Megaflo / boiler / Nest combination not producing hot water

Discussion in 'Engineers' Talk' started by ERJ, Nov 11, 2022.

  1. ERJ

    ERJ New Member

    We have moved into a house which has a system involving a Vaillant boiler, indirect Megaflo and Nest. The Megaflo has been producing the hot water since we moved in (which we didn't realise) but the fuse blew the other day. We've now replaced the fuse. The boiler engineer was able to 'boost' the hot water from the boiler to fill up the Megaflo via the Nest, and by locking the two-way valve, but that hot water has now run out and we can't get the boiler to fill the Megaflo again. The Megaflo itself also doesn't appear to want to produce hot water. Clearly something has gone wrong somewhere (the Nest isn't telling either to produce hot water?) but we're a bit stuck. Any advice would be welcome on whether there's something simple we're missing, or do we need to get an engineer in (in which case, what kind as neither the boiler engineer nor the electrician seemed to really understand the system). Thank you!
     
  2. MGW

    MGW Screwfix Select

    Oh dear, I had the same problem, the heating and ventilation engineer admitted he did not have a clue on the wiring, lucky I did.

    But there are many ways to wire a boiler, and heat store, and domestic hot water, some rather complex, the heat store can be simply for domestic hot water (DHW) or it can be used to combine different heat sources. My brother-in-law had LPG, solar panels, and a wood burner heat the water heat store, and the central heating then worked from that.

    But this house the heat store is only used for DHW, and there is very little control, in summer just time.

    Most central heating follows a plan, we call them C, S, W, Y etc. There are variations with each plan, in the main C plan has no motorised valves, however this house is C plan and has two. But in the main W is only used when built into boiler, Y plan uses a three port valve, and S plan uses two x two port valves.

    Megaflo is a make, and it often means there is more pressure in the heat store, being the same as mains pressure, and they need regular testing. But not always.

    I am no plumber, I am an electrician, I need to understand how the plumbing works, to get the electrics to do what is required, but I don't play with pipes.

    With Y plan default is DHW, so seems unlikely you have Y plan, so seems most likely S plan, so there is likely two motorised valves, one for DHW and one for CH, the sensor/thermostat/timer/programmer which every name you want will likely just open the valve, and inside the valve there is a micro switch which will fire up the boiler.

    On the valve there is likely a lever to open the valve to assist bleeding the system, it normally does not open the valve enough to work micro switch, and can be latched open, latching the DHW open will likely mean when the central heating runs the DHW will heat up. Not a cure, but will help as temporary fix.

    If the electrical system opens the valve, that lever normally goes slack, so you can tell if open or closed.

    But really need to know more, pictures likely best way, and how many and how many ports each valve has.

    At a guess, and I can only guess, the immersion heater has failed, so you now have only the boiler to heat DHW. When a fuse ruptures it normally needs more than simple replacement, some thing caused it to rupture.
     
  3. ERJ

    ERJ New Member

    Thank you - this is really helpful! Yes, it sounds like we have an S plan. There is definitely a two-way valve coming from the Megaflo, not sure whether there is also one coming from the boiler but seems likely.

    The thermostat (Nest) does presumably open the two-way valve. The boiler mechanic we had round last week 'forced' the two-way valve open and then boosted the hot water - that appeared to produce hot water (not clear whether the hot water was produced by the boiler or by the Megaflo itself as he also switched that on) but it didn't last long and now won't do that. Based on what you've said, presumably it will have been the Megaflo producing the hot water as the micro switch won't have been activated by the valve being forced open. That makes sense.

    I agree something must have caused the fuse to blow. But I don't think that's the problem - or maybe it is but there's another problem too! It seems that the DHW was never heated by the boiler since we moved in, only the Megaflo. So I think something is preventing the boiler from producing / supplying DHW (it doesn't have any problem producing CH). I wondered (after a weekend of reading up!) whether the diverter vale in the boiler might need replacing? As the boiler is working (to produce CH) but not producing DHW. Does that seem likely? I'll try to take some photos later.

    Thank you so much for your help, much appreciated.
     
  4. ERJ

    ERJ New Member

    (*diverter valve, not vale!)
     

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