How Inspectors Find Damaged Heat Exchangers
By Charlie Bellefontaine
Sep 5, 2020
This photo shows a heat exchanger removed from the furnace. It is easy to see the holes in this tubular heat exchanger now that it is removed. The problem for home inspectors is that we don't removed the heat exchanger. We cannot view this heat exchanger when it is inside the cabinet. Let's see what we CAN do.
We use special tools and equipment to help in making this determination. We use a combustion analyzer to measure the flue gasses. There has to be an understanding of some science behind using some tools like this. So let's see what makes a furnace safe and the science behind it.
Gas / Air Mixture
The Heat Exchanger
Air Pressure Differentials
Things that keep furnaces safe.
Fuel, heat, oxygen and a chemical chain reaction are needed to make a fire. If everything is mixed perfectly, there will very little byproducts of the fire. One bad byproduct of incomplete combustion is Carbon Monoxide (CO). If the gas pressure is too high, or too low, high levels of CO will be created. This also happens if combustion air is limited. This CO is measured in the exhaust gasses of the vent, or flue. We measure the CO in the exhaust gasses and make sure the flame is burning properly and producing very little CO.
Gas Air Mixture
The heat exchanger is a piece of metal that separates the gasses produced from the fire from the air that is circulated through the house. Heat exchangers are NOT hermetically sealed. There should not be any additional cracks, holes, or any opening in the heat exchangers. The fire produces hot gasses that heat up the metal of the heat exchanger. The blower fan blow the air that we breath around the hot metal of the heat exchanger absorbing that heat and bringing it to the rooms in our home. If the heat exchanger is in tact, it is impossible to get CO into the air we breath.
The inducer fan creates negative pressure on the inside of the heat exchanger. The blower fan creates positive pressure on the outside of the heat exchanger. If there is a crack, hole, or any opening in the heat exchanger, the air should flow into the heat exchanger instead of bad gasses entering the circulation air. We measure the amount of oxygen after the fire. If there is an opening, the oxygen levels will be higher than normal. Our machines calculate the excess air inside the flue gasses and when they are too high it indicates an opening in the heat exchanger.
NO! We Need All Three Safety Features Working!
So if one fails, the other two will make it safe; right?
The results of CO entering the home are catastrophic and deadly. If any of the three items are not working properly, we need to fix the problem, or replace the furnace.
A common issue on serpentine heat exchangers is the eyelets get damaged and allow the heat exchanger to separate. The arrow pointing to this eyelet is damaged and the heat exchanger is now open. We caught this with our combustion analyzer because the circulation air was blowing additional oxygen into the exhaust gasses. This triggered our alarms. CO was NOT entering the home at this time. This furnace was still replaced because any crack is considered dangerous. Our clients were able to negotiate for a new furnace.
These are photos of secondary heat exchangers found on high efficiency Carrier furnaces manufactured from 2012 and earlier. Looking at these photos, it is obvious that the heat exchangers are in pretty bad shape. But again, you cannot see any part of the heat exchanger unless the heat exchanger is removed or the blower fan is removed. Even if the blower fan is removed, you will only be able to see the underside of the secondary heat exchanger.
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