Economy 7 Immersion Heater Timer

[economy777]

I have one electricty meter,
two consumer units - one 24 hour, the other economy7,
two immersion heaters - one from each CU.
The economy7 heater therefore switches on when the economy7 period starts, probably midnight or soon after and keeps the water hot until say 7am with a thermostat.
I want to fit a timer so that the overnight heating occurs only from say 5am till 7am.
A Horstmann timer looks like it would work BUT to keep its clock working it would have to be wired into the 24 hour CU circuit. This would make the economy7 wiring to the immersion heater redundant.
Is that the way to do it?
My understanding is that all electricity used in the cheap period is charged at the lower rate so that's not a problem.

 

[Taylortwocities]

Why do you need/want to do this? When the water has reached a certain temperature (and you can adjust it), the thermostat switches the power to the immersion off. It doesn't stay on heating the water all night.

 

[EFLImpudence]

It depends which Horstmann you have in mind.

There isn't much point installing a timer for what you want to achieve.
As you say, it is controlled by the thermostat so when hot enough it will switch off.
If the water gets too hot turn down the thermostat.

 

[Taylortwocities]

As a follow-up

You can achieve this with an immersion timer with voltage free contacts.
The timer would be powered by the 24-hour supply and the volt-free contacts would enable the off-peak supply to the immersion. Its tricky to wire, if you don't know what you are doing...

But it would take a long time to get a return on the investment as properly insulated tank will not lose much heat in a couple of hours. Per the above, save yourself the bother and back the stat off a few degrees - it will have the same effect for less trouble!

 

[AndyPRK]

I wouldn't recommend a mechanical Horstmann as you have little control over the timings. You still need a good 5 hours or so on time.

They do however have a battery backup. Not sure how it would cope being used every day though.

As said, they may have volt free contacts to over come the problem. or just use the 24 hour supply and trust the timer! (something I would avoid if I could).


If you run out of hot water in the evening I think you would be better off spending money on:

A) A timer to give you ½ Hour boost in the evening
or
B) A boost timer where you turn it on and it automatically goes off after 30 mins (or whatever you select)

 

[JohnD]

an immersion heats about one litre per minute. Slower in winter when the incoming water is colder. So if you have a 120l cylinder with an element at the bottom, it will take about two hours (but the element is unlikely to reach quite to the bottom).

With economy 7 or similar tariff, it is usual to heat the lower element off peak, and only turn on the top element for a small top-up if you run out. The top element alone will not give enough for a bath. I like Andy's (B) with a push-button variable timer..