"Thump" when the CH Hot Water is turned on - Help

[macketty]

Hi! First post here, although I've been reading a lot of the helpful posts for some time, so I'm hopeful someone can help.

Every time our central heating system turns the Hot Water on, there's a loud "thump" from the pipework which I assume is an airlock. I've bled all the radiators (even though they're all hot and the noise only occurs when the hot water is on), but no air comes out.
I've had the auto air vent off from the feed into the hot water tank to confirm it was working and that water was available from the feed and expansion tank. All seemed ok there to.
Tried flushing the system from the drain cock to see if I could force any air back up to the expansion tank, but nothing seemed to happen.
The noise has been around for a while, but the effect of the airlock is more apparent now as No 1 daughter is back from Uni and the demands on the hot water for showers has suddenly increased! I assume the airlock is preventing the hot water from the boiler circulating properly to the tank and therefore there isn't as much hot water available.

Getting frustrated! Any help would really be appreciated.

Mac

 

[seco services]

i wouldn't say it was air causing it.
is it only one thump when the hw first turns on ?
if so i would try looking at the mot valve etc.

 

[JohnD]

there's a loud "thump" from the pipework which I assume is an airlock.

I think the thump is more likely the gas igniting in the boiler.

Stand next to the boiler and turn the boiler stat up and down to see.

When was it last serviced?

If you divulge the boiler make and model someone will know more.

I am not a pro.

 

[macketty]

Thanks for the quick response.
Karl.
Don't think it's the valve. I replaced the head about a year ago. The noise seems to be after that in the system. The valve seems quiet. Is there any way of eliminating it though?

JohnD.
The noise only happens the first time the valve diverts the flow to the hot water tank. The boiler does "fhrwumpf" when it ignites, always has done. The noise is definately from the pipework after the valve.

Thanks. Mac.

 

[JohnD]

If you divulge the boiler make and model someone will know more.

 

[gassafeman]

Do you have a Honeywell 3 port valve?

It happens a lot when the MV pulls back to HW only.

The head is electrical, the valve body has the mechanical parts.

Paul

 

[seco services]

Is there any way of eliminating it though?

only feeling it when it happens.

The noise only happens the first time the valve diverts the flow to the hot water tank.

Thanks. Mac.

now you say that i'm 100% with the valve being faulty
sounds like its thumping shut.
when the valve go's to hw only it is pulled back to close the heating port by a spring then hw the valve is in its relaxed position.
i would go for a faulty spring etc and the pump starting up forcing the valve shut.

 

[macketty]

JohnD.
Sorry meant to include the Boiler details: Potterton Profile, about 19 years old. Seviced last year.

Gassafeman.
Yes, Honeywell V4073A 3 position vlave.

Thanks. Mac

 

[macketty]

Hi Karl.
Follow what you're saying. Must admit, I hadn't credited the valve with being able to generate such a noise. I'll investigate more in the morning when the system's cool and see if I can feel the thump in the valve. Presumeably, if I remove the motorised head and move it by hand, I should be able to tell if it's the valve?

Mac.

 

[macketty]

Gassafeman.
Seems as if you're concuring with Karl? (Sorry, didn't read all of your post!)

Mac

 

[gassafeman]

Seco has explained what I would have posted. Change the body.

EDIT. Leave the head in place and set your system up to test it as per Seco's advise.

Paul.

 

[seco services]

I remove the motorised head and move it by hand, I should be able to tell if it's the valve?

Mac.

i doubt it very much not by hand.
you can always remove the metal lid and watch the valve as it closes etc when the pump first starts up.

you'll have to put heating on first just to get the valve to the heating position so you can watch it fall to hw only.

 

[macketty]

Thanks Paul.
I've got a spare valve body (it was cheaper to buy a complete valve last year, rather than just the head - never did understand that!) which I'll dig out tomorrow. You sound very definite. Is this common?

Mac

 

[gassafeman]

Very common.

You could test it right now...

Get your heating on with no demand for HW
Turn your heating off.

Create a demand for h/w (Turn up cyl stat if neccessary)

Feel the valve body when its 'shuts' to deliver h/w.

Paul

 

[macketty]

Thanks Paul.

Just ferreted under the floorboards and I think you're probably right. All that faffing about looking for airlocks! suupose a case of a little knowledge, etc, etc. Guess tomorrow's job is all mapped out now. Drain the system. Change the valve body. Fill-up and chase the new airlocks!

One question though: If I've understood what you're saying, is it not the head that's got a knackered spring and not the valve body assembly? Playing with the spare, new, valve body here, there doesn't appear to be any return spring in this assembly. Or have I got the wrong end of it completely?

Mac.