Gas & Oil Furnace Leaking Water When Heat/AC is On: Why & What to Do

If your furnace is leaking water when the heat or AC is on, here’s what you should be doing to remedy the situation.

A furnace, whether gas or oil, is not supposed to leak water. This is why a lot of homeowners, probably including you, are at a loss as to what to do when it starts to leak.

This is why we’ve put together this comprehensive guide so you’ll know what to do when it starts dripping. This article can help you solve the problem and do preventive measures so that it won’t happen again.

We’ll talk about the different types of furnaces. More importantly, we’ll tell you about the different types of problems that they bring. In case you didn’t know what type you have, we’ll also help you do so. We’ll even discuss the differences between high-efficiency condensing and conventional furnace.

Why should you fix the problem? This type of leaking presents problems and dangers. We’ll tell you what they are to convince you that this is an emergency that you need to address now.

What should you do with water dripping from the furnace vent pipe? We’ll tell you exactly what to do. More importantly, we’ll show you how to fix a leaking furnace. Of course, this is not a recommendation for you to do it DIY unless you’re an experienced DIYer with the right tools and skillset needed for the job. We’ll tell you why it’s better to hire a professional.

It’s not enough that you fix the problem. You should also know how to maintain it to prevent a repeat. Hiring a professional can help you educate yourself on proper maintenance, but we’ll also give you maintenance tips that you can apply.

We’ve been talking about hiring a professional. The question is – how much? We’ll give you some numbers based on national averages, but we’re also recommending that you ask for quotes directly from contractors. On that note, we’ll tell you of a nifty trick that can give you around 4 of the best quotes that you can choose from.

Different Types of Furnaces – Different Types of Problems

Basically, there are 2 different types of furnaces:

  • High efficiency

Also known as condensing, you can easily tell if this is the type that you have by looking at the exhaust pipe. Is it a PVC pipe? If yes, then you have a high efficiency furnace.

Another way to check is to take a look at its Annual Fuel Utilization Efficiency rating. If it has a rating that’s 90% or higher, then it’s high efficiency. This is because furnaces that have this rating will have a cool exhaust and will condense.

Most of the newer models will display the Annual Fuel Utilization Efficiency rating. Look at the yellow sticker that shows the energy guide and the rating should be there. If there’s none, just check the vent pipe.

  • Standard efficiency

Otherwise, you have the standard efficiency type. You can double check. You’ll see that the exhaust pipe is made of metal. This means that it doesn’t condense. You can also check the yellow sticker and you should see that the Annual Fuel Utilization Efficiency rating is 89% or below.

High efficiency furnaces are prone to leaking due to condensation which is the usual reason behind the leaking. This type works by extracting the heat from the gas caused by combustion and they do this for longer than the standard efficiency variety. This cools down the gas which will eventually lead to condensation.

This shouldn’t be a problem in most cases. This is because the condensation goes straight to the drain. However, there could be issues that prevent this from happening like the following:

  • The tubing or drain is clogged.
  • There’s breakage in the condensation line.
  • There’s something wrong with the condensate pump.

There’s a different set of issues with a standard efficiency. Heat is quickly extracted and vented, so it’s unusual if there’s leaking due to condensation. If there is, it’s probably using a flue or exhaust pipe that has the wrong size.

In addition to condensation, here are the other possible reasons behind the leaking:

  • The humidifier that you use for the entire house could be leaking.

This is an issue if it’s connected with your furnace.

  • The drain system inside the furnace is clogged.

This can be an issue if the furnace has the same internal drain as your air conditioning unit. If the AC is running and there’s a clog, the water will back up to the furnace.

  • There’s a problem with the secondary heat exchanger.

This only applies with the high efficiency type. If it’s damaged, then you need to replace the furnace itself.

Whatever the potential issue could be and whatever type you have, it’s a must that you have it checked by a professional. A trained professional can spot the problem the right way. This can help because he can get started on solving the problem at once and the right problem will be solved.

Gas Furnace

It’s also important that we talk about the gas furnace. Unfortunately, a lot of homeowners still use their old furnace which either runs on gas or oil. This is not to say that a gas furnace is dangerous per se. There are good modern options offered by companies like Goodman and Rheem. It’s just that most of the older furnaces run on it and that can lead to a lot of dangers.

For starters, it can cause carbon monoxide poisoning. It can contaminate your home with carbon monoxide along with methane. Needless to say, this affects the quality of your air indoors.

It can also cause a gas leak or fire. Obviously, no homeowner wants this to happen. It’s very hard to come back after such events, especially if they will lead to loss of life.

