Chimney crumbling - Liner possible ?

We have a 60+ year old house with a chimney that is dedicated to the exhaust from our gas hot air furnace and gas hot water heater. Every year we have had to "patch" the exterior bricks and mortar in the upper section of this chimney. It is apparent the gases are now causing major corrosion of the lining of this chimney.
What is the cheapest remedy to this problem ? Is it possible to use a stainless steel or even a fiberglass liner inside of the existing flue ?
Any ideas greatly appreciated !

 

It's possible to install certain types of dedicated metal exhaust pipes for gas appliances within an existing chimney.

Probably the best source of information is going to be installation manuals from hot air furnace and gas water heater manufacturers.

The type of exhaust piping depends on the appliance and what temperatures are involved.

For example, tankless water heaters generally require rigid stainless steel pipe but a gas fireplace insert uses flexible ducting such as "Simpson Dura-Vent".

In any event, the manner in which this is done should be carefully considered. The damage you are seeing may be due to the existing flue liner having deteriorated and the culprit being excess heat rather than a chemical reaction. If this is the case then you may have a fire hazard.

Personally, I'd call in a heating and/or a plumbing contractor to look at the problem.

 

no question, for the past 3 years, after re-pointing and fixing brick that was disintegrating, it is definitely a CHEMICAL reaction, not heat. The moisture at the top of the chimney has mixed with the flue gases to create an acidic condition that has destroyed the mortar between the terracotta joints....causing the acidic gas to seep into the mortar of the surrounding brick...and even to destroy the brick itself !
I think the cheap solution is a 6" round stainless steel liner....but I'm not sure it will fit within the existing flue....an oval liner would be better, but it's TWICE the cost as round !

 

Moisture and the Acid condition is bad news. Masonry does not stand up well to this. If you have a high lime mortar, it's even worse.

You definitely need to have a stainless liner installed (or 2 separate exhaust ducts) depending on what the conditions are. Normally you don't want 1 exhaust duct for 2 separate gas burning appliances. I'm not sure if that would even be legal.

If you go the route of just lining the flue, make sure that the liner is properly terminated above the brick with a chimney cap that will prevent re-entry of exhaust gas. In addition, the exhaust ducts from each appliance should be properly sealed where they enter the flue so that no exhaust gasses can escape into the home.

 

If a damper is used for each of these, I don't see the 'big deal' with regards to both going out the same duct....the hot water heater in particular is just so infrequent in terms of exhaust.
When you say 'lining the flue', are you referring to some sort of liquid coating or what ? I've been wondering if, after a good cleaning, it would be possible to coat the innards with some hi-tech polymeric material.

 

Check with your local Building Department. In most locals a chimney is not considered a gas appliance exhaust.

No, I'm talking about installing a Stainless Steel (or other approved metal) duct inside the clay flue pipe. As I stated above, a brick/clay chimney is not a valid gas appliance exhaust pipe. Gas appliances emit CO2 which as you know when mixed with water (either rain or condensation) results in acid. You can not apply a liquid sealer of any kind to the inside of a clay chimney to correct the problem.

The metal liner must be an approved design for the gas appliances and must be securely fastened and sealed at the connection to the appliance exhaust pipe. Only at approved locations such as wher the exhaust pipe connects to the water heater ( allows draft so that exhaust can flow ) can there be any gap into the house.

Hire a pro - you need to get it done properly for you own safety.