I need to drain our Megaflo to replace the immersion heater(s). The system is a mains-fed one, ie no tank in the loft. I just want to check the procedure for draining and re-filling the cylinder, please.
Turn off cold water feed.
Open hot tap and let as much water drain out as the air pressure thingy inside the Megaflo expels.
Connect hose to drain and drain it.
Question: The Megaflo will need to fill with air when the water drains out. Where does the air come from? Is there an automatic valve somewhere, or do I need to open something? Or does it get sucked in through the open hot water tap?
Filling:
Close drain valve, and open cold water feed.
Anything else???
Do these things come with an instruction book? If so, I have no idea where ours is, assuming that the builders ever left it.
Ah, many thanks! I eventually tracked down the installation manual, which says this (just in case somebody else needs to know at some time in the future):
8.4 DRAINING THE MEGAFLO UNIT
Switch off the electrical supply to the immersion heater(s) and shut down the boiler on
indirect units. Turn off the mains water supply to the Megaflo unit. Attach a hosepipe to the drain
c*ck having sufficient length to take water to a suitable discharge point below the level of the unit, at
least one metre below the unit is recommended. Open hot water tap nearest to the Megaflo to relieve
the system pressure. Open drain c*ck. If water fails to drain from the Megaflo vent the unit by
manually opening the Temperature/Pressure Relief Valve.
8.5 DESCALING IMMERSION HEATER(S)
Open the cover(s) to the immersion heater housing(s) and disconnect wiring from immersion heater(s).
Remove the thermostat by carefully pulling outwards from the immersion heater. Unscrew immersion
heater backnut(s) and remove immersion heater from the unit. A key spanner is supplied with the
Megaflo unit for easy removal/tightening of the immersion heater(s). Over time the immersion heater
gasket may become stuck to the mating surface. To break the seal insert a round bladed screwdriver into
one of the pockets on the immersion heater and gently lever up and down.
Carefully remove any scale from the surface of the element(s). DO NOT use a sharp implement as
damage to the element surface could be caused. Ensure sealing surfaces are clean and seals are
undamaged, if in doubt fit a new gasket.
Replace immersion heater(s) ensuring the lower (right angled) element hangs vertically downwards
towards the base of the unit. It may be helpful to support the immersion heater using a round bladed
screwdriver inserted into one of the thermostat pockets whilst the backnut is tightened. Replace the
thermostat(s) by carefully plugging the two male spade terminations on the underside of the thermostat
head into the corresponding terminations on the element.
Rewire the immersion heater(s) in accordance with Diagram 8. Close and secure terminal cover(s).
8.6 REFILLING SYSTEM
DO NOT switch on the immersion heater(s) or boiler until the system has been completely refilled.
Close the drain tap. With hot tap open, turn on mains water supply. When water flows from the hot
tap allow to flow for a short while to purge air and to flush through any disturbed particles. Close hot
tap and then open successive hot taps in system to purge any air. The electrical supply can now be
That's right. The cold feed shutoff and pressure reducing valve are usually located close by. Shut it. Open a hot tap untill the expansion volume has exhausted, and the water stops flowing. This can take a surprisingly long time! Time it to co-incide with a bath, so as not to waste the hot water! Then, there ought to be a drain c*ck plumbed in just at the cold feed, at the bottom of the cylinder ( colour-coded blue ring ). Attach a hose, and drain down. Leaving the hot tap open will speed this up, as air can enter this way. Otherwise, hold open the Temp / Pressure relief valve ( directly above the cold inlet, grey colour-coded ring, cleary marked as Temp /Pressure relief ) untill the gurgling stops and the cylinder is empty.
It will naturally fill with air during draining-down, either via the hot tap, the manually opened T/P relief, or by air bubbling back up the drain hose. Don't worry about it.
No. This will re-generate the air volume too, which is something you need to do occasionally ( anually here ) anyway, as per the instructions on the front.
Yes. There's an installation manual in pdf format here:
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don't thhink there's a user manual, other than the instructions for re-generating the air volume which is on a label on the cylinder. They should have left a Benchmark Log Book ( service history, basically ) with the installation.
The hot draw-off is at the top. The draw-off pipe extends some short distance down into the cylinder, to allow for the expansinon volume.
Regardless of how low the tap is, it will only drain as far as the end of the draw-off pipe, or untill the pressure in the expansion volume is exhausted. This is not low enough to replace the immersion elements, which are mounted on the side of the megaflo cylinder.
replying to GB, Terry Taylor wrote: I have an unvented megaflow DD145HE in which I need to replace the bottom heating element. I have good basic plumbing skills and a fully qualified electrician to assist. Looking at this thread it seems fairly straightforward, any reason why I should not tackle it?
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