# economic way to heat water



## Ollie11 (21 Jul 2008)

hello. any thoughts on the most most economical way to have a constant supply of hot water in the home? available options are gas central heating on full or part time (with the thermostat turned down) or the immersion on to "sink" full time.


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## dem_syhp (21 Jul 2008)

It's cheaper to heat the water tank with the gas.  However - I haven't compared that to heating with just the sink option.  I've checked the appliance calculator on the esb web site and they don't have it listed.  I don't have easy access to my meter to check.  Based on the appliance calculator to heat one tank a day from cold with the electric emersion is 68 euro per 2 months!  

However, really you're comparing the cost of heating one tank with gas to heating the tank with electricity.  It's easy to check the meter for the time to heat up the tank with each.  It will be approximated with the electric (take away roughly the other appliances by looking at the average) 

For me there is a 5x difference in cost, but that's comparing the figures on the esb site for a full taken with my own metering for 15 min with the gas - doesn't heat it fully, but sufficient for me. So it's not an apples to apples comparison - but good enough that I'm using gas! - My boiler is 10+ years old, really not that efficient, but even with that, far cheaper.  

If you want hot water check the following:
Check thermostat in hot water tank - no point in over heating water
Check insulation - pipes, lagging jacket - or factory insulted tank far better - this won't reduce price of heating tank, but less re-heats required. 
Just heat the water, don't heat the radiators - can easily switch between them. 

Ideally:
Is solar an option?  
If not, get a high efficiency gas condenser boiler (20-30% more efficient)
Consider a boiler that heats the water on demand instead of a hot water tank.  More expensive than standard boiler - but may save you in the longer term.


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## rockofages (22 Jul 2008)

We have night saver electricity and our immersion heater is on a timer. It's 2kW ("sink") and comes on every morning for 30 mins, so it consumes 1kW/h per day (2kW*0.5 an hr). On night saver that's 7c a unit, or 7c a day, by 365 = €25.55 a year. That's enough for 2 gravity fed showers per day and bit left over for general use.

If the immersion was set to "bath" (5kW) it would cost €63.88 a year.

I don't believe it's cheaper to heat water by gas any more (unless the central heating is on anyway) as you end up heating the water in the whole system, which is let cool down again and therefore a waste.


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## Leo (23 Jul 2008)

rockofages said:


> I don't believe it's cheaper to heat water by gas any more (unless the central heating is on anyway) as you end up heating the water in the whole system, which is let cool down again and therefore a waste.


 
A lot of people seem to consider the tank of hot water provided when the heating is on anyway as free for some reason!

New energy efficient washing machines only use a cold feed, as it is generally more efficient to heat the water in situ on demand rather than have it sitting in a tank for hours. It also loses heat in the piping run.
Leo


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## aircobra19 (23 Jul 2008)

If you are running the heating, you don't run it any longer to heat the tank at the same time. Maybe I'm missing something.


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## Leo (23 Jul 2008)

It requires energy to heat a tank of water, this is the same amount of energy whether you are using gas or electricity. When you run this off the central heating, the temperature of the water being circulated to the rads drops as it travels through the coil in the water tank. So you end up needing to leave the central heating on that bit longer to raise the temperature of the house to the required level.
Leo


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## z109 (23 Jul 2008)

Have you thought about getting a timer on the immersion?

Then you can set it to heat water a few times a day when you think you need it.

One of the ones with the fiddly multiple settings (the clock ring with the push in tabs) is probably better as you might want it on three or four times a day.

Also, check what temperature you are heating the hot water to - plumbers seem to leave the default at almost boiling, which surely can't be that efficient.


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## aircobra19 (23 Jul 2008)

Leo said:


> It requires energy to heat a tank of water, this is the same amount of energy whether you are using gas or electricity. When you run this off the central heating, the temperature of the water being circulated to the rads drops as it travels through the coil in the water tank. So you end up needing to leave the central heating on that bit longer to raise the temperature of the house to the required level.
> Leo


 
Ah, that makes sense alright. I've never noticed that myself because the house heats so fast. But of course it must take more energy.


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## DavyJones (23 Jul 2008)

Most houses have the ability to just heat hot water only from the central heating boiler. I.E the radiator circuit is shut down. I rarely use my immersion. Next time you have it on watch your meter spin.


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## Caveat (23 Jul 2008)

yoganmahew said:


> Have you thought about getting a timer on the immersion?


 
We did this. Very quick, cheap, easy job.


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## mickpyrmont (7 Aug 2008)

DavyJones said:


> Most houses have the ability to just heat hot water only from the central heating boiler. I.E the radiator circuit is shut down. I rarely use my immersion. Next time you have it on watch your meter spin.


 
If you turn of all the rads and turn on the boiler does this use the same amount of gas as if you had a seperate setting on your boiler for turning off the central heating circuit?


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## moondance (7 Aug 2008)

I don't know if this is an option for you but in my apartment gas is used to heat the water "on demand" so when you turn on a hot tap the water is only heated as you need it. I find it efficient for me anyway!


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## DavyJones (7 Aug 2008)

mickpyrmont said:


> If you turn of all the rads and turn on the boiler does this use the same amount of gas as if you had a seperate setting on your boiler for turning off the central heating circuit?



