I was called out recently to a house in Petts Wood where the hot water pipe coming out of the top of a Megaflo had started buzzing. It was loudest when nobody was using any hot water, and it stopped as soon as a hot tap was turned on. The customer was worried something was about to fail.
It's a common problem on older Megaflo cylinders, and the fix is usually simple.
Got a Megaflo or unvented cylinder problem in Orpington, Bromley or south east London?
I'm a G3-qualified engineer and service and repair all makes of unvented cylinder. Cylinder service £140, no VAT to add.
How older Megaflos handle expansion
When water heats up, it expands. In a sealed, mains-pressure cylinder, that extra volume has to go somewhere. Most modern unvented cylinders have a separate expansion vessel on the cold supply. Many older Megaflos don't. Instead, they have an air gap (often called the “bubble”) trapped inside the top of the cylinder. As the water heats and expands, it squashes that pocket of air, which acts like a cushion and keeps the pressure steady.
What happens when the air gap is lost
Over time, the air gradually dissolves into the water and is drawn off through the taps. Once it's gone, there's nothing left to absorb the expansion. As the cylinder heats up:
- the pressure inside rises
- the pipework and valves on the hot outlet start to vibrate, which gives the buzzing or humming
- opening a hot tap releases a little water, the pressure drops, and the noise stops, until the cylinder heats up again
You might also notice the tundish dripping or water discharging when the cylinder heats, because the expansion relief valve is doing the job the air gap used to do.
How it's fixed
The air gap is recharged by partly draining the cylinder through its safety valve, with a hot tap open so air can get back in, then refilling and testing it. On the Petts Wood job, that was all it needed. Once the cylinder had heated back up, the pipe was quiet.
This involves the cylinder's safety valves and hot water under pressure, so it should be done by a G3-qualified engineer, not as a DIY job. It's normally done as part of an annual cylinder service.
When it's a sign of something else
If the air gap keeps disappearing within months, or the noise carries on after it’s been recharged, the cause might be:
- a faulty pressure reducing valve letting mains pressure creep up
- a sticking expansion relief valve
- or simply a cylinder that's reaching the end of its life. Older Megaflos are often 20 to 30 years old now.
Need it sorted?
If your Megaflo is buzzing, humming or dripping at the tundish, I can recharge the air gap and check the safety valves as part of a full cylinder service for £140, with no VAT to add. If it turns out the cylinder is past its best, I'll tell you straight. Replacements are supplied and fitted from £1,550.
Need it looked at?
If your cylinder is losing pressure, dripping at the tundish, making noises or not giving you hot water, I can come and find out why. Annual cylinder service £140, or £285 with your heat pump. No VAT to add.
Cylinder getting on a bit? If it's 15 years or older and giving you trouble, a replacement often makes more sense than repeated repairs. Unvented cylinders supplied and fitted from £1,550.