How to Drain a Gas Boiler

A man works on a boiler.
  • 1-3 hours
  • Beginner
  • 15-25
What You'll Need
Length of hose
What You'll Need
Length of hose

If you use a boiler to heat your home, you will periodically need to drain the boiler in order to clear out the accumulated sediment. Completing this task will make the boiler run more efficiently. However, you should be aware that a boiler doesn’t need to be drained too often. You should only drain it approximately once every year for maximum efficiency. If you drain it too often, you can introduce minerals that cause scale inside the boiler, as well as oxygen, which can eventually cause corrosion in the boiler.

Step 1 - Make the Boiler Safe

Before starting the drain, you will need to make the boiler safe for the operation. To do this, shut off the water supply either at the tap on the water line or at the mains.

Next turn off the power to the boiler. This will ensure you don’t burn out the heating element during the draining.

Finally, shut off the gas to the boiler. This isn’t absolutely necessary but stands as an extra precaution. When the power to the boiler is shut off, the gas shouldn’t fire up. However, turning it off affords an extra layer of security.

Step 2 - Attach the Hose

You will see a water gauge for the boiler’s water level, a PSI gauge for the pressure of the steam, and a low-water cutoff relay. You will also see a pressure valve, a spigot and a larger valve, which is the surface blow-down valve. You will be using them all in the draining process.

Start by attaching the length of hose to the spigot with the other end of the hose running into a large bucket. The bucket is where the water will drain. Open the pressure valve with a wrench and turn the spigot to drain the water from the boiler.

Step 3 - Drain the Sediment and Refill the Boiler

The water that drains from the boiler will almost certainly be a dark color. Allow it to drain completely before you close the spigot. Empty the bucket outside and disconnect the hose. Turn the water back on and allow the boiler to fill to its proper level.

Place the bucket under the surface blow-down valve and open it with the wrench. Allowing it to drain will take out the remainder of the sediment and dirt that has built up in the boiler since its last cleaning. Expect plenty of sediment, which can be quite thick in appearance. Close the valve once the water from the boiler begins to run clear. Empty the bucket outside.

Step 4 - Turn the Boiler Back On

Before turning the power to the boiler back on, use the wrench to close the pressure valve. Turn the gas back on and light the pilot according to the manufacturer’s instruction, which should be located on a sticker on the boiler itself. When you’ve accomplished this, turn the power back on.

It will take a while to heat the water in the boiler after draining. You will not have heat during that period.