# Immersion - How best to use it



## BazFitz

Hi Folks

My immersion comes on for 3 hours during the night.  That just about gives us enough hot water for the day.  Lately we've started putting it on for a bit in the evening as well.  Some people have advised me to just stick the timer on all the time on the basis that it's actually cheaper to maintain a tank at a certain temperature rather than heating a cold tank up every day.

Does anyone know what the best approach is (i.e. the most economical)?

Thanks.


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## michaelm

BazFitz said:


> Some people have advised me to just stick the timer on all the time on the basis that it's actually cheaper to maintain a tank at a certain temperature rather than heating a cold tank up every day.


That sounds like nonsense to me.  I would be disinclined to leave it (anything bar the fridge & freezer) on at night.  Do you even have a night-saver electricity meter?  If not then you're not saving anything.  I just use it as needed, it heats up quickly.


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## z107

It's cheaper to heat it up as needed, rather than maintain a constant heat. When you use some of the water, the new cold water has to be heated up anyway.

Think about a kettle, would it be cheaper the keep this boiling for when you want a cup of tea, or just heat it up as needed?

In light of recent (largely unreported) events in Japan, I've really being keeping an eye on energy usage. Energy is probably too cheap.


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## BazFitz

michaelm said:


> That sounds like nonsense to me. I would be disinclined to leave it (anything bar the fridge & freezer) on at night. Do you even have a night-saver electricity meter? If not then you're not saving anything. I just use it as needed, it heats up quickly.


 
Yes, we have a night saver meter.  It seems to take quite a while to heat up though and I'd prefer to have hot water on demand rather than having to manage a limited amount.

Thanks for the advice, Folks.


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## Shane007

Turn it off completely. It is the most expensive way to heat hot water. Use your oil or gas to heat hot water. You will have more hot water in 1/4 of the time.

Immersions have a 3kw element therefore it is the same as boiling water in the kettle. It costs the same.


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## BazFitz

Shane007 said:


> Turn it off completely. It is the most expensive way to heat hot water. Use your oil or gas to heat hot water. You will have more hot water in 1/4 of the time.
> 
> Immersions have a 3kw element therefore it is the same as boiling water in the kettle. It costs the same.


 
Not an option but thanks for your input.


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## SparkRite

Immersions are VERY efficient, when kept free of limescale and any other contaminants, they can be around 95% and upward efficient as practically all the energy inputted is converted to heat and directly transferred to the water.

Regardless of heating source it is vital that the cylinder is well insulated.


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## DGOBS

They maybe efficient as in the heat transfer rate etc, but they would still be the dearest way to heat water when compared with gas/oil


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## alexandra123

On an analysis I done a couple of weeks back on gas
It costs me flat rate of 20 cent for 15 minutes of hot water using 1 radiator
It costs me 70 cent for 1 hour of heat supplied to all the radiators (7) at 21 degrees.

Does anyone know how much it costs to heat the water through the immersion? I dont use mine as it takes nearly an hour to heat up.


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## Guest105

DGOBS said:


> They maybe efficient as in the heat transfer rate etc, but they would still be the dearest way to heat water when compared with gas/oil


 

I would agree with you there, I disconnected 2 electric showers in the house and connected them directly to the hot water tank which is heated by oil. I cut my electricity bill by over a third.

Mind you the price of oil has since rocketed but I still think it is cheaper to heat the water by oil rather than electricity.


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## browtal

There must be a problem with your immersion.  When I heat the sink section it takes about 20 minutes.  I have a good supply of hot water for several hours.
I use timer for the above. It is on for 30 minutes in the morning. I have it timed for 30minutes at 5pm for dinner preparation.
Even if you like continuous hot water, using a timer for 30 mins. every few hours would give you a constant supply.
Showers need considerably more. I dont find the immersion efficient to heat for showers.  Electric showers are very efficient.  Browtal


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## rockofages

alexandra123 said:


> Does anyone know how much it costs to heat the water through the immersion? I dont use mine as it takes nearly an hour to heat up.



As stated above, 3Kw/h on bath, meaning it'll use 3Kw per hour. 2Kw/h on sink. How much are you paying per Kw/h for your electricity? Usually around 16c.

OP, 3 hours is way to long to heat water on the immersion heater. How big is your cylinder?

Typically it should take 30 to 40 mins to heat an average cylinder to 65 degrees C.


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## BazFitz

rockofages said:


> As stated above, 3Kw/h on bath, meaning it'll use 3Kw per hour. 2Kw/h on sink. How much are you paying per Kw/h for your electricity? Usually around 16c.
> 
> OP, 3 hours is way to long to heat water on the immersion heater. How big is your cylinder?
> 
> Typically it should take 30 to 40 mins to heat an average cylinder to 65 degrees C.


