immersion heater controller on unvented system

[mulls]

Hi,

I would like to add a controller/timer to my unvented water heater (which has not actually been installed yet). The unit has 2 immersion heaters each rated at 3Kw. I think this means the controller would have to have a switching capacity of 25 amps (6000W/240V=25A) but I can’t seem to find one that will cope with this. Some seem to have the capacity to control 2 immersion heaters but it seems that one is always connected and the other would be used as a boost. Also, the controllers I have seen appear to be geared up to run on off peak cheap electricity – which I don’t have.

Can anyone recommend how I should proceed. I’m thinking of having just one heater connected via a controller and manually control the other heater via its fcu but I’d rather just set the timer and forget about it until maintenance time.

Thanks.

 

[Steve]

Your cylinder is designed for 2-rate electric, yes. But you can use both heaters as follows:

Use a basic timer to power up a 2 pole contactor (fancy relay). This contactor will then control 2 circuits at 15A each - wire one 'mersion circuit to each contactor pole. But this is definitely a job for an electrician unless you know how to wire a contactor up.

By the way, 6kw is a big load - nearly an electric shower - please be sure you have these on a thermostat - wired with the timer so both heaters operate together and dont defeat each other. The water will heat up in no time. You might want to leave out the second heater full stop though.

 

[kai]

Just use the bottom one, keep the top one as a backup element only on a separate 13amp DP Switched FCU, on a separate circuit from the bottom one.

 

[toasty]

If it is a big cylinder, you will need them both to heat it from cold.
For example a 210litre unvented will take about 90mins even at 6Kw, a 300litre will take about 2hours (4hours at 3Kw)

I think I'm right in saying that the dual element cylinders don't have an indirect heating coil in them (i.e they are electric only - you can't use your boiler to indirectly heat the water)

If this is the case, I'd seriously reconsider, the cost of electricitity is higher than that of gas, and unvented cylinders do produce a lot of hot water, so cost a lot to run, ours (210litre) gives a great shower, and fills a bath in no time, but costs a lot of money in gas to heat it, I'd dread to think of the equivalent cost of electricity.

If you don't have gas then fine, but if you have the option of electric or gas, I'd opt for gas.

Regarding the timers, a contactor is a good idea, another would simply be two timers each rated at 13amps, use one to boost when required.

 

[pjo123]

I've got a 145litre all electric megaflow, also with two emersions.

I have the bottom one on a timer that comes on for just 1 hr overnight on economy 7. Only had it installed for a week, but so far this has been ample for providing hot water throughout the day for a family of 4 with hot water still available at bed time.

I have the top one on a seperate circuit so should we ever need to turn it on to boost during the day, I don't have to disturb the timer.

 

[mulls]

This is great feedback - thanks very much everyone. I think to start with I'll go for just having the one immersion on a timer and leave the other (on a seperate circuit) for top-up if/when needed - as per kai & pjo123's suggestion. I might go for the contactor at a later date though if things don't work out.

BTW, yes electric is expensive but I have no gas option.

Thanks again.