How To Fill A Boiler System With Water (3 Easy Methods)

If you are living in an extremely cold place where you can’t even find or get wood to burn to make the inside of your house warm enough, a boiler system can be a lifesaver. It keeps your central heating system intact. But as a boiler system saves you from the cold you need to save it too by regular maintenance and keeping the pressure between 1 to 2 bars. You need to learn all the tips and tricks to keep it running like how to fill it up or drain it to clean. 

To fill up the boiler, turn off the power supply of your boiler. Drain it out completely, vent the air from the radiators, let it cool, then turn on the valves underneath the boiler until the pressure rises to 1.5 bar in the pressure gauge, and turn the valves off. Now you can turn on the boiler to heat your house. 

How To Fill A Boiler System With Water

All these steps require some plumbing experience because they can be tricky at times as there are lots of adjustments that need to be done during the whole process. Or otherwise to have to call your plumber and pay them money to help you with the simplest tasks that anyone could have done basically.

How To Fill A Boiler System With Water:

How To Fill A Boiler System With Water

Step 1

The first thing you have to do is shut off all those radiators, and vent the air out starting from the downstairs and then the upstairs ones until they are fully bled off. Turn the key clockwise to shut the air bleed off on the radiator. Let the system cool down fully before proceeding with the next step. 

Step 2

Now go back to the airing cupboard area. There will be a feed valve to the system that you must have turned off earlier, so turn that on now and push out any air from it. After ten minutes turn that off. 

Step 3

When you are done venting it’s time to open the feed valve unhitch will let the water through and increase the pressure until the pressure gauge bar reaches between 1.5 to 2 which is the ideal pressure for any boiler. Then close it down and turn on your boiler like usual. 

How To Drain A Boiler And Refill 

Draining, refilling, or checking down the pressure system of a boiler is a fairly easy task to do but you may require to have some previous plumbing experience to get this job done. But first, you need to gain some knowledge about how the pressure system works which we already discussed earlier. Now let’s get to the main discussion point:


Before you drain out the system, go around and turn all the radiator TRVs on fully. So they’ll all drain out nicely. Now turn off the power to the boiler and open the front cover. Now turn off the power button. Turn off the power of the ball switch to close the front cover. To isolate the boiler from the rest of the heating system you also need to turn off the drain valves with a four-millimeter Allen key. The slot will be pointing across the pipe. 

Now attach an eight-millimeter hose to the drain valves and use a bucket to catch the water. Open the drain valve using an eight-millimeter ratchet spammer. When you do that the water will start draining and you have to wait for an hour or so until it fully drains. 


If the pressure in your boiler dropped below 0.5 bar your boiler will stop working and you need to refill the system with water or when you drain out the water from the boiler for cleaning you need to refill it. So to do that, you need to find a filling loop below your boiler, open one valve and then the second valve. 

When you open the second valve water from the mains will start filling up the system all the time to control the pressure gauge. Sometimes it takes just a few seconds to refill the system. The arrow inside your pressure gauge must not reach the red scale and should stay between 1/1.5 to 2. The ideal pressure of a boiler is 1.5. When your boiler is full, close both the valves. Sometimes the filling loop is located directly on the boiler. 

How To Add Water To The Steam Boiler 

To make your boiler run smoothly it is important to check the pressure of your boiler and do regular maintenance. When the water level of your boiler runs low you need to fix it immediately. So let’s discuss how you can add water to your steam boiler:

  • First, shut off your boiler stem fully and give it time to fully cool off; you must not add water when the boiler is hot 
  • After some time check the pressure gauge and if it shows that the water level is between an inch at the center now is the time to add water to it
  • Now gentle pull out the handle of its water feed valve and check the pressure gauge as it fills the boiler
  • When the gauge meter will reach the right level push the water feed valve inside so it stops filling and now you can turn on your boiler as usual

How To Add Water To Radiator Heating System 

To add water to your radiator you need to shut it off, drain it out fully, and let the radiator cool down. Check the pressure gauge if it indicates low pressure or not. If there is no pressure then you can feed water to your radiator and what you need to do is:

  1. Go under the boiler where there will be a filling loop; this is usually the pipes under the boiler
  2. There are two valves underneath and you need to open both of them; this way your radiator heating system will start filling up
  3. Keep checking the pressure gauge until it reaches the necessary level and then turn off both the valves 
  4. Lastly, turn on your radiator 

How To Refill A Pressurized Heating System

To refill a pressurized heating system after draining it down, start by closing all the air vents and any drain off by turning them clockwise using a wrench. Before you fill up, it’s important to add central heating in a bitter to a radiator. This is to prevent corrosion from building up within the system.