 
Thanks, ROA.

To be honest, I've no idea how big the cylinder is.  It looks pretty standard to me (i.e. similar in size to others I've seen - about 4 feet high and 2 feet in diameter).


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## bluemac

I heat my tank for 1.5 hours 7.30am - 9am to get the night save rate.
this gives hot water all day and enough for a good deep bath at night but if we use a bit more water during the day I do have to boost it on an evening for 15 mins for the bath.

As with other posted i change to a timer to stop us using the electric shower and it has cut out power usage by a good bit.


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## Complainer

bluemac said:


> I heat my tank for 1.5 hours 7.30am - 9am to get the night save rate.
> .


Are you sure this will get you the night rate? From [broken link removed]


> The hours during which the day and night charges apply for non quarter-
> hourly metered customers are controlled by time switches at the
> customer’s premises. The day rate is applicable to kWh metered between
> 8.00 a.m. and 11.00 p.m. (GMT). As no alterations are made to time
> switches at the beginning and end of summertime, the hours during the
> summer time are one hour later (i.e. 9.00am to 12.00am). For quarter-
> hourly metered customers day rates apply 8.00 am to 11.00pm summer
> and winter.


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## rockofages

BazFitz said:


> Thanks, ROA.
> 
> To be honest, I've no idea how big the cylinder is.  It looks pretty standard to me (i.e. similar in size to others I've seen - about 4 feet high and 2 feet in diameter).



In that case it should take no more than about 40 mins max. Bear in mind that the immersion heater will never be able to heat the tank all the way to the bottom.



			
				Complainer said:
			
		

> Are you sure this will get you the night rate?


It will in the summer (ie now), but not in the winter as Nightsaver ends at 8am during the winter (ie when the clocks change).


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## Shane007

rockofages said:


> As stated above, 3Kw/h on bath, meaning it'll use 3Kw per hour. 2Kw/h on sink. How much are you paying per Kw/h for your electricity? Usually around 16c.
> 
> OP, 3 hours is way to long to heat water on the immersion heater. How big is your cylinder?
> 
> Typically it should take 30 to 40 mins to heat an average cylinder to 65 degrees C.


 
Not quite true. A standard 30 gallon (136 litre) copper cylinder will require 8.72kW to heat from 10°C to 65°C (∆T 55°C) in one hour, therefore a 3kW immersion will take just under 3 hours to heat completely. In 30 to 40 mins, you will have only heated 46 litres of water from cold to 65°C.

To put this into prospective, a shower will have a flow rate of approx 8-10 litres per minute, whilst a power shower will have a flow rate of double this.

A 26kW oil or gas boiler will have all 136 litres heated to 65°C from cold in approx 20 mins.


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## hippy1975

BazFitz, a friend of mine was telling me for years that leaving the immersion on constantly was the way to go, I tried it 3 years ago and haven't turned it off since as it has made no major difference to my elec bill and the constant hot water is so handy.  Try it for a month or two and see what you think. I think it's more efficient in the long run because you're only heating the amount of water you have used rather than possibly letting a whole cylinder of water go cold to then heat it up from scratch a few hours later.   Give it a try,


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## z107

That was a very interesting post Shane007. 
Do you believe hippy1975's post makes sense?


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## villa 1

Leaving the electric immersion on constantly. Hopefully the immersion thermostat won't fail, which could lead to catastophic circumstances with collapse of polythene cold water storage cistern. See case in the UK a couple of yrs ago,


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## Leo

You could get one of [broken link removed] to measure the power usage of both options. That way you'd know for sure.
Leo


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## SparkRite

Shane007 said:


> A 26kW oil or gas boiler will have all 136 litres heated to 65°C from cold in approx 20 mins.




ABSOLUTELY NOT!!

A 26Kw boiler cannot transfer anywhere near the amount of energy required to raise the temp of 136L of water by 55deg. in 20mins using a standard coil heat exchanger found in a typical domestic cylinder.

I think you are assuming that 26Kw's of heat energy is being directly "applied" to the water, which , of course, it is not.


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## hippy1975

Yeah hopefully it won't fail, don't think it should though, my friend's has been on for about 15 years now (except hols, they do turn it off when they go away)


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## Shane007

SparkRite said:


> ABSOLUTELY NOT!!
> 
> A 26Kw boiler cannot transfer anywhere near the amount of energy required to raise the temp of 136L of water by 55deg. in 20mins using a standard coil heat exchanger found in a typical domestic cylinder.
> 
> I think you are assuming that 26Kw's of heat energy is being directly "applied" to the water, which , of course, it is not.


 
Energy required to heat 136L by 55°C does require an input of 26kW to raise in 20 minutes.

