Help for a newbie.

[The Soulman]

Hello all, I'm a newbie to the site and at this point I'd like to state that although I am pretty good at taking things apart / putting back together I am by no means a plumber.

I'm hoping that you guys might be able to help me to diagnose a problem with my boiler.
The boiler in question is a (12 ish yrs old) Vaillant 242 Turbomax, I'll try and keep this short, so here goes,
last week I lost hot water, (Heating still working fine) going from past experience I suspected the diaphragm.
I removed the old diaphragm only to find that it seemed in decent condition,(No holes) since I had bought a new one I replaced it anyway.
This is the first time I have done anything to the boiler myself, and I'm happy that I followed the correct procedeure when replacing the diaphragm.

When I tested for hot water by opening one of the wash basin taps half way (As I do usually) the boiler did not fire / supply hot water,
(I was not really surprised at this given the decent condition of the old diaphragm)
however, when I closed this tap and opened a bath tap fully, the boiler fired up no problem, after trying all the hot taps in the house, I discovered that with a tap at half open the boiler would not fire up, but at full open would fire up no problem.

I may have been able to get hot water last week before replacing the diaphragm by opening the taps fully, but as I only usually open them half way I'll never know one way or the other.

So guys, its over to you the experts, obviously there is some relation to the flow of water / the boiler firing, but what is it ?
is it some sort of sign that there is a problem somewhere else in the boiler, maybe a component beginning to fail ?
your help and advice would be very much appreciated,
Regards,
The Soulman.

[susmil]

probably one of three things
1) basket strainers perish colapse and reduce the flow of the water check it out in the manual there is a metal replacement set
2) also you may need to replace your diverter valve as the pistons do deteriorate you can get kits or a recon one if you have to, best to buy a new one.
3)scale
Its all about diagnosis, any monkey can keep on changing parts
best to call a pro.....

good luck
sus