Immersion heater thermostat problems

I've just replaced the hot water cylinder because the old one was leaking. It's a vented indirect cylinder, heated by solid fuel in winter and immersion heater in summer.

I think there is something wrong with the new thermostat, as even on the hottest setting it cuts off at 40 degrees. Unfortunately I can't just pull it out and send it back because of the angle of the sloping roof - I would need to remove the tank again - a struggle getting all the pipe connections to line up.

So what I am trying is using an external thermostat, clamped to the tank about 1/3 to 1/2 way up. Initially I am using a spare probe type from a short immersion heater, pushed in through the foam insulation tangentially so that the tip lies alongside the copper cylinder.
This works with partial success, but the range of operation is so wide that it is wildly erratic in controlling the temperature. The point at which it clicks on/off is over 20 degrees, so I can have 40 degrees or 80 degrees actual temperature, but it's very hit or miss getting 60.

What I am now thinking of is fitting a proper sensitive tank thermostat, of the kind that clamps flat against the tank after cutting out a rectangle of foam. Will such a thermostat operate the immersion heater directly, or does it need a relay? I can find ones rated at 10 amps, but that doesn't really seem enough to switch a 3Kw heater.

Has anyone any thoughts or advice to offer?

I'm afraid 10 amp switching isnt enough for a 3kW element......that works out at 13 amps, give or take.
You can use a relay to do what you want but its rather unorthodox!
John

Thanks John, I suspected that. I just thought someone might have heard of a thermostat that could take a higher current. I'll use a relay then.

I know it's unorthodox, it's just because I really can't face taking the tank out again, and putting it back to test a replacement.
So no fundamental objections to the method then, with the currant proviso.

I had considered bypasing the thermostat and just using the timer to limit heating to say 3 hours or whatever got it up to temperature. But I imagine that really would be potentially unsound even dangerous.

No, that's not a good idea but even so, the expansion pipe should deal with the overheating!
Obviously you need a relay with a 250v coil, capable of switching 15 amps or better still, higher if you can get one. The last time I used a relay like this it was an RS one with an octal base, but that was ages ago.
John

Oops?

You have not given the cylinder size!

But if it was a standard 36" x 18" then a standard immersion heater element is only 27" long and will not heat the lower half of the cylinder very much.

Therefore to control the temperature of the top part you need a stat well within the 27" length of the element!

Tony

At the end of the day, I'd have to suggest taking the tank out again.......but
I do sympathise with the nightmare.
You could then fit a dual element immersion heater, with a high quality thermostat (and heed Tony's advice).
John

I would go one step further and say that no one should EVER fit a cylinder were the immersion element cannot be withdrawn!

In fact I go a second step further ( at my extra expense ) and always ensure that any boiler or cylinder that I fit can be replaced just by undoing compression joints!

Tony