Questions on: Immersion switch replace with timer

[SimonH2]

Sooorrry I meant RCDO, like that which is protecting the outdoor sockets. My understanding is they give the best of both functions.

RCBO actually, but yes you are correct that they combine both functions. The big but there is that you need a non-RCD way in the CU and not all CUs have that option. Good luck finding a DP one (or more likely, DP switching with SP overcurrent detection) - they simply do not exist for most makes/models.

Switch AND timer?
The reason I'm uncomfortable about this is the 'idiot' factor. Switch & Timer in series makes it easy for 'someone' to deliberately or accidentally switch the rocker switch when the immersion is required and expected, leaving no hot water for those occasional very early mornings when I have to be on the road at 4 or 5am.

Yes, but there's always that option - someone can press buttons on the timer, or they could ...
If you can't trust people in the house not to switch the immersion off when it should be on then you've bigger problems.

As to putting the switch in the garage, well that introduces a whole new scope for problems. As it is, my late father in law's house has the boiler in the garage and the electrics are really "bodged" in that BG simply spurred off from a socket, put an unlabelled "switch" (FCU) next to the socket, and it's nowhere near anything to do with the heating. When relatives were staying in the house shortly after he died, they found themselves without any hot water or heating when someone switched it off while trying to figure out how to turn the garage light off !

While it's too late now, it's occurred to me that you don't actually need a timer at all ! Unless the cylinder is really badly lagged, there's no penalty in just leaving the heater on all the time to operate on it's stat. Simple, hot water any time, no timer to fiddle with or get wrong.
Long gone are the days of "putting on the water" several hours in advance of wanting a bath
That's assuming no economy 7 like tariff - which you haven't mentioned and would have different solutions anyway.

As it is, if you immersion element fails in one of it's more common failure modes, you'll find yourself with several circuits in the house off - and unable to switch them back on. You have been warned. Without the DP switch on the immersion, my late FIL's house would have no heating, no power to the sockets, no power to the cooker, and no power to whatever else is on the RCD !

 

[kai]

The double pole switch at the immersion end is essential - complies with the current regulations - the flex outlet plate can easily be replaced with a DP switch with Neon and flex outlet on the same plate.

 

[ban-all-sheds]

Unless the cylinder is really badly lagged, there's no penalty in just leaving the heater on all the time to operate on it's stat.

And I really don't see what the quality of the insulation on the cylinder has to do with the relative costs of electricity and gas:

My guess it is because he doesn't know that energy from electricity costs about four times as much as energy from gas.

JohnD
Our gas boiler is ancient and inefficient. I've added extra lagging on top of the existing yellow urethane foam cylinder insulation, and want to use try using the immersion more as an environmental measure than a cost equivalent.

 

[SimonH2]

And I really don't see what the quality of the insulation on the cylinder has to do with the relative costs of electricity and gas:

And I didn't make any connection between them
What I suggested was, unless the cylinder is so badly* lagged that it has high* standing losses, then leaving it on all the time won't cost significantly more than running it on a timer - and avoids all those "oh we hadn't planned for that and there's no hot water" moments.
EDIT: I can't imagine I'm the only one here old enough to remember the days of "putting the water on" in preparation for having a bath.

But to directly address the point you make, there are situations where lecky is cheaper. If it is an old and inefficient boiler, then it'll be burning gas all the time (pilot light - yes there are plenty of those about). If heating is off, and hot water usage is low, then it may well be better** to just turn off the boiler altogether.

I only have one data point to go off, but it's one I've personally measured. Just after installing the thermal store in the flat, I also had the house empty at the same time. I was able to do comparisons to see if there was any truth about "thermal stores are wasteful because of the standing losses". Running the store off the immersion heater and allowing it several days to settle down to equilibrium, I was able to compare it's standing losses (roughly 80W) with those of the modern combi in the house which without it's eco mode on consumed the equivalent of about 160W worth of gas to keep the DHW HE warm. Of course, turning on eco mode would virtually eliminate standing losses for the combi, at the expense of not having hot water "on demand" - but it does show that things aren't always as simple as "gas=cheaper".
What I didn't measure, and perhaps should have done, was the standing losses for the flat using the boiler. I suspect they would have been considerably higher due to it being an old pilot light model and burning gas all the time even when not needed.

* These being relative, not absolute terms
** By whatever definition of "better" you choose to use