Immersion heater help!

[lolaVie]

We live in a solely electric apartment so no gas and no boiler. Hot water comes from an immersion heater with a large water cylinder which was working absolutely fine until
yesterday afternoon.

It has a boost immersion which we often rely on through the day. From the outside the immersion feels hot to touch and so I feel as though it must be working. However after hours of boosting when we turn on the hot tap we get cold water for about 10 seconds, 5 seconds of warmish water, then back to cold again. Happened out of the blue yesterday after our usual hot water working as normal.

DH thinks he may have left a hot water tap running through the day which is the only thing I can think - if he emptied the cylinder how long would it take to refill and reheat as this seems too long?

would prefer not to call a plumber out obvs if the advice is just to leave it X number of hours for the water to heat up again because it was emptied.

Google says 2-3 hours.
Why do you use boost though? Is there not a timer? Could the thermostat have been knocked so it’s not heating hot enough even though you are boosting?

At the absolutely maximum, assuming you have a gigantic water cyclinder, I would expect the water to be fully hot within three hours.

As an aside, can I just ask if you are aware how extortionate those things are to run?

I was monitoring an old house last September before yet more electricity increase, and it was £1.70 per hour.

Once the water is actually hot the amount PH lowers, but it's a very costly method of water heating.

We had to have the immersion on 24/7 when we moved into our new house (as we had no oil and wanted the tank cleaned) and our first month's electricity (big but energy efficient house, two people, July 22) was £450.

My last month's electricity (March 23) was £96, for comparison.

I'm in a flat that's all electricity, no gas either. Do you not have Economy 7 to heat water overnight when its much cheaper? I almost never use the boost in the day as it's SO expensive to run.