Some advice on operating this type of Horstmann Electronic 7 thermostat. These control the heating of domestic hot water tanks and are very common in the UK and probably elsewhere.
Thanks to MyBuilder for the picture of my type.
There are apparently two heaters in the water tank this controls, bottom and top. The idea of the Electronic 7 is to work both heaters safely, and not allow them both to be on at the same time.
According to advice on the Electrician's Forum in 2010 the Electronic 7 offers a timer that lets the bottom heater run off the cheap rate electricity. This rate is usually only offered overnight, perhaps between 11pm and 6am in a British winter. This lower base heater is apparently slower as it "may take 3 hours" to heat the tank from cold. Thus it's not much use to only put it on for 50 minutes or so.
The top heater is there to more quickly boost the hot water if you run out, and thus seems to be a bit more powerful. It can usually be put on for an hour at a time via the "Boost" button, and apparent somehow uses only normal rate electricity(?). It only directly heats the top third or so of the tank, but should provide enough hot water after an hour for one bath. Indeed, it does.
Now here's the confusing thing. If you turn off the Timed heating (i.e. big rocker button to 'Timed' and slider-switch to 'Manual', as seen above), then the red 'Timer' light on the box will still come on. What seems to be happening here is that this light also comes on to indicate the 'cheap rate time'. It being 'on' thus does not always mean 'the water is heating'. Just that the Timer can only be used at this time. When this 'Timer' light is on, you will then be unable to turn on the 'Boost' button and its light. This is presumably a safety thing, as it prevents both heaters being on at the same time and (I guess) overheating the tank. However, it may be rather confusing to some first-time users.
So, knowing this, it seems to me that it may be more expensive to run the bottom heater for four hours at off-peak each night, compared to a one-hour morning boost at peak cost. If a one-hour boost is all you need, and perhaps then not even every day. That said, I'm uncertain if the top heater has a different power wattage than the lower one. If it has then the cost advantage could be less. Check your boiler make.
Anyway the problem with this mutually-exclusive timer/boost arrangement is that if the tank is cold and you needed a bath at 5.30am, you would actually have to wait until 7am. ('Boost' button is not enabled until 6pm, then heating takes an hour from cold = hot bath only available at 7am). Obviously this is not viable for people who need to get to work quite early, and who like a bath beforehand.
The solution: Simply nudge the main clock-time forward on the Electronic 7. Press the 'hour set' button a couple of times and fool the clock into thinking it is running three hours ahead. In other words, when it is actually 4.30am your Horstmann's clock will then read 7.30am. The device has been fooled into thinking it is out of off-peak / cheap-rate time. It will now let you press that 'Boost' button just after you wake up. The 'Boost' light will come on, and you'll have a piping hot bath ready an hour later.... after some breakfast and light reading.