Braided Appliance Inlet Hose: Clear a Hot Water Airlock in the UK
A standard braided appliance inlet hose the same type used to connect a washing machine is one of the most effective tools for clearing a plumbing airlock yourself. It lets you safely bridge the high-pressure mains cold supply to the blocked hot water pipework and force the trapped air out in a matter of seconds.
Quick Answer
To clear a hot water airlock with a braided hose: turn off both hot and cold appliance valves under the sink, connect the hose between them, open the hot valve, then open the cold valve for just 3–5 seconds. The mains pressure forces the trapped air bubble back up through the hot pipe and out via the loft header tank. This method works on gravity-fed systems with a hot water cylinder not combi boilers.
Why Use a Braided Hose for Airlocks?
In homes with gravity-fed hot water tanks, air bubbles can get trapped in the pipes after the system has been drained creating a blockage that stops hot water flowing even though the cold supply is perfectly fine. A braided hose fixes this because:
- It harnesses mains pressure: Mains cold water runs at significantly higher pressure than a gravity-fed hot system, giving you enough force to shift the air pocket.
- It back-flushes the system: The surge of cold water reverses the flow in the hot pipe, pushing the trapped air up and out through the header tank in the loft.
- It creates a leak-free connection: The threaded 3/4" BSP ends screw securely onto standard appliance valves far more reliable than the old cloth-over-tap method.
Signs You Have a Hot Water Airlock
Airlocks only occur in gravity-fed hot water systems homes with an insulated hot water cylinder in the airing cupboard and a cold water storage tank in the loft. If you have a combi boiler with no cylinder, this is not your problem. Look for these symptoms:
If you also have low pressure in your upstairs shower at the same time, that points strongly to a gravity-fed system with a loft tank issue rather than a localised airlock.
Gravity-Fed vs Combi: Which System Do You Have?
Check your airing cupboard. If there's a large insulated hot water cylinder (a tank wrapped in foam or a jacket), you have a gravity-fed system and airlocks are possible. If your boiler heats water instantly on demand with no separate cylinder, it's a combi and a loss of hot water with a combi points to a different fault entirely, such as a failed diverter valve or combi boiler not producing hot water.
Where to Buy a Braided Inlet Hose
You probably already have a spare behind the washing machine. If not, they are very cheap usually £4 to £10 and available from any plumbing or DIY merchant. Any standard washing machine or dishwasher fill hose will do the job.
| Retailer | Stock Status | What to Expect | Link |
|---|---|---|---|
| Screwfix | In Stock | Available lengths Selection | View Range |
| Toolstation | Available | Standard 3/4" collection | Check Stock |
How to Clear a Hot Water Airlock Using the Hose Method
The easiest place to cross-connect is at the washing machine isolation valves under your kitchen sink or in the utility room. You'll need one braided appliance hose and nothing else no tools required. If you want to reseal any threaded connections afterwards, keep some PTFE tape to hand.
Locate the Hot & Cold Washing Machine Valves
Find the two isolation valves beneath your kitchen counter one red (hot) and one blue (cold) which feed your washing machine. Turn both levers to the OFF position, which means the lever sits at a right angle across the pipe. This isolates both lines before you connect the hose.
Screw One End onto Cold, One onto Hot
Take your braided inlet hose and check that the rubber washers are seated inside both end connectors these are what make the seal. Screw one end tightly onto the cold valve outlet and the other end onto the hot valve outlet. Hand-tight is usually enough; the rubber washer does the sealing.
Open the Pathway Into the Hot System
Turn the red hot valve to the ON position (lever in line with the pipe). Water won't flow yet because the cold valve is still closed. This step simply opens the route from the hose back into your hot water pipework.
3–5 Seconds Only Then Test
Turn the blue cold valve ON for just 3 to 5 seconds, then turn it back OFF. The high-pressure mains cold water shoots through the hose and into the hot pipe, forcing the air pocket up and out through the header tank in the loft you may hear gurgling from above. Disconnect the hose, reconnect the washing machine, and test the hot tap. If the flow is still weak, repeat with one more short burst.
If you notice slow drainage elsewhere in the house after running water through the system, check whether you might also have a slow-draining sink that needs attention separately.
💡 Important: Keep Bursts Short
- 3–5 seconds maximum per burst: Leaving the cold water running into the hot line for too long risks overfilling the cold water storage tank in the loft. If the ball valve isn't working perfectly, it could overflow and flood your ceiling.
- No washing machine valves? Try the cloth method wrap a wet cloth tightly around the spout of a mixer tap and turn both hot and cold levers on at the same time. The mains pressure can sometimes push air back up the hot side, though it's less reliable than the hose method.
- Standard fitting size: UK appliance hoses use 3/4" BSP threads, which match the standard isolation valves fitted under most UK sinks. No adaptors needed.
- Airlock keeps coming back? A recurring airlock usually means a faulty ball valve in the loft tank is allowing air into the system. That's a job worth getting a plumber in to check.
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Frequently Asked Questions
Why is my hot water not working but cold is fine?
In a gravity-fed system, a trapped air bubble is the most common reason your hot tap runs dry whilst cold flows perfectly. The air pocket creates a physical blockage in the hot pipe. Cross-connecting a braided hose between hot and cold forces the airlock out using mains pressure.
Does the braided hose method work on a combi boiler?
No. Airlocks from trapped air are almost exclusively a problem in gravity-fed systems with a hot water cylinder. Combi boilers run both hot and cold sides on mains pressure, so if you have no hot water with a combi, the fault is more likely a diverter valve or low system pressure see our guide to combi boiler not producing hot water.
What size hose do I need to clear a plumbing airlock?
Any standard braided appliance inlet hose with 3/4" BSP threaded ends will work. This is the universal fitting used on washing machine and dishwasher fill hoses across the UK and matches the standard isolation valves found under most sinks.
How long should I run the cold water when clearing an airlock?
Short bursts of 3 to 5 seconds only. Running cold water into the hot line for longer than this risks overfilling your loft cold water tank. If the ball valve isn't seating perfectly, this can cause water to overflow into your ceiling. One or two short bursts is usually all it takes.
What if I don't have washing machine valves under my sink?
Try the cloth method: wrap a wet cloth tightly around the end of a mixer tap spout and turn both hot and cold levers on together. The mains cold pressure can sometimes push air back up the hot side. It's less reliable than using a hose but worth trying if you have no accessible appliance valves.
Can an airlock come back after you clear it?
Yes. If the fault that caused it isn't resolved, the airlock will return. A faulty ball valve in the loft cold water tank is the most common culprit if it's letting air into the system each time the tank refills, you'll keep getting airlocks. A plumber can replace the ball valve quickly and cheaply.
How do I tell if I have a gravity-fed system or a combi boiler?
Check your airing cupboard. If there's a large insulated hot water cylinder (a tank in a foam jacket), you have a gravity-fed system and airlocks are possible. If your boiler heats water on demand with no cylinder, it's a combi. If you're not sure, also check whether you have low pressure upstairs gravity-fed systems often have noticeably lower pressure on upper floors.