Megaflo Repairs in
London
Specialist Megaflo and unvented cylinder repairs by G3-qualified engineers. 24/7 emergency response for tundish discharge, pressure loss, immersion faults and expansion vessel failure.
Tundish or PRV discharging?
Switch off the cylinder power and call us immediately — this is a safety warning.
Book a Megaflo Repair
Same-day and emergency appointments available.
24/7 emergency dispatch — call now
What is a Heatrae Sadia Megaflo?
The Megaflo is an unvented mains-pressure hot water cylinder — not a boiler. It stores and heats water at full mains pressure, delivering consistent hot water to every outlet without a cold water tank in the loft.
Mains-fed storage
Cold water enters directly from the mains. No header tank required. Stores 100–300 litres, supplying every tap and shower simultaneously at full pressure.
Electric or indirect heating
Heated by immersion elements (direct) or a boiler coil (indirect). Most London homes use an indirect Megaflo with an immersion as backup.
Pressure & temperature control
An expansion vessel absorbs thermal expansion. A PRV limits inlet pressure. A T&P relief valve protects against dangerous overheating.
G3 regulated by law
Unvented cylinders must be serviced and repaired by a G3-qualified engineer under Building Regs Part G. Unqualified work invalidates your insurance.
Megaflo & cylinder installation has its own dedicated pages
This page covers repairs and emergency fault diagnosis. For new installations, replacements and vented-to-unvented conversions, see our dedicated installation pages with full sizing guidance, pricing and project information.
Megaflo Repair Costs
Clear, upfront pricing — no hidden extras. Fixed quote after diagnosis.
Same-day repair — parts carried on board
Indicative Pricing
£180–£450
Diagnostic visit & common repairs included
Fixed-price quote after survey — no hidden extras, no call-out charge.
No call-out or diagnostic fee
Indicative Pricing
£150/hr +VAT
Hourly rate for complex or multi-fault repairs
Fixed-price quote after survey — no hidden extras, no call-out charge.
Common Megaflo faults
Megaflo faults are mechanical and hydraulic — not boiler-related. Here are the issues we most commonly repair across London:
PRV dripping or discharging
Overpressure in the system or a waterlogged expansion vessel. Never cap this pipe — it is a critical safety device.
Expansion vessel waterlogged
The internal diaphragm has split. The PRV lifts repeatedly as a result. Needs re-pressurising or replacing.
No hot water / cold water only
Immersion heater element failure or thermal cutout (TCO) has tripped. TCO requires manual reset by a G3 engineer.
TCO keeps tripping
A failed thermostat is allowing water to overheat past 90°C. Thermostat and TCO should always be replaced together.
Tundish discharging continuously
T&P relief valve or PRV is lifting. Switch off cylinder power immediately and call us.
Weak hot water pressure
Blocked inlet strainer, partially closed isolation valve, or limescale restricting the mains inlet.
Lukewarm water / slow recovery
Failing or scaled immersion element, or thermostat set too low. Should be 60–65°C.
Banging or kettling noises
Limescale on the immersion element causes localised boiling. Very common in London's hard water postcodes.
Cylinder leaking at base
Corroded immersion heater boss or failed compression fittings. Severe cases may require full replacement.
Emergency: tundish or PRV discharging
Switch off the cylinder's power supply and call us immediately on 020 7458 4949.
What's included in a Megaflo service?
Heatrae Sadia recommend annual servicing by a G3-qualified engineer. Our full Megaflo service includes:
- Test T&P relief valve — lift and reseat
- Inspect and test the pressure reducing valve (PRV)
- Check and re-pressurise the expansion vessel
- Test the thermal cutout (TCO)
- Inspect tundish and discharge pipework
- Check all pipework connections for leaks
- Inspect immersion elements for scale and deterioration
- Issue written service certificate
What London clients say
"Our Megaflo started discharging from the overflow. RARE came within 90 minutes, diagnosed a failed expansion vessel and replaced it the same visit. Exceptional service."
Sarah B.
Knightsbridge
"No hot water on a Sunday — TCO had tripped. RARE reset it, replaced the thermostat and had us back up and running by midday. Outstanding."
Tom F.
St John's Wood
"Annual Megaflo service carried out brilliantly. They found the expansion vessel losing pressure before it became a problem. Neat, professional and on time."
Nadia W.
Marylebone
Megaflo Repair Questions Answered
Common questions about Megaflo repairs, faults and emergency callouts in London.
Still have questions?Related reading from our Knowledge Hub
Authoritative Sources & References
- SourceHeatrae Sadia — Unvented Cylinder Manufacturer
Heatrae Sadia — The official manufacturer page for Heatrae Sadia unvented cylinders, including the Megaflo range — the UK's leading unvented hot water cylinder.
- SourceMegaflo Eco — High Performance Unvented Cylinder
Heatrae Sadia — Official product specification page for the Megaflo Eco unvented cylinder, covering capacity, heating options and installation requirements.
- .gov.ukApproved Document G: Sanitation, hot water safety and water efficiency
gov.uk — Building Regulations Approved Document G covers cold water supply, water efficiency, hot water supply and systems — including G3 requirements for unvented hot water storage cylinders.
Cylinder beyond repair? It may be time for a replacement
If your Megaflo is over 10–15 years old, repeatedly failing, or showing signs of internal corrosion, a replacement may be more cost-effective than continued repairs. See our dedicated Megaflo installation page for replacement options, sizing and pricing.
Serving London's most iconic neighbourhoods.
Our engineers are established across prime London — near landmarks including Berkeley Square, Harrods, Eaton Square, Regent's Park and Hampstead Heath.
Megaflo repairs London — related guides
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Central Heating
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Power Flushing
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Boiler Installation Costs
Full London pricing guide
Boiler Replacement Guide
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PPM Membership
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Megaflo vs OSO
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Reviewed by RARE Plumbing London Engineering Team
Gas Safe Registered Heating Engineers · Reviewed 3 September 2026