Induced-draft or forced-draft burners increase the efficiency of the furnace over natural-draft burners. Let's make sure we understand the terms.
relies simply on atmospheric pressure and the venturi effect of the gas burner to move the air. Excess air has to be available both at the burner and at the draft hood to ensure full combustion and good draft as it moves up the chimney. With natural draft (also sometimes called gravity venting), we have to keep the chimney warm enough to maintain the upward flow of exhaust products. The pressure in the venting system is negative with natural draft burners.
An induced-draft burner uses a blower to pull air into the burner, and through the combustion chamber and heat exchanger. The fan then pushes the flue gases out through the vent. This creates negative pressure in the furnace, and may create positive or negative pressure in the venting systems.
Sidewall-vented, induced-draft furnaces have significant positive pressure pushing exhaust gases outside. Induced-draft furnaces designed to vent into chimneys or B- vents, for example, are baffled so the exhaust gases are at atmospheric pressure (considered negative) when they enter the chimney. These can then be manifolded with natural-draft water heaters, for example. Sidewall-vented systems cannot.
Induced-draft fans are also called
aust blowers, power vents
. Systems with induced or forced draft fans are sometimes referred to
3. Forced-draft burners
A forced-draft combustion system uses a fan to push combustion air into the burner, through the combustion chamber, through the heat exchanger and out through the vent system. All oil burners and some gas systems use forced draft. Forced-draft burners cannot usually be manifolded with any other type of burner, although two forced-draft burners can be coupled. Some forced draft systems produce negative vent pressure (conventional oil fur-nace, for example) and some produce positive vent pressure (high efficiency gas furnaces, for example.)
A natural-draft system has no fan.
An induced-draft system has a fan
the burner and heat exchanger but before the vent.
A forced-draft system has a fan
the burner, heat exchanger and vent.
The induced-draft fan sits in the path of the hot products of combustion. The forced- draft fan sees room air before it goes into the furnace. Induced-draft fans pull things through the furnace; forced-draft fans push things through the furnace.
Manufacturers began to put induced-draft fans into their furnaces because they improved efficiency by wasting less house air. The beauty of the induced-draft fan system was that we no longer had to rely on gravity to exhaust the flue gases. Therefore, we didn't need excess air. In fact, we don't need a draft hood at all anymore. We could push them right out through to the vent system.
Heat exchangers were designed so that they wouldn't slow the products of combustion as they moved through the furnace by convection. If we slowed down the products by making the heat exchanger too restrictive, we could lose our draft up the chimney. Putting in an induced draft system eliminates that problem, too. Induced draft fans are common in mid-efficiency furnaces.
We spoke earlier of needing 15 cubic feet of
and 15 cubic feet of
to burn one cubic foot of natural gas. With an induced draft fan, we only need the 15 cubic feet of combustion air, more or less.
To summarize, there are three areas where efficiency can be improved over natural draft by using an induced-draft blower:
We don't have to use so much extra air, so we reduce the amount of room temperature air we throw outside.
We can use a heat exchanger that is more restrictive (able to take more heat out of the exhaust products as they move by) because we don't have to worry about maintaining the momentum of our exhaust gases.
The induced-draft fan also makes a great sock in the chimney. We don't need an automatic vent damper anymore because when the fan is off, it virtually eliminates airflow through the venting system.
What about the pilot?
Since the chimney is blocked off when the furnace isn't running, instinct tells us we need an electronic ignition system. Some manufacturers tried standing pilots on induced draft furnaces but had corrosion problems.
Recently, some manufacturers have tried again, making sure the pilot combustion gases do not go through the heat exchanger when the draft fan is off. Instead, they vent up the front of the heat exchanger faceplate and out through vents in the top of the furnace cabinet. Time will tell if this approach is successful.
We need an alternative to a standing pilot. This
includes intermittent pilot, hot surface ignition and direct spark ignition systems.
We need an induced-draft fan that can stand up to the high temperatures and moisture content of exhaust gases.
We need 120 volts of electricity to operate the fan.
We need a switch to tell the fan when to come on and when to turn off.
We need automatic safety controls to make sure the fan can move gases through the venting systems. This may be an
to make sure the fan has come on before the burner starts up. We don't want the burner on with the fan off. All the combustion products will spill back into the house.
It could also be a
that senses high temperatures at the front of the burner. If it detects exhaust products or flame rolling out the front of the burner, it will shut the furnace off. This is sometimes called a
Is life getting more complicated? You bet. Let's look at how the furnace works now.
Sequence of operation for furnace with an induced-draft fan
1. The thermostat calls for heat. 2. A switch closes to start the induced draft fan. 3. The air-proving switch senses the air pressure from the fan and delivers energy to the ignition module if everything's okay.
4. The ignition module starts a
. This is something new that we didn't need with an automatic vent damper and a natural draft furnace. With an induced-draft system that shuts off the vent tightly, we want to get rid of any unburned gas in the furnace before we light things up. The ignition control module contains a timer to run the induced-draft fan for 15-30 seconds, to clear any unburned gas that may have accumulated in the furnace.
5. Once the purge cycle is completed, the main flame is lit. 6. From here on, the operation is just like a conventional gas-fired furnace. Once the house-side air is warm enough, the house-air fan will come on.
7. The thermostat is satisfied. 8. The main burner shuts down. 9. The induced-draft fan shuts down. Note: In some cases, the induced-draft fan runs for a while in a
to make sure all the products of combustion are exhausted.
10. The house air fan operates until the fan limit switch senses that the house-air side of the heat exchanger has dropped back down to about 90F, and then the fan shuts off.
So now that we've eliminated the off-cycle losses and improved the steady state efficiency somewhat, we're able to achieve seasonal efficiencies (AFUE) of 75% to 82%. Life is getting more complicated, electronically and mechanically, but we are saving fuel.