High-efficiency Condensing vs Conventional Furnace

There’s some confusion between high-efficiency condensing and conventional furnace. Both of them bring in the cold air where it passes through the unit’s air filter. The unit’s circulating fan will circulate the air. Both also have a gas burner controlled by an electronic ignition.

furnace filter

One thing that differentiates high-efficiency condensing vs conventional is that the former has a secondary heat exchanger to complement the primary heat exchanger that’s also present in the latter. The secondary heat exchanger absorbs most of the residual heat that passes through the primary heat exchanger. This is why it can afford to use a plastic pipe for its vent.

The high-efficiency condensing type also has supplementary technology like a heat recovery ventilator that pulls in more fresh air. Air quality is also improved because most of the models come with a high-efficiency air filtration media.

They can also differ on the blower motor used. The conventional type typically uses a permanent split capacitor. While the condensing type also uses that, some models also use the electronically commutating motor that offers a direct current and variable speed features.

Another important difference is that the condensing type has insulated blower compartments. In addition, the condensing type also features a more reliable and efficient electronic ignition system.

Why is My Furnace Leaking Water When the AC is On?

If it’s only leaking whenever the air conditioner is running, then that should give you a good idea on what the problem is. It’s not the furnace. It’s your air conditioner that’s causing the leaking.

To better understand this, you have to know that the AC’s job is not just to cool the house. It also dehumidifies it. This is why moisture that’s caused by the humid air is absorbed.

This shouldn’t be a problem because the AC unit has a condensate drain line. This is where the moisture is drained outdoors. This can get clogged. There could also be a leak. This can leak water not just in the immediate area but in the surroundings as well.

The misconception that the furnace is leaking instead of the AC is because of how the drain line of the AC is positioned on or around the furnace. This is why you see water under the furnace.

Do the following to determine once and for all that it’s the AC that’s leaking:

  • Verify that the AC unit is running.
  • Take a look at its condensate drain pan and line. Try to spot leaks.
  • Once you find leaks, contact a professional to fix them.

In addition to the clogged drain line and damaged drain pan, here are the potential problems that need professional assistance:

  • The condensate pump is broken.

This is a consideration if the AC is in the basement together with the furnace. This setup requires a condensate pump. Its main responsibility is to pump water outdoors. Needless to say, it’s not going to do a good job at it if it’s broken. The water won’t be pumped outdoors which will lead to leaking.

  • The air filter is dirty.

A clean air filter will allow good airflow through the AC unit’s evaporator coil. If it’s dirty, then it will block the airflow. Without good airflow, the evaporator coil will be cooled until it freezes. The problem is when it starts to melt. The drain pan won’t be able to accommodate the water. This will lead to leaking.

This can be prevented by regularly replacing the air filter. You should be changing it every one to three months anyway.

  • The refrigerant level is low.

This has the same effect as a dirty filter. If you think that your air conditioner is not that efficient in cooling your home whether it’s winter or summer or you can hear noises that sound like hisses or bubbles, then your refrigerant level is probably low.

If you spot these issues, it’s time to call in professionals.

Water Dripping from the Furnace Vent Pipe

Here’s the bad news. It’s actually a very common issue. Your furnace, especially if it’s high efficiency, will eventually develop leaks. But don’t worry, because it’s just a condensation from the pipe most of the time.

Condensation can be caused by several factors. Here are the most common:

  • Size of the flue vent.

It can’t be too big. If it is, then a big quantity of air will be in circulation. The warm vapors will experience difficulties going out the vent because there won’t be enough updraft from the bottom.

This will lead to the warm vapors getting trapped, with cool air pressing downwards. The warm vapor will eventually cool which will lead to condensation and leaking.

  • Using an existing flue after upgrading the furnace.

It’s not unusual for homeowners to upgrade their furnace if they feel that the one they have isn’t efficient anymore. Majority of them don’t upgrade the flue in the process because it’s still in good shape. While it may seem like a logical idea, it can lead to problems later on.

The problem is it can’t keep up with the more efficient furnace. The flue becomes cooler, which can lead to condensation.

Generally speaking, leaking due to condensation is avoided by ensuring that the whole system works together into pushing the warm vapor out of the vent before it condenses. This won’t happen if something is wrong. The warm vapor will cool before it’s able to exit. This leads to the condensation.

While condensation is a simple issue that may seem harmless at first, it can lead to worse issues like damages to the flue vents and the furnace itself. You can do something like install a condensation trap but it’s better to just hire a professional for a more long-term solution.

