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How to Drain a Central Heating System


Given the role of the central heating system in providing hot water and heating to your home, you wouldn’t want it to be inefficient or stalling due to poor maintenance. As a result, learning how to drain it for maintenance and cleaning purposes should be at the top of your central heating maintenance bucket list.

Why is it important to know how to drain a central system?

Learning how to drain your central heating system stretches far and beyond maintenance purposes. For instance, to remove a radiator, install a new one, add an inhibitor, or remove accumulated sludge and limescale, you must first drain your central heating system.

Simply put, some heating system repair works can’t proceed without you having drained the whole system down.

Knowing how to drain your central heating system will enable you to maintain your central heating system in peak condition. Having your central heating system working at full capacity will ensure your home never goes without hot water or heating.

In this step-by-step guide, you are going to learn how to drain your central heating system correctly. Before we delve into the specifics, let’s first touch on some of the tools you’ll need to gather for this task.

But there’s no need to get agitated though, most of these tools are already in your possession, so you need not worry about purchasing a truckload of new equipment.

What tools do you need to drain your central heating system?

Here is a list of the things you will need to drain your central heating system successfully.

  • An adjustable spanner
  • A hosepipe
  • A jubilee clip
  • Flathead screwdriver
  • A bucket for collecting water
  • Radiator key
  • Towels

Easy, right? Well, now that you’ve assembled your work equipment, you are ready to dive in and drain your central heating system.

How do I drain a central heating system with a drain-off valve?

Step 1. Switch off the central heating system

As always, your safety comes first. Therefore, the first thing you should do when draining your central heating system is to switch it off and allow it to sufficiently cool down.

Switching off the central heating system and allowing it to cool for 1 to 2 hours is a safety precaution that will ensure the hot water in the system does not scald you.

The method of turning off your central system will largely depend on the type of system you have as well as the age of your boiler.

To directly turn off your central heating system using the boiler, locate the on/off switch, often marked in red. Turn this switch to the off position, and you can confirm that the system is off by checking the small screen on the boiler.

On the other hand, to use a boiler timer, locate it in its position near the boiler. The boiler timer consists of a circular dial on a large square box. Alternatively, the boiler timer can be inbuilt into the boiler. Either way, if the boiler is ‘On’, proceed to turn the timer dials to show between which hours you want your heating system to be off.

To use a central heating thermostat, lower the current room temperature by several degrees, and the system will turn off. If you have a smart thermostat, you can install a suitable mobile app that will allow you to turn off the central heating system from your phone.

What if your boiler operates on solid fuel? Simply extinguish the fire and allow the boiler to cool off, and you’ll be good to go.

Step 2. Cut off the water supply

The second step when draining your central heating system is to shut off the water supply to your boiler. Cutting off the water supply to the boiler will ensure no water enters the system while draining it.

Shut off the water intake valve located on the cold water pipe above the boiler to cut the water supply. To close the valves, use the flathead screw to turn the screw heads clockwise until they are tight.

Alternatively, you can cut off the water supply at the mains water stopcock by closing the tap or the valve using the flathead screwdriver. This method, however, will cut off the water supply to your entire house, and you should only use it if you are comfortable with this.

On the other hand, if you have a combi boiler, the process is straightforward, as you have to switch off the boiler and wait for it to cool down.

Step 3. Locate the radiator with the drain-off valve

After switching off the system and cutting off the water supply, locate the radiator with the drain off valve. This radiator is often located somewhere on the first floor or basement of your house, so it shouldn’t be hard to trace it.

If you have more than one radiator in your basement or on the ground floor of your home, there is no need to panic, though. You can identify the drain-off valve by an attachment that looks like a pipe opening positioned on one of the two valves on either end of the radiator.

Once you’ve identified this radiator, take a hosepipe and attach it to the drain-off valve. Use the jubilee clip to fasten the hosepipe to the drain-off valve securely. Even though a jubilee clip isn’t necessary for situations where the garden hosepipe perfectly fits the drain-off valve, we still recommended you use it as a precaution against slipping of the hose and leaking.

To be on the safe side, consider laying down a couple of absorbent towels beneath the drain-off valve just in case it does leak during the draining process.

Next, trail the hosepipe outside the house towards a drain and away from your flowerbed or lawn. This ensures that the central heating inhibitors in the drained water won’t harm your plants as some of its constituent chemicals are harmful to plants.

It is also essential to ensure the hosepipe doesn’t discharge the drained water on the road or street, especially during winter, as the water may freeze and result in accidents.

Not having a hosepipe around shouldn’t pose a big problem, though. You can simply place a bucket under the drain-off valve to collect the water coming out of the boiler. However, the process will be lengthy and tedious as you have to temporarily shut off the drain valve when it fills up and empty it.

Step 4. Open the drain-off valve

With the hosepipe securely held in place and leading to a safe place on the other end, it is time to open the drain-off valve and start draining your system.

Been wondering what is the use of the adjustable spanner, right? Well, this is where it comes into play. Use it to turn the nut next to the drain-off valve in an anti-clockwise direction to loosen it. Loosening the nut will open the drain-off valve, and you’ll begin to hear water rushing through the hosepipe. If you are using a bucket, it will start to fill up with water.

Step 5. Bleed the radiators

To fasten the draining process, consider bleeding all the radiators in your home. Bleeding the radiators will hasten the draining process and suck out air trapped in the system. This way, the radiators will evenly distribute heat once the system is back in operation.

To bleed the radiators, open all the radiator valves in your house, starting with ones on the uppermost floor. Wait for about 10 to 15 minutes and open the valves on the radiators on the lower floors. This approach will help hasten the water draining process.

To know if your central heating system has completely drained, check the amount of water coming off the hosepipe or drain-off valve. If there is little or no water coming out, then the system has been successfully drained, and you can proceed to do the maintenance or repair work you intended to do.

Step 6. Refill the central heating system.

Once you’ve done whatever you needed to do, it is time to refill your central heating system. But before you refill the system, close all open bleed valves and the drain-off valve. When this is complete, detach the hosepipe from the drain-off valve, and at this point, no water should be dripping from it.

After detaching the hosepipe, turn on the mains water supply or open the closed water intake valve, and the system will begin to refill. The next step is to bleed the radiators again, starting from those on the ground floor and moving upwards.

Once done, turn the power supply back on and check the performance of the central heating system. If need be, you may have to bleed some of the radiators again.

How do I drain a central heating system that has no drain-off valve?

The process for draining a central heating system without a drain-off valve is slightly different from the above method.

First, you’ll start by turning off the heating system and allowing it to cool. Next, you’ll need to cut off the water supply.

The third step involves isolating the radiator from the heating system.  To achieve this, you will need to close the regulator and the lockshield valves on either side of the radiator. Closing the regulator valve is simple and requires you to rotate it in a clockwise direction.

To close the lockshield valve, remove the plastic cap and use a pair of pliers to tighten the valve. Make sure you use a top tip to note the number of turns you made, as you will need to re-tighten it by the same number later when refilling it.

Next up is to bleed the radiators to release trapped air and speed up the draining process.

Once done, release the coupling nut on the regulator side and place a bucket and towels under it. Then using the adjustable spanner, rotate the nut in an anti-clockwise direction to loosen it. As it relaxes, water will start pouring into the bucket. Once the bucket is full, tighten the coupling nut, empty the bucket, and repeat the process.

If, however, you find the process tiresome, you can attach a hosepipe to drain the radiator using a speed fit tap.

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