Why Does My Gas Furnace Leak Water In The Winter?

Gas furnaces that leak water in winter are usually high-efficiency gas furnaces. When gas furnaces began to be more efficient they also began to condense the flue gases, draining moisture back down the flue pipe into the furnace.

You may or may not have noticed your gas furnace leaking water in the winter. A gas furnace leaking water in summer is generally an air conditioning problem with condensation.

Water problems with gas furnaces began primarily when furnace efficiencies approached and exceeded 80%. With early model gas furnaces that had pilot lights continually burning they most often vented into a brick chimney in combination with a gas water heater.

The temperature of the flue gases going up the chimney stayed hot until they exited the top of the chimney. The chimney was always very warm and dry from the bottom to the top from all that wasted heat.

Some gas furnaces vented into metal flue pipe called B-Vent. This double-walled pipe reduced the clearances needed in construction and provided a solution to venting where a chimney didn’t exist. B-vent heated up pretty good and water coming back down the pipe was not too common.

But it could happen when proper venting was not adhered to or followed. Improper venting could be things related to the wrong size B-vent, to many elbows, or not enough slope.

Presently, we have three basic types of gas furnaces:

1. Natural draft (less than 79% efficient)
2. Induced draft (79% or slightly higher)
3. 90% + efficiency (recuperative furnaces)

Natural Draft Gas Furnaces

A natural draft furnace uses the draft created by a chimney to pull the flue gases from the furnace. Most of us are familiar with lighting a fire in a fireplace and getting a slug of smoke in the room until the chimney heats up and the smoke starts flowing out the chimney.

That’s the draft capability of a chimney. And just like a fireplace is a huge loss of heat for a house, so is a natural draft furnace when installed within the insulated walls of the home.

The picture above is a natural draft furnace. Notice how simple it is with no inducer motor. At the top is a draft hood that is constantly sucking air from the home and being drawn up the chimney even if the furnace isn’t turned on.

If a natural draft furnace is replaced with an induced draft and the new flue pipe was just stubbed into the chimney like its predecessor, then condensation could be taking place in the chimney. That condensation is acidic and will destroy the mortar between the bricks in the chimney.

The only way to solve the condensation problem is to install a chimney liner. The liners come in a kit that includes a cover to seal the chimney top and a liner top cap to keep out the rain and the birds.

If you love to do things for yourself (DIY), be sure to understand the principles involved and promise to get a permit from your local building department; you can purchase (ad) chimney liner kits from Amazon.

As furnace efficiencies increased, more of that heat that went up the chimney was recouped by more recuperative coils inside the furnace. The potential for leaks came with the recuperative coil and the plumbing needed to drain the condensation.

To me, it’s an amazing technology. The geniuses behind developing these higher-efficiency furnaces have gas-heating furnaces hitting 97% efficiency. Think about that for a moment. Only three percent of the heat created goes up the chimney compared to more than 20%.

What Is An Induced Draft Gas Furnace?

Drafted-induced furnaces have an additional fan incorporated into the flue that induces the exhaust out of the heat exchanger and up the chimney—ironically called an inducer assembly, inducer fan, or inducer blower.

The inducer assembly is necessary because manufacturers made the heat exchanger tighter to capture more heat, making the furnace more efficient. So, the inducer fan is assisting the natural draft of the chimney.

With the heat of the flue gases being lower, these gases go up the chimney at a lower temperature. The flue gases would then cool before reaching the chimney’s top.

As the flue gases cooled, they would reach the dew point, condense into water, and drip back down the chimney.

The science behind this reaction is a little deeper than what I have just told you, but my intent in these articles is never to get too deep. For more on “dew point”, Go here.

So now that you understand that a gas furnace under the right conditions can actually create water, it might offer some clues as to why your furnace is leaking.

You can’t stop this rust unless it’s treated the right. Sanding, scraping, and painting will not keep this cancer from going right on through the metal.

The title of this article specifies that these leaks are happening in the winter, so that rules out condensation from running your air conditioner. Air conditioners pull moisture from the air as your furnace circulates the warm humid air from your home.

The AC coil picks up this moisture, and it settles in the drain pan of your air conditioner’s evaporator coil near the furnace. This moisture usually drains out of a furnace coil in a 3/4 inch (usually white) PVC pipe and then goes into a drain or a small pump near the furnace.

90%+ High-Efficiency Gas Furnaces

So now you have to determine what efficiency furnace you have. Is it vented in a metal pipe or PVC pipe? The furnace below is a Goodman 97% efficient natural gas furnace.

My customer didn’t want to dispose of his old Rheem heat pump, so we tied it into this furnace and made it into a dual-fuel system.

If your furnace is vented in PVC like the image below, then there are all kinds of places it could be leaking. Each joint in the PVC is a potential leak. Inside the furnace is where a lot of leaks occur. These leaks are a threat to the metal cabinet that holds your furnace together.

My image above doesn’t show the condensation pump (sorry I spelled condensation wrong on the lower right) very well so just below is an image from Amazon that you can click on to check prices and compare in case you’re suspecting your pump is wore out.

This Little Giant pump has many good ratings, and I’ve used it for years. You can check out the current pricing and availability here on Amazon.

These pumps work best when used with the (ad) proper drain tubing, which is also available on the Amazon website. It’s important to remember that drain tubing that is terminated to the out of doors has the potential to freeze in the winter. A frozen tip just outside the home will plug the pump, overflowing and possibly damaging your home, depending on its location.

These pumps work best when used with the proper drain tubing. It’s important to remember that drain tubing that is terminated to the out of doors has the potential to freeze in the winter. A frozen tip that hangs just outside the home will plug the pump, causing it to overflow and possibly damaging your home, depending on its location.

If you are not having regular maintenance done or taking the door off your furnace at least once a year, you could be the owner of a giant rust bucket. The condensation that drips back to your furnace is acidic. It will rust a hole through any metal that it touches inside that furnace cabinet.

I have used a product by (ad)Permatex called Rust Treatment, which is available on Amazon. It’s a simple paint-on with a brush application. Let it set for about 15 minutes, and apply a second coating. The Permatex turns a nice-looking black color, changing rust into a protective primer coating.

Not only can a condensation leak rust out the inside of a furnace, but it will also etch out a concrete floor and make permanent marks in other materials if it’s leaking out a pipe joint away from the furnace.

Leaks are not always a symptom of a bad connection in the PVC or drain pipes within the furnace. Recuperative coils have been known to be defective. If the water seems to be coming from deep inside the furnace, that must be fixed.

Finding water leaks is sometimes a nightmare. Water likes to travel down the most inconspicuous places. It leads one to think the leak is in one place while it’s in a completely different location.

Hopefully, your furnace is still under warranty if it’s a 90+ efficient furnace. The repair cost for recuperative coils can be quite expensive, If your warranty is good then even the labor will be high because it incorporates almost completely dismantling the furnace to get at the coil.

Conclusion

The installer created the leak in the image above (97% efficient furnace). A poorly fitted drain pipe at the trap inside the furnace. Not all defects, especially in systems like HVAC equipment, are defects made in a factory.

So many things in a good heating and air conditioning system are dependent upon a knowledgeable installer.

You may benefit by reading an article I wrote about what questions to ask when buying a new HVAC system.