Why Is My Furnace Leaking Water?

Generally, we give little thought to our furnaces, unless they aren’t working properly. They’re one of the backbones of our homes and we assume they’ll do what they are designed to do. Unfortunately, they are mechanical objects and occasionally break down.

One type of furnace problem that no one wants to experience is a leaking furnace – especially if it is gushing, rather than trickling! If you see water appearing at the base of your furnace, you may want to call your HVAC professional to assess the problem and help you to fix it. Water can damage floors and walls and cause unhealthy mould and mildew.

First, however, determine what type of furnace you have so you can provide proper information. If there is a white PVC vent pipe coming out of the top or side of your furnace, you have a high-efficiency condensing furnace, which has an annual fuel usage efficiency (AFUE) rating of 90 or above. If the vent pipe is metal, you have a conventional furnace with an AFUE rating below 90.

Next, you need to take some action. Shut off the system, either with the switch or the circuit breaker, if you can’t locate the switch. If possible, thoroughly soak up all the water that has leaked out because water damage occurs quickly.

Whether or not you seek assistance, you’ll need to assess the problem. Each type of furnace has its own weaknesses to consider.

High-efficiency Condensing Furnaces

Condensing furnaces normally create condensation as part of the heating process (hence their name). They have two heat exchangers to carry away moisture that condenses and is carried away through a condensate line. The most common reasons they leak are:

  • Blocked condensate drains. Check your drain trap, which collects dirt and water over time. If it is blocked, it may cause leakage. You can easily clean it out with a shop vacuum.
  • Condensate pump problems. The condensate pump pushes water through the lines, away from the furnace. Unless you’re a good mechanic, this is a mechanical fix that requires your HVAC professional.
  • Issues with the condensation line. You may have a clogged or broken pipe that is no longer removing condensation, so it is pooling at the base of the furnace. Your HVAC professional can repair these pipes or lines.

Conventional Furnaces

The most likely reasons for conventional furnaces to leak are:

  • Leaky humidifier. If your furnace has a built-in humidifier, water regularly flows in and out of it to allow moisture into the air. Locate the humidifier, which is usually on the outside of the furnace, and look for leaks or clogging in the drain line, the water tap line, the casing or the water feed tube. If you find a leak, it’s time to call your HVAC professional.
  • Improperly designed vent pipe. When a flue pipe works properly, it carries away gases quickly before they have time to cool and condense into moisture. If it is too big or has no slope, it won’t work as planned. Check to see if your pipe has no slope to it or has water leaking from it. It will need to have the proper diameter and design for your furnace, so contact your HVAC professional for assistance.
  • Furnace is leaking when A/C is on. Usually, this is actually an air-conditioning problem, not a furnace problem. However, the a/c unit generally sits next to or on top of the furnace unit, so it seems as if the furnace is the culprit when it is probably a clogged or leaky a/c drain line. Confirm that the a/c is the culprit by ensuring that you are, indeed, running the a/c and not the furnace. If the a/c is on, check the condensate line and the drain pan to isolate the source. Then, it’s time to call your HVAC professional.

All Furnaces

Check for these other potential causes of leaks:

  • Plumbing issue. Your leakage problem may have nothing to do with your furnace; it may result from a clogged or broken pipe causing a leak in the drainage system. Your pipes will need to be repaired or replaced.
  • Clogged filter. If you don’t change the air filter in your furnace regularly, it can restrict air flow to the furnace coil. Sometimes, this can leak and cause water to pool around the unit. Check your filter every quarter to see if it needs replacing.

Here’s hoping you won’t need these tips and suggestions any time soon, but if you do, your HVAC professional is available to lend you a helping hand.

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