Megaflow problem - is it thermostat?

TheGreenFrog

I have had an intermittent problem with my hot water which is now no longer intermittent! I have a megaflow system - what happens is that when the hot water is switched on, the boiler fires up but then shuts down after about a minute. This is a probem restcited to the hot water, as central heating seems to work fine. I have taken the zone valve off, manually opened the valve and then watched the operation of the head unit when I switch on the central heating. It seems to work fine - the lever moves across to press the switch and the boiler fires up. When the boiler stops about a minute later, the head unit is still in the same position (i.e. pressing the microswitch). In addition, when the boiler does fire it is only after a long rest (so I switch the hot water on, the boiler fires, then goes off. If I then immediately switch off the water (zone head unit to off) and switch on again (with head unit switching on the microswitch) the boiler does not fire up).

So I am wondering whether the problem could be the thermostat on the megaflow - is the above behaviour consistent with that?

vacheron

This looks to be a thermal store, so there will be a thermostat (cylinder stat) on the tank.

If this is set too low, it will switch off the boiler once it thinks the tank has reached temperature and your zone valve will close (as the thermostat is connected directly to the zone valve).

it could also be that the thermal store is already full of hot water, so the return water coming back to the boiler is already hot, this will cause the boiler to go into overtemperature lockout and temporarily switch off iresepective of the position of the zone valve (there should be lights or codes on the boiler to indicate this). this would also explain why there is a preset delay before it will try to fire up again.

When the central heating is on, the return water comes back to the boiler via the radiators, so it is much cooler when it reaches the boiler, hence why thermal lockout isn't happening with CH calling (but your hot water will still be heated).

TheGreenFrog

https://forums.moneysavingexpert.com/discussion/comment/81968121#Comment_81968121

This problem has only started happening recently (and I have had the megaflow for 20 years) and the zone valve remains open when the boiler swicthes off (with power flowing through the microswitch). So it sounds as though your second explanation may be the problem - I will investigate. Thank you

vacheron

👍

Another thing is that thermal stores (as owner of one too) is they are renowned for collecting sludge at the bottom of the tank, restricting flow from the exit port which can cause boiler overheating, but you’d probably see that in the central heating performance too.

Rodders53

When was the boiler {make and model of boiler and fuel type may aid diagnosis} and Megaflo (unvented HW tank) last serviced?

The Megaflo will have two thermostats (one for / maybe built into) the immersion element. The other will switch the zone valve motor on/off. The zone valve microswitch calls for heat from the boiler, and the circulating pump to run. You describe that as happening correctly.

Does the return pipe from the Megaflo coil get hot (or even the flow pipe between the zone valve and cylinder) as this sounds, to me, more like a low flow issue = a blockage in the HW circulation pipework. Perhaps the zone valve spindle is moving without the actual port opening fully? {Although that may not be very likely?? as you ststed you manually opened it with head removed.}

TheGreenFrog

https://forums.moneysavingexpert.com/discussion/comment/81968201#Comment_81968201

Its a Heatrae Sadia CL300. The boiler is oil-fired and I get it serviced annually (last time a few months ago). I don't think I have ever had the megaflow serviced although I do vent it every 12-18 months (probably due again - could that be the problem?).

The problem was noticed because the water was not heating up. When I took the zone head off the valve was stiff to open and I have loosened it a bit. But I thought that if valve wouldn't open then head unit cannot trip the microswitch = boiler won't fire?

The boiler overheat did trip at start of the prblems (with lights to prove) but no overheat lights since that first time.

WIAWSNB

https://forums.moneysavingexpert.com/discussion/comment/81968209#Comment_81968209

I was going to ask if the actual zone valve was smooth in operation. It obviously should be, and swing easily from open to close with a distinctive clunk at each end - it should be finger-doable.

So, I would suspect the valve here. Being stiff, it would likely have caused wear within the 'head', and probably a fair amount of gear 'play'. That means that the motor thinks it's moved the valve, and the microswitch still gets tripped, but the gears that move the actual valve simply take up half the movement with this wear, so the valve is left only partially open.

I guess you can test this (you may already have) by removing the head and using pliers or a spanner to make completely sure the valve is fully open - reaches the end 'clunk'. Then turn on the DHW.

TheGreenFrog

https://forums.moneysavingexpert.com/discussion/comment/81968230#Comment_81968230

Yes I did that, and the boiler still cut out (with head unit still in on position and power flowing through the microswitch). Valve too stiff to turn by hand but managed it with pliers.

Rodders53

HW cylinder coil pipes? Do they (both) heat up to boiler stat set temperature (60-70C) or stay relatively cool (I find 60+C is too hot to keep hand on for any length of time).

I asked for the boiler (now known to be an Oil burner) make and model. Don't think my Grant condensing oil boiler would cut out and automatically reset like you describe yours doing (it's a manual reset button on Grant boilers) …

Have a word with your service technician for guidance on the boiler and why this is happening, perhaps?

TheGreenFrog

Yes I will get in touch with the boiler man. Boiler is a Clyde CK10. There are temp gauges on the boiler outflow and inflow pipes - but boiler is cutting out when inflow temp shows only about 50c (with outflow about 75c) and when water tank not anywhere near full heat and with zone valve still calling for heat. So I don't know what is going on.

And the one time the overheat trip function definitely kicked in I had to manually reset the boiler.

TheGreenFrog

https://forums.moneysavingexpert.com/discussion/comment/81968340#Comment_81968340

Yes the coil pipe does heat up to >70c. But only for a couple of minutes before the boiler shuts off.