Outdoor Furnace: When A Complete FLUSH Is Urgently Needed?

Outdoor Furnace: When Is A Complete Flush  Urgently Needed?

Many outdoor wood boiler owners are NOT AWARE that they must completely flush their outdoor boiler water at certain times. Here are the standards from industry experts:

1 - You must completely FLUSH your outdoor wood burning boiler at least every four years. If you have had your outdoor boiler for more than four years and you have never flushed it, you MUST do a flush now. (See below for a simple flush procedure.)

2 - If you have not properly treated your outdoor wood furnace water with Liquid Armor water treatment in the last 12 months, you MUST do a flush now.

3 - 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 waterline, and not above. Wow!  DO NOT BUY CHEAP IMITATION WATER TREATMENTS!!! It will destroy your boiler.

Our Liquid Armor water treatment was specifically formulated to protect your exact grade of steel with great precision and safety

- 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.

4 - If you do your annual sediment flush, and your water does not run clear, you must 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 stove!


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

2.) Remove all firewood, coals, and ash. Take this opportunity to completely scrape out the 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 some 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 HERE at OutdoorBoiler.com.

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 the pump back on. 

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

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

12.) Turn the pump off, and close the bypass at the 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 those are cheap imitations and can actually DAMAGE your boiler. (We literally discovered a series of products that destroy furnaces. Do NOT accept any cheap imitations!)

15.) Turn the 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 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.