How to drain a central heating system

The two big reasons why you should drain your central heating system

Many homeowners and landlords might only drain their central heating system in an emergency such as a leak or a more cumulative fault – like when sludge builds up in your system. It can also be necessary if you need to fit a new radiator.

However, did you know that aside from these types of issues, many people drain their central heating system every few years as part of a maintenance practice to keep it running smoothly and efficiently?

Draining your central heating system in an emergency

Let’s assume you have a leak or a build-up of sludge – it may be the middle of Winter, you’ve got small children or an elderly relative staying with you and you need to get your system working again fast. We’ve prepared a guide to help you drain your system step-by-step.

What you need to know before you drain your central heating system

To learn how to drain your central heating system, you have to know a little about how boilers work. Go to the link for the full low-down, or we’ll summarise it quickly for a combi boiler here:

All types of boilers use the basic process of burning fuel and sending the heated water through to radiators and taps.

Combi boilers supply both heating and hot water to your home without any extra tanks or cylinders – they combine them, hence the name. Combi boilers have two independent heat exchangers – one connects to your radiators and the other one connects to your hot water supply.

Because combi boilers provide hot water on demand, they’re constantly on standby. Once a request for hot water or heating is made, a combi boiler burns its fuel, which then causes the heat exchanger to heat water.

Combi boiler systems are built with control valves that operate in different directions depending on whether hot water or heating is required – they cannot provide both at the exact same time.

What tools will I need to drain my central heating system?

Make sure you get the following items ready before you start:

• Spanner
• Screwdriver
• Towel
• Jubilee clip
• Radiator key
• Hosepipe
• Buckets

How to drain down your central heating system

Your central heating system provides you with a warm and happy home, so let’s treat it with care. Here’s how to drain it, step by step:

Step 1. Switch off your system

Safety first! Before you begin the process of draining, switch off your boiler completely. After this, wait until your radiators and pipes have cooled down 100%.

Step 2. If you have a solid fuel boiler, put out the fire

If your boiler burns solid fuel, If your boiler burns solid fuel, take the appropriate steps to extinguish the fire safely before you go any further. You should also wait until your boiler is completely cold.

Step 3. Now cut the water supply

The next step is to cut off the water supply to your boiler. This way, no water will come into your boiler or your system while you drain. Most boilers have their own stop tap or isolation valve.

If you have a combi boiler, the hot water is supplied directly to it, so you’ll need to close the mains stop valve, which is usually under your kitchen sink. If you have a hot water cylinder, you’ll have a pipe going from the cold water storage cistern to the hot water cylinder, with a valve on it.

Step 4. Attach a hosepipe to the correct radiator’s drain-off valve

Look for the radiator in your house with a drain-off valve. Look for it on the right or left-hand side at the bottom. The valve is often coming off at a 45-degree angle. Once you’ve located the radiator with the drain-off valve, grab your garden hose and clip it around the valve with the jubilee clip. Make sure the other end of the hosepipe is in a bucket, or running outside (but not on your plants, as a chemical radiator inhibitor may have been used in your radiators).

Please note: If you don’t have a drain-off valve, we’ve included a guide for you below.

If you need to, tighten your jubilee clip with a flat-head screwdriver. If you don’t have a hosepipe, you can collect the liquid directly in a bucket, but you’ll need to shut off the valve and empty the bucket every so often.

Step 5. Start draining your upstairs radiators

Now it’s time to go round your property and make sure each radiator’s valve is open. Then come back to the radiator with the attached hosepipe and open up the drain valve.
All the water will start to drain out.

Step 6. Open bleed valves to speed up the process

You can speed the process up by, bleeding all your radiators. Start with the upstairs ones. You’ll hear the air being sucked into the system. Leave the water to drain for about 15 minutes.

Now go to a downstairs radiator and try slowly opening a bleed valve. If water starts coming out, quickly tighten it up again and wait a bit longer. But if you can hear the air going into the system, fully open the bleed valve and then go and open the same valves on your downstairs radiators. This ensures that you get every last drop of old water and sludge out of your system.

Step 7. Close all valves back up

Once you’ve fully drained your system, tighten up the bleed valves again. Close the drain-off valve and take off the hosepipe – keep a bucket or towel handy to mop up.

