Note: This is part II of a three-part series of articles excerpted from the ASHI@HOME education program.
In part I, we defined high-efficiency furnaces, discussed how to identify them, and reviewed their components and sequence of operation. Now, let’s look at some of the things to watch for when you inspect a high-efficiency furnace.
Combustion air supply – There is nothing unique here. If combustion air comes from inside the house, we need 50 ft. per 1000 BTUs. That is basically a wide-open basement. If the system is in a confined room, we need two openings: one near the floor level and one near the ceiling level. Each opening should have 1 in. for every 1000 BTUs.
Illustration: Combustion air supply
Check for adequate air supply.
If the furnace is starved for air, it may not start or it may back draft.
If the combustion air comes from outdoors, check for pipes that are leaking, disconnected, sagging or cracked.
On the exterior, check for a poor location or obstructed inlet. Poor locations include inlets that are close to other exhausts, or are too close to ground and might be covered by snow (where that’s an issue).
Burners – Again, we’re looking at normal things here. The flame should be stable and mostly blue.
We don’t want to see it lifting off or floating.
We don’t want to see it coming back out the front of the furnace.
On the sealed combustion unit, you might have trouble seeing much. Look for rust on the burners or burners that are misaligned.
We recommend running the system for at least 10 to 15 minutes to make sure it does not shut down on high-temperature limit.
Some furnaces have multistage or modulating burners. Here’s a question many home inspectors have not considered:
• For multistage burners, does the house air fan speed change as the firing rate changes? The answer is yes.
•On a related note, does the induced draft fan speed change as the firing rate changes?
• The answer is yes for most manufacturers.
Photo: Misaligned burners
Ignition system – Most high-efficiency furnaces use a hot surface igniter (HSI), although they can use a direct spark or intermittent pilot for ignition.
From an inspection standpoint, you will not be able to see much. The igniter either works or it doesn’t.
If the igniter has failed, the furnace will not start.
You may see the furnace trying to start up repeatedly. Typically, if there is a problem, the flame sensor will shut down the furnace in two to eight seconds after it tries to start.
Heat exchanger – Heat exchangers on high-efficiency furnaces are subject to all the same problems as conventional heat exchangers.
Rust, cracks and holes are common problems.
Clogging is another problem.
Secondary and tertiary heat exchangers can be very restrictive. Any carbon/soot buildup can clog them. Flushing the heat exchangers on high-efficiency furnaces may be part of regular servicing.
Flame rollout, short cycling and water or rust below the heat exchanger may all indicate heat exchanger problems.
Condensate system – Leakage is the big issue on condensate systems. Sometimes that’s because pipes are split or disconnected. Sometimes it’s because things are clogged. The evidence usually is easy to spot.
The other common problem with condensate systems is an inappropriate discharge point. There should be an air gap so that nasty sewer gases cannot be drawn back into the system. For example, condensate systems should not discharge into a plumbing stack.
Photo: Heat exchanger: rusted through and cracked.
Induced draft fan – These fans are much more reliable in modern furnaces than they were in first-generation mid- and high-efficiency furnaces.
Evidence of problems includes squeaking or grinding noises that suggest a bearing problem. Splashing and gurgling on startup may indicate a clogged condensate system.
Photo: Induced-draft fan
Draft-proving system – Air-proving switches make sure that the combustion air and venting system is working properly.
We need to be sure the fan is moving air through the system. There might be differential pressure switches or an air-proving switch. There is not a lot to look at from an inspection standpoint. If draft is not proven, the furnace will not operate.
Home inspectors are not going to troubleshoot an inoperable furnace; just report it.
There’s still more that can go wrong with high-efficiency gas heating systems. In Part III, we’ll cover control board, venting system, house air fan and safety devices.
Illustration: Draft-proving switch
Note: This article is an excerpt from the ASHI@HOME training program.
ASHI Past President Alan Carson has been a pioneer in home inspection since 1978. His work includes home and commercial building inspections, inspection training and the HORIZON report writing systems. He has developed many educational programs, most significantly the ASHI@HOME training program.