Unit 49 Gas Furnaces

If the drain for a 90% efficient furnace and the drain for the air conditioning coil are run with a common drain, the drain should be sized?

For the furnace drain, which is larger.

How does a thermocouple work?

Temperature difference between the hot and cold junction generates voltage.

Why must condensate pumps used with condensing furnaces be approved for furnace condensate use?

The condensate from a furnace is mildly acidic and can damage the materials in some condensate pumps.

Why are draft inducer fans necessary in today's furnaces?

The flue gasses cannot make their way through the new heat exchangers on their own.

What controls the amount of gas entering the burner?

The gas manifold pressure and the burner orifice size.

What controls the indoor blower motor on an 80% induced draft furnace?

The integrated furnace control board.

How does the standing-pilot system provide a safety check for the continually lit pilot?

The pilot flame impinges on the thermocouple.

How did serpentine heat exchangers compare to older heat exchangers?

The serpentine heat exchangers required a heat-induced fan, causing negative pressure.

How can a technician determine a major component failure on a mid-efficiency furnace?

The technician can use the self-testing feature on the control box.

The temperature rise of a furnace is?

The temperature difference between the air entering the furnace and leaving the furnace.

Why are the 90 percent condensing furnaces unique?

The venting of this type of furnace is made of PVC and is easy to seal airtight.

For what reason are Category II and Category III furnaces rare?

The venting system is made of a material that is no longer available.

What is the purpose of the gas burner venturi?

The venturi mixes the gas and primary air by making them swirl as they pass through.

What is the purpose of the heat exchanger?

To separate the combustion gas from the circulating air while transferring heat from the combustion gas to the circulating air.

What is the purpose of the roll-out switches?

To shut the gas valve off if flames roll out of the combustion area.

What is the purpose of the draft switch?

To verify that the draft is working before energizing the igniter or energizing the gas valve.

What type of vent is used with 80% Category I furnaces?

Type B double wall gas vent.

Available furnace cabinet configurations include?

Up-flow, down-flow, horizontal, low boy, multi-position.

Why is it possible to use smaller pipes in higher efficiency units?

Volume and temperature of flue-gases are lower.

How does venting compare on 80 percent and 90 percent efficient heating units?

80% furnaces use type B vent and 90% furnaces use PVC pipe for venting material.

What components are added to make a two stage furnace?

A two stage gas valve, two draft proving switches, and a two speed induced draft fan.

Which of these technologies is NOT used in 80% furnaces?

Adding a condensing recuperative coil.

How does a power burner differ from an atmospheric burner?

An atmospheric burner has a fan used to force air into combustion.

A precisely drilled hole that controls the amount of gas entering a burner is called?

An orifice.

Heat exchanger designs from oldest to newest are?

Barrel shaped, clamshell, serpentine, tubular.

The two most common categories of gas fired furnaces are?

Category I and Category IV.

The difference between 80% efficiency furnaces and a 90% efficiency furnaces is that 90% efficiency furnaces?

Have an extra recuperative heat exchanger that condenses water out of the flue gas.

Which type of burner port should be used for maximum efficiency?

In-shot.

The correct operating sequence for an induced draft 80% furnace is?

Inducer starts, igniter glows, gas valve opens, indoor fan starts.

The different types of atmospheric gas burners include?

Inshot, drilled port, slotted, and ribbon.

What type of vent is used with 90% Category IV furnaces?

PVC.

What is the difference between a a 90% furnace with a sealed combustion system and a typical 90% furnace?

Sealed combustion units require combustion air to be piped in from the outside.

Why are standing pilot natural draft furnaces no longer manufactured?

They cannot provide the minimum 78% AFUE required by DOT.

When will installation technicians most likely install an upflow furnace?

When there is little floor space.

What is required when installing a 90% furnace that is not required for an 80% furnace?

A drain for the condensate from the recuperative coil.

A furnace that is composed of a heat exchanger compartment and a blower compartment is considered?

A gas forced-warm-air furnace.

A Category I furnace operates with.

A high temperature, negative pressure vent.

A Category IV operates with?

A low temperature, positive pressure vent.

Which of the following is true about the installation of condensate pumps on high-efficiency furnaces?

A pump must be approved for use with condensate.

A stainless steel coil that is built like a refrigeration coil and allows a high-efficiency unit to move into the 90+ efficiency range is called?

A recuperative coil.

How does the induced draft blower improve furnace efficiency of a natural draft furnace?

Combustion gasses do not have to be as hot to correctly vent.

Why do gas furnace efficiencies jump from 80% to 90%?

Efficiencies over 80% cause condensation but the flue gas is still too hot for PVC vent pipe until the efficiency reaches 90%.

In all furnace cabinet configurations, the return air?

Enters at the fan housing end and exits the heat exchanger end of the furnace.

The four categories of gas fired furnaces are based on?

Flue gas temperature and pressure.

What type of burner do most modern furnaces use?

Inshot.

How is a concentric vent used in a high-efficiency furnace?

It allows a single outdoor exit point for the vent and combustion air pipe.

How does a thermocouple provide pilot safety?

It generates voltage to hold in the pilot safety solenoid.

What did the removal of the pilot light, improvement of the heat exchanger, and the addition of an induced-draft blower do for the mid-efficiency furnace?

It improved unit efficiency to 80 percent.

How does air flow in a standing-pilot, natural-draft system?

It relies on heat to increase the buoyancy of the air to pull air into the combustion chamber and force flue-gases into the proper vent.