VCW advice please

I am to visit one of these with HW going hot/cold/hot. Not seen many of them - I'm too young
Will check for modulation, and sludge.

Modulation - I know it uses a diaphragm pump - manual says blow down the tube to test.

• Q1 - Is it a step thing straight to min gas pressure or a continuous variation?
  Q2 - Has anyone measured the pressure out of the pump?
  Q3 - I presumed modulation works off the NTC - but one of our owld contributors here said they didn't, can anyone confirm or deny??

The dhwhe:
If it's ye olde blacke thinge, is it likely/feasible to clean it or will it leak everywhere?
Not a very hard water area so not expecting limescale even if the disc wasn't changed in the DV, when the DV was changed recently (£300, owch!)

I know it's a diy forum so if anyone would prefer to answer off, it would be appreciated. Email in profile

is it a vaillant, i'm thinking it is and i,ve been to a lot of the 242's that do that and there all set at about 8mb (or is that the 142's i cant remember) when on the badge it says max 5.3mb. its just one screw, you will need to snip a loop of wire to get to it i(if no-one has done it before) then its the screw on the right hand side. (sorry if i'm getting confused with another boiler)

the dhw h/ex block up, if stainless one fitted these are easy to remove and clean, older types had the black plastic box cylinder type that leak, you can buy a kit to convert black one to stainless hrpc no 475286 £120 retail.

The dia pumps you can gently suck open the gas valve to go to high modulation, the pump fails because the hose pipe elbow clips crack and suck in air (same clips as for aps for baxi solo 3 hrpc no 439626 ), have had some of the gas valve bundy tubes block before though.


the modulation works of the ntc and is variable via diaphragm on gas valve

Thanks chaps, good info.
So with the black plastic h/e's, are they unlikely to block, or likely to leak if disturbed, or...?
I can see an "It didn't leak before you came" situation!

The black ones dont tend to block up, either way feel the inlet and outlet pipes, if the in is red hot from main heat ex but out is cold and burner is going on/off it means there is a blockage.

You can check the modulation via the diaphragm pump with the brightness of the green light on the control panel and of course the burner pressure and flame height. Diaphragm pump voltage modulates from 1 volt to 6 volts (terminals D1 and D2 on electronics board). I'd check the NTC resistor (250 - 3000 Ohms) and the DHW overheat stat (NO). I've spare s/h diaphragm pumps (I've never known one to fail) and other VCW bits if you need anything.

As for the old black plastic DHW H/Es, the secondary coils inside are microbore copper. These H/Es tend to leak around the top or bottom manifolds, although scale may then block a leak, so I would think more than likely to develop a leak if you try cleaning.