Buzzing Central Heating = no heating

Can someone confirm my suspicions.....
New house, well 5 years old, and CH has stopped working though hot water is ok.
When turn CH on I get a buzzing from the airing cupboard. Noise is coming from a white box ie :
myson power extra mpe322 ipx2 6w 36mA motor load max ambient 52 'c water temp 5' to 95' 2 ports ie < ch dhw > porta portb
This has a manual override lever which is a bit stiff but can be moved up and then to one side to sit in a slot. If I do this the radiators then get hot.
I now understand that this box is an actuator and sits on a valve. So I figure this actuator is the problem unless it is the valve that is sticky. So removed actuator from valve turned CH and it starts its repeated buzzing again. Also the valve does not appear sticky. So I think it is the actuator except I am confused in that when I move the manual override lever - remember actuator is not connected - then the heating appears to start ie I hear the pump start up. The lever did not appear to turn the whole in the centre of the acutator. I thought this should happen ie that the valve would need to be turned. The valve appears to stay in an 11'0'clock position which I think means supply both HW and CH.
I have had a look for the actuator and seen the myson mpe322 at :
http://www.discountedheating.co.uk/cgi-bin/sh000001.pl?ACTINIC_REFERRER=http%3a%2f%2fwww%2ediscountedheating%2eco%2euk%2fshop%2facatalog%2fsearch%2ehtml&REFPAGE=http%3a%2f%2fwww%2ediscountedheating%2eco%2euk%2fshop%2facatalog%2fOnline_Catalogue_Thermostats_306%2ehtml&WD22&PREVQUERY=REFPAGE%3dhttp%253A%252F%252Fwww%2ediscountedheating%2eco%2euk%252Fshop%252Facatalog%252FOnline_Catalogue_Thermosta s_306%2ehtml%257C%257C%257Chttp%253A%252F%252Fwww%2ediscountedheating%2eco%2euk%252Fcgi%2dbin%252Fss000001%2epl%253FSS%253Dmyson%252Bactuator%2526ACTION%253DGo%252521%2526PR%253D%2d1%2526TB%253DA%2526SHOP%253D%257C%257C%257C%26PREVQUERY%3dREFPAGE%253Dhttp%25253A%25252F%25252Fwww%2ediscountedheating%2eco%2euk%25252Fshop%25252Facatalog%25252FOnline_Catalogue_Thermostats_306%2ehtml%25257C%25257C%2525 C%2526PREVQUERY%253DSS%25253Dmyson%25252Bactuator%252526ACTION%25253DGo%25252521%252526PR%25253D%2d1%252526TB%25253DA%252526SHOP%25253D%252B%2526RANDOM%253DNETQUOTEVAR%25253ARANDOM%2526PAGE%253DSEARCH%2526SS%253Dmyson%2526TB%253DA%2526GB%253DA%2526ACTION%253DSearch%26RANDOM%3dNETQUOTEVAR%253ARANDOM%26PAGE%3dSEARCH%26SS%3d322%26TB%3dA%26GB%3dA%26ACTION%3dSearch&PN=Online_Catalogue_Myson_277%2ehtml% 3am#am
but this just seems to refer to valves and not acutators.
Am I right in thinking its the actuator thats at fault and can anyone recommend where to obtain a replacement? Thanks.
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Zordiac wrote:

Yup. Its a 'motorised valve'
Current drives the motor, the motor opens the water flow AND swiches on the pump.
The override lever also does the same.
You can get spare motors at nearly all plumbing supply places - take the old one in.
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I have had a look for the actuator and seen the myson mpe322 at :
http://www.discountedheating.co.uk/cgi-bin/sh000001.pl?ACTINIC_REFERRER=http%3a%2f%2fwww%2ediscountedheating%2eco%2euk%2fshop%2facatalog%2fsearch%2ehtml&REFPAGE=http%3a%2f%2fwww%2ediscountedheating%2eco%2euk%2fshop%2facatalog%2fOnline_Catalogue_Thermostats_306%2ehtml&WD22&PREVQUERY=REFPAGE%3dhttp%253A%252F%252Fwww%2ediscountedheating%2eco%2euk%252Fshop%252Facatalog%252FOnline_Catalogue_Thermostats_306%2ehtml%257C%257C%257Chttp%253A%252F%252Fwww%2ediscountedheating%2eco%2euk%252Fcgi%2dbin%252Fss000001%2epl%253FSS%253Dmyson%252Bactuator%2526ACTION%253DGo%252521%2526PR%253D%2d1%2526TB%253DA%2526SHOP%253D%257C%257C%257C%26PREVQUERY%3dREFPAGE%253Dhttp%25253A%25252F%25252Fwww%2ediscountedheating%2eco%2euk%25252Fshop%25252Facatalog%25252FOnline_Catalogue_Thermostats_306%2ehtml%25257C%25257C%2525C%2526PREVQUERY%253DSS%25253Dmyson%25252Bactuator%252526ACTION%25253DGo%25252521%252526PR%25253D%2d1%252526TB%25253DA%252526SHOP%25253D%252B%2526RANDOM%253DNETQUOTEVAR%25253ARANDOM%2526PAGE%253DSEARCH%2526SS%253Dmyson%2526TB%253DA%2526GB%253DA%2526ACTION%253DSearch%26RANDOM%3dNETQUOTEVAR%253ARANDOM%26PAGE%3dSEARCH%26SS%3d322%26TB%3dA%26GB%3dA%26ACTION%3dSearch&PN=Online_Catalogue_Myson_277%2ehtml %
Try http://tinyurl.com if you want to post long addresses, most peoples newsreaders will split the lines making it impossible to refer to the link.
Colin
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Thanks chaps for the advice. I went into the local Plumb Center and picked up an acutator for only 35 inc vat. Fitted it and now the CH is working fine.
Only concern is that this is the 3rd actuator in 4 years. First one in was duff and now I've replaced the second. Also when I contacted another local plumbing merchant the person I spoke to said that their relative was on their 4th actuator in 6 years! So are Myson products duff? I noticed with the one I bought that there was no mention of any guarantee.
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     snipped-for-privacy@hotmail.com (Zordiac) writes:

Hum, that's what I paid for a whole mid-position valve at a local plumbers merchant. I noticed the motors alone (not the whole head) for 16 somewhere (Wickes or B&Q probably).
--
Andrew Gabriel

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