Left the immersion on - for 2 weeks!

JDxtra

I had some plumbers in doing work in my house and they finished two weeks ago.

However I just realised they left the immersion on - possibly from when they were testing the hot water. This means it was on for at least two weeks. And yes, it was set to 'bath'...!

Does the element of an immersion heater run 24/7 - or does it auto switch off after a set temp is reached?

I'm wondering what kind of damage this could do to my ESB bill! :eek:

ircoha

JDxtra wrote: »

I had some plumbers in doing work in my house and they finished two weeks ago.

However I just realised they left the immersion on - possibly from when they were testing the hot water. This means it was on for at least two weeks. And yes, it was set to 'bath'...!

Does the element of an immersion heater run 24/7 - or does it auto switch off after a set temp is reached?

I'm wondering what kind of damage this could do to my ESB bill! :eek:

Iff the cylinder is well insulated and there was no usage from the cylinder, the hit will be minimal.
OTOH, an uninsulated cylinder with loads of hot water used, the hit will be maximal.

Yes it does cut out once a set temp is reached

fred funk }{

I know a few people who leave the immersion on all the time. As pointed out, if the tank is well insulated the heat loss is quite low.

leeroybrown

Don't worry too much JDxtra. As the previous posters have pointed out, once the cylinder is reasonably well lagged/insulated and you weren't running off huge volumes of extra water it shouldn't be dramatically more. There is a point beyond which it doesn't cost any more to leave it on permanently.

zuroph

is the immersion integral lagged? IE that green foam stuff stuck on the tank? i work in bathroom sales, and would usually recommend just leaving on the immersion, if the tank is lagged this way. company i worked with tested this extensively, and it cost less to run this way.

you can check at what temp the immersion cuts out by lookin on the head of the immersion, there will be a little screw with arrow that points to the set max temp. wen the temp drops below this, the immersion turns on and heats back up the few degrees to the temp then shuts off. this is much better than having to heat from cold repeatedly.

byte

Interesting. I didn't know this.

ircoha

zuroph wrote: »

is the immersion integral lagged? IE that green foam stuff stuck on the tank? i work in bathroom sales, and would usually recommend just leaving on the immersion, if the tank is lagged this way. company i worked with tested this extensively, and it cost less to run this way.

you can check at what temp the immersion cuts out by lookin on the head of the immersion, there will be a little screw with arrow that points to the set max temp. wen the temp drops below this, the immersion turns on and heats back up the few degrees to the temp then shuts off. this is much better than having to heat from cold repeatedly.

I dont buy this, for the following reason:

Heat loss is a function of the temperature differential across the insulation

By keeping the immersion on all the time the temp diff is at the max all the time so the heat loss is always at the max.
Similarly as water is heated it expands and some will come up the pipe at top of cylinder beyong the insulation so the hotter the water the more the heat loss.

ircoha

ircoha wrote: »

I dont buy this, for the following reason:

Heat loss is a function of the temperature differential across the insulation

By keeping the immersion on all the time the temp diff is at the max all the time so the heat loss is always at the max.
Similarly as water is heated it expands and some will come up the pipe at top of cylinder beyong the insulation so the hotter the water the more the heat loss.

I tend to agree with this, I ran an immersion 24/7 for a while and just found that it raised the temperature in the airing cupboard!

JDxtra

Thanks very much for the helpful replies guys.

Regards,
Jonathan.

10-10-20

ircoha wrote: »

I dont buy this, for the following reason:

Heat loss is a function of the temperature differential across the insulation

By keeping the immersion on all the time the temp diff is at the max all the time so the heat loss is always at the max.
Similarly as water is heated it expands and some will come up the pipe at top of cylinder beyong the insulation so the hotter the water the more the heat loss.

I've got to agree there.
The more heat you have in the tank and hotpress, the more you have to loose.
Hence, install a timer which is set to come on at the times when there is demand.

jacko

In reply to ircoha/dolanbaker/10-10-20, I've got to agree with zuroph. The amount of extra energy needed to warm water from cold to the required temperature would offset any energy loss you would get in a well insulated tank.

for example, if you want warm water in your tank every morning and evening you'd set your timer accordingly. this would mean heating the tank from cold both times which would take a fair bit of energy. (Most of the energy is used heating the water from cold to 30°)
If your immersion is constantly on then the amount of energy needed to keep topping the temperature up would be a lot less than from cold. Remember the thermostat will knock off the immersion when the set temperature is reached, it will turn it back on again when the temperature drops to a certain value)

All this is assuming your tank is fully insulated and temperature gauge is set correctly. Also assuming your water usage from the tank is average i.e. used morning and evening

You'll always use more energy if you have the immersion on 24/7 simply because insulation only slows down the heat loss and if you always have the tank at the preset temperature (60c or whatever) you will lose more energy than if you allow it to cool overnight.

ircoha

jacko wrote: »

In reply to ircoha/dolanbaker/10-10-20, I've got to agree with zuroph. The amount of extra energy needed to warm water from cold to the required temperature would offset any energy loss you would get in a well insulated tank.

for example, if you want warm water in your tank every morning and evening you'd set your timer accordingly. this would mean heating the tank from cold both times which would take a fair bit of energy. (Most of the energy is used heating the water from cold to 30°)
If your immersion is constantly on then the amount of energy needed to keep topping the temperature up would be a lot less than from cold. Remember the thermostat will knock off the immersion when the set temperature is reached, it will turn it back on again when the temperature drops to a certain value)

All this is assuming your tank is fully insulated and temperature gauge is set correctly. Also assuming your water usage from the tank is average i.e. used morning and evening

Re

(Most of the energy is used heating the water from cold to 30°)

its time for some basic math here.
http://www.engineeringtoolbox.com/heat-capacity-d_338.html

The formula referred to in the above link shows that the heat required to raise the temp of a substance is a function of the heat supplied, the heat capacity of the substance and the change in temperature.

[The starting and ending temp dont matter as long as they are in the normal range 0 to 100]

This link shows that for water the heat capacity is 4.19 kJ/kg.k regardless of temp

http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

This link shows how to convert from kJ to kWh {3600 kJ = 1 kWh]
http://www.shec-labs.com/calc/unit_conversion.php?dim=7&common=T

The point I am making is that the same amount of energy is required to increase the temp of water by a degree, regardless of the starting temp.

As I already stated, all other variables being constant, heat loss is a fn of the temp differential
see http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html

so the higher the temp differential the higher the heat loss

slavetothegrind

It is all about the heat loss figures,
We install fully insulated 220 litre water heaters gas fired and they have very low heat loss when not being drawn from.
I installed one in a stables as part of a horse wash system late one winters evening in a two thirds open shed uninsulated ( barn ), when we had it tested and at 60 deg. celcius we turned it off. It was 11pm and the air temp was minus 2.
The next morning the gauge on the unit read 57 degrees celcius.
So if really well insulated the always on theory can hold water!! ( sorry:o)