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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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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
  • check_circleMegaflo, Gledhill, Telford, Stelflow
helpThe Causes

Why Hot Water Pressure Drops

Why hot water pressure drops while the heating coil is a separate circuit Mains water passes through the inlet control group into the cylinder and out to your taps. The boiler or heat pump heats the water through a sealed coil that your hot water never passes through. Two separate circuits Cold mains in Mains supply Inlet control groupPRV · strainer Cylinderstored hot water coil Hot taps Boiler / heat pump heats the coil only sealed heating circuit — your hot water never passes through it Weak flow at the hot taps Look at the blue path: strainer, pressure reducing valve, expansion vessel, or the outlets themselves Runs out fast / slow reheat Look at the orange path: scaled coil, 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.
Text description of this diagram

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

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.
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.
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.
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.
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.
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.
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.
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.
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.

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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.

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
callCall Gary — 07717 846247