Central heating "Thump", Help!

[MickyMann]

Hi everyone, I hope someone can help solve this one.
We've a 12 year old (at least) Worcester RSF280 Combi system (not condensing) which has, over the last couple of months, developed a "thud" seemingly in the house pipework when the CH pump starts up and it seems to be getting more noticeable.
It's not unlike water-hammer when a cold-water tap is turned off, except that it happens when the pump starts. Moreover, it isn't apparent when the system is heating the house in the early morning, it only happens when the system is hot and the room-stat is calling for cyclic off-on to maintain temperature.
The system sequence seems OK insofar as the fan and diverter operate then after a 12 second delay the pump starts (usually accompanied by a "thud") then after a few seconds the igniter starts and the boiler fires up.
The radiators have been bled repeatedly with little gas being present each time.
When cold the system pressure sits around 0.8bar and when hot, never exceeds about 1.4bar and it's lived quite happily like this for years.
I thought I'd sorted it out today when out of desperation I depressurised and part-drained the system to check the expansion vessel charge pressure and found it to be lower than the gauge could measure (<0.3bar) so I pumped it back up to about 0.6bar and refilled and re-pressurised the system; it seemed great for a while but as the system had been off for about an hour, when it'd been running a while and everything heated up, the "thud" returned.
I'm loathe to call in a professional at this stage as after talking to neighbours who've had various boiler problems (but none like this) I get the idea that many seem to fault-find by a "swop-it-and-see" routine iincurring substantial charges for unnecessary parts in the process which as a retiree I can ill afford.
Surely someone must have come across a problem like this and can advise, please!
Mick

 

[Breesey]

You're convinced it's the pump, you can change the pump yourself!

 

[DP]

Will not be the pump.

 

[MickyMann]

You're convinced it's the pump, you can change the pump yourself!

I'm not convinced about anything, that's why I'm asking; I'm naive enough to think that someone out there has experienced this problem before and will give me a definitive "It's xx that's causing the problem".
Mick

 

[MickyMann]

Will not be the pump.

Any ideas what it might be then? diverter valve, diverter motor? although I guess if you knew you'd have mentioned it.
Mick

 

[DP]

I do know the answer but component is within the boiler and that is all I can say on this forum.

although I guess if you knew you'd have mentioned it.

In fact if I did not know the answer, I would not have posted. Pump just spins regardless of the demand type, so will not be the pump

 

[MickyMann]

DP, am I to assume your response is because I'm not a "GS fitter" and if I were to consider a DIY repair, would not be "qualified" to work on a purely wet-plumbing/electrical issue?

Hmmmm
FYI, if I call in a "professional" to do work for me, I like to know in advance what he'll be doing and WHY he'll be doing it and more especially, I want to know that HE knows why he's doing it (I am paying for his expertise after all); I know too many people who've had supposed "pro's" rip them off by coming in and changing parts one at a time until the problem is fixed (certainly not just plumbers), and I'm especially wary of anyone who uses the "closed-shop" excuse to withhold the information that I believe I've a right to know to make an informed decision.

Anyway, it’s fixed now, and the person who fixed it was helpful, knowledgeable and not afraid to tell why the problem was happening, what he'd done to determine the cause and exactly what was required to fix it. Exactly what I expect of someone I'm expected to pay.
If any genuine DIYers want to know what caused the thud I'd be happy to pass on this information (and in case you're worried it didn't involve gas or venturing into the combustion chamber).

 

[mogget]

I have to admit I am curious as to the nature of the fault - as long as it doesn't involve any 'prohibited' information!

 

[oilhead]

What is the point of your post?
I would immediately have suspected the diverter valve. It is a common problem on Worcester oil combis when heating takes over from hot water, and the pump 'helps' the diverter open to the flow pipe.
On a gas boiler, this would not be a prohibited part to diy.
So I assume that you feel you have had bad service in the past, but this is no reason to treat contributors on a forum with the contempt you have.
One day, you may really need help, and there are many people on here who often give good FREE advice to help those who wish to help themselves. If you adopt the same attitude to tradesmen at your house, then I'm not surprised you get bad vibes.

 

[SimonH2]

So I assume that you feel you have had bad service in the past, but this is no reason to treat contributors on a forum with the contempt you have.

I suspect it may have been DP's "I know but I won't tell you" posts that triggered it.

But to be honest, just how often are people slagging off the "no idea, lets change <random but expensive part> and see if it changes anything" type of people that are prevalent in so many trades (plumbing and vehicles seem to be the worst - at least by reputation) ? The guy is obviously keen to have some idea, so that should he find himself with someone like that he'll be able to challenge them.

 

[MickyMann]

Oilhead....The point of my post???
I was asking so that...
(1) if it was user fixable I would know where to look and not have to replace on a trial and error basis (like those I mentioned).
(2) if it wasn't, then I'd know what to expect when a "competent person" turned up and not employ him if he'd had to "phone a friend" for advice.
What's wrong with that?
As you would've "suspected the diverter valve......common problem...". that is the type answer I was looking for but didn't receive from anyone, even with the proviso "it's a gsf job only".

Sorry, I don't consider my response to DP as contemptuous, merely an explanation of my reasoning.
You said "One day, you may really need help" well in this instance that's exactly what I was asking for and didn't get it, what I got was an "I know but I'm not telling" answer with no explanation, surely THAT'S a contemptible way to respond and from someone with 5000+ posts!

Exactly as SimonH2 said.

Mogget: It was the diverter. The mechanism, not the motor. It was operating much more slowly than a clean/new device and despite the visible lever giving the appearance of having completed it's changeover, the port vane was still moving inside. After the repair I wired it up on a bench and tested it; sure enough, the moves took a fraction longer than the 12 seconds that the boiler control allows, hence the "thud"
And if DP or anyone else wonders whether I'm "competent" to do this on a workbench, yes, I am, as an Electronics Development Engineer (retired) I consider myself probably more competent than most.

I hope this answer is of use to anyone who asks in future (it would've been nice for some who already knew this to answer for me in this way).

Mick

 

[MickyMann]

A (hopefully) final note on this.
Might I be so bold as to point to rule 10 of The forum Rules, which has the following link " //www.diynot.com/forums/plumbing/diy-gas.8090/ "

Quote from the 3rd last paragraph on that page....

"Water carrying parts of a boiler unrelated to gas or airflow, can be worked on without gas training, though you still have to be competent, for example electrically."

Mick