I have a Vaillant ecofit pure and my hot water is no longer

The switch is probably for an immersion heater, which is a useful backup in this situation.

What is the device you used to use "summer mode"?  Is it a programmer?  Is it possible that you've unwittingly disabled the hot water function?

Where are the controls that you think your father might have messed with?

Where are the controls that you your daughter changed?  On the boiler, or on a programmer?

OK.  Please bear with for a minute or two...

This is conjecture on my part, but an educated guess.  If there was a fault with a (MZV (motorised zone valve), or the cylinder thermostat, before switching to summer mode, it could have been masked by the fact that you had the heating on some of the time.  Turning off the heating means that the boiler relies on those two components, and one being faulty would mean the boiler receives no "heat demand" for the hot water.

I can tell you how to check for this, if you can attach a photo of the area around the hot water tank/cylinder, or tell me the make and model of any MZV that you have there.

The Grundfos thing is a circulating pump.  Not far from that you should find a MZV.  There might be more than one.

OK, but if the immersion heater is working then the water will heat up from that.  Whilst this gives you hot water, it doesn't fix the problem.

The photo shows a circulating pump, and it's quite old, not new at all.

I'll send you images of one brand of MZV...

This is a 2-port valve.
This is a 3-port valve.

OK.  You have a Honeywell 3-port MZV.

Yes I can - I think you haven't seen my response.  Are you viewing this topic on the web site?  If not then I recommend it!

I'll explain what the MZV does...

The purpose of the MZV is to open the flow from the inlet to the outlet, and then to provide the signal (a heat demand) to the boiler to ignite. If the MZV malfunctions, then the boiler won't have a heat demand.

A 3-port MZV responds to a signal (a "call") from two different thermostats - room and cylinder. Those thermostats can malfunction too. This chain of signals originates at the programmer. So, a quick and easy observation is to see how the MZV valve behaves, and work back from there.

On the end of the silver box, under the black cable, there's a thin metal lever in a slot. Please can you attempt to push the lever along the slot, and let me know if it feels floppy, or do you feel resistance? If you feel resistance, push the lever to the end of the slot and release it - does it spring back?

OK.  This is a strong indication that the valve is stuck in the "heating" position.  Therefore it won't pass on the heating demand to the boiler.

Usually the valve can be eased open, but you can't do with the lever.  I can tell you how to do it if you feel up for it.

OK good. Here are the steps.

1. Power off the heating system, for safety.
You can leave the immersion heater switched on.

2. With a suitable Torx bit or flat-bladed screwdriver, loosen by a couple of turns the screw that holds the actuator cover of the silver box in place, and lift off the cover. On later models the cover is tethered by an earth cable, which can be a nuisance when it hangs in the way.

3. With the same Torx tool or screwdriver, loosen the two screws in the 1 o'clock and 7 o'clock positions on the baseplate of the actuator (when viewed with the short side of the casing at either 12 o'clock or 6 o'clock).

4. At this point, the actuator will lift away from the brass valve, but you might need to do some wiggling to persuade it. I advise against prising it off with any tools, as you might bend the baseplate.

5. With the actuator removed, try to wiggle the spindle of the valve with fingers. It has only about 35 degrees of travel. It should be possible to move it just with your fingers.

If it moves freely, then it tends to indicate that the intermittent behaviour of the valve is caused by a failing actuator. If you can't easily turn the spindle with fingers, then it's sticking - you can try to wiggle it free with a tool, but don't use anything with serrated jaws that might rough up the brass surface. A small adjustable spanner is good, but don't force the valve spindle - just gentle wiggling.

If the valve was sticking the actuator motor might have become damaged, but you can turn the power back on with the actuator unmounted, and check that it operates.  If the actuator is not faulty, the boiler will fire up.
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Let me know if you have trouble with any of the above guidance.

I shall await your news :-)

With the actuator lifted off the valve body, you should be able to move the lever, by slowly pushing it, all the way to the extreme of the slot.  It should then spring back with a buzzing noise.

The spindle of the valve is the brass "sticky thing" poking vertically up.

That can happen when there is wear in the mechanism - the gears are a bit cheap, and the motor pinion is made of brass.  it's possible that the motor has seized - they can be replaced quite cheaply.  Or you can fit a new actuator, which has the advantage of having a new mechanism.

However, before you buy anything, you need to determine whether or not the valve has seized.  Can you turn the spindle with fingers?

I see the photo - thank you.  Please see my previous post :-)

Yes.  The spike.

If you can't easily turn the spindle with fingers, then it's sticking - you can try to wiggle it free with a tool, but don't use anything with serrated jaws that might rough up the brass surface. A small adjustable spanner is good, but don't force the valve spindle - just gentle wiggling.

Pliers aren't well-suited for turning any spindle.  You will need a small spanner or a small adjustable wrench.

The alternative is simply to replace the entire valve - they come with a new actuator as an assembly.  However, that becomes a plumbing task, not just an electrical one, and involves partially draining the heating system (not the hot water tank).  At this point I suspect you'll be better off calling a heating engineer to do the work.  They might succeed in freeing off the valve, to buy you some time before it needs to be replaced, but spring is a good time to have this kind of thing done.

To be clear, the part that turns has two flat sides to it.

You're welcome.  I am about to go to appointments, so do you have any other questions before I go?

Thank you for your rating, and your kind bonus. I'm glad that I could help.

I hope you enjoyed your experience with Just Answer.

Have a wonderful day :-)

I have a friend in Wellington so I know how foreign the UK central heating systems seem to you!

When you first wiggled the spindle with the small spanner, was it very stiff?  And then it became looser as you repeated the wiggling?  It has only a very limited range of movement, but it needs to be loose enough to move with your fingers.  If it does that then it's likely that it will last many more months.  Perhaps years.  But you need to exercise the central heating occasionally to discourage the valve from sticking again.

If you turn the spindle fully clockwise (when viewed from above), but leave the actuator unmounted, and activate the central heating, then the boiler will heat your hot water and not the radiators.

There are three possible scenarios here:

1. The valve was sticking, and you free it off.  Such a valve can continue to operate for many years, or at least many months.  The actuator isn't damaged or worn, and you re-mount and then everything works.

2. As per 2, but the actuator is faulty or worn, and you have to replace the actuator with a new one.  They can be purchased separately, but are disproportionately expensive compared to complete valve.

3. The valve was sticking, and you can't free it off sufficiently, so it's days are numbered.
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BTW, a sticking valve can be a sign of CBPs (corrosion by-products) in the system - they accumulate and make the valve spindle stick.  CBPs are another topic really.