Troubleshooting: Don’t Forget About the Furnace

There are two potential ways a furnace can deliver extra water and cause problems for an onsite system

Since the focus of this series is on potential sources of extraneous water, I am trying to highlight areas where water can be added to the system in large enough quantities to negatively affect system operation. One of those areas often overlooked is the furnace. There are two potential ways a furnace can deliver extra water that can cause problems for the septic system: by running with an automatic humidifier and with the condensate from the operation of high-efficiency furnaces.

If you live an area like I do in Wisconsin you are painfully aware your home is dry for several months out of the year, which can result in effects ranging from static electricity that causes shocks every time you touch someone to nosebleeds and other health issues — not to mention the drying out any wood flooring in your house. Many people have installed a whole house humidifier to operate with their furnace during the cold months. These humidifiers utilize the air heated by your furnace to disperse humidity to your entire home.

There are two basic types: evaporative and steam humidifiers.

With an evaporative humidifier, warm air passes through the humidifier's evaporator pad, a ceramic-coated pad saturated with water. The hot air absorbs that moisture and moves it throughout your home. Evaporative humidifiers are powered by the furnace's fan and so must be installed in the ductwork. These are known as bypass units, meaning they are installed on the cold-air return. Air is supplied to the humidifier from the furnace's outgoing warm-air supply. During warmer months when your air conditioner is running, a damper in the bypass vent pipe can close off airflow through the humidifier.

A steam humidifier operates by heating water in a canister and converting the water to steam that is then forced through the ductwork. Although these units also are installed in ducts, many do not need to be connected to your furnace to run. These fan-powered models are installed directly on the warm-air plenum. The built-in fan pushes the moisture into the outgoing warm-air flow.

The ones that concern us are the evaporative type, although the ratings on a gallon per day basis would not seem to be a problem. Ratings depend on the area of the house and the humidity setting, from 1-2 gallons per day to 10 gallons per day. Any excess water does go to the floor drain, and those with self-cleaning features can potentially deliver higher quantities every time the cleaning function is used. So if the homeowner has this feature, suggest that a furnace professional check to make sure the unit is operating properly — and not delivering excess water to the septic system — and that the shut-off valve or damper is working. If it needs replacement, the steam humidifier might be a good option.

The second way furnaces can deliver excess water is condensate. With the way high-efficiency furnaces operate, the amount of condensate delivered usually is not cause for concern; it’s a few gallons per day depending on operation. The major problem seen with the condensate is not the amount of water, as the floor drain does not go to a sump where it mixes with other water but goes directly out the house sewer. Since only small quantities are delivered, in cold regions this slow, intermittent drip can freeze in the pipe, causing blockage over time due to ice buildup. The solution is to install a sump or container to store and deliver higher amounts of water (a gallon or two).

About the author: Jim Anderson is connected with the University of Minnesota onsite wastewater treatment program and is an emeritus professor in the university’s Department of Soil, Water and Climate. Send him questions about septic system maintenance and operation by sending an email to


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