How to Fix a Cracked Furnace Heat Exchanger

A man works on a furnace.
What You'll Need
Sheet metal
Iron repair solution
Metal cutters
Carbon monoxide alarm
Safety glasses

If you find that your furnace heat exchanger has trouble getting enough heat and power, then you may find that it has developed a crack or hole. Through this crack, combustion fumes can escape from the exchanger, and can build up in the room. This is dangerous, as one of the gases produced by the heat exchanger is carbon monoxide, a deadly poison which is virtually unnoticeable. In order to solve this problem, most people choose to replace the furnace heat exchanger, rather than run the risk of it developing more corrosion. Doing a repair such as this is a risk, as you could be exposing yourself to the dangers of carbon monoxide poisoning. However, if you need to perform a quick fix, it can be done.

Step 1 - Check the Furnace Heat Exchanger

Before you can do any repairs, you should make sure that you check over your furnace heat exchanger thoroughly. Remove it from the furnace, and examine it in the light. Look for signs that the metal has been overheated, or symptoms of long-term metal fatigue. Examine the crack itself. Look for evidence of rust, wiping down the surface with a cloth if necessary. Take steps to address the cause of the problem, as well as performing the repair. You should also look for signs of discoloration around the crack, as this suggests that a large amount of carbon monoxide has been pumped out into the atmosphere. If you find this, you need to call in a professional at this point.

Step 2 - Make the Repair

Once you are ready to begin the repair itself, put on your safety glasses and gloves. If you have a steel exchanger, then cut down a piece of sheet metal to cover the crack, plus 2 inches all the way around. Fire up a welder, and use the liquid metal to form a glue-like line around the edge of your sheet metal. Press this onto the furnace heat exchanger, and allow it to dry. Weld the sheet metal some more as it dries. If you have an iron heat exchanger, then you need to find an iron repair liquid, which uses chemical reactions to seal holes in iron. Pour a little bit of the chemical on the hole, and leave it to dry until it seals around the exchanger.

Step 3 - Aftercare

Now that you have fixed the heat exchanger, replace it in the furnace. Operate it for as long as you need to, but it is important to get the exchanger replaced as soon as you are able to. You should add a carbon monoxide alarm to the space above the furnace. This will give you a clear warning if fumes are leaking from your furnace, and protect you from possible poisons. Furnace heat exchangers are a vital part of your furnace, so altering them could also invalidate any warranty that you have on the furnace.