Screeching noise from Vokera Mynute 28SE

[pantouphles]

Looks like a couple of other members have a similar issue* and a British Gas engineer is stumped so looking for some more suggestions ...

I have a Vokera Mynute 28SE with a cylinder in S plan configuration. The boiler fires, igniting the flames, then it makes a screeching sound for 10 to 20 seconds, the pressure rises then both the noise and pressure subside together. When the boiler is on supplying either CH or hot water there are no issues at all - radiators warm, pumps operating quietly etc.

The noise only seems to occur when the boiler has power but is off - ie there is no requirement for heat or hot water per the controls and all LEDs on the control box are off. It is also sensitive to the boiler thermostat setting, the higher the setting the more likely it is to happen. One night the boiler was firing every eight minutes, followed by the screech until I turned off the power to get some sleep.

The BG engineer heard the sound, agreed it was abnormal but couldn't find anything wrong. Any suggestions greatly appreciated.

* //www.diynot.com/forums/viewtopic.php?p=34233#34233 and
//www.diynot.com/forums/viewtopic.php?p=1952257#1952257

 

[mysteryman]

Is there a bypass? Many installers fail to understand that the purpose of pump overrun is to dissipate residual heat from the boiler. An S Plan prevents this because both 2 port valves will close when there is no demand. A bypass sending the water back to the boiler does not dissipate residual heat. This is why a Y Plan is better - there is always a port open, and if you - correctly - have one radiator on lockshields, you do not need a bypass. Contrary to what some say, a Y Plan is no more difficult to wire, and is simpler to plumb in, as you have one less motorised valve and no bypass.

A pump overrun boiler on an S Plan with no bypass is trouble, and can wear the pump out prematurely.

 

[pantouphles]

> Is there a bypass?
The manual says "An automatic by-pass is incorporated in the boiler and systems should be designed to ensure that with only one radiator turned on a flow rate of at least 350 litres/hour is achieved through the boiler." I understand that with a lower flow rate a manual by-pass should be installed - I haven't found one so assume (bit dangerous) it wasn't needed.

From your explanation I'm guessing that your hypothesis is:
1. the boiler is trying to dissipate heat to ensure safe operation
2. the water is flowing from the boiler and returning through the bypass without going through the cylinder or radiators
3. the water is therefore retaining heat
4. the boiler is failing to dissipate heat so is in fact accumulating
5. a safety mechanism is operating due to the accumulating heat
6. the safety mechanism is making the noise
Have I understood you correctly?

If I have, then the root cause is to do with the bypass. We've only been in this house for 6 months and AFAIK the boiler has been running successfully for approx 10 years so something must have failed or accumulated in the last six months.

Could it be that this automatic bypass has failed?

 

[pantouphles]

The central system has been fixed so I'd thought I'd post the causes and resolution.

Causes:
1) The valve to the hot water was broken. The valve was physically shut but demanding supply from the boiler. The boiler and pump would fire when the temperature fell below the temperature set by the boiler thermostat. The pump would push water at the shut valve and presumably round the bypass. The temperature would rise, trip the thermostat and the boiler would stop firing and switch off the pump. While the pump was pushing water round the bypass and the shut valve it made the shrieking noise.
2) The dry side of the expansion vessel had lost pressure so when the boiler fired and valves were close the pressure fluctuated wildly and over a couple of weeks the pressure gauge fell even though there was sufficient water.
3) The pump was very dirty so was fairly inefficient.

Solution:
1) Replace broken valve
2) Re-pressurize expansion vessel
3) Replace pump and power flush system