The Ides of October are upon me as I begin to write this article and, oh my, a magnificent blanket of frost covers my backyard. I can imagine HVAC service technicians will soon be dispatched with fists full of no heat job tickets.

Companies that service all makes rely on technicians being able to diagnose and repair a variety of cranky residential furnaces, boilers, heat pumps, packaged units, fireplaces and even commercial products such as unit heaters and rooftop units to name just a few. There’s a mountain of knowledge—no, a universe of knowledge—and experience required to deal with everything out there under the HVAC sun.

I like to watch HVAC technicians who post YouTube videos of their diagnostic adventures. Some repairs prove to be almost absurdly challenging, usually involving several problems needing to be solved before the real core issue presents itself.

One Youtuber regularly starts his troubleshooting journey from behind the wheel of his stationary service vehicle advising viewers of the type of equipment waiting for him at the impending service call, and he then shares his thoughts on what the issue might be.

Invariably, the brand of the installed product noted on the job ticket comes under his close speculative scrutiny as he drives to the customer’s home. All the while, viewers are being treated to a litany of things that commonly go wrong with this Brand X unit featured in the video.

I’m sure any of us who have done any kind of HVAC service work also run potential scenarios related to Brand X’s no heat failure, or any Brand’s no heat or poor performance failings for that matter. And many times we’re right, because Brand X might be famous for a particular type of failure.

Yet, once on the jobsite, our premonition of a speedily resolved service event is turned upside down when Brand X has failed for an entirely different, unexpected reason. This presumptive diagnosis of Brand X is a phenomenon known as confirmation bias, that is, diagnosing problems solely based on the information provided in the service ticket, without first visiting the jobsite.

By forming conclusions prematurely technicians may overlook critical details and misdiagnose the issue, ultimately affecting the quality of service and customer satisfaction.

In Defense of Brand X

There is no argument on my part that some products, in the universe of products, demonstrate the concept of value engineering all too well and too often provide a mountain of grief for their owners and servicers.

However, I would argue that efforts to make such products both competitive and affordable in the marketplace can benefit consumers and contractors alike provided solid industry practices and manufacturer’s installation instructions are followed.

I installed a Brand X air conditioner at my sister’s house, in fact, the unit I chose was stenciled from a well-known manufacturer of HVAC equipment. Why? Because it was affordable for her. The unit lasted 22 years: the condenser fan motor and contactor were replaced over the time by a local servicer that also provided yearly maintenance for her. Not bad, Brand X.

In one Youtuber video, he arrived at a service call to find the limit control had failed open. He expected a failed hot surface igniter, and brought one into the basement with him, only to find the igniter was in good condition.

A control board flash code eventually led him to testing the limit. Not having the replacement part handy, he left the job to source a new component at a local supply house. Once replaced, the unit started working again. He changed the nearly plugged air filter with what looked like a MERV 4 air filter the homeowner had on hand and then left the job.

A furnace limit control failure is a serious matter. Even though the Youtuber had more service calls waiting, further investigation should have been conducted.

Perhaps, off camera, the homeowner was encouraged to book another service appointment, we will never know.

Here’s what I think should have happened:
• The blower wheel and secondary heat exchanger should have been inspected for cleanliness.
• Based on the ductwork I observed in the video, a temperature rise test was necessary. Any limit failure ought to dictate a temperature rise test.
• Test static, working and manifold gas pressure.
• The gas meter should have been clocked to verify input.

Modern gas furnaces typically use a 24 volt “limit on a stick” control.

This control will reset automatically, but it is not an operating control. In reality, this safety control only has to work once, yet this type of control will reset a number of times, how many times is anyone’s guess.

The service technician most often has no way of knowing how many times this control reset itself before failure. Unlike the old-style 120-volt helical fan limit controls that too often allowed cycling on the limit for extraordinary lengths of time, possibly years! Today’s new limit controls are more conditioned to detect overheating and to act faster thus helping to avoid heat exchanger failures.

Confirmation Bias vs. Pressure Switches

It’s true that in older condensing furnace designs the metal pressure switches were prone to corrosion caused by water vapour from the flue gas migrating into the switch, thus rusting holes into the casing.

Today’s plastic switch body housings along with improved placement location preventing such migration means the pressure switches are remarkably robust. However, the pressure switch retains its villain status to the point where anyone can purchase a new one from online retailers.

Modern HVAC equipment is sophisticated. Control boards monitor so many system functions that sometimes failure mode readouts—be they flashing LEDs or smart thermostat coded events—that technicians with confirmation bias have this reinforced by another fallibility called cognitive incapacitation.

Faced with various types of equipment using unique failure readouts, many codes that techs have probably never seen before, all requires absorbing new information from 100-page manuals being read in poorly lit basements. Ultimately techs can become overwhelmed and simply start throwing parts at the offender.

