Are Blocked Condensate Drains Causing Problems With Your High-Efficiency Furnace?

Posted on: 21 January 2021

High-efficiency furnaces, also known as condensing or 90% furnaces, are capable of heating homes and commercial buildings using far less gas and energy than a conventional furnace. However, they are also more mechanically complex than conventional furnaces, and this can make troubleshooting problems more difficult if you don't have professional help.

If your high-efficiency furnace is not heating your home or business as it should, the problem may be caused by blocked condensate drains.

What Causes Condensate Drain Blockages In High-Efficiency Furnaces?

High-efficiency furnaces have more efficient heat exchangers than conventional furnaces, which allows them to draw more heat from the combustion chamber as it burns gas. As a result, the exhaust gases produced by high-efficiency furnaces tend to be very cold. This causes a significant amount of liquid condensation to form inside the furnace.

Liquid condensation prevents efficient combustion if it is allowed to remain inside the furnace, so high-efficiency furnaces are fitted with condensate drains, which funnel the unwanted liquid into a dedicated drain pan. Soot and other solid debris created by burning gas can accumulate inside these drains over time, preventing the liquid from flowing freely away from the furnace.

However, high-efficiency furnaces have another problem. Because the exhaust gases they produce are so cold, the liquid condensation produced by the furnace can freeze as it travels through the drains, causing serious blockages. If the condensate drain lines fitted to your furnace run outside your building, this problem is especially likely to occur during the winter months.

What Happens If Your High-Efficiency Furnace Has Blocked Condensate Drains?

If the condensate drains become blocked, either by frozen liquid or accumulated debris, the condensation that forms inside the furnace will not be removed quickly enough and may start to back up into the furnace itself. When this happens, the furnace's safety shutoff switches are activated, and the furnace will fail to ignite when you switch it on.

If the condensate drains are completely clogged, the furnace will not work again until the drains are fully cleared. If they are only partially clogged, your furnace may work intermittently once the condensation has completely drained away, only to deactivate again when liquid levels inside the furnace become too high. 

How Are Blocked Condensate Drains Repaired?

If your high-efficiency furnace is deactivating without warning or will not ignite at all, you should call in a professional HVAC repair service to inspect the furnace and its condensate drains. If blocked condensate drains are causing your furnace's problems, these services can unblock the drain lines quickly and safely, getting your furnace back up and running.

If your condensate drains are becoming blocked on a regular basis, your chosen HVAC service may recommend that your condensate drains be rerouted or redesigned. Angling the drain lines at a steeper downward angle can allow condensation to flow away more quickly, preventing future blockages. They may also recommend fitting a small heating element to the drain lines to prevent freezing problems.