compressUnvented Cylinder Fault

Megaflo Losing Pressure? Here’s What Causes It

Hot water pressure dropped off while the cold is still fine? That is a specific fault pattern with a short list of likely causes — and it is rarely the cylinder itself. G3 qualified across Orpington, Bromley and Kent.

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buildRepair, Not Replace

money_offNo Call-Out Fee

paymentsGetting It Diagnosed

check_circle£140 G3 service with full diagnosis
check_circleStrainer cleaned as standard
check_circleIncoming mains pressure measured
check_circleRepairs quoted at fixed price first
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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.

Call 07717 846247 Book online

helpThe Causes

Why Hot Water Pressure Drops

Two separate circuits

The water you use: mains to taps
Sealed heating circuit: never mixes with it

Weak flow at the hot tapsLook at the blue path: the strainer, pressure reducing valve, expansion vessel, or the outlets themselves.

Runs out fast or slow to reheatLook at the orange path: a scaled coil, the immersion, thermostat or heat source. Flow at the tap is usually unchanged.

Flow and recovery are different faults on different circuits. Weak flow points at the inlet control group or outlets; slow reheat points at the coil or heat source.

Read this diagram as text

Cold mains water enters through the inlet control group, which contains the pressure reducing valve and strainer, then into the cylinder, then out to your hot taps. That is the blue path your water actually travels. Separately, the boiler or heat pump heats the stored water through a sealed coil inside the cylinder, shown in orange — your hot water never passes through that coil. So the two symptoms have different causes: if flow at the hot taps is weak, look at the blue path, meaning the strainer, pressure reducing valve, expansion vessel or the outlets themselves. If hot water runs out quickly or reheats slowly, look at the orange path, meaning a scaled coil, the immersion heater, the thermostat or the heat source — and in that case flow at the tap is usually unchanged.

The important clue is whether the cold is still fine. If cold taps run at full pressure and only the hot has weakened, the problem is almost certainly between your incoming main and the cylinder — not with the water supply itself.

Blocked strainer in the inlet control group

One of the most common causes I find. The strainer is a fine mesh filter that catches debris from the mains, and over years it silts up and throttles everything downstream. Cleaning it is a routine part of a proper service, which is exactly why cylinders that get serviced rarely develop this.

Failing pressure reducing valve

The PRV sets the pressure your cylinder sees. When it starts to fail it usually drifts downward, and hot water pressure quietly declines over months in a way people adapt to without noticing until it is bad.

Scale build-up

Hard water areas like Orpington, Bromley and North West Kent see genuine scale accumulation in the control group and on immersion heaters. It restricts flow and reduces heat transfer at the same time.

Expansion vessel waterlogged

A vessel that has lost its air charge fills with water. That affects pressure stability and often shows up alongside a dripping tundish — two symptoms, one cause.

Genuinely low incoming mains pressure

Occasionally it really is the supply. That is why measuring the incoming pressure is the first thing I do rather than the last — there is no point replacing valves to fix a problem that is outside the house.

helpFAQ

Cylinder Pressure Questions

Why has my hot water pressure dropped but the cold is fine?expand_more

That pattern points at something between the mains and the cylinder rather than the mains itself. The most common cause by far is the strainer in the inlet control group filling with debris. Second is a failing pressure reducing valve.

Is it the cylinder itself?expand_more

Rarely. The stainless steel vessel almost always outlasts the controls around it. Be cautious of anyone quoting a replacement cylinder for a pressure problem before they have checked the strainer, the pressure reducing valve and your incoming mains pressure.

Could it be scale?expand_more

Yes, particularly in Orpington, Bromley and North West Kent which are genuinely hard water areas. Scale builds in the inlet control group and on immersion heaters, and it is a routine finding on cylinders that have not been serviced.

How much does it cost to fix?expand_more

Diagnosis is included in a £140 G3 service visit. Cleaning a strainer is part of the service; a replacement pressure reducing valve or combination valve is quoted at a fixed price agreed before I start.

Will a service fix it?expand_more

Very often, yes. Cleaning the strainer and checking the inlet control group is standard on every service I do, and that alone resolves a good proportion of pressure complaints.

Could it be an airlock?expand_more

On an unvented system, genuine airlocks are much rarer than on gravity-fed systems because everything runs at mains pressure. It is usually restriction rather than air.

Would a power flush help?expand_more

No — that treats the heating circuit, not your hot water supply. Different system entirely. Anyone suggesting a power flush for low hot water pressure has misunderstood the problem.

Can you fix it the same visit?expand_more

Usually. Cleaning the strainer is part of a standard service, and I carry the common valves. If it needs a part I do not have, I quote it and come back.

Is my cylinder too old?expand_more

Age alone rarely causes pressure loss. The stainless vessel typically lasts 15 to 25 years and it is the controls around it that fail first — and those are all replaceable.

Get Your Pressure Sorted

£140 G3 service with full diagnosis, no VAT, across Orpington, Bromley, Sevenoaks and Kent.

call07717 846247

calendar_monthGet in Touch water_heaterMegaflo Servicing & Repair

Check First

Two Things Worth Checking Before You Call

Neither needs tools, and either could save you a visit.

Is it every hot tap, or just one?

If only one tap or shower has weakened, the problem is at that outlet — a blocked aerator, a scaled shower head, or a failing thermostatic cartridge. That is a much smaller job than a cylinder fault.

Is the cold genuinely still strong?

Run a cold tap fed from the mains, usually the kitchen. If the cold has dropped too, the issue is your incoming supply rather than the cylinder, and the water company may be worth a call first.

Has it dropped suddenly or gradually?

Sudden usually means something has failed — a valve, or debris that has moved. Gradual over months points at scale build-up or a pressure reducing valve slowly drifting downward.

Any other symptoms?

A dripping tundish alongside low pressure often means one cause, not two — typically an expansion vessel that has waterlogged. Worth mentioning when you call.

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.

See cylinder replacement Call 07717 846247

photo_cameraThe Usual Culprit

This Is Nearly Always It

A silted strainer, cleaned as part of a standard service.

Unvented cylinder inlet strainer removed for cleaning — The strainer mesh and brass cap removed — the most common cause of low hot water pressure

More Cylinder Guides

Megaflo Pipes Buzzing or Humming? The Air Gap Explained Tundish Dripping on an Unvented Cylinder? What It Means No Hot Water From an Unvented Cylinder? Common Causes Unvented Cylinder Pressure Problems: Why Hot Water Loses Pressure