Tuckers Air Conditioning, Heating & Plumbing Blog

Combustion Problems With High-Efficiency Furnaces


Modern furnace technology has allowed for heating systems with high efficiency that surpass anything built before. Where furnaces once averaged efficiency ratings of around 80% AFUE (meaning 80% of the fuel turns to heating energy and 20% escapes as exhaust), special high-efficiency condensing furnaces can score around 97% AFUE. This is because these high-efficiency furnaces are condensing furnaces. Condensing furnaces contain a second heat exchanger where the exhaust from the first heat exchanger is condensed to release even more heat.

Although a condensing furnace isn’t the ideal heating system for all homes and budgets, in the right home they’re a fantastic way to enjoy savings. But they can have some specific repair issues to watch for, specifically when it comes to combustion. We’ll look at a few of these combustion troubles below.

Clogged condensate drain

The burners in a condensing furnace may fail to ignite because the condensate drain that removes the liquid condensate from the furnace becomes clogged up. You may associate a clogged condensate drain with an air conditioner, and the situation here is similar.

When this drain is blocked, it triggers a pressure switch that shuts off the burners. This can also occur if the condensate drain freezes, which is possible in our cold winters. Condensate drain problems can come and go, but you shouldn’t let them; the issue needs to be fixed at the core so it doesn’t come back.

Clogged flue vent

Any furnace has a flue to allow the exhaust from the combustion process to safely escape from the unit and out of the house. A standard furnace has a large metal flue to allow the exhaust from the heat exchanger to leave. With a high-efficiency condensing furnace, the exhaust goes out a smaller PVC pipe, which may become more easily clogged. This will also trip the pressure switch and shut off the burners.

Broken pressure switch

We’ve already mentioned the pressure switch twice. It’s a vital part of the safe operation of a furnace since it detects if there’s enough airflow to allow for combustion. But the pressure switch can also break so it ends up shutting off the burners when nothing else is wrong. The switch may get stuck, in which case you’ll need to have professionals repair it or replace it.

Blocked air intake pipe

Moving away from the flue and the release of exhaust, there’s issues with lack of airflow through furnace intake of a two-pipe system, which makes it harder for the burner flames to ignite. A two-pipe system means the condensing furnace draws the air it uses for combustion not from the air in the house, but through a pipe that accesses outdoor air. Check outside to see if something like a bird’s nest has blocked this pipe. If you don’t see an obvious obstruction, call for professional repairs.

No matter what type of furnace repair in Frederick, MD you need for your high-efficiency condensing furnace, you can trust our experts to handle it. A natural gas furnace must have professionals service it. Amateur tampering is potentially dangerous, so please leave the diagnosis and repairs to our HVAC technicians.

Tuckers Air Conditioning, Heating & Plumbing serves Washington D.C., Montgomery & Frederick Counties. Schedule furnace repair when you need it.