When Is A Complete FLUSH Required?

When Is A Complete FLUSH Required for Your Outdoor Wood Boiler

(Ultimate Summer Shutdown Checklist Part 4 of 10)

An outdoor wood boiler must be flushed every four years at a minimum, but possibly more often if the boiler water is not properly treated.

Sediment Flush

First, please note that all outdoor boilers must have a "Sediment Flush" every year. This is not a complete flush, but rather just a simple 5-second flush from the bottom drain valve to remove any sediment buildup. 

The water will look muddy at first, but this should clear up within five seconds and look completely clear. If the water is still cloudy after five seconds, continue to flush until the water runs clear, even if this means allowing all the boiler water to drain out completely. Sediment will destroy not only your boiler but also your heat exchangers.

Please note - Do not completely drain an outdoor furnace that has any hot fire or coals in it. Remove all firebox contents (firewood, coals, ash) before a complete flush. This is not necessary for a 5-second flush.

Four-Year Flush

If you have properly treated your outdoor wood boiler water faithfully every year and obtained "SATISFACTORY" water analysis results from the OutdoorBoiler Lab every year, you only need to flush your wood stove every four years. You may want to remember this based on the presidential cycle, or every four years from the year you purchased your boiler.

If you have owned your outdoor wood stove for more than four years, and have never flushed it, NOW is the time to do so. 

Be sure to only fill your boiler with fresh water that has NOT been treated by a water softener. If you have a water softener in your home, please bypass the water softener when you fill the boiler. Softened water contains sodium (salt) and just imagine what road salt does to the steel on a car. If a water softener is used in the home, the laboratory will often be able to tell, because the water's condition will result in an "UNSATISFACTORY" test result.

Emergency Flush

If you have not treated your wood stove properly in the last 12 months, it is recommended that you flush your boiler immediately, regardless of the time of year. Follow the recommended "Flush Procedure" here.

If you have ever used a cheap imitation "water treatment", it is critical that you completely flush the outdoor boiler water now. PLEASE NOTE: Some manufacturers have found that their recommended water treatment ACTUALLY CAUSED DAMAGE to boiler steel.

We have seen this - corrosion occurs BELOW the waterline, not above. Wow! DO NOT BUY CHEAP IMITATION WATER TREATMENT!! It will destroy your boiler.

And with the current EPA ban on outdoor wood burning boilers, remember that once your boiler fails, you cannot replace it. Our Outdoor Boiler Liquid Armor water treatment was specifically designed to shield your exact grade of steel with maximum precision and safety. Click here to buy our Our Outdoor Boiler Liquid Armor.

Failed Test Flush

If you get an "UNSATISFACTORY" water analysis test result, you will be prompted to retest your water for the second time. If you fail a second time, you will be instructed to drain 50% of the water, refill with fresh water, and submit another sample. If you fail a third time, you should flush your boiler immediately. Click HERE for the recommended "Flush Procedure".

Antifreeze Exception

If you use antifreeze in your system, you will need to call our technical team for a more customized analysis of when and if you need to do a complete flush.

Keep your furnace HEALTHY. A freshwater flush when needed will ensure many years of saving money on your heating bills with your outdoor wood burner!


If you have any questions or concerns, Click here to contact OutdoorBoiler.com.