Discharge from High Efficiency Furnaces, Water Softeners and Iron Filters into Onsite Wastewater Treatment Systems

Questions often arise regarding water use utilities that discharge clean water into onsite sewage treatment systems.  In response to this issue, the University of Minnesota has published a paper outlining potential problems and solutions:

The Problems with High Efficiency Furnaces, Water Softeners and Iron Filters Discharging into Onsite Sewage Treatment Systems High Efficiency Furnaces

High Efficiency Furnaces – These furnaces operate at a high efficiency and therefore save on energy use. One of the results of the heating process is that condensation occurs in the unit. When this condensation builds up water slowly trickles out of the unit and into the plumbing that is often connected to an onsite system. This water can cause freezing problems in the onsite system because of the slow steady flow. In addition, this water is clean and therefore does not need to be treated. When the furnace is in operation this water typically trickles out of the unit totaling 5‐10 gallons on a cold day.

Water Softeners and Iron Filters – Water softener and iron filter recharge water adds a large volume of water to the system ‐ typically 30 to 80 gallons per cycle. This is water that does not require treating. A growing concern with water softener recharge water is that it may cause an increase in the amount of solid material staying suspended in the liquid layer (effluent) in the septic tank ending up in the drain field trenches or a mound. These solids may shorten the life of the soil treatment system increasing the chance of drainfield or mound failure. This water softener discharge concern has conflicting results in research studies, but it does appear that scum layers are often absent in tanks where the water softener recharge water enters the septic tank.


  1. Be sure to check with the local government unit before any changes are made to the onsite system.
  2. Route your furnace, water softener and iron filter discharge out of the onsite system.
  • It can be directed into the collection system of footing drains. This may require the installation of a sump in the basement to collect this water.
  • This water can be day‐lighted to the surface as long as it does not directly discharge into a water body or it can go into an existing drywell or abandoned drain field. If it is day lighted remember that this water contains salt and can be hard on vegetation.
  • Install a small separate section of drainfield to deal with this water (no tank is needed). In most cases 20‐50 feet should be sufficient.
  • If only the furnace water is being added this can go into the onsite system but a sump or other device to collect the water must be used so water is not trickling out, causing freezing problems.
  1. If rerouting is not an option, a good solution for everyone is to minimize the amount of salt and water used by the softener or iron filter.
  • Reduce the total volume of water used in the home
  • Adjust the water softener or iron filter to recharge less frequently. Adjusting the frequency can be done by lengthening the time between recharges on a timed unit or increasing the volume of water passing through the unit before recharging on a metered unit.
  • Add heat tape in the pipe receiving the furnace discharge. This will help prevent freezing.

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