Energy required to raise in 1 hour = Volume x ∆T x 0.001166
therfore 1 hour = 136 x 55 x 0.001166
therefore 1 hour = 8.72kW
therefore 26kW = 20 minutes

If you are saying that a 26kW boiler does not produce 26kW then you are wrong, it does. Gas boilers are not my fortee, but oil is and a 15-26kW oil boiler can be set to produce 26kW by adjusting the fuel to air ratio. You maybe confusing efficiency with output. If a boiler is say 85% efficient, it will still output 26kW of energy, however, the combustion efficiency to produce this will only be at 85%. A 26kW condensing boiler with an efficiency of 97% will still only produce 26kW output but it will use less fuel to do so.

With regard to beyond the boiler to the cylinder, I am talking about using all of that 26kW to heat the cylinder on it's own and not using any of it to simultaneously heat radiators or any other form of space heating. You may lose a very small amount due to exposed uninsulted pipework between the boiler and the cylinder (approx 10% loss with a lot of exposed pipework).

Then onto the cylinder itself. Modern cylinders are well insulated and also have highly efficient rapid heat recovery coils. Most modern coils are finned to increase their surface area and therefore increasing their heat transfer rate. 
Granted, with a 30 year oil uninsulated copper cylinder with 30 year oil standard coil, it may take longer, but not that much longer. Even I give a loss of 20%, this will only add a few minutes to my heat up time.

Whatever way you want to turn this, upside down, inside out, an input of 3kW will not stack up to an input of 26kW and yes, it is an input of 26kW. We can detract some kW transfer for unserviced boilers, poorly running ones, but I am talking about in use as per manufacturers instructions as you are talking about an immersion that is unaffected by limescale, etc.




umop3p!sdn said:


> That was a very interesting post Shane007. Do you believe hippy1975's post makes sense


I would completely agree with Villa1 on this. Leaving on the immersion is dangerous as if the built in thermostat fails and they regularly do, many dangerous outcomes can evolve.

Whether is is better or not will depend entirely on how much hot water you use and on how quickly you use it. If you use a little often, then I would say it is cheaper, but if you use a substantial amount at various intervals, then I would say it is more expensive.

My advice is leave it off, zone your heating system so that you can heat hot water independently from your heating system. If works out far cheaper in the long run.

If this is not an option, install a timeclock to the immersion heater and only leave on for the required amount of time.


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## DGOBS

Hi Shane, but what about the heat transfer rate of the older cylinder, wouldn't the boiler just start to short cycle! as the 26kw/h output is unable to be transfered via the coil and circulate on the return knocking out the burner

But suffice to say, the boiler would be much quicker than an immersion (and cheaper to run)

If people are so worried about heatup times for water, running cost, and waste where are all the combi boilers??????


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## Shane007

Yes very true. My point is a 26kW boiler is large enough to heat 136L in 20 minutes. An older cylinder's coil will not have the capacity to transfer the heat quick enough and the boiler will be on top of the heat very quickly. 
It is difficult to comment on every installation and can only assume certain parameters, preferably modern ones. That said it will not be an enormous additional time to heat an older cylinder.


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## trav

I did an actual comparison of a gas/oil boiler compared to an immersion   heater in my own house - real values as opposed to theoretical.  Seems   in most cases the immersion is the winner from a cost point of view.    Results at 

[broken link removed]


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## SparkRite

Shane007 said:


> Yes very true. My point is a 26kW boiler is large enough to heat 136L in 20 minutes. An older cylinder's coil will not have the capacity to transfer the heat quick enough and the boiler will be on top of the heat very quickly.
> It is difficult to comment on every installation and can only assume certain parameters, preferably modern ones. That said it will not be an enormous additional time to heat an older cylinder.




My point exactly!!

You can have a 100KW boiler, if you like, and it will not be capable of transferring that energy to the water (136L) in 20 mins or even close to it in a normal domestic installation. So therefore an immersion, running at approx. 96% efficiency (and above) may be just as cheap to heat a normal hot water cylinder.

I'm sure you will agree Shane007 that just having the heat coil in a boiler circuit is tantamount to a short circuit in an electrical circuit.

Note: "Tantamount"


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## onlineprint

leaving the immersion on all the time is :

1. Dangerous -  I done this and the wires on the top of the tank MELTED, we were very lucky.
2.Meter readings in Ireland are not regular and are mostly estimated, you should read the meter when you have immersion turned on and check the amount it uses, it is quite high
3. I found my ESB Bills have went down since I stopped leaving immersion on 24/7.


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## Shane007

onlineprint said:


> leaving the immersion on all the time is :
> 
> 1. Dangerous - I done this and the wires on the top of the tank MELTED, we were very lucky.
> 2.Meter readings in Ireland are not regular and are mostly estimated, you should read the meter when you have immersion turned on and check the amount it uses, it is quite high
> 3. I found my ESB Bills have went down since I stopped leaving immersion on 24/7.


