Gas Furnace Leaking Water: Is That Considered Normal? Why Does It Happen?

Gas Furnace Leaking Water

Gas Furnace Leaking Water

Why is my gas furnace always leaking water? Is that considered normal? This is a question we hear throughout the year, and the answers vary depending on the type of furnace, furnace orientation (upflow, downflow, and horizontal), location of furnace, and whether or not an air conditioning cooling coil is installed. This article lists a number of reasons why a gas furnace leaking water is not normal, and Sun-Aire Comfort System will identify and correct that problem for you.

Types of Gas Furnaces

Gas furnaces are generally divided into two types, non-condensing and condensing. If your furnace has a metal gas exhaust pipe it is non-condensing, if there is a PVC gas exhaust pipe it is condensing. Older non-condensing furnaces had design efficiency as low as 65%, with current models at a minimum of 80%. Condensing furnace efficiency start at 90% and go up from there, with the Carrier Infinity furnace having one of the highest ratings at 98.5%. Regardless of the furnace name or efficiency, all gas furnaces can leak water.Water in Non-condensing

Gas Furnace

A gas furnace leaking water can occur regardless of what type furnace your have. Non-condensing furnaces have two classifications, Standard-Efficiency and Mid-Efficiency. Standard-Efficiency furnaces use natural draft (non-mechanical) to deliver combustion gases to the atmosphere, with exhaust stack temperatures between 350*F and 450*F. Mid-Efficiency furnaces use a flue fans assist to help with the natural, with stack temperatures between 275*F and 300*F.

During combustion of natural gas, more than two pounds of water vapor is produced for ever one pound of gas that is burned. With approximately 20,550 Btu’s in one pound of natural gas, a 100,000 Btu input furnace would produce about 10 pounds of water vapor for every 60 minutes of combustion. That is a lot of water, and both classifications of non-condensing furnaces are designed to push that water vapor out the top of the gas exhaust pipe.

Most Standard-Efficiency non-condensing furnaces use natural draft to push gas exhaust from the combustion chamber (heat exchanger) to the outdoor atmosphere. The exhaust gas contains a significant amount of water vapor, carbon dioxide, carbon monoxide, and small amounts of nitrogen dioxide and sulfur dioxide. The warm gases and vapors are between 350*F and 450*F and should create a strong updraft and exit the exhaust pipe, we call this negative pressure.

Sometimes the wrong conditions exist and the water vapor cools down rapidly in the exhaust pipe, immediately condensing and producing water moisture. Instead of warms gases and water vapor rising, cold air is falling in the pipe, we call this negative pressure. Over a period of time the moisture and exhaust chemicals remaining can damage the exhaust pipe, furnace components, and the furnace heat exchanger. When carbon dioxide mixes with water it creates carbonic acid, a weak acid that causes corrosion over time. The damaged components can reduce furnace efficiency, be costly to repair, and involve life safety issues.

Conditions that interfere with the non-condensing furnace’s ability to move all combustion gases and water vapor out of the exhaust pipe are due primarily to positive pressure in the pipe that is just enough to overwhelm the initial negative pressure. There are two primary conditions that cause positive pressure, (1) exhaust pipe restrictions, and (2) exhaust pipe heat dissipation.

Exhaust Pipe Restrictions will cause the pressure in the flue to rise, creating positive pressure and inhibiting proper exhaust. Improper flue exhaust can result in your furnace leaking water. Some causes of Exhaust Pipe Restrictions are,

  • The flue cap is missing.
  • Improperly installed vent connector that reduces exhaust velocity.
  • Flue obstructions, which can range from critter nests to improperly, bent sheet metal at flue pipe joints.
  • Flue pipe diameter is too small for the Btuh capacity of the furnace.
  • The flue pipe horizontal and vertical length is too long for its diameter and furnace capacity.
  • Too many elbows are installed in the flue pipe.
  • Flue pipe rise is less than 1/4” per foot. The pipe cannot be installed level or sloping down in the wrong direction.
  • The vertical to horizontal pipe ratio is not 1:1. The lineal feet of vertical and horizontal pipe should be the same.
  • Top of the flue pipe is not high enough above the roof.
  • The flue pipe is installed too close to a vertical wall.

Exhaust Pipe Heat Dissipation is governed by one of the laws of Thermodynamics, heat seeks cold. The following conditions cause the exhaust gas heat to lose too much heat before reaching the atmosphere, so the gases and water vapor cool too rapidly and condense in the pipe.

  • The furnace is too small for the size of the exhaust pipe, so the combustion gases and water vapor have a low velocity.
  • If single wall exhaust pipe is installed in unconditioned spaces such as a crawl space, attic, or cold basement. Exhaust pipe should be B-Vent (double walled metal pipe) when installed in unconditioned spaces.

Mid-Efficiency furnaces with a fan assist can strike out for many of the reasons listed above under Exhaust Pipe Restrictions and Exhaust Pipe Heat Dissipation. Most technicians consider a Mid-Efficiency furnace with a fan assist as a natural draft furnace. The fan assist is meant to help with the natural draft, not pressurizing the exhaust pipe with a power vent fan found in condensing furnaces and discussed below.

