Boiler says 70C but water is tepid?

[Christmasinbed]

Paging @PigletJohn My newish Vaillant boiler says 70C on the display but the water's only coming out tepid (despite giving it time to heat up). It's attached to a Megaflow. Does this mean a thermostat has gone? Zero chance of getting a heating engineer this close to xmas..

When did it last work correctly?

The Megaflow takes time to heat up. Is the programmer set to "HW on?" Since when?

Also, go and check that hot water is not spilling out if its overflow, and no taps are running.

Modern boilers usually have ample power, so it is unlikely, but possible, that your radiators are taking so much heat that the Megaflow is not getting enough. This can be addressed by setting the timer so the HW starts half an hour or so before the CH.

Can you post a picture of the megaflo and associated pipework? Im a heating engineer and also happy to help!

Also a picture of the front of the boiler might be useful.

I'm not using the heating so the rads aren't an issue. I manually operate the water 'on' using the control box. I usually let it heat for 30-45mins & this is plenty for a hot bath. It was last working perfectly yesterday. No water was coming out of the overflow and no taps were on. There's only me in the house using it. It's now shut off but when it's on, I can see a drip in the megaflow pressure relief valve when the boiler water is on. I'm just this minute using the immersion heater in the megaflow so I can have a bath and the relief valve isn't dripping (boiler off).
It's very hard to photograph the front of the boiler because it's located in the eaves but it's a Vaillant Ecotech plus 630 and when I switch on the water the display shows the temperature 70C

Megaflow & boiler are in separate rooms, 'L' shaped apart from one another.

Common thing to cause this issue is that people turn the heating thermostat down on the front of the boiler when in fact this knob controls the temperature of water running not just through the heating but through the megaflo. It could be that this is turned down. Turn the control knob up if you have one. Older ecotecs have two control knobs to the left and it will be the top one. Set it at least to 60 degrees. The hot water one (the bottom knob) does nothing on your boiler.

If you don’t control knobs or if they are turned up above 60 come back and we can look at other potential issues

If it was fully cold, it will take longer to heat. You say it is dripping so it may need the bubble to be regenerated or some other repair. Do you have a regular service person who is qualified on unvented cylinders?

The electric immersion heater is much more expensive and will take much longer to heat it.

Hi, @Droven

I see you're on to it.

@PigletJohn two brains still better than one though!

😆

@Droven & @PigletJohn Hi there, I came back to update. Ignore the NC. So, the boiler temp is set at 70, it wasn't that. What was causing the problem was the Honeywell valve box that sits external to the boiler. I think it's called a diverter valve? The heating one was working perfectly, the hot water one was only moving a little and then getting stuck - hence letting a small amount of warm water through. We replaced it and it's now working fine. I'm not impressed with Honeywell valves, I can't tell you how many we've had replaced. It seems they only last 1 -2yrs. This time the chap fitted a cheaper alternative and we'll see how long it lasts..
As for the megaflow, yeah it needs a service and the air pocket re-charging. The chap who came out frowned at the megaflow and said the tunstall? & valve before it both likely need replacing, but he looked unconfident so I didn't let him touch it. I'll find someone qualified in megaflows.

Sometimes I think it'd be really handy to just qualify in heating myself so I can sort these things out without hassle.

Honeywell valves are usually very good.

They comprise two parts, that can be changed separately. On top is the motor and switch. The moving parts can wear, but it can easily be removed and replaced. Underneath is a brass valve with moving spindle passing through a water seal, that moves a rubber ball to block one of the outlets. This is a plumbing job, and it needs the system to be drained, at least partially, and refilled afterwards. As it has moving parts it can seize or jam, and eventually wear out. I'd say around 15 years life is common.

If the circulating water is dirty (common if the radiators are old and you do not have a magnetic filter) the moving wet parts will have a shorter life (so will the pump).

Do you know if both parts of the valve were replaced, both times? A new valve is easy to recognise because it will be bright shiny brass, not tarnished.

Yeah completely new valves have been replaced every time. Honestly I lose count. We must've gone through 10 in the years I've lived here. I wonder whether it's our hard water. The system has been cleaned/flushed 3 times in the years we've been here and we do have a magnetic cleaner. I've a pile of old Honeywells in the corner of the boiler cupboard. I think I'm going to pull one apart and have a look inside.