 
I completely agree. Leaving on 24/7 is over-relying on immersion thermostat and they regularly fail. I have changed many immersions burnt out that melted the cylinder insulation surrounding them and some that actually caught fire.



SparkRite said:


> My point exactly!!
> 
> You can have a 100KW boiler, if you like, and it will not be capable of transferring that energy to the water (136L) in 20 mins or even close to it in a normal domestic installation. So therefore an immersion, running at approx. 96% efficiency (and above) may be just as cheap to heat a normal hot water cylinder.
> 
> I'm sure you will agree Shane007 that just having the heat coil in a boiler circuit is tantamount to a short circuit in an electrical circuit.
> 
> Note: "Tantamount"


 
As DGOBS previously stated, efficiency is not the main factor here, but kW output is. Most decent cylinder coils can rapidly heat a cylinder, giving a much greater volume of hot water in far less time than a 3kW immersion can. The immersion heater of 3kW or many of 2.5kW @ 96% efficiency will give an heat transfer of 2.88kW and 2.4kW respectively. A simple home test will easily throw light on the issue. Heat your cylinder with boiler for 20 minutes. Turn on tap and time hot water until goes cold. Then, heat your cylinder for 20 minutes with immersion. Turn on hot tap and time until it goes cold. I think you will find the difference of time will be substantial, in favour of the heat via the coil. Then cost can be easily worked out in relation to volume of hot water versus cost of each source of energy.

With regard to agreeing with boiler circuit being "tantamount" to a short circuit in an electrical circuit, I am afraid my English fails me here to understand what this means. If it means that it compares a boiler short cycling to an electrical short circuit, then this is not true. The two are very different. 

Apologies if I am misunderstanding your point.


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## meadow

trav said:


> I did an actual comparison of a gas/oil boiler compared to an immersion   heater in my own house - real values as opposed to theoretical.  Seems   in most cases the immersion is the winner from a cost point of view.    Results at
> 
> [broken link removed]



Interesting project trav.

One thing that caught my eye was the following

"_The gas reading was complicated a bit by the fact that I do not have a bypass valve on the pump, so when the system is running there should always be a flow-path for the water to save putting too much pressure on the pump.  This means normally leaving the smallest radiator on which obviously reduces efficiency.  I ran a subsequent test with this rad off to get a reading for a similar system with a bypass valve – possibly at the expense of my pump’s life-expectanc_y."

I use the gas to heat my hot water during the summer. I turn off all the radiators to do this as this was the approach advised by my gas installer who converted my set up from oil to gas a few years ago, in the process installing a new boiler and pump.

I would generally turn on the gas heating for an hour a day and it is just heating the tank as all rads are turned off.

Last year my pump failed and needed to be replaced. It was only 5 years old. My installer said that it was a very good German make and they usually last much longer than this. He was at a a loss to explain why it had failed so early. I asked if turning off all the rads during the summer would cause a problem and he said it would not. He didn't mention anything about a bypass valve for the pump. I don't know if my pump has one or not. How could I check this ?

From your post it would seem that turning off all the rads can cause a pump to fail earlier  if there is no bypass valve. Is that correct ?

The 2 options I would have would therefore be to leave one rad on and heat water using gas or else heat water using the immersion.

I may switch to using immersion instead as cost of replacing the pump was expensive but did not realize the pump failure could be linked to my practice of turning off all rads in summer.


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## mum2009

I would like to know what hippy1975 's electricity bill is like.
Would the element not burn out?
What do you do for holiday times? leave it on while away?
it seems lunacy to me.... total waste of money


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## Guest125

What is a bypass valve? Where is it in the system? 1" or 3/4" copper from the boiler to the copper cylinder without any rads on shouldn't cause any excess strain on the circulation pump. I use the oil with all the rads off in the summer. It's a 10 year old pressurised sealed system and it works perfectly.


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## Pamkly

Hi, I have fitted a timer switch to the imersion, comes on for an hour 3 times a day, morning, midday and evening and I have hot water all day. Beats forgetting to turn it off. Pamkly


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## trav

meadow said:


> From your post it would seem that turning off all the rads can cause a pump to fail earlier  if there is no bypass valve. Is that correct ?



Sorry about the late reply - was away from the forum.  In answer to your question, if you have the most common setup I don't think this would be your problem.  The gas boiler is heating your water so there is a flow path at least through the immersion coil.  The difference in my system is that I have a thermostatic valve on the immersion feed which closes when the tank is up to temperature, so can completely shut off the pumped water.  These are not normally fitted, but if you want to check it looks like a radiator thermostatic valve on the top 3/4" pipe going in to the immersion coil.

A pump failing early could also be due to swarf/debris in the piping which wasn't flushed before use, or running the pump dry.


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