How To Completely Flush Your Outdoor Wood Boiler?

An outdoor 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 build up.  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 a boiler 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 boiler water faithfully every year, and obtained "SATISFACTORY" water analysis results from the Hawken Lab every year, you only need to flush your boiler 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 boiler for more than four years, and have never flushed it, NOW is the time to do so. Follow the "Flush Procedure" below.

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. Many times, our lab can tell when a customer has a water softener because our sensitive lab equipment can measure this, and the result is an "UNSATISFACTORY" test result.

Emergency Flush

If you have not treated your boiler 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" below.

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 discovered that the water treatment they themselves recommended and sold, ACTUALLY CAUSED DAMAGE to boiler steel. We have seen this - corrosion occurs BELOW the water line, and not above. Wow!  DO NOT BUY CHEAP IMITATION WATER TREATMENT!! It will destroy your boiler.

And with the EPA ban on outdoor boilers, remember that once your boiler fails, you cannot replace it.  Our Outdoor Boiler Liquid Armor water treatment was specifically formulated to protect your exact grade of steel with great precision and safety

Failed Test Flush

If you receive an "UNSATISFACTORY" water analysis test result, you will be advised to retest your water. If you receive a second failed test result, you will be given instructions on what to do next - drain 50% of the water, refill with fresh water, then submit another water sample. If you receive a third failed test result, you should flush your boiler immediately. Again, follow the recommended "Flush Procedure" below.

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.


If you have any questions or concerns, please do not hesitate to contact via any of the following contact methods at this link HERE.

 Flush Procedure

1. Allow fire to completely burn out and allow water jacket temp to drop to 80 degrees or lower.

2. Remove all firewood, coals and ash. Take this opportunity to completely scrape out firebox of all ash and creosote that removes easily.

3. Connect a hose to the bottom drain valve of the boiler. This is found in the bottom rear of the boiler so as to be at the lowest point of the water jacket. Direct the hose to where you want the boiler water to drain. Note that GX Series boilers have two drain valves and both should be flushed during this process. 

4. Turn off the pump. The power to the furnace can be shut off from the breaker if desired to achieve this.

5. Clean the filter cartridge. Put filter assembly back together completely. Replace filter cartridge as needed - it is a stainless steel mesh screen and is reusable, but will not last forever. Replacement filter cartridges are available at

6. Open the drain valve. Allow the water to completely drain out of the boiler. This will take some time. Take note of how many seconds the water looks muddy - it should only look like muddy river water for a few seconds and then turn clear.

7. Once completely drained, close the drain valve(s).

8. Refill the boiler completely with fresh water. Bypass any water softener. Do not add water treatment to this water.

9. Turn pump back on. 

10. Open any bypasses around heat exchangers, and the filter.

11. Allow pump to circulate water for a minimum of 24 hours. Make sure all ball valves are open so that this fresh water flushes out all pipes.

12. Turn pump off, and close bypass at filter.

13. Drain boiler again by opening drain valve(s).

14a. Refill the boiler completely with fresh water. Bypass any water softener. At this time, be sure to ADD Liquid Armor water treatment to this water according to proper specifications - ONE gallon of water treatment for every 200 gallons of water jacket capacity. We recommend one gallon of water for every 150 gallons of water jacket capacity for the initial fill.

14b. Liquid Armor Water Treatment is available at this link HERE - do NOT use any other water treatment product as none of these are authorized for use in your boiler, and some have even been found to INCREASE the rate of corrosion. (We literally discovered a series of products that destroy furnaces. Do NOT accept any cheap imitations!)

15. Turn pump back on and allow this to circulate the water for a minimum of 24 hours.

16. Take a water sample and submit it to the Outdoor Boiler Lab for FREE testing. In approx. 4-8 weeks, you will receive your water analysis test results via email (don't forget to put your email address on the water sample bottle). Most test results come back "SATISFACTORY". Follow the test results instructions, if any.

17. For more information on FREE water analysis and testing, click HERE.