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How does a high-efficiency furnace work?

by Manage My Life
April 26th, 2007

How does a high-efficiency furnace work?

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Manage My Life
So far we have been exhausting products of combustion as gases. Even with induced-draft furnaces, the products of combustion are 250F to 300F when they leave the house. We have been careful to keep the exhaust products warm enough to prevent condensation from occurring inside the furnace. The condensate from natural gas is slightly acidic, and can rust out furnaces well before their time. Condensation in exhaust products is to be avoided in conventional furnaces.

However, in our quest for maximizing efficiency, we threw caution to the wind and looked for ways to suck even more heat out of the exhaust products. This meant that we were going to cool them down to the point where condensation would occur in the furnaces and exhaust flues. These furnaces would have to tolerate condensation and be able to get rid of it.

Why is condensation such a big deal?

As gases cool, they release a tremendous amount of heat as they change their state from gas to liquid (creating condensation). One of the main products of combustion of natural gas is water. Let's look at the

latent heat of vaporization

of water. We know it takes one BTU to raise one pound of water one degree Fahrenheit. If we heat that same pound of water from room temperature (70F) to boiling (212F), we're raising the temperature by 142F. This means we had to add 142 BTUs to it.

Now, take that one pound of water at 212F and convert it to steam at 212F. How many BTUs do we have to add? Believe it or not, the answer is about

970

BTUs! We have to throw 970 BTUs at the water to create steam, even though we haven't raised the temperature. This is called the

latent heat of vaporization

. This works in reverse, too. Condensing 212F steam into one pound of water

releases

970 BTUs.

When we think about how we can capture heat from the products of combustion from a furnace, and we remember most of that is water anyway (burning natural gas generates water and carbon dioxide), this becomes kind of exciting. As we cool exhaust products, they will release heat to us. However, when we cool down to the point where we convert from water vapor to water liquid, we get 970 BTUs for every pound of water! The next generation of furnaces, those we call

high-efficiency

, is designed to allow condensation within the furnace.

How do we get more heat out of the exhaust products?

The trick to grabbing more heat out of the exhaust products so they'll condense is to keep them in contact with the heat exchanger longer. Heat exchangers work on a very simple principle. Heat flows from an area of high energy (where it's hot) to an area of low energy (where it's cool). The greater the temperature difference on either side of the heat exchanger, the faster the heat will move. But as long as there is a temperature difference, heat will move from the hot area to the cool area. Since the exhaust products are always warmer than room temperature, the secret is to make very long heat exchangers.

High-efficiency furnaces, in effect, do have very long heat exchangers. Most of them have two or even three heat exchangers.

Most

high-efficiency

furnaces use an induced-draft fan, the same as the

mid-efficiency

furnaces. The difference is that the path from the burner to the induced-draft fan is much longer and the exhaust products are exposed to much more heat exchanger surface. This means that much more heat is transferred into the house air. As the exhaust gases flow through this long route, the gases cool to the point where they condense. By the time the exhaust gases leave the furnace, their temperature is only 100 to 150F. At this point, we need to do two things:

1. We need to make heat exchangers resistant to the slightly acidic condensation.

This is normally accomplished with stainless steel heat exchangers. This is one reason there are often two or three heat exchangers rather than just one very long heat exchanger. The first heat exchanger is conventional galvanized steel. No condensation takes place in the first part of the furnace because the exhaust products are so hot.

As the exhaust gases move to the second and third heat exchangers, they cool and are likely to condense. Here we use stainless steel, because it's much more resistant to the condensation than is galvanized steel. On some high-efficiency furnaces, the two heat exchangers are combined into one long stainless steel tube. Although the material costs are higher, it simplifies construction of the furnace.

2. We have to collect the condensate, and drain it out of the furnace.

Half-inch or 3/4-inch diameter clear plastic tubing is usually used for condensate. It has to slope down toward a drain, since the condensate only flows by gravity. Some manufacturers use a trap in the condensate line to prevent air being drawn into the furnace through the condensate line.

The condensate is discharged outside or into a floor drain or laundry tub. It is not considered good practice to drain condensate through the floor slab into the soil. In some areas, it has to go through a

neutralizer

to take away the acidity. The neutralizer is a salt bath the condensate passes through on its way to the drain.

You should find out whether a neutralizer is required in your area. The condensate is not as acidic as vinegar, wine or soft drinks, but some municipalities require that it be neutralized anyway.

A condensing furnace operating continuously for 30 minutes can generate up to a quart of condensate.

Heat exchangers work best where there is a big temperature difference across the heat exchanger. As we get farther along the heat exchanger, the products of combustion get cooler, and the rate of heat transfer to the house air slows down. The house air gets warmer as it moves along the heat exchanger. Manufacturers have found that it's best to have the coolest (room temperature) air pass over the coolest part of the heat exchanger (the end.) As the air is warmed, it moves along to the warmer parts of the heat exchanger. The house air sees the hottest part of the heat exchanger, just before it leaves the furnace.

The efficiency rewards

A condensing furnace or partially condensing furnace can enjoy seasonal efficiencies of 85% to 95%. Some claim even higher efficiencies. The sequence of operation is effectively the same as for the induced-draft mid-efficiency furnace.

High-efficiency furnaces are even more efficient when they're warming up because there is lots of condensation when the heat exchanger is cold. This is the opposite of a conventional furnace because we're using the

latent heat of condensation

(the reverse of the latent heat of vaporization).

Because high-efficiency furnaces are so efficient during their warm up, when the heat exchanger is cold, their seasonal efficiency can be even slightly higher than their steady state efficiency!

Any drawbacks?

The condensate collection system creates an additional layer of complexity.

The long and restrictive heat exchanger paths also result in clogged heat exchangers, since condensate can accumulate in the heat exchangers and attract dirt and soot.

You have to be careful venting induced draft appliances with positive vent pressures. Interconnecting them with natural-draft appliances may cause spillage throughout the natural-draft appliance's diverter hood. Only induced-draft systems specifically designed to end up with atmospheric pressure at the chimney, can be used with natural-draft appliances but you can't always identify them by looking. Many high-efficiency furnaces have positive vent pressures.
by Manage My Life
April 26th, 2007
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