Controlling My Office Water Heater

[tmetford]

I want help to create a circuit that allows the heater element of my office water heater to run for a single heating cycle only rather than continously repeated reheating cycles.
In more detail this means that I want the heating element to be switched on, say by a mains timer switch, heat the water tank until the thermostat switches the heater element off and then for the mains power supply to be automatically disconnected so that when the water temperature falls and the thermostat reconnects, the power supply will then no longer be available to reheat the water; at least until the mains timer switch turns it on again.
My intention is to help reduce energy consumption at my office where one tank-full of hot water is sufficient for a day, it doesn’t need reheating. Having said that, a manual over-ride would be a useful addition.
I have drawn up a circuit and constructed a low voltage prototype of the device but, whilst it looks okay on paper (to my novice-level eyes), it doesn’t work and I want to find out why.
I’ve attached the circuit diagram (eeeeek!) and hope you will help.
Cheers, Tim

 

[davenn]

Hi there Tim
welcome to the forums

ok so why make it so complicated ? why not just use single timer and have it set to switch on a couple of hours before you get to the office.
Work out how long it takes to fully heat the water and then have the timer turn the power off after that till the following day. You can still put in a switch to override the timer if need be.

Is there really a need for all the LAN control ?

cheers
Dave

 

[tmetford]

Hi Dave and thanks for your comments.
Just as you rightly suggest I've had it set up using a single timer switch for several years now and this works fine but please read on:-
The LAN control is there to enable me to control the office heaters from home because my schedule at the office varies on a daily basis and I'd rather not waste electricity by having the heating come on to a preset schedule. Similarly, I now want to extend this control to the water heater and the LAN control will enable me to disable the attempt by the mains timer to turn the water heater on if I'm not coming in that day.
Lastly, my reason for creating a circuit that allows only a single heating cycle comes as much from my interest in learning about electronics as it does from a desire for energy efficiency! So, providing such a circuit can be achieved using basic electronic components, I'd really appreciate your help in this project.
Thank you in anticipation.
Tim

 

[duke37]

A simple circuit is attached. The 12V relay should be energised for only a moment.
The contacts on both relays will need to take the full load current.

 

[tmetford]

Thanks for that and it looks so much simpler than my idea! However, I need to check my understanding of your circuit.
Looking at the attached copy:-
1. Does the box at "A" represent the coil of a 12Vdc relay which, in turn, would close the uppermost circuit at "B"?
2. Does "D" represent a 240Vac relay coil which would close the lower switch at "C" to maintain the power to the load?
3. Does the 240Vac at "E" come from the mains isolator?
4. Why should the 12Vdc relay be energised only for a moment? - I can understand that leaving it powered on for too long could maintain the power supply into a second heating cycle. The reason I ask is that the mains timer switch has a minimum "ON" period of 1 minute and I want to know if that could be unsafe or damage the relays?
Many thanks for your help.

 

[(*steve*)]

1. yes
2. yes
3. yes
4. Because as it is energised the other relay will turn on and continue to power the heater. When the thermostat turns off, that relay will also drop out preventing power from reaching the heater when the thermostat closes again. Leaving it on for a minute would be fine. Remember that both relays need to be capable of switching the entire current to the heater.

 

[duke37]

I took your idea and separated the control and latch functions, otherwise they are similar. I should have said the the 12V needs to be applied for only a moment rather than should be applied.
I sketched the diagram and scanned it. How did you add the red lettering?

 

[tmetford]

Thank you, Duke, for your ingenious adaptation and I'm looking forward to putting it to work!
I opened your sketch in an image editing programme (Corel PHOTO-PAINT X5) and drew on it using an electronic pen and tablet thing connected to my PC. The same can be done in the programme "Paint" that comes as standard with Windows 7.
Thank you once again for providing the breakthrough with my project.

 

[tmetford]

Would it be a good idea to install a diode across the coil of the relays?
On an earlier project I was advised to connect a reverse biased diode (e.g. 1N4148) across the teminals to prevent back-emf from damaging the coil if the relay switched off the load.
I know how to do this for the 12Vdc relay coil, but I don't know if it's needed on the 240Vac relay coil or how to go about it?

 

[duke37]

Under no circumstances put a diode across the AC relay, if you look, you will see that it is directly across the mains. If you are lucky, a fuse will blow, if you are unlucky, the exploding diode will hurt you.
AC relays have a copper shorted turn over part of the pole piece to limit the back EMF and provide a more constant pull as the AC changes polarity.

 

[tmetford]

Thanks once again, I'm glad I asked!
Cheers
Tim

 

[TBennettcc]

Tim,

What software did you use to create the first schematic you posted?

 

[tmetford]

Hi TBennettcc,
I used CorelDraw X5. This is great design software though, as with any progamme, there's a learning curve if you're going to create diagrams from scratch. I use it only because I'm familiar with it and, though I'd recommend it as a design programme, I must add that it doesn't come with any pre-designed templates or symbols for creating electronic circuits. I'm sure you'll find a set of conventional electronic symbols to make circuit design process quick and easy, it's just that I haven't looked for anything like that.