Heating and Cooling Solutions

Pressure Switches

Dear Comfort Advisor,

My gas furnace is operating intermittently. Some days it does well, other s I have to reset it to keep the heat on. It has had maintenance within the last several months but worries me as it will get colder. Any ideas?

Dear On Again/Off Again,

Intermittent heat issues with furnaces can be challenging to diagnose. They will inevitably behave themselves while my technicians are there, and then continue irregular operation later in the day. A primary component that is suspect would be the pressure switch (es) that most furnaces use. These sense the pressure developed by the inducer fan which pulls flue gas through the heat exchanger. This pressure is quite small and varies from furnace model and manufacturer. They look like a small metal disk with one or two small tubes connected. Many newer furnaces have diagnostic lights on the control board that will signal a pressure switch error. Many times pressure switches get replaced when in reality it was another component, a lack of maintenance, or vent issue. The switch itself is fine and just doing its job.

The pressure switch is considered a safety device and simply monitors air flow through the burner and vent system. As the snow starts to fall and the wind blows, snow can be built up on the vent termination. This happens more on a wall outlet than a roof outlet but be aware any obstruction of the venting will affect the operation of the pressure switch. Any big snowfall will create a flood of emergency phone calls by people with no heat. We convince them to go outside (Brrrr) and clean the snow off the vent before we dispatch a technician. Most folks call back and thank us for the tip. If you have a roof termination that looks like a white “cone” it brings in combustion air as well and exhaust gas out. You should prepare yourself with a snow rake or some method of keeping it free of snow.

Although there is nothing really to maintenance on a pressure switch, the lack of maintenance to the rest of the unit directly impacts the function of this safety switch. High efficiency furnaces (90% AFUE and above) produce condensate water when they operate. This water drains through a number of hoses, into a trap, and then discharged to a sanitary drain. The small amount of particulate in this water will eventually clog up the tubes. When the water backs up the pressure switch does its job and shuts off the furnace. A common occurrence is the water will eventually drain and then the unit starts up and heats like a champ until the water slowly backs up again.

On older furnaces, 15-20 years old, there are gasket surfaces and seals that leak and this reduces the amount of negative pressure that the switch needs to operate. Hoses will also deteriorate and leak within the furnace from the slightly acidic nature of the condensate water. Very long exhaust vent lengths can also be a problem because they are an added resistance to the inducer fan that must develop enough negative pressure. Every furnace has a “vent table” that states specifically what diameter pipe, length of pipe, and number of elbows allowed. If the installer did not play by the rules, then intermittent lock outs will be unavoidable.

Our team of professional technicians can help you get to the bottom of the problem. We use the latest diagnostic equipment and many years of experience to keep you and your furnace happy.

As always, estimates and good advice is free at AirWorks.

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