Problems to Look Out for

Yes, there are several problems to look out for. Fortunately, not all problems require professional assistance. These problems can be solved by an experienced DIYer:

  • The Armaflex of the suction line is missing. The Armaflex serves as insulation so it’s going to be a problem if it’s missing. Fortunately, it’s just a matter of replacing it.
  • The drain line is not pitched as it should be. It should be sloped downward. It could have been moved.
  • The boiler drain is leaking. If it’s leaking from the bottom to on the floor, it’s probably the drain. It could be damaged or it could be clogged.
  • The condensate pump is somehow unplugged.

On the other hand, you need the assistance of a professional for these issues:

  • The humidifier has leaks. It can also be overflowing.
  • The pipe or its fittings could be damaged. Their joints may have separated.
  • There’s something wrong with the evaporator coil. It could be wrapped in dirt or worse, it could be damaged.
  • There’s something blocking the pump tubing.
  • There’s something blocking the condensate trap. You can also check if something is blocking its pan or drain. Speaking of the pan, it can also have cracks or damaged gasket.
  • The condensate pump is damaged.
  • The indoor coil is malfunctioning so it becomes frozen. The ice will eventually melt which will cause leaking.
  • There are damages to the tubing or other parts of the furnace.

When in doubt, always call a professional.

Furnace Maintenance

A furnace is one of those things that are not too demanding when it comes to maintenance. Unfortunately, this is why a lot of homeowners totally forget about it.

Maintenance is very important if you want to avoid expensive repairs in the future. In fact with proper maintenance, you could avoid the problems listed above.

You have to do it once a year during the fall season. This is perfect timing before the cold autumn starts to creep in.

Here’s how to do it:

  1. Shut it down first.

You need to turn off the power and the fuel supply. Turning off the power is just a matter of switching the switch from on to off. On the other hand, look for the shutoff valve to shut down the fuel supply.

  1. Clean the combustion chamber.

Obviously, this can get really dirty. It’s going to accumulate soot in addition to dirt since this is where fuel and air are mixed.

There will be carbon buildup. This is removed using a wire brush and a lot of elbow grease. An industrial-grade vacuum is then used to suck out loose particles. While cleaning it, make sure to also check if there are damages and rust.

  1. Inspect the flue.

Check if there are holes. Smaller holes can be covered using foil tape. If it’s corroded already, the flue needs a replacement.

If you hire a professional for maintenance, he will also inspect the barometric damper and adjust it accordingly. This controls the chimney draw.

  1. If you have an oil furnace, you need to replace the filter.

It blocks small particles so that they won’t clog the nozzle. This is important because a clogged nozzle can stop the furnace from functioning.

  1. Replace the air filter.

You have to know that the indoor air that you and your family breathe passes through here. This is why you have to replace the air filter regularly.

  1. Ensure efficiency.

Professionals use a combustion analyzer in calculating the efficiency of the furnace. He will also check the color of the frame produced. The oil nozzle will also be replaced. If you have a gas furnace, the burner tubes will be cleaned as well using a vacuum.

For best results, have a professional do the maintenance for you.

How Much Does it Cost to Fix a Leaky Furnace

There are several factors to consider but you can expect the repair cost to be anywhere from $20 to $1500. The national average is close to $300.

You’re in luck if you just have to replace the thermostat. Base cost is just around $20 although it can go as high as over $200. The sensor of the flame may also require repair or replacement, and that can cost as high as $250. The ignitors can be faulty. The cost is typically around $350.

Worst case scenario is the motor itself is damaged. The repair bill can go as high as $1500.

At this point, you have to consider whether you should repair or replace it. As a general rule, just replace it if it’s more than 15 years old. If the repair cost is about half of what a new furnace cost, it may be more logical to just buy a new one. You can also consider buying a new one if repair needs are becoming more and more frequent.

Why Hire a Professional

Don’t let the cost discourage you from hiring a professional. In most cases, it’s the more logical choice. Here are the benefits of hiring a professional:

  • You’re taking advantage of his expertise.

There’s no need to spend hours or even days trying to find the source of the leak or the problem. A trained professional can even tell you what the problem is just by explaining the situation and a quick check will confirm the issue. This way, he can do a proper diagnosis and provide the right solution.

A lot of homeowners end up spending more money because they make the issue worse. Since they don’t really know what to do, they end up creating more damage.

Also, every day of delay is a day of more damages caused by the leaking. This is why the expenses will pile up.

They then end up hiring a professional to fix the bigger issue. They end up spending more money than they would have spent if they hire a professional in the first place.

In addition, you get to prolong the life of your furnace. You don’t end up needing to buy a new one.

Getting Quotes from Competing Pros

You actually don’t have to spend a lot of money to hire a professional HVAC contractor. All you need to do is to ask for quotes from competing professionals. The easiest way to do this is to use the form that you see on this site that gives you access to our free service that will have competing contractors sending their best quotes for your consideration. Try it today!

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