When a Megaflo cylinder stops giving hot water, the cause is usually one of a few simple things: the immersion heater has been switched off, its thermal cut-out has tripped, or a thermostat or element has failed. This guide covers how to check each one, where the reset button is, and which replacement part fits which Megaflo. It also covers the immersion heaters for the larger Megaflo Commercial cylinders. Everything here is taken from Heatrae Sadia's own installation manuals and data sheets.

Safety first: the immersion heater terminals are live at mains voltage. Always switch off at the isolating switch before removing any cover. Heatrae's manuals say only competent persons should work on the cylinder, and a Megaflo is an unvented cylinder, so anything beyond resetting the cut-out belongs with a qualified installer. Never bypass a thermal cut-out.

Which immersion heaters does your Megaflo have?

Look at the data badge on your cylinder. The model tells you whether it is direct (heated by electric immersion heaters) or indirect (heated by a boiler through an internal coil). Our guide to direct and indirect cylinders explains the difference.

  • Megaflo Eco direct: titanium immersion heaters, usually two, an upper and a lower. The 70 litre has one, and some larger direct models carry three or four. Each immersion has its own thermostat and its own manually reset thermal cut-out.
  • Megaflo Eco indirect: one Incoloy immersion heater as an electric backup to the boiler. There is a thermal cut-out on the immersion and a second one on the boiler side, wired through the 2 port motorised valve supplied with the cylinder.
  • Rating: each Megaflo Eco immersion is 3kW at 240V (2.8kW at 230V), on an AC supply only.
  • Warranty: Heatrae covers Incoloy immersion heaters for 2 years and titanium immersion heaters for 5 years. For how element materials differ, see which immersion heater is right for you.

Megaflo not heating? Work through these checks

This is the fault list from Heatrae's Megaflo Eco manual, in the order it suggests.

Hot taps run cold

  • Immersion heater not switched on. Check it is switched on, including any timer.
  • Immersion heater thermal cut-out has tripped. Reset it by pushing the button (see the next section).
  • Indirect only: the programmer is set to central heating only (set a hot water programme), the boiler is not working, the indirect thermal cut-out has tripped (reset it, then check the indirect thermostat), or the motorised valve is not connected correctly.

No water at all from the hot taps

  • The mains water supply is off (check the stop cock), the strainer is blocked (it sits in the pressure reducing valve and can be cleaned), or the cold water combination valve has been fitted incorrectly.

Very hot water running continuously from the T&P relief valve

  • Heatrae lists this as a thermal control failure. Switch off the power to the immersion heaters and shut down the boiler, but do not turn off the water supply. Once the discharge stops, the thermal controls need checking and any faulty one replacing.

Milky water

  • Not a fault. Water from a pressurised system releases tiny air bubbles as it flows, and the milkiness clears after a short while.

How to reset a Megaflo immersion heater

Each Megaflo Eco immersion heater thermostat has a built-in thermal cut-out. If the thermostat fails and the water overheats, the cut-out switches the immersion off, and it stays off until someone resets it by hand.

  • Switch off the electricity supply to the immersion heater at its isolating switch.
  • Undo the large screw on the control cover with a flat-ended screwdriver, then lift the cover from the bottom until it comes away.
  • Press the thermal cut-out reset button on the thermostat. Direct models have a cut-out on each immersion, so check them all.
  • Refit the cover by tilting it, lining up the top two lugs with the holes in the housing, then hinging it down until it snaps into place. Tighten the screw, then switch the power back on.

If it trips again, the cut-out is doing its job and something else is wrong, usually a failed thermostat. Do not keep resetting it, and never bypass it. A qualified installer should test the thermostat and element. The usual replacement for a direct Megaflo Eco is the 95612720 combined thermostat and thermal cut-out.

Thermostat settings

  • The Megaflo Eco immersion thermostat is factory set at 60°C and adjusts from 10°C to 70°C. The maximum working temperature is 70°C.
  • In very hard water areas Heatrae recommends limiting it to 60°C to 65°C, to reduce limescale building up on the element. Heatrae's lifetime cylinder warranty excludes damage caused by scale.

Megaflo immersion heater part numbers

Check your cylinder's model before ordering, as the older Megaflo ranges use different elements. Use only the recommended Heatrae Sadia replacement: the manual warns against fitting any other immersion heater.

