How to Drain a Boiler? Simple Guide + Benefits

Boiler maintenance is often neglected until the boiler breaks down. However, this can be quite costly as it could cost thousands to fix or replace a broken boiler. 

Most homeowners are not aware that they need to manually drain their boiler every few months. If you don’t, the water inside may develop mold or corrosion which could lead to expensive repairs down the road.

In this blog post, we will provide you with some tips on how to drain a boiler.

Why should you drain a boiler?

So you bought a new house and the boiler is okay but probably old such as steam or hot water heating system. You know you need to replace it, but before you do so, you should drain the boiler. Why would anyone want to drain a boiler? Well, here are some of the reasons why this may be necessary:

Signs It’s Time to Drain Your Boiler

  • The pressure is too high and needs to be released.
  • There may be some sediment that has built up in your tank which could clog up pipes and valves if it’s not cleaned out regularly.
  • Your boiler may have had some leaks which also need to be drained because any wet areas can turn into mold if they’re not dried out quickly enough.

There are other reasons why draining your boiler may be necessary such as to get rid of any debris which has built up in it or if you’re switching from oil or gas heating to electric because the pipes need to be cleaned out and the fumes will need clearing out afterward before turning back on.

Here’s how to drain a boiler:

  1. Turn off the main water supply to the boiler
  2. Drain the system through the tap on the cold-water storage cylinder until no more steam comes out of it – this should only take a few seconds as pressure should be reduced in the system. If there is a lot of air left in the system, use a length of hose down into any sinks or buckets and discharge that into them instead. 
  3. Wait 15 minutes before removing any remaining air from your Type ‘B’ pressure vessels by slowly opening one of its taps and allowing water to discharge directly into buckets/sinks again – don’t fill these vessels above the highest tap or any valves that are fitted higher than the lowest tap.
  4. Once you’re sure there’s no air left in your Type ‘B’ pressure vessels, turn the water back on to them and open each one until no more steam comes out of it – this should be at least 20 minutes with a system boiler, so if it isn’t using a length of hose down into any sinks or buckets and discharge that instead.
  5. Open up all other taps and allow the boiler to refill its system before operating normal heating again. If you have a combi boiler or don’t want to keep going around doing this every time, get an installer to install a high-level switch for the oil boilers, gas condensing boilers, or a high-temperature limit switch for the electric ones.

Benefits of Draining a Boiler:

Here are the benefits of draining your boiler:

1. Removes sediment build up in the bottom of a boiler that may reduce water heating efficiency

2. May allow you to use a lower-cost heating oil that can be dirtier and requires more frequent draining

3. Removes sediment that may be blocking the steam vents of a superheater, resulting in low steam pressure or no steam.

4. Removes water that has a high mineral content and may cause the heating surface to deteriorate as it can form a crust over time.

5. Removes water that has been overheated and may cause corrosion to the heat exchange surface

6. Removes water that has been left in the boiler when the fuel tanks have run dry and may result in damage to your oil burner or steam boiler

7. Removes water that has been heated to a very high temperature which produces alkaline conditions that can damage heat exchange surfaces

8. Removes water that has been heavily soiled with ashes or cinders that can cause damage to heat exchange surfaces

9. Removes water that has been heated to the point where it creates an acid condition that can cause corrosion.

10. Removes water with a very high lime content that can accumulate on heat exchange surfaces and cause corrosion

11. Removes water with a high nitrate content which results in corrosive conditions

12. Removes water with a high ammonia content which can cause corrosion problems

13. Removes water that has not had the opportunity to dry out after a cool spell and may contain a large number of dissolved gases which can damage the heating surfaces

14. Removes water that has been sitting in a low spot of a boiler and has not circulated, creating an environment where rusting can take place

How often should you drain your boiler?

It’s best to drain your boiler while heating system experts recommend that it should be done annually over the winter. Pipes, valves, and insulation can reduce water flow, so draining an annual supply of sediment-heavy water is a good idea. Another huge benefit is the elimination of mineral deposits or lime buildup in your pipes and valves, which can lead to serious heating problems. By adding a couple of gallons of vinegar before draining the boiler each year will prevent mineral buildup in both pipework and radiators! 


This blog post has shown you how to drain a boiler and we hope that it helps the next time your heating system needs service. If there is anything else we should write about, please let us know!

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