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You can download the Installation Guide, the Service Instruction Guide and the User Guide (all as pdf files) from the manufacturer's website HERE. Just scroll down to the "Discontinued Gas Boilers" section, right-click the manual that you want and select "Save Target As... " from the menu.

One reason for the lock out is that a flame is not detected within 8 seconds of the start of the ignition cycle. There are a number of things that can cause this. So, have you tried following the faultfinding charts in the Maintenance and Service Guide for the boiler? To do this, you may...

I am not aware of any automatic flue dampers - for use with domestic gas fires - that are available in the UK. They are available is the US (e.g. Flue Sentinel), but the cost of importing one and having it installed (assuming it is possible) may be prohibitive. Also, such a device would have...

JDC, You can usually find brand new Potterton Suprima PCBs on eBay, for a fraction of the RRP. For instance HERE, or from a search like THIS.

There's a couple of free pdf documents that may help: Pipe Sizing - Basic Principles and Pipe Sizing - Tabulation Method

From THIS thread: "It is normal for the diverter valve not return to its resting position when the cylinder stat (or programmer) stops calling for DHW before the demand for CH ends. The diverter valve is held in place by the 240V that is applied to its Grey wire, from the programmer's 'DHW...

Pipemaster - made by Antex: They're available from HERE (code YP01B) and a number of other online suppliers (enter Pipemaster into your preferred search engine).

Sulphidation filters, like THESE, are used in areas where the gas supply has a high concentration of hydrogen sulphide, which reacts with copper pipework to form a black film (a copper sulphide deposit) on its inner surface. Flakes from the deposit can block injectors and the bypass of oven...

As the 'Replies' and 'Latest Post' columns are not being updated in this forum, authors will be unaware that anyone has responded to their post.

Not all thermostatic radiator valves (TRV's) have a bi-directional flow capability, so should be connected in the flow to the radiator. Are yours connected in the return from the radiators?

It is most likely to be a problem with the expansion vessel. For more information please see the pdf document HERE, or search these forums (there have been many posts concerning expansion vessels).

There's an article that explains how to size gas pipes HERE Edit: That's odd, this reply appears here twice but is not listed in the replies column.

There's an article that explains how to size gas pipes HERE

fjh, You might try disconnecting everything from the PCB except for the mains supply. If the fuse still blows when the power is switched on then you will know that the PCB is faulty. Otherwise, you may be able to identify the cause if the fuse only blows after a particular external component...

Mex, What type of fuse are you using as a replacement? It should be a 3.15 Amp Anti-Surge (T) fuse. Some suppliers refer to these as "Slow Blow" or "Time Delay" fuses. If you've been using Quick-Blow (F) fuses instead, then that would explain why they keep failing. Incidentally, I know...

You should be able to download an instruction manual from HERE. If you cannot find your boiler model there, contact Vaillant and they will send you a copy of the manual by post (usually free of charge).

Thanks Alan for correcting my mistake. Several years ago I dismantled a 3-port valve to try to understand how it worked. I assumed that the motor was asynchronous because I did not think that the inherently low starting torque of synchronous motors would be able to overcome the stiffness of...

When in DHW only mode, with the cylinder stat calling for heat, the Grey wire should be floating (not connected to any power source) - please see the second diagram from the top, in my last post. Hence, there should be no voltage present on the Grey wire, other than a small amount of noise...

It does make sense, and further suggests that your programmer and 3-port motorised valve are working correctly. A brief description of the operation of your CH system, with diagrams, may make things clearer. If the programmer's power supply was interrupted, the initial state of your system...

I'm not familiar with your boiler, but if its thermostat is of a bimetal construction, which includes an internal spring to provide hysteresis (a lag between the on and off temperatures), then the stat is the most likely cause of the problem. The springs do break or lose their springiness with...