Summary

Megaflo, manufactured by Heatrae Sadia, is one of the most widely installed unvented hot water cylinder brands in the UK, covering both the standard Megaflo and Megaflo Eco ranges (indirect, coil-heated from a boiler or heat pump) and the SystemFit range (direct electric immersion models). Unlike a boiler, there's no multi-digit fault code system to read — diagnosis is based on observing physical symptoms (discharge, pressure, temperature, flow) and working through the specific components an unvented cylinder has that a vented (header tank) cylinder doesn't: the expansion vessel (or internal air gap on older designs), the pressure reducing valve on the cold feed, the temperature/pressure relief valve, and the tundish/discharge pipe.

Getting this diagnosis right matters because the most common genuine callout — water discharging continuously or intermittently from the tundish — is very often wrongly assumed to be a faulty PRV when the actual cause is a depleted expansion vessel or lost air gap, meaning the PRV is doing exactly what it's designed to do (relieving pressure it has no other way to accommodate). Replacing the PRV without addressing the expansion system fixes nothing and the discharge recurs.

This is also a safety-critical appliance: the relief valves exist specifically to prevent a dangerous overpressure/overtemperature event, and discharge should always be investigated as a system fault, never simply ignored or the valve re-set/blocked off.

Key Facts

Quick Reference Table

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Symptom Most likely cause First check
Continuous or intermittent drip/discharge from tundish, cylinder otherwise working normally Expansion vessel lost pre-charge, or internal air gap depleted (older models) Check vessel pre-charge pressure against data plate spec
T&P valve discharging, water noticeably hotter than set temperature Thermostat failure, overheat Check thermostat and ECO (energy cut-out) status — do not simply reset
PRV discharging, system pressure reading high Pressure-reducing valve on cold feed failed/incorrectly set Check and reset/replace pressure-reducing valve
No hot water, indirect (boiler/heat pump coil) model Heating system not firing, or diverter/motorised valve not opening to coil Diagnose heating system side — confirm boiler firing and valve operation
No hot water, direct electric (SystemFit) model Failed immersion element or thermostat Test element continuity, check thermostat
Slow reheat time Scaled coil (hard water areas), undersized coil for demand, or low flow rate from source Check water hardness/scale history, confirm coil flow rate
Low flow/pressure at outlets, cylinder otherwise fine Pressure-reducing valve set too low, or blocked inlet filter Check PRV setting and inlet strainer/filter
Audible banging/knocking from cylinder area Water hammer, often from expansion vessel or pipework, not the cylinder itself Check expansion vessel and pipe restraint — see water hammer
Discharge at tundish but no identifiable fault found Possible nuisance trip from a borderline-high but technically acceptable system pressure Monitor system pressure over a full heating cycle before condemning a component

Detailed Guidance

Diagnosing tundish discharge — the single most common callout

Before replacing any valve, confirm which relief device is actually discharging — the PRV (pressure) or the T&P valve (temperature and pressure) — since they point to different fault categories. For PRV discharge, the overwhelming majority of genuine cases trace back to the expansion accommodation system: check the expansion vessel's pre-charge pressure with a tyre-type pressure gauge at the vessel's Schrader valve (with the system depressurised), comparing against the figure on the cylinder's data plate. A vessel reading significantly below spec, or reading zero (indicating a ruptured diaphragm), cannot accommodate thermal expansion and will force the PRV to discharge repeatedly as the water heats and expands with nowhere to go. On older Megaflo models using an internal air gap rather than a separate vessel, the equivalent fault is a depleted air gap, resolved by following the manufacturer's recharge/drain-down procedure rather than a vessel replacement.

Diagnosing T&P valve discharge — always treat as a potential genuine overheat

Unlike PRV discharge (usually a mechanical expansion-system fault), T&P valve discharge should always prompt a check of the actual water temperature and the thermostat/overheat cut-out status, because this valve's job is specifically to catch a genuine runaway overheat condition that the normal thermostat and cut-out failed to prevent. Check whether the separate energy cut-out (ECO) — a non-resettable safety device distinct from the normal working thermostat — has tripped; if it has, this confirms the working thermostat failed first and allowed the water to overheat before the ECO caught it. Never simply reset a T&P valve and move on without identifying why it discharged.

Pressure-reducing valve vs pressure relief valve — don't confuse the two

These are two entirely different components with similar names, and confusing them is a common source of misdiagnosis. The pressure-reducing valve sits on the cold mains feed, before the cylinder, and reduces incoming mains pressure down to a safe working level for the cylinder. The pressure relief valve (PRV) is a safety device on the cylinder itself that discharges if pressure inside the cylinder exceeds its set point. A failed pressure-reducing valve allowing excessive mains pressure through can be a root cause of PRV discharge, but the two components are checked and serviced separately.

Servicing and typical service intervals

Unvented cylinders require periodic servicing by a G3-competent engineer, which should check the expansion vessel pre-charge, the operation (not just presence) of the PRV and T&P valve, pressure-reducing valve setting, and general condition — annual servicing is the commonly recommended interval, aligned with many manufacturers' warranty requirements. A cylinder with no service history and an unknown expansion vessel condition should have this checked as a priority on any fault callout, since it's the most common root cause across the symptom categories above.

Frequently Asked Questions

Can I just replace the PRV if it's discharging, without checking anything else?

No — this is the single most common misdiagnosis on this appliance. A discharging PRV is very often working correctly in response to an upstream fault (most commonly a failed expansion vessel), and replacing the valve alone will not stop the discharge recurring. Always check the expansion vessel/air gap system first.

Is it safe for a customer to simply ignore occasional dripping at the tundish?

No — any discharge indicates the system isn't accommodating pressure or temperature correctly and should be investigated promptly by a G3-competent engineer. It should never be capped, blocked, or routed somewhere less visible rather than fixed, as this defeats a genuine safety device.

Does a Megaflo have a digital display I should be checking for an error code, like a boiler?

No, not in the way a gas boiler does. Standard and indirect Megaflo/Megaflo Eco models have no fault-code display at all — diagnosis is entirely symptom-based as covered in this guide. Some electric SystemFit models have a basic thermostat status light, but this is not a comprehensive code system and should not be relied on as the primary diagnostic tool — ****.

What qualification do I need to work on a Megaflo cylinder?

Installation, commissioning, and any work on the safety-critical components (PRV, T&P valve, expansion vessel, pressure-reducing valve, tundish) requires Part G3 competency under the Building Regulations — typically evidenced by a recognised unvented hot water systems qualification. General plumbing competency alone is not sufficient for this specific appliance category.

Regulations & Standards