Draining a central heating system is sometimes necessary to carry out maintenance or repairs. It involves removing water from radiators, pipework and sometimes the boiler itself. When done properly, it helps prevent leaks, avoids airlocks and ensures safe access to internal components.
In this blog, we explain what draining involves, when it’s required and how to do it correctly. We also cover what equipment is needed, how to refill the system and what to watch out for once the job is complete. Whether you’re managing a home, a rental property or a commercial building, this guide will help you understand the process and decide whether it’s best handled by a qualified engineer.
Paine Manwaring provides heating and plumbing services to homeowners, landlords and commercial clients across Sussex and Hampshire. Our engineers are experienced in working with all types of central heating systems. If you need help draining your system or upgrading your heating, get in touch with our team.
When Should You Drain a Central Heating System?
Draining is usually required when:
- Replacing or removing radiators
- Carrying out pipework or boiler repairs
- Flushing out sludge and debris
- Leaving a property unoccupied for an extended time
Removing the water from the system creates a safe environment for further work and protects the boiler and pipework from internal damage.
What Equipment Do You Need?
Before you start, you’ll need:
- Radiator bleed key
- Hosepipe, plus a jubilee clip
- Adjustable spanner
- Bucket or trays
- Towels to protect flooring
- Screwdriver
- Protective gloves
Make sure all items are prepared in advance so you’re not caught out once draining begins.
Safety Considerations
It’s important to take precautions:
- Turn off the boiler and allow it and all pipework and radiators to cool completely
- Shut off the water supply to your boiler. If you have a combi boiler, close the mains stop valve, usually located under the kitchen sink. If you have a hot water cylinder, close the isolation valve on the pipe from the cold water storage cistern.
- Lay down towels or trays under valves to protect flooring from spills
- If in doubt, contact a professional. Avoid continuing if you’re unsure how your system is configured. Mistakes can result in scalding, flooding or damage to the boiler.
Step-by-Step Central Heating System Draining Guide
Follow these key steps to safely drain your entire central heating system:
Step 1: Locate the drain-off valve
This is usually located at the bottom of a downstairs radiator. Attach a hosepipe to this valve and secure it with a jubilee clip. Make sure the other end of the hose leads to a bucket or an outside drain. If using a bucket, prepare to empty it regularly.
Note: If your system has used a chemical inhibitor, avoid draining onto grass or garden beds.
Step 2: Open the drain-off valve
Use a spanner to open the drain-off valve by turning it counter-clockwise. Water will begin to flow out.
Note: The drain-off valve is part of the system pipework and is used to release water from the system.
Bleed valves, found on radiators, are used to release trapped air.
Step 3: Open the bleed valves on radiators.
Begin with the upstairs radiators. Use a radiator key to slightly open each bleed valve – this allows air into the system, speeding up the draining process. You should hear a ‘hissing’ sound as air enters.
Once the hissing stops upstairs, move to the downstairs radiators:
- Slightly open a bleed valve using a radiator key.
- If water comes out, close it again and wait a bit longer.
- If you hear air being drawn in, or nothing comes out, it means the system has drained in that area. You can then open the bleed valve fully and repeat this on the remaining downstairs radiators.
Step 4: When draining is complete.
Once no more water comes out the hose, close all radiator bleed valves. Next, close the drain-off valve and remove the hose carefully, catching any leftover water with the bucket or towels.
Step 5: Optional: Flush the system. If you’re doing maintenance or adding an inhibitor, now’s a good time to flush radiators or clean sludge using a hose or power flush machine.
Important: To flush effectively, make sure the drain-off valve is open and radiator bleed valves are open. This allows water and sludge to flow out freely and prevents airlocks in the system. If these valves are closed, flushing won’t work because the water cannot circulate or drain properly.
How to Refill a Central Heating System
Once your central heating system has been fully drained and any flushing or maintenance tasks are complete, the next step is to refill it safely. Refilling restores pressure and reintroduces water throughout the system. The process must be handled carefully to avoid introducing air or causing imbalances.
Follow these steps to bring your system back into working condition:
- Make sure the drain-off valve is closed.
- Turn the water supply back on.
- Begin bleeding radiators again, starting with those on the ground floor.
- If you have a sealed system, watch the pressure gauge on the boiler and stop at around 1.5 bar.
- Restart the boiler and ensure all radiators heat up properly.
- Check all valves and joints for any leaks.
Common Problems and How to Fix Them
Even when draining and refilling are done correctly, a few common issues can occur as the system settles. The table below outlines typical faults, their causes and how to resolve them. If you’re unsure or the issue persists, professional help is recommended.
| Issue | Cause | Action |
| Air trapped | Radiators not bled properly | Bleed again until air stops |
| Leaking valve | Loose or worn fittings | Tighten carefully or replace |
| Low pressure | Refill loop not open | Check filling loop and open |
| Cold radiators | Air or sludge system | Flush or bleed radiators |
| Error codes | Pressure or circulation fault | Contact your engineer |
Preventive Maintenance Tips
Once your system is up and running again, it’s worth considering how to keep it in good condition long-term. Regular maintenance reduces the likelihood of breakdowns and helps extend the lifespan of your heating system.
- Avoid draining more often than needed
- Arrange annual servicing to catch issues early
- Install a magnetic filter to reduce sludge build-up
- Don’t over-bleed radiators, as this may introduce air into the system
Central heating maintenance helps reduce faults, lower heating bills and extend equipment lifespan.
When to Involve a Professional
Not all systems are suitable for DIY maintenance. Contact an engineer if:
- You’re working with a sealed or pressurised system
- The drain-off valve is damaged or missing
- You cannot build or maintain boiler pressure
- Radiators remain cold after refilling
Paine Manwaring’s experienced team offers fast and reliable support across all heating systems. We work with homeowners, landlords and commercial clients and provide everything from radiator upgrades to boiler replacements.
Book a Professional Heating Service
Knowing how to drain a central heating system is helpful, but the process can go wrong if done incorrectly. If you’re unsure about your system or encounter problems during draining, it’s best to speak to a qualified engineer.
Paine Manwaring offers domestic and commercial heating services across Sussex and Hampshire. We provide routine servicing, emergency repairs and full system upgrades.
To arrange a visit or speak to an expert, contact our team.
Related Services:
Paine Manwaring is an established, professional and highly experienced supplier of plumbing, heating and electrical services to domestic and commercial customers throughout Sussex, Surrey and Hampshire.
For more information about our heating, plumbing and electrical services which we provide for both homes and businesses, get in touch with a member of our expert and friendly team today on 01903 237522 or enquiries@painemanwaring.co.uk (Worthing) or 01243 784711 or info@painemanwaring.co.uk (Chichester).