vaillant combi - CH but no hot water

Hi, Im trying to diagnose a problem with a combi boiler.

All my experience to now involves open systems so this is a bit new to me.

The boiler works fine for the heating (there are three rads in this studio flat), but it refuses to fire up when I demand hot water.

Now, my simple knowledge leads me to surmise that the fault lies in one of 2 places:

1) with the mechanism that detects hot water demand
2) with the circuitry that fires the boiler to heat the water.

Questions are:
1) Is my simple, top level diagnosis correct, or are there other things which might be affecting lack of hot water?
2) Precisely how do combi-boilers detect hot water demand?
3) What are the normal steps to follow to try to diagnose and fix the problem?

Many thanks

Read this especially #1

1) There are other things
2) They vary
3) It depends...

to Slugbaby: If I had that information, I would have supplied it. I was going to ask another question, that being: how does one identify the model of a Vaillant boiler? There dont appear to be any identifying marks on or in the boiler and (as you would expect) I have have no manual for the boiler

to ChrisR: could you be a little more expansive? If Ive not provided you with enough information, pls let me know what other information you need.

Its a natural gas boiler btw.

I dont know precisely when it started happening, lets say about 2 months ago.

Nothing changed to the best of my knowledge to cause the problem.

One thing which I have done recently (long after the problem started) was to bleed the system. There was a great deal of air in the radiators, so much so in fact, that I had to open the cold water valve to fill the system with water again. That valve is now closed again and the heating is working fine.

That really is all the information I have Im afraid, if you can help me but need more details, Im happy to try to provide them.

Rgds

P.S. If it helps, there are 2 PCB's mounted at 90degrees to each other in the boiler housing. They are marked:130391 and 130390. This leads me to believe then that the boiler is from the TURBOmax range i.e. VUW242EH or 282EH

The very old Vaillants have the data plate mounted on the lower front chassis.

The next generation had the data plate just underneath the combustion chamber on the right hand side.

After that hinge down the control box/front panel assembly - that data plates on the back.

wicked, thanks v much Gasguru, I will check it out.

to Slugbaby: If I had that information, I would have supplied it. I was going to ask another question, that being: how does one identify the model of a Vaillant boiler?

Click to expand...

Saying it was a Vaillant would have been a start.
Is there a red and white control knob for the water temp under the boiler?
If so on the end of that is the water section which has a replaceable diaphragm in it. This pushes a pin up and operates a servo valve. Look over the top of the servo [to the right of the control box] to see if you can see a cam moving when the tap is turned on.

irfanb they're as different as different cars - like "My Ford won't start " wouldn't be much use..

The circuit boards identify it - it's a Turbomax 242e or 282e- an early one before the /1 versions.
Black panel, green knobs.

The diverter valve - right hand side brass lumpy thing behind the pump, has a diaphragm which responds to water pressure/flow when you open a tap. Diverts boiler water to tap water heat exchanger and switches boiler on.

If you have hw when ch is working it may just be the electric switch, rhs of brass thing , either turquoisy or black if it has been replaced. If you operate switch manually boiler should start.

Would change the whole diverter valve at that age.

Wheee awesome, thanks v much for your help ChrisR.

Appreciate that you need to know the model and thanks for your help in determining it, you are indeed correct, it has a black panel with green knobs.

We never have hot water but its useful to know that its the diverter valve that contains the diaphragm which detects the pressure change.

I'll check and report back with my findings.

Thanks all
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