Megaflo Eco direct

Megaflo Eco indirect

Older Megaflo cylinders

Every element and thermostat we stock is on our immersions and elements page.

Replacing or descaling a Megaflo immersion heater

This is a job for a qualified installer. In outline, Heatrae's procedure is:

  • Turn off the mains water, isolate the electricity supply and turn off the boiler.
  • Run a hosepipe from the drain cock to a discharge point lower than the cylinder, open a hot tap near the cylinder, and open the drain cock to drain it.
  • Remove the immersion housing cover and disconnect the wiring from the thermostat.
  • Unscrew the immersion heater with the 95607861 immersion heater spanner and lift it out carefully.
  • To descale rather than replace, remove the scale carefully. Never use a sharp tool, as it can damage the element surface.
  • Make sure the sealing surfaces are clean and the gasket is undamaged. If in doubt, fit a new immersion heater gasket (Heatrae part 7035165).
  • Refit the immersion heater without trapping the gasket, then rewire, check, and close and secure the cover.

Heatrae recommends a service each year, timed to match the boiler service. The immersion heater boss also doubles as an inspection opening for looking inside the cylinder.

Megaflo Commercial immersion heaters

The Megaflo Commercial range runs from 400 to 2,500 litres. On the direct models the immersion heaters are not factory fitted: you choose them separately to suit the electrical supply and the reheat time the building needs. Each direct cylinder has an upper and a lower boss.

Single phase or three phase

  • Single phase: 6kW and 9kW.
  • Three phase: 12kW, 18kW, 24kW, 30kW, 36kW and 45kW. Heatrae's range goes up to 54kW.
  • The maximum rating each boss can take depends on the cylinder size. The element allocation and maximum kW tables in the Megaflo Commercial installation manual (linked below) give the figures for each size.

How long will it take to heat?

Heatrae gives a simple formula for direct heat-up time: minutes = litres x temperature rise ÷ (total kW x 14.3). Its worked example is a 500 litre cylinder heated through 45°C with 57kW of elements: 500 x 45 ÷ (57 x 14.3) = about 27.6 minutes. Halve the kW and the time doubles, which is why element choice matters so much on a commercial job.

Wiring and settings

  • Every Megaflo Commercial immersion heater has a thermostat in the centre of the heater plate and a cut-out fitted beside it. Both must be wired in series with the operating coil of a contactor, which is not supplied.
  • The manual's wiring notes specify two external contactors: the one controlled by the thermostat must be rated for 10,000 cycles, and the one controlled by the non-self-resetting cut-out for at least 30.
  • Set the cut-out to 80°C and the thermostat to no more than 70°C. Above 70°C the cut-out can trip as a nuisance. The maximum working temperature is 80°C, and in very hard water areas Heatrae suggests 60°C to 65°C.
  • Heatrae recommends servicing every 12 months, and periodic descaling of the elements in hard water areas, which means draining the cylinder.

Browse the full range on our Megaflo page, from the domestic Megaflo Eco to cylinders such as the Megaflo Commercial 500 litre direct. Choosing a new cylinder? See choosing the correct Megaflo cylinder.

Megaflo manuals

Heatrae Sadia's own documents, free to download, with wiring diagrams, reset button locations and full spare parts lists.

Quick answers

Does a Megaflo have an immersion heater?

Yes. Direct Megaflo Eco cylinders are heated by their immersion heaters, usually two. Indirect models have one as a backup to the boiler. On Megaflo Commercial direct cylinders, the immersion heaters are bought separately.

Where is the Megaflo immersion heater reset button?

On the thermostat, under the control cover over the immersion heater. Switch off the power before removing the cover. On a Megaflo Commercial indirect cylinder, the thermal cut-out reset sits under a cap on the controls.

What tool removes a Megaflo immersion heater?

Heatrae's own immersion heater spanner, part 95607861. The cylinder must be drained first.

Titanium or Incoloy?

The direct Megaflo Eco uses titanium elements and the indirect uses an Incoloy one, each with its own thermostat and fixings, so order the part listed for your model.

Not sure which part you need?

Call us on 01934 253600, or send a photo of your cylinder's data badge or the old immersion heater on WhatsApp to 07340 354023, and we will match the part for you. Free delivery over £150 ex VAT.