Vibration in central heating system

19 May 2004
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United Kingdom
This problem started happening only recently. It happened for a week or so last month then stopped and now it's started again.

The installation is a Worcester Bosch boiler installed some 5 years ago in a gravity system. The system was converted to a closed system when a megaflo was installed about a year and half ago. In all this time everything was working absolutlely fine until now.

To explain when the vibration sound begins. The HW heating is off by this time and only the CH is on. When temperature is reached (the CH light goes off on the programmer), and the boiler shuts down and that's when the buzzing vibration sound begins until the pump stops after a minute or so (guess it's normal for the pump to run for a bit after the boiler shuts off).

When the boiler shuts down and the pump is still running, should both control valves (CH & HW) be closed? When the vibration occurs I've pushed the manual override on the CH valve to 'open', the vibration and sound stops.
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It sounds as if an automatic bypass valve should of, but has not, been fitted. This allows the water to flow around the system even when all TRVs and zone valves are closed.
Matthew I think there may have been a by-pass valve before the megaflo was installed.

This megaflo unit came as factory pre-assembled with all the pipework, valves, pump and various other gauges, release valves etc. all neatly attached around the megaflo and also with programmer, room stat and expansion vessel (for converting to a closed system). The whole system was fitted by a qualified unvented system plumber. I seem to recall that Megaflo had said that the system was designed to work with two valves only, perhaps there is some other mechanism built-in to do what a by-pass valve would normally do?
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Thanks for the link greemyamochop. That's exactly the system I've got installed. I've checked my megaflo and there is the bypass valve illustrated in the diagram on pg 30 (it has a green plastic top that moves quite freely with a dial and numbers!?). When in normal operation and CH heating is on with boiler on, the exit (return) side of the by-pass valve is not hot. When the boiler shuts off with the pump operating in overrun, the return side gets hot. I guess this indicates that the by-pass is working correctly.

I had the Megaflo engineer check the system a week ago, but frustratingly the noise had stopped a couple of days before he showed up and couldn't be demonstrated, so he just checked that the expansion vessel was OK, changed the HW valve(!) and left. A week later and the noise returns!

There is a section on pg 22, in the manual at the link, on the bypass and how it is setup, which I guesss would have been done by the installer at the time of installation. Could it be an annoying intermittent fault with a component or just some residual air in the system that keeps moving around?
A lot of engineers i know when commissioning just close the bypass fully then open it a crack to allow just enough water through, obviously not the correct way.

Try turning your pump down to speed 2 and follow the commissioning steps in the PDF then youre basically doing with the bypass what you were doing with the zone valve.

So, its probably open but not enough!!

Let me know...
hmmm...I've read the section on calibrating the by-pass and although it's concise and looks a simple task, the whole setup with all the pipes, valves and dials looks intimidating so I'm a bit nervous about undoing the valve in case I'm in a situation where I find I'm out of my depth and have made things worse - Heatrae and the council were quite specific about who can install or service an unvented system. When the system comes on and I hear the noise I'll check the temp of the outlet pipe of the by-pass and see if it gets hot when there is a noise. If it isn't getting hot, then I guess we've most probably found the cause. In any case I'll run it past the techinical people at Heatrae after the bank holiday and see what they advise - if they say I can do it and it will not invalidate the warranty then fine.

The more I think of it, the more it seems like a malfunction of the by-pass.

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