Step 8. Refill your central heating system

The process to refill your radiators starts with closing all the valves you opened.

Our Home experts add inhibitor to the new influx of water to slow the build-up of limescale, corrosion and sludge. You can find it online or in all good DIY shops. Have a read of our guide to know more about central heating inhibitor explained.

Now go back to the isolation valve on your combi boiler and open it back up. The water should refill.

Once your system has completely refilled to the required pressure, switch your boiler back on and check that all your radiators are getting hot. Bleed your radiators again to ensure there’s no air in the system, and check there are no leaks below your radiators.

How to drain central heating system without drain valve

If you don’t have a drain-off valve on any of your radiators, the process is a little different and you’ll need to purchase a special tap fitting from a DIY store.

What you’ll need:

• Speedfit tap / speedfit draining fitting from your local / online DIY store
• Pliers
• Adjustable spanner
• Hosepipe
• Bucket
• Old towels

Step 1. Turn off your system and cut the water supply

Before you start, switch off your central heating system and wait for the radiators to cool down.

If you have a combi boiler…
…turn it off and wait for the whole system (radiators and all) to cool.

If you have a conventional boiler…
…first isolate the water and then switch off the boiler.

Step 2. Isolate the radiator from your heating system

If you don’t have a drain valve, you need to separate the radiator from the system by closing the two valves on each end of the radiator.

On one end is the regulator or thermostatic radiator valve (TRV). All you need to do here is rotate it in a clockwise direction. On the other end is the lockshield valve – for this you need to remove the plastic cap, grab your pliers and close the valve tightly.

Step 3. Bleed all of your radiators

To speed up the process of draining, you need to let all the air out of your system by opening the bleed valves on all your radiators.

Step 4. Loosen the union nuts

Now comes the main task:

• On the regulator / TRV side, release the coupling nut
• Place your bucket and old towels under it to catch any spills
• Use your adjustable spanner to rotate the nut anti-clockwise until the water starts pouring into the bucket

Step 5. Attach a hose and drain your system

With the radiator valve in the ‘off’ position, attach your hose to it and start draining your central heating system. You need a special fitting called a ‘speedfit tap’ or ‘speedfit draining fitting’ to attach the hose to the radiator. Make sure the measurement is correct for your radiator pipe when you buy.

Once your speedfit tap and hose is connected, you can then drain your system.

How long does it take to drain the central heating system?

It depends on the type of system you have and whether you have a drain-off valve. If you have a drain-off valve it can take 20 mins to 1 hour to completely drain a central heating system. To help speed up the process, systematically open the bleed valves on all of your radiators.

How often should you drain your central heating system?

To keep your central heating system running smoothly, drain your central heating system once a year.

Look forward to another year of smooth running heating

 

Draining your central heating system once a year is a great way of avoiding problems with your boiler and radiators.

If you run into problems with your current boiler, we’re working with BOXT who offer a speedy installation service– you could be enjoying your new system as soon as the very next day*.

Get an estimated new boiler quote and find the perfect boiler for your home.

FAQs

How do you drain and fill a central heating system?

1. Switch off your central heating system
2. Wait for all radiators and pipes to cool
3. If you have a drain-off valve on one of your downstairs radiators, attach a hose and open the drain valve
4. Bleed all your radiators from top to bottom to speed up the process
5. Once your system is completely drained, you can refill it, adding inhibitor to keep it clean

How long does it take to drain the central heating system?

It depends on the type of system you have and whether you have a drain-off valve. If you have a drain-off valve it can take 20 mins to 1 hour to completely drain a central heating system. To help speed up the process, systematically open the bleed valves on all of your radiators.

How often should you drain your central heating system?

To keep your central heating system running smoothly, drain your central heating system once a year.

How do I drain my central heating without a drain valve?

You’ll need to purchase a special speedfit tap fitting from a DIY store and use that to attach a hosepipe to your radiator and drain off the water.

How much does it cost to drain a central heating system?

It’s inexpensive to drain your central heating system if you do it yourself. You may need to purchase a few cheap items from a DIY store to get the job done.

*Subject to engineer availability