Do this by simply removing the radiator blank and pouring a bottle of Laksaw using a funnel. Go to your boiler and it should have a filling loop on the pipe. Open the filling loop slightly and bleed all the downstairs radiators first. Once you’ve done this, bleed all the radiators upstairs, and if you’ve got a towel radiator make sure you do this last. 

Because it’s generally the highest point of the system. Once all your radiators are filled cause a filling loop with the system pressure at 1 to 1.5 bar. It may help if someone monitors the system pressure whilst you’re filling up. So you don’t over-pressurize the system. After you’ve done this you can turn the boiler back on and fire back up the heating system. Always check your radiator to make sure they’ve come back.

How To Add Water To A Weil Mclain Boiler

When the pressure gauge of your boiler comes down to 0.5 you know it’s time to add water to your Weil McLain boiler and to do that:

  • The Weil McLain boiler has an auto-feed for the bypass which is a typical autumn feed with five heads called 911S. This is the backflow preventer and an auto-feed on the side and a bypass valve at the bottom
  • In this configuration, if this is working to bleed the system all you need to do is open the loose key valve at the rack and the auto-feed should provide automatic makeup water when you’re doing that
  • During this process the bypass valve should be in the off position it will force the water through and if you want to bypass just open it and get the boiler pressure from 12 to 15 pounds 
  • Make sure in a two-story building the pressure should be under 12 pounds and in a three-story building the pressure should be between 19 to 20 pounds 

Frequently Asked Questions (FAQ)

How Do I Add Water To My Boiler Heating System?

To add water to your boiler heating system turn off the power, turn off the radiators, let them vent the air pressure, let it cool, turn the valves under the boiler to fill out your boiler until the pressure reaches 1.5 bar, turn off the valves, and lastly turn on the power of your boiler. 

How Do You Fill A Water System After Draining?

After drawing a water system, let it cool down. Then look under the boiler. There will be two valves and you need to open both of them until the pressure gauge reaches 1.5 bar and turn the valves off. Then just turn on your power and all the radiators.

Can You Add Water To A House Radiator?

Yes! You can. When you bleed off your radiator the pressure drops. So you need to increase that pressure slowly. So add water to the opening and closing valve and add water to the radiator while bleeding the air out. 

How Do I Manually Add Water To My Boiler?

To manually add water to the boiler let it cool down first. Then look under the boiler where you’ll see two valves. So open them and wait until the pressure increases to 1.5 bar. After that, close both valves and turn on the power. 

How Do You Fill A Radiator With Water?

To fill the radiator and bleed the air outtake out the bleed nipple and introduce water into this and the water will come up the pipe that is on the side. It will also force out any water that is inside out of the bleed valve. When it is full, put the bleed nipple back to seal it off. Also, seal off the pipe so no air goes in. 

Do I Need To Add Water To My Heating System?

Yes, you do. Because when you drain out the whole system for cleaning or maintenance purposes you need to add water into your system so it runs perfectly. 

Why Won’t My Boiler Fill Up With Water?

Usually, if there is any wall on the pipes or the valves your boiler will fail to fill up with water. So turn it off, vent out the radiators, drain the system and check for the cause of your problem or any leak. 

Final Verdict

Maintenance of your boiler doesn’t have to be a headache if you know the right ways to maneuver it and keep the pressure in check regularly. Its pressure must not go too high or too low; otherwise, you’re going to face problems. You must have some plumbing knowledge that is required for boiler maintenance otherwise you’ll be in a big mess. 

I'm Jimmy, the creative mind behind Drawing from extensive expertise in home improvement, I founded this platform to help individuals transform their living spaces. My dedication lies in empowering homeowners with dependable advice, ensuring that every DIY project not only elevates your home but also enhances your lifestyle. Join me as we redefine homes, one project at a time.

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