Why You Need to Drain Condensate for High Efficiency Furnace?

High-efficiency furnaces have two heat exchangers, primary and secondary. The purpose of the second heat exchanger is to get more heat from the exhaust before it is vented. As a result, high-efficiency furnaces require condensate drains because their exhaust gases cool enough to allow the moisture in them to condense from steam into water droplets. Yes, water is a byproduct of the combustion/burning of natural gas and propane.

The chemistry equation of the burning of natural gas molecules (methane/CH4) is: 

\(CH_{4}+2 O_{2} + heat = CO_{2} + 2H_{2}O\)

As a result of condensing from steam to liquid water in the secondary heat exchanger, the moisture isn’t vented, and so it must be drained away. Oh, and the moisture is acidic - that’s an important part of the story too. 

That’s the quick, short answer. Read on for the rest of the explanation for why 90% furnaces need condensation drains. Much of the content takes the form of a High Efficiency Furnace and Condensate Drain FAQ.

What is a High Efficiency Furnace?

A high efficiency furnace is any furnace that has an AFUE rating of 90% or higher. 

Terminology Tip: AFUE is an acronym for Annual Fuel Utilization Efficiency. The way that the AFUE is determined is by measuring the heat output of a gas furnace compared to the amount of fuel that it burns during that time.  

For example, let’s say you’ve got a 100,000 BTU/hour furnace that is 93% efficient. Its burner creates 100,000 BTUs per hour of running - though, of course, your furnace rarely runs for an hour straight. The 100,000 BTU number is called its “input.” But because it is only 93% efficient, 93,000 BTUs of heat get pushed into your home. This is called the furnace’s “output.” The other 7% of the heat is lost along with the vented combustion gases.

Did you know? Every furnace is required to have a yellow sticker that displays the furnace’s efficiency rating.

High Efficiency and Condensing Furnace: Is there a difference?

No. High efficiency furnaces are also named condensing furnaces due to the condensate that is produced when the furnace is running.  A high efficiency furnace uses a second (aka secondary) heat exchanger to transfer the air from the exhaust (condensed) gases to reach a higher efficiency rating.  This type of furnace requires a special type of venting as well as a condensate drain with the condensate drain lines running the drain.

Today’s most efficient gas furnaces from all top makers have ratings of 97% or higher. Lennox makes a furnace with near-99% efficiency.

What Is a Condensate Drain?

The condensate drain system includes a drain pan, drainpipe and, of course, some type of drain or pump. If your home has a floor drain and the furnace is in the basement, the drain line can empty into the floor drain. If there’s no floor drain, then the condensate flows into a reservoir that is emptied by a condensate drain pump.

The system is also used to drain away condensation on the evaporator coil in the furnace used for air conditioning.

Since a high efficiency furnace produces so much condensate, a condensate drain is necessary in order to safely remove the acidic water (condensate) away from the furnace and home.

Clogged condensate drain problems: A leaking furnace or air handler is one of the more common maintenance-related problems homeowners deal with. Condensation carries debris to the pan where it can also develop mold or algae, and the result is that the pan drain becomes blocked, and the water overflows the system. The Pick HVAC pros explain how to unclog a condensate drain line

How Much Condensate Does a High Efficiency Furnace Produce?

A 90% or higher efficiency furnace produces around 0.8 gallons of condensate per hour of running.  This equates to about five or six gallons per day of acidic water from condensation each day.  Most of the time, the drain and drain line runs to outside the home and the condensate just seeps into the ground, harmlessly.  

What Does it Mean that the High Efficiency Furnace Condensate is Acidic?

Carbon dioxide and H2O are byproducts of gas combustion. In the exhaust, they combine to form carbonic acid. On the 14-point pH scale the condensate from a high efficiency furnace will usually range between 3.0 and 5.0, but it can be more acidic.  To further explain how the 14-point scale works, 0 is at the far acidic end and 14 is at the far alkaline end.  A pH level of 7 is a considered neutral solution.  

The acidic condensate that is produced by a high efficiency furnace can cause damage to the plumbing in your home due to its corrosive nature.  Older plumbing (pre-PVC) will need to be replaced if you have a high efficiency furnace.  There have been some technological advancements for neutralizing the acidic condensate.  There are some “condensate neutralizers” that are acid-resistant plastic tubes filled with limestone and it lessens the acidic properties of the condensate. 

Because the condensation is acidic, the secondary heat exchanger in a high-efficiency gas furnace is always made from stainless steel to prevent corrosion. Any pipes or vents the condensate may come in contact with must be PVC or stainless to prevent corrosion issues.

What Do You Do If Your High Efficiency Furnace is Leaking?

If you notice water leaking from your furnace or pools/puddles on your floor, this indicates that the condensate might have caused corrosion in the drain pan, some of which are made from galvanized steel rather than PVC plastic. It could also be a simple clog. Either way, the issue should be immediately addressed – the pan replaced with a non-corrosive pan and/or the pan cleaned and the drain line flushed. If the issue happens when the furnace is heating your home, rather than when the AC is running in summer, then the water will be acidic and can damage whatever it comes into contact with.

How Do You Maintain a Condensate Drain?

Proper maintenance for a condensate drain is very important in order to keep it working as it is supposed to.  Here are the necessary steps that all homeowners should do if they have a high efficiency furnace:

  • The trap in the drain should be cleaned out two to four times a year. Simply rinse it out and make sure that the water flows through the trap easily. If the water doesn’t flow smoothly you will need to clean the drain with hot water.  Difficult blockages may require bleach or white vinegar. Bleach will also lessen the possibility of mold in the drain, but it can damage any vegetation outside your home that the drainage water makes contact with.
  • The drain lines that run to the condensate drain should be inspected regularly too. The best drain lines for a condensate drain are flexible hoses, but some also use PVC piping.  Any leaks of water may indicate that a line should be replaced.

How Do You Clean Your Condensate Drain?

Cleaning your condensate drain on a regular basis is a prudent way to ensure that everything on your high efficiency furnace is working properly. Below are the steps that one should follow in order to properly clean a condensate drain:

  1. Turn off the main power to your furnace by switching off the breaker box.  Also, turn off your thermostat.
  2. Next, locate the condensate drain pan.  It will most likely be located behind a removable access panel on your furnace.  Check out your owner’s manual, if needed.
  3. Check the condensate drain pan to see if there is any standing water in the pan. Standing water in the drain pan will indicate that the drain pan or line is clogged.  Clean up any standing water with a rag, mop, or a wet/dry shop vac vacuum.
  4. Using soap and water, completely clean the condensate drain pan.
  5. Find where the condensate drain outlet is located.  If it is clogged a wet/dry vacuum can often clear the blockage.  If needed, a plumber’s drain snake can be used to clear the drain outlet.
  6. Finally, flush the drain with a cleaner, such as white vinegar, diluted hydrogen peroxide, or hot water with some dish soap. Allow the cleaning agent to stay in the drain for at least 30 minutes. After 30+ minutes, flush the drain with water and make sure that the drain is working freely.

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