Water in Condensing Gas Furnaces

Condensing Furnaces, also known as High-Efficiency Furnaces are generally in the 90% to 97% efficiency range. A number of furnace manufacturers are reaching 98% efficiency, such as the Carrier Infinity at 98.5%. As noted in the beginning of this article, a High-Efficiency Furnace was have a PVC exhaust pipe, and a power exhaust fan that pressurizes the exhaust pipe.
As with the non-condensing furnaces, the condensing furnace has a primary heat exchanger that captures most of the heat generated during gas combustion. The condensing furnace has a high efficiency because of a secondary heat exchanger and sometimes a third heat exchanger. The primary heat exchanger passes its combustion gases and water vapor to the additional heat exchange(s) where the laws of thermal physics make increased efficiency happen.

The secondary heat exchanger reclaims the latent heat of condensation, which occurs when the condensing flue gases containing water vapor change back to liquid water. This is called latent heat of condensation, in which latent or hidden heat is given off when the water vapor turns to water, but remarkably no change of temperature happens. Due to the capturing the latent heat of condensation, the stack temperatures range between 110*F to 120*F, roughly 300*F less than those of Standard-Efficiency furnaces.

High-Efficiency condensing furnaces turn the majority of combustion water vapor to water, and use tubes and piping to remove the water. Inside the furnace there are two drains, (1) one from the secondary heat exchanger (2) and a drain from the flue gas exhaust pipe. The drains inside the furnace deliver the water to an internal condensate trap, and that trap attaches to external condensate pipe.

The condensing type furnace has another feature that differentiates it from non-condensing furnaces, how it gets combustion air. Non-condensing furnaces have slots in their front covers to provide combustion air. High-Efficiency furnaces have a somewhat closed and sealed system that brings combustion in through a specific opening. Pipe is attached to the combustion air opening and should, in the majority of installations, extend outside the building to bring in fresh air for combustion. Piping outside air into the furnace can bring moisture into the cabinet, a problem that magnified if the piping in not installed properly.

A majority of furnace manufacturers today produce what is referred to as multipoise units, meaning they can be installed in different positions. The different positions for furnace installation are upflow, downflow, and horizontal flow, which designate the direction of the air as it leaves the furnace. As an example, a downflow furnace will use a blower (a squirrel cage type fan with a motor) to push the air down and into the duct system under a floor.

During the past few years the trend has been away from furnaces designed to blow air in just one direction, in order to produce the multipoise units noted above. As with most manufacturers, Carrier Corporation multipoise furnaces come out of the factory oriented for up flow. Based on the make and model, the upflow units require slight modifications to be installed in the downflow or horizontal flow.

In case you have a recently installed High-Efficiency furnace that is oriented in the downflow or horizontal flow position, it is possible the installer failed to modify the condensate trap location and collector box. The modification requires relocating the unit internal condensate trap, unplugging one condensate collector box and plugged another one, and rerouting condensate tubing. If the trap is not relocated or incorrectly relocated, the furnace will leak water. In some cases condensate will end up in the induction fan, which is the power exhaust fan that pressurizes the exhaust stack, and a sloshing water sound can be heard. If the induction fan is inundated with water, it will not operate correctly and the furnace will shut down.

If the condensate trap is in the right place, the outlet of that trap must have a connection to deliver the water to the outside of the furnace. There are several components for this outlet trap connection, with the rubber elbow and open spring clamps being the most important. Without outlining the details, the elbow and clamps fit into a very small and tight area, and sometimes the clamps are misplaced. In short, a leaky 1/2” elbow can be the end result, which means you will have water in the furnace.

The PVC exhaust pipe and combustion air pipe must be installed properly, or they will introduce water into the furnace, and in some cases can take the furnace off line. As with most of the problems found in all types of furnaces having water in them, the blame can most often be placed in the failure to follow the manufacturer’s instructions by the contractor doing the job.

If the PVC exhaust pipe has a section that is installed horizontally, it must be graded 1/4” per foot back to the furnace, collected in the collection trap and tubes, and then sent outside the unit. Grading the horizontal section of the exhaust pipe away from the furnace will allow the reverse grade to act as a trap and collect water. If enough water collects in this trap, it will eventually slug the furnace, overflow to the induction fan, and shut down the furnace.

Combustion air from outside of the building is better for your furnace, its evaporator coil, and air infiltration into your house, all subjects of future articles I will write. The piping for combustion requires something at the top, and something at the bottom. If combustion air piping does not start at a concentric vent kit or sidewall kit, then it will penetrate a roof or wall, and must have a set of elbows look down to prevent water from rain or sprinkler systems from entering the pipe. If water does enter the combustion air pipe, there should be a tee installed at distinct locations to prevent water from entering and leaking from the furnace.

Gas Furnace Leaking Water From Evaporator Coil

An upflow gas furnace will have the evaporator coil installed on top of it. The evaporator coil is the indoor coil for air conditioning. During the cooling cycle the evaporator coil removes humidity from the interior space. The humidity becomes water inside the evaporator coil and the water leaves the coil through a pipe that exists outside of the house. If the evaporator coil is not properly installed and the drain pipe configuration is not per the manufacturer requirements, the coil can leak water into the furnace. We will discuss this in another article.

The conclusion of the matter is this, if your furnace is leaking water call a professional. In the Springfield, Missouri area, call Sun-Aire Comfort Systems at (417) 352-0975. Eliminating water from coming into, lingering in, and leaking from your furnace, will extend the furnace life and make it more efficient. A gas furnace leaking water is not normal, and can be corrected quickly. In the Springfield, Missouri area, call Sun-Aire Comfort Systems at (417) 352-0975, for timely and reasonable correction of your gas furnace leaking water.

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