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Indirect (Boiler Fed) Megaflo Doesn't Stop Heating

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techweb

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Hi,

I noticed yesterday that when my HW is on, that the boiler runs constantly... I even tested this morning after it ran last night so no water usage overnight and it ran for 30 minutes before I turned it off on the controls. No dripping from Tundish.

My suspicion is the thermostat in the Megaflo which presumably is demanding heat continually. The boiler temp is 60c so it isn't going to overboil the water the cylinder as it reaches an equilibrium.

I'm not G3 qualified so cannot do any real troubleshooting myself but the three way zone valve appears to be operating correctly and hot pipe out of this is then fed into the two way zone valve. There is no resistance on the lever on the two way valve which indicates to me that it has power, presumably by virtue of the HW being on on the controls and the cylinder stat calling for heat.

I'll ask my plumber to take a look but my suspicion is the cylinder stat at this stage unless there is a flaw in my logic. Could a faulty 2 way zone valve cause it to remain in the open position and demanding HW heat continually? I think not as otherwise it would continue running when the HW was turned off on the controls.

Thanks...
 
If the boiler stat and the cylinder stat are both set the same, or if the boiler stat is set lower than the cylinder stat then the cylinder stat will never be satisfied and the boiler will continue to run, the boiler stat should be set at least 5C above the cylinder stat set opoint.
 
John.g said:
If the boiler stat and the cylinder stat are both set the same, or if the boiler stat is set lower than the cylinder stat then the cylinder stat will never be satisfied and the boiler will continue to run, the boiler stat should be set at least 5C above the cylinder stat set opoint.
Click to expand...
Thank you, and I understand this, but nothing has changed. This appears to have just started happening.
 
Or the 2 port stuck open / micro switch engaged
 
If the cylinder stat is satisfied the valve will close, stiff resistance when pulling on the manual lever, if the boiler then keeps running then its the end switch in the valve but I think you said the lever is loose which means the valve is still powered open. Maybe just switch off all power and check the valve is closed, stiff lever resistance.
Then restore the power but just programme off the HW, valve should remain closed.
 
Last edited:
techweb said:
OK, so mine stops as soon as I turn off on the main control.
Click to expand...

So that means the store isn’t upto temp need to upp the boiler temp to 70 and cylinder temp to 60/55
 
techweb said:
Indeed, or of course that the thermostat is faulty.
Click to expand...

Wouldn’t say it’s that as it’s sensing the temperature they just normally fail

The temp probe is fully in the dry pocket ?
 
ShaunCorbs said:
Wouldn’t say it’s that as it’s sensing the temperature they just normally fail

The temp probe is fully in the dry pocket ?
Click to expand...
Well, i've eliminated the control valve or leak. If the stat fails, it may not tell the boiler shut off. I will adjust the stat down on the cylinder tomorrow but like I said, this hasn't been touched. I have my plumber coming to do some other work so will have him take a look and troubleshoot correctly.
 
techweb said:
Well, i've eliminated the control valve or leak. If the stat fails, it may not tell the boiler shut off. I will adjust the stat down on the cylinder tomorrow but like I said, this hasn't been touched. I have my plumber coming to do some other work so will have him take a look and troubleshoot correctly.
Click to expand...

Boiler temp been knocked down ?
 
techweb said:
Not been touched and per a suggestion on this thread I even increased it to see if it would stop.
Click to expand...

What boiler do you have ?
 
techweb said:
Valiant EcoTech 637. It's a Y system, with a 3 port valve into a 2 port then into the UVC.
Click to expand...

And the display for heating is reading 70 c ?
 
techweb said:
Valiant EcoTech 637. It's a Y system, with a 3 port valve into a 2 port then into the UVC.
Click to expand...
The default position of a MPV is HW which should sent 230V to open the 2 port valve with HW programmed on and a call from the cylinder stat, the 2 port valve end switch then sends 230V to fire the boiler, the two port valve should close when the cylinder stat is satisfied or when the HW is programmed off, I think you said it does close and the boiler shuts down when the HW is programmed off so looks like the cylinder stat. If it doesn't close when the HW is programmed off then it might be getting 230V from the MPV orange wire.
 
Just a reminder that thermostats on unvented cylinders, such as Megaflos, must be serviced and repaired by someone who holds a current unvented hot-water systems ('G3') qualification.
 
Chuck said:
Just a reminder that thermostats on unvented cylinders, such as Megaflos, must be serviced and repaired by someone who holds a current unvented hot-water systems ('G3') qualification.
Click to expand...
Although the process of swapping it is straightforward, I will have my plumber who is suitably qualified firstly confirm my fault determination and then swap the stat.
 
techweb said:
Yes, although I took it back down after the test to what it was before, 65.
Click to expand...

And you let it run for a couple of hours at 70 ?
 
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