Immersion heater

amstel2

I get hot water from an electric immersion heater/tank that was installed about 5 years ago (the old one was over 50 years old & had given excellent service but finally stopped working). The chap that installed said the most economical way to use an immersion is to leave it on all the time as it costs a lot more to heat a tank of water from cold than to let the thermostat maintain the temp.

I tested what he said I left it on for a month & the following month put it on for about an hour morning & night as I needed it. To be honest going by the meter readings it was slightly more economical leaving it on as he said. With the added connivance of constant hot water.

However, I have now heard it can be dangerous to leave an immersion on all the time in case the thermostat fails & the the water would keep getting hotter which could cause the tank to explode. Is this right.

Also the tank is insulated with a blue poltsyrine type coating the installer said their was no need to fit an additional jacket. Also i am not on Economy 7.

Thanks

CashStrapped

What your plumber said is a bit of a myth. The answer can get long winded and complicated.

Firstly, You have a new tank, which is probably well insulated. Your new tank will probably be losing much less heat than your old one. So you maybe were comparing the new tank with the old one which is skewing your view of things.

If you leave the immersion on, the new tank will still lose small amounts of heat throughout the day, and the immersion will continuously top it up. As the new tank is well insulated, this may amount to very small amounts of electricity, but it may still add up over time.

If the tank was poorly insulated then it would be a different story. It would be like leaving a kettle on all day just so it is ready and boiling when you need it in the evening.

The principle is the same, but the amount of energy lost (hence energy used to maintain the temperature) will be much smaller. With a super well insulated tank, the difference between the two will be minimal, but heating the tank only when you need it will be slightly cheaper.

Just ask yourself the following. Would you keep the tank heating itself if you were out most of the day, ready for when you needed it? Sounds Plausible? Then expand it. You’re going away for a week. Would you heat the tank from Monday to Sunday, just so it is ready for when you needed it on Sunday evening? Expand that to a year and so on!

So basically as I said, as long as you are using the hot water every day, the difference should not be too much. But heating it only when you need it will be cheaper in the long run.

What may save you more money is turning down the immersion thermostat so it did not heat the water to a too high a temperature. So the lowest temperature for a comfortable hot shower/bath without mixing lots of cold into it.


Re the overheating:

The immersion element will have a thermal cut out separate from ther thermostat. This will trip the element if it got too hot. So you should not need to worry.

Cardew

Post above is 100% correct; we go through the same question and answer session every year.

However, I have now heard it can be dangerous to leave an immersion on all the time in case the thermostat fails & the the water would keep getting hotter which could cause the tank to explode. Is this right.

I think this is probably someone getting confused with a tragic incident about 7 years ago.

http://www.theguardian.com/uk/2008/jan/08/uknews4.mainsection

In almost every case if an immersion heater is faulty it stops heating. However it can rarely happen that the thermostat would fail and carry on heating. In that case the water would boil and siphon into the cold water tank.

In the tragic case above, the cold water tank in the loft was of a plastic construction and split and the boiling water cascaded through the ceiling onto the baby's bed below.