In such situations, technicians should fall back on essential knowledge common to all HVAC products: the sequence of operation.

This is a typical sequence for a single stage gas furnace:

1. Call from thermostat (R to W1)
2. Control board runs a self-check routine, it may exercise some relays and look for:

• • All pressure switches open
  • Limit control closed.
  • Flame Rollout closed.
  • No flame detected.

If no faults exist, then the draft inducer is energized.

1. Pressure switch closes.
2. Igniter warm up period.
3. Gas valve energized.
4. The control board flame sensor circuit confirms ignition.
5. Blower fan energized.

If the control board indicates a pressure switch error at the start of the cycle, the pressure switch must be closed; nothing is going to happen. There are only three basic pressure switch (PS) errors:

1. PS Closed at the start of the cycle.
2. PS Doesn’t close at the start of the cycle.
3. PS Opens during the cycle.

Why would the PS be closed at the start of the cycle?

1. The switch may be faulty.
2. There is a wiring short circuit.
3. Strong wind outdoors pressurizing the vent system.

Why would the PS not close at the start of the cycle?

1. The draft inducer isn’t running or not running fast enough to produce the deep negative needed to close the PS.
2. The venting is blocked.
3. The venting system is too long over its maximum equivalent length, too many elbows, undersized pipe.
4. There are sags in the vent or sections with improper slope allowing water to accumulate thus blocking the vent.
5. The drain is blocked allowing the vent or draft inducer to fill with water.
6. The furnace heat exchanger is defective.
7. The vent terminal is blocked by debris, snow or ice, improper termination.

Why would the pressure switch open during the cycle?

The PS will often open during the cycle as the reasons for not closing at the start of the cycle manifest themselves once combustion is enabled. It is worth examining the vent terminal before making any assumptions about the cause of a no heat.

Is the vent terminal installed appropriately? Could the ice buildup on a nearby fence impede efficient venting and trip the PS from time to time (intermittent failure)?

In one situation a saw a vent pipe was sloped towards the vent termination elbow. Installed in November, the system ran until New Year’s Day before the accumulation of water at the elbow prevented the pressure switch from closing. You could hear the water sloshing around in the vent as the draft inducer tried to close the PS.

Manufacturers specify how much deep negative the draft inducer needs to develop at the start of the cycle assuming the vent and drain systems are installed according to the installation instructions.

Said pressure can only be determined using a low-pressure manometer. For example, the manufacturer might specify a deep negative of – 0.92-in. water column (w.c.) to close the switch.

Once the combustion cycle starts, pressure in the system will increase (move closer to zero). The opening pressure (trip point) might be specified at – 0. 77-in. w.c. with a range of +/- 0.04-in. w.c. The PS may designate an (OP) Operating Pressure including a manufacturer specified range, often +/- 0.04-in. w.c.

Before connecting a manometer, be sure to verify all the physical restraints that will impair proper venting are not the issue. Only then can a manometer identify a faulty switch.

Maybe it’s not the flame rod

More often than not, the flame rod needs servicing or replacement—no confirmation bias there. But, whenever servicing the flame rod it’s best to take a before and after DC microamp reading to be sure the true problem has been corrected.

Burners must be clean, especially the cross lighters, be sure to check for additional problems such as badly rusted burners. Why is the burner rusty? Contaminated combustion air often plays a role in burner degradation.

Just a seemingly insignificant amount of debris in one part of the cross lighter can cause flame failure regardless of the virtues of the flame proving circuit.

Other sophisticated components such as electronically commutated (EC) fan motors and electronic control boards are fundamentally voltage dependent. Check for voltage at the device such as Line terminal on the control board.

Voltage drops are caused by wiring faults, loose connections, poor grounding or other defective components that must be repaired. Electronic devices starving for proper input voltage are subject to erratic output operation leading to the most frustrating kind of no heat calls.

Whether a low-cost Brand X or a highly rated premium HVAC product found its way onto a technician’s no heat call list, every technician should have a basic understating of how the device is meant to operate.

Furthermore, a second-rate installation will launch any HVAC product into the orbit of premature failure.

Technicians should take the time to evaluate the entire system from gas meter to furnace drainpipe looking for external culprits leading to internal breakdowns.

Companies servicing all HVAC brands should consistently offer their service staff comprehensive training opportunities. By ensuring technicians are familiar with the diverse range of equipment installed in the area, companies can enhance diagnostic accuracy and repair quality.

Additionally, developing, refining and consistently reviewing company policies related to installation and commissioning criteria is essential.

Providing support and feedback mechanisms will foster a culture of continuous learning and improvement in your business, ultimately benefiting both technicians and customers. <>

Ian McTeer is an HVAC consultant with over 35 years of experience in the industry. He was most recently a field rep for Trane Canada DSO. McTeer is a refrigeration mechanic and Class 1 Gas technician. Any questions or comments, Ian can be reached at imcteer@outlook.com.