I have a direct vent gas fireplace made by Heatilator Mdl # GNDC36, SN GA255662 originally installed in 1997 which recently during heavy wind driven rain took so much water through the vent which vents horizontally out the side of a masonry chimney. The water came back through the vent and laid on top of the fire box as well as dripped down under the box and came out under the solid
    oak strop flooring in the family room causing it to cup.
    What is the cure for stopping this from happening again. Has the design of the exterior vent box changed recently and should that be replaced?

  4. To me this sounds like this part of your “sudden occurrence”Insurance claim.I would contact a Heatilator repair company in your area.
    The design of the cap on the outside cannot be modified, or it could void the UL listing on you entire fireplace system.
    I have concerns if this entire unit should be replaced under your insurance claim.I would contact Heatilator for a written opinion on repair or replacement of your fireplace.

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