immersion heater timers

I had anticipated you might use the thermostat integral to the immersion heater, which would require a 3rd wire.

I expected a momentary switch across the integral thermostat to energise the relay coil. The major consequence is the heater is powered up when the momentary switch contacts are made even when the internal thermostat is not calling for heat.

Your method of having an external cylinder stat would remove the issue, and would require the internal thermostat set to a higher temperature to minimise interaction between the two stats.

I did consider that, but thought it would be kinder to the switch contacts to not stick full load through them at each startup. Also there was the consideration that it might be desirable to allow the immersion to cycle on its stat.

Yup, I think I mentioned that at the end of the description - perhaps it should be more prominent?

Momentary switches in the style of other domestic switches seem hard to get hold of. If you want a big green button kind then there's quite a choice. I guess a stairway plunger switch would do the job!

Yes, it is there. I missed it in my quick read!

Grid switches don't look too industrial.

You can get a few - some are labelled "Bell", but there are others just with Push or Press:

I probably ought to make that more "visible" since its kind of a show stopper!

They seem to like the term 'retractive' rather than 'momentary'

Thanks, I hadn't come across the term. Even Screwfix have them!

Sort of, but not dangerous and ought to be obvious!

I would hope anyone implementing the solutions would have read the article a few times.

If only the OP had said more clearly exactly what he is trying to achieve, we could come up with an off the shelf solution that only requires minimal DIY skills.

For example, if he just has a single box on-off switch (hopefully with a big red neon!) then swapping for the CPC / Toolstation push button "boost" timer will achieve what he wants (even if it's not what he actually asked for).

The "Boost" capabilities are ok, but most don't have any feedback. So if you want a capability to get a full reheat from any unknown state, you could end up with a situation that is either wasteful to run, or does not always do the job properly.

(i.e. a nearly hot 150L cylinder may only need 15 mins of heating, compared to a completely cold one, that could take 3 hours. So you would have to use a boost time of 3 hours as a default and accept that for the

Dangerous no, although could be expensive and not do what it says on the tin.

By way of example, A mate of mine wanted to rent his house out, so before he did that, we made sure the heating system was working well, and stuck in TRVs on most of the rads since the system did not have any initially. A couple of years later he got someone to manage the letting for him, and she promptly instructed a plumber to go fit TRVs to the rads that were "missing" them... Needless to say, those were the hall rads that shared a space with the main prog stat.

No amount of explanation would convince her that she had just wasted the landlords money on work that did not need doing, and potentially put up the gas bills for the tenants!

In the end I gave up and just told the tenants to set the ones on the rads to max, and leave them there, and control the temp with the main stat.

You can hope :-)

I had not thought of across but before like this

It depends on how long you keep your finger on the button for. Even Johns suggestion will have half the current running through the push switch.

It should have on the drawing NO contacts not NC contacts on the cylinder stat.

Drawings are usually done in a non calling situation.

It's showing the starting condition state - which is non calling. The cylinder stat will remain in that state until the set point is reached, and it opens it's n/c contacts, which will allow the relay to release.

This is a three state system though. Off, calling, and satisfied. In the satisfied state there will be a period where it's not possible to put it back into the heating state because the cylinder stat is open.

Yup, that would work - I could not quite visualise from Fred's description how it could ever disengage.

Although on that one you could stick a current limiting resistor in series with the start switch...

(just trying to see if there is a way of using the relay's coil to limit the switch and cylinder stats peak current)

The relay is only energised as long as the heater is powered. That is the circuit I had envisaged. The downside is the switch takes full current for a split second and then some unknown fraction before release.

I was thinking this as most 'retractive' switches have a limited current capability.

A resistor could be placed between the heater and relay coil. Where the switch applies power directly to the relay. Once the relay is energised only the holding current is required. Unfortunately the resistor would give of heat.

It may be possible to use a capacitor with the right reactance to ensure a sufficient holding current but might need some knowledge of the relay inductance. Power consumption could even go up if the LC combination was close to resonance at 50Hz but I don't think that is likely, the inductance must surely be too low?

True. Less of an issue with decent insulation. An easy option could be to add one of the cheap LCD thermometers with a thermistor on a couple of metres of wire. "Four time boost options" looks like the default now.

Exactly. Also your strap-on thermostat would have to be mounted on bare metal revealed after chiselling out a section of the foam insulation.

Once pressed the heater would be permanently on, and temperature cycled using the internal thermostat. It doesn't provide a one-shot heating cycle.

Just like any tank/cylinder stat.