Immersion Heater Info.

How long would it take to heat up completely a bog standard copper cylinder with a cheap immersion heater.
Plus can you tell me how long that water should retain it's heat for bathing.
When i had new heating fitted 4 years ago both were replaced but only cheap parts were used. As my boiler is out of comission temporarily I am using the immersion. I find the water is NOT really hot enough for bathing 7 hours after having it on for 1.5 hours at 65 degrees.
Any recommendations please.

caveman2 said:

I find the water is now really hot enough for bathing 7 hours after having it on for 1.5 hours at 65 degrees.
Any recommendations please.

Click to expand...

I don't understand the problem - you run the immersion for an hour and a half, and after 7 hours there's still enough hot water to run a bath. What are you worrying about?

In any case, nobody can answer your question precisely as the answer will vary depending on the length and power rating of the immersion heater, capacity of the cylinder, temperature of water in the cylinder when cold, and how well insulated it us.

My apologies. I meant it is NOT hot enough.

caveman2 said:

My apologies. I meant it is NOT hot enough.

Click to expand...

Again, it's still impossible to answer as we know nothing about the cylinder. However, expect it to take a LOT longer to reach a given temperature than if it were being heated by the boiler through the cylinder coil. Depending on its length, the immersion may not even heat the entire cylinder.

If you're worried about having hot water later in the day then either run the immersion again to top up the HW in the evening, or run it for longer in the morning. As you've set the thermostat to 65 degrees (I assume this was the one inside the immersion can, and not strapped to the cylinder? The latter controls heat from the boiler only) then the immersion heater will switch off and stop drawing power as soon as the set temperature if reached. If the water in the cylinder falls below that temperature, the immersion will kick back in to maintain the set temp.

tell us how the cylinder, and the hot pipes around it, are insulated.

The cylinder has the manufacturers foam insulation but the pipes are not insulated.

caveman2 said:

The cylinder has the manufacturers foam insulation but the pipes are not insulated.

Click to expand...

If you have a model number then I'm sure you'd be able to find static heat loss information on the manufacturer's web site, assuming it's a current model or they have archived documentation.

That doesn't change the fact that you may not be running the immersion heater for long enough to heat the tank properly. I'd suggest you go from running it for 1 1/2 hours to 2 1/2 hours and see what difference this makes. You should also be able to get a good indication of how much of the water in the cylinder is being heated by feeling the exposed copper parts.

depending on the layout of the pipes (especially if there is one that goes straight up from the top of the cylinder) they may be losing quite a bit of heat, from the hottest part.

BTW as far as I recall, an hour and a half will not fully heat a cylinder. And as most immersion heaters do not reach right to the bottom of the cylinder, they will not heat enough water for a bath anyway. When my boiler was out of action and I had to use the immersion, I had to top it up with cold to get an adequately full bath. However the water stayed hot for 16 hours inside its insulating jacket after turning off the heater.

Presumably you have checked that there is no dripping hot tap or other use of hot water.

In the absence of further info from the OP, let's assume the cylinder holds 120 Litres, and the heater is long enough to heat 100 Litres of water.

Water has a specific heat capacity of about 4.2 Kilojoules per kilogram per degree Kelvin, so it would take about 6 Kilowatt Hours of electricity to heat 100 Litres of water from 15 degrees centigrade to 65 degrees centigrade.

As a standard immersion heater is 3KW, so it would take 2 hours to heat 100 Litres of water. After an hour and a half, it will only have reached about 53 degrees.....

If the cylinder and pipework loses heat at an average rate of, say, 300 Watts, it will have lost 2.1 KWH , or about half of it's energy input in seven hours, so the average water temperature in the cylinder would be about 35 degrees C

An immersion heater only heats the water above it in the cylinder. The water below it stays relatively cool.

Some immersion heaters are only about 300mm long, only projecting far enough into the cylinder to heat enough water for a quick shower relatively quickly. Others almost reach the base of the cylinder, and will heat almost the whole cylinder over a longer time.

With the best will in the world, a short heater probably wouldn't heat enough water for a bath , even if it was left running 24/7.