Megaflow CL170 intermittent noise/vibration

8 Jan 2012
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United Kingdom
Our Megaflow CL170 makes a regular noise/shudder when one of the hot water or central heating is on. It happens about every 10 minutes and seems to be becoming more frequent.

I cannot pin-point exactly where the shudder occurs but it seems to be around the bottom of the tank either where the cold feed or the flow in is.

We have lived in this house for 6 months. Before xmas we had the boiler replaced to a Worcester Greenstar RI24 and had the central heating and boiler pipework power flushed.

We have recently followed the procedure to recharge the air volume as some water was dripping through the tundish, which we assume was coming through the pressure and temp relief valve.

I do not know enough to know whether this is a problem with the just the megaflow or if the boiler system is involved.

It is a bit worrying though - hopefully someone can offer some useful advice. Thanks.
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Megaflo leaking is normal, it is flawed design. The marketing of it is superb, never mentions this part of the design, which is why pretty much every other unvented on the market is superior.

I'm assuming the clonk is ever since the 24Ri was 'installed'.

The Ri needs a water bypass fitted to enable the pump to overrun. If you haven't got one, or it is shut, there will be a clonk each time the boiler stops.

Alternatively, one the motorized valves may be the wrong way round (theres an arrow on the side).
Ok - many thanks.

The clonk probably only has been since the new boiler - I can't remember it before.

Is the water bypass part of the boiler or is it fitted in the region of the megaflow? We will go back to our 'installer' and question why it is clonking and see where we go from there.

Is the clonking likely to be damaging anything?

Would our previous bolier - a baxi barcelona - have had one of these water bypass? i.e. Why no donk before?

Many thanks if you have answer - there is a lot I don't know!
More information...

I made sure the hot water and central heating were off last night but the clonking continued, (so I was wrong earlier to say it was when the HW/CH was on). I double checked in the middle of the night too that the boiler was not on.

So, it would appear the clonk must eminate from the cold water supply to the megaflow? But why would this be doing anything either - no water was being used anywhere in the house.

Does the megaflow cylinder try and fill itself up all the time, even if the HW is off to maintain pressure? Is water is getting out somewhere, either from the tank through the PTR, leaky tap, leak?

Any help appreciated.
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I think I have located the problem...

After identifying it was a problem with the cold water, I found some info on water hammer / vibration in pipes and possible causes.

We have a downstairs toilet that lets by. To see if this was the problem I turned the water off to this toilet and hey-presto - no banging anymore - result.

I will have to fix the toilet but hopefully that is all.
I am surprised that you were looking at the cylinder when you were already aware of a defect on your toilet.

Its important to prevent damage to your boiler that its fitted with an auto bypass. You should check in the FAQ to see what they look like and see if you have one. If not ask the installer! It should have been included in the quotation installation! Its not optional ( but not required when a three port valve is used. )

Tony Glazier

DIYnot Local

Staff member

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