Residential And Light Commercial Systems

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1

List the four categories of gas fired furnaces based on flu gas temperature and pressure.

Category I: Vent gas temperature greater than 140 degrees F above the flu gas dew point

Category II: Vent gas temperature less than 140 degrees F above the flue gas dew point

Category III: Vent gas temperature greater than 140 degrees F above the flue gas dew point, the positive pressure vent requires a sealed vent system

Category IV: Vent gas temperature less than 140 degrees F above the flue gas dew point

2

Explain the difference between a Category I furnace and a Category IV furnace.

Category I: Vent gas temperature greater than 140 degrees F above the flue gas dew point, which avoids excessive condensate production in the vent.

category IV: Vent gas temperature less than 140 degrees F above the flue gas dew point, Which may cause excessive condensate production in the vent.

3

List the five cabinet configurations.

five styles: Up Flow, Lowboy, down-flow, or counter flow, horizontal, and multiposition.

4

Why are the standing pilot natural gas furnaces no longer manufactured?

...

5

What changes were made in furnace design to achieve 80% efficiency?

Furnace manufactures achieve an efficiency 0f 80% by eliminating the pilot light, improving the heat exchanger efficiency, and adding an inducer draft blower.

6

What is the difference between an 80% efficient furnace and a 90% efficient furnace?

High efficiency or condensing type gas furnaces are rated at 90% AFUE or better. They differ from the 80% AFUE mid-efficiency furnaces in that an extra ex-changer (secondary heat ex-changer) is added to extract more heat from the flue gases.

7

Discuss the evolution of the heat ex-changer.

The earliest heat ex-changers where basically barrels with a hole in the bottom and a hole in the top. They were adequate for gas separation, but were rather inefficient heat transfer apparatuses. Large quantities of the relatively unrestricted flue gas found their way up the vent without ever contacting the surface of the heat ex-changer. This translates into lost heat.

8

Discuss the operation of an atmospheric burner.

The gas shoots out of the orifice in a small quick stream which creates a negative pressure around it and draws in primary air through the primary air openings. The burner is shaped like an hourglass on the end. It has a larger diameter at the face where the gas and the air enter, a small diameter a short distance away, and returns to a large diameter at the burner's main body.

9

Explain why furnace efficiencies jump from 80% to 90%.

When the first 80% furnaces were being introduced, there were still many furnaces around with efficiencies in the 50-60% range. The 80% furnaces represented a midpoint in both efficiency and cost between the traditional standing pilot, natural draft furnaces and the higher 90% condensing furnaces. Today the 80% furnaces represent most manufacture's bottom line.the minimum efficiency is 78%.

10

What is the purpose of the orifice?

An orifice is a precisely drilled hole that meters the correct amount of gas into the burner

11

Why is it not a good ideal to drill out orifices in the field?

Since the orifice size is what really matters, most people refer to the spud as the orifice. the specified sizes are very small and they must be drilled accurately. a slightly large hole lets in too much gas and over-fires the burner. A hole that is not drilled straight can cause even more problems. The space between the orifice and the venturi on most atmospheric burners is open to the air. It has to be open so that primary air can be injected by the jet action of the gas emitting from the orifice. If the gas stream does not aim straight inward, it may hit the side of the venturi; or worse, the gas stream may miss the venturi altogether. This causes poor ignition and even explosions.

12

What components are needed for a furnace to operate as a two stage furnace?

The distinguishing features of variables capacity furnaces are the components that allow for two firing rates including two stage gas valves, two speed induced draft fan, two draft proving switches, and the use of a two stage thermostat.

13

Why are some condensate pumps not approved for furnace condensate?

Not all condensate pumps are approved for installation on condensing furnaces because furnaces condensate is mildly acidic, typically in the pH range of 3.2 to 4.5. When a condensate pump is required, select a pump that is approved for condensing furnace applications.

14

Can the furnace drain and the air conditioning drain drain be run with a common drain?

Do not normally combine a furnace condensate drain and evaporator condensate drain. Pressure produced in an up-flow coil plenum can pressurize the drain and the secondary heat ex-changer in the furnace. This stops the furnace from draining, pressurizes the heat ex-changer, and causes the furnace to lock out on it's pressure switch safety.

15

List the different types of gas burners.

Atmospheric gas burner draws it's air in from surrounding atmosphere and requires air both at the point of gas entry and at the point of combustion.

Power burners normally get all their air at the point of gas entry. A power burner also draws it's air from the surrounding atmosphere, but a blower forces it in mechanically.

16

Why do most furnaces today use burners?

One of the most wasteful periods of a furnace operation cycle is the warm up time when the burners are firing and the fan is not on. Increased cycling leads to decreased efficiency. The furnace run time can be extended and the cycling reduced by using a two-stage gas valve

17

What is the temperature rise of a furnace?

...

18

What part of the furnace does the air travel through first in all furnace cabinet configurations?

The air always travels through the blower compartment first, before going to the heart ex-changer. The reason is fairly simple; the fan motor would overheat otherwise.

19

Why should technicians be familiar with standing pilot,natural draft furnaces since they are no longer manufactured?

Because they are still in use

20

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

It has adjustments for both the low fire and high fire manifold pressures.