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A look at the heat exchanger
So, what is a heat exchanger?
It is a (usually metal) separation between the burning gasses and air (or water) forced by that easily transfers heat between the gasses and the air being heated.
It can be pipes inside which the gas is burned as on the left.
It is a (usually metal) separation between the burning gasses and air (or water) forced by that easily transfers heat between the gasses and the air being heated.
It can be pipes inside which the gas is burned as on the left.
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Below left is a clamshell type heat exchanger. It is made by placing 2 halves made of sheet metal with the gas passages stamped onto them together and riveting and folding to hold them together. These types of heat exchangers are used in 80% efficient furnaces that have a vent temperature of above 140F. Usually quite a bit higher. High efficiency furnaces have 2 heat exchangers. The primary heat exchanger removes heat, but not to the point that water condenses. The secondary heat exchanger is made of stainless steel, removes more heat and condenses water from the gas. This water is highly corrosive. It contains carbolic acid and small amounts of sulphuric acid. The stainless steel should not corrode from contact with the acids. The slideshows on the lower left show different views of a high efficiency (90% + efficiency) heat exchanger. The clamshell primary has a large cross section where the burner feeds burning gas, and reduces as the passes thru the tortuous turns as the gasses cool and take up less space. The secondary heat exchanger looks similar to an air conditioning coil. this part of the heat exchanger removes more heat but more importantly, condenses the water in the gas that was steam. This releases latent heat to get all the heat possible out of the gas. Vent temperatures of these furnaces can be as low as 80F. The video below compares efficiencies of 3 types of furnace. All these furnaces must have some method of drawing off the condensate for disposal. |
The video below gives a tour of the gas high efficiency heat exchanger.
Below is a video demonstrating how the heat exchanger in an 80% efficient furnace picks up the heat from the flame.
This heat exchanger was in a furnace that was underfired. The gasses condensed in the heat exchanger and rusted it out. This could also be caused by operating the furnace with a low return air temperature.
This heat exchanger was on a furnace that was overfired. You can see by the discoloration of the pipes caused by high temperature. |
The above are all failed heat exchangers
The left above photos show the results of a rapid expansion of burning gas inside the heat exchanger. The upper right is a rusted out heat exchanger, the lower right is a cracked heat exchanger that has been brazed back together. Brazing or welding a heat exchanger is illegal in all states that I know. It is not effective as the heat exchanger cracks next to the welds.
The center photos are cracks due to metal fatigue.
In all these cases, the furnace either would not run or could have produced carbon monoxide.
The center photos are cracks due to metal fatigue.
In all these cases, the furnace either would not run or could have produced carbon monoxide.