You are shutting down the radiator system the same,  although it is a pain to shut off the radiators (you need only shut down one valve on rad to stop circulation). This will save you money as the amount of water to heat has been decreased.


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## Voyager (7 Aug 2008)

I have a question about using the immersion heater that can't be answered on the ESB website. If you leave the immersion on all the time it is only being used to keep the water hot and not heating up a tank from cold (except for the first time you turn it on that is). Is there any way to calculate if it is more efficient to leave it on all the time than to heat the tank from cold even if only a few times a week? The ESB calculator assumes that you are heating the full tank from cold.


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## zohan (7 Aug 2008)

Voyager, 
I was just going to ask the same question!  
At the minute we leave it on all the time (no timer switch), would it be worth our while getting a timer installed?


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## DavyJones (7 Aug 2008)

is this any help?. 1 BTU (British termal unit) is the amount of energy needed to raise the temperture of 1 pound of water by 1 degree F.

1 BTU = 3.412 watts
1 pound = .45359237 KG
1 litre of water = 1.01 kilograms
Degree F = 5/9 degree C

Standard cylinder volume is approx 135L

I think if these are applied in the right way you will get your answer.

Any mathematicians?


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## z109 (7 Aug 2008)

Voyager and zohan, I don't have the capability to work out the figures, but unless you have a tank that loses zero heat through radiation (is lagged to death), you will lose out by constantly topping up the temperature.


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## z109 (7 Aug 2008)

One other consideration with a tank is to look at the temperature the water is being heated to - by default many tanks seem to be set very high - 80C on my father-in-laws. As most of the hot water ends up flowing down the sink, this is a waste. There's usually a screw switch under the plastic cover at the top of the immersion coil that can be adjusted to reduce the maximum temperature the water is heated to. Also a handy thing to do if you have small children to prevent scalds.


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## DavyJones (7 Aug 2008)

Cylinder temperature should be no more than 60C.


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## Leo (7 Aug 2008)

DavyJones said:


> Any mathematicians?


 
Do we want to get really complicated and start adding in usage patterns  and temperature loss over time from the cylinder? So I'm certainly not volunteering... 

I think the best way to find out for sure is to get an energy meter and monitor for a standard week with each method. You'd need to get this wired in to the immersion circuit though.
Leo


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## DavyJones (7 Aug 2008)

Leo said:


> Do we want to get really complicated and start adding in usage patterns  and temperature loss over time from the cylinder? So I'm certainly not volunteering...
> 
> I think the best way to find out for sure is to get an energy meter and monitor for a standard week with each method. You'd need to get this wired in to the immersion circuit though.
> Leo



I agree, Thats why I didn't even attempt it


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## aircobra19 (7 Aug 2008)

DavyJones said:


> Cylinder temperature should be no more than 60C.


 
Why 60?


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## DavyJones (7 Aug 2008)

aircobra19 said:


> Why 60?




Because its not hot enough to scold  , but it is hot enough to kill off any bacteria and germs that may want to live in your cylinder.
Ofcourse you can have any temp you are comfortable with.  In public buildings it should be no higher than 60C.


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## j26 (7 Aug 2008)

DavyJones said:


> is this any help?. 1 BTU (British termal unit) is the amount of energy needed to raise the temperture of 1 pound of water by 1 degree F.
> 
> 1 BTU = 3.412 watts
> 1 pound = .45359237 KG
> ...



I could be way out, but I'm calculating that at about 73Kw to raise a tank from 20 to 60 celsius


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## dem_syhp (7 Aug 2008)

I think you might have a small problem with your maths 

135 litres  = 136.35 kg
136.35 kg = 300.6 pounds
~300 BTU to raise tank by 1 deg f  
if looking at 20 to 60 - this is a difference of 40 degrees celcius, or 22.22 degrees F
So to raise tank by 22.22 F = 6666w, or 6.6 Kw.  

How does this compare to the ESB web site: 9.25 to heat a tank, but this is a 120 litre tank and they may be heating it higher.  They also gave a 2 month figure that I divided by 8, possibly should be 9.  

However, none of this answers the question - the real problem is how much heat is lost during the day and hence needs to go back in to heat up the tank.  This is down to the insulation.


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## DavyJones (8 Aug 2008)

dem_syhp said:


> I think you might have a small problem with your maths
> 
> 135 litres  = 136.35 kg
> 136.35 kg = 300.6 pounds
> ...



Where are you getting the 22.22 degrees F from, please?

Oh and by the way I meant 125L not that it matters.
The best way to use your immersion imo is to turn it on 30 mins before you need it. who needs a full cylinder of hot water on stand by, heat as much as is needed.


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## dem_syhp (8 Aug 2008)

Right, I've been thinking about this - now I'm not an engineer - so open to be proven wrong.  I've no figures here - just an explanation.  

But, the rate of change of temperature of an object is proportional to the difference between its own temperature and the ambient temperature.  (Newton)

We all know this - if you make a cup of coffee and won't drink it for a couple of min, you're better putting in the milk first to minimise the amount it cools down.  Versus waiting the couple of minutes and then adding the milk - it will have cooled down more (greater temperature differential) and then when you add the milk, if you compared it to the first cup - it's cooler.  

So, if the tank is hot the temperature loss will be at it's greatest - so while you're only heating it by a couple of degrees each time, your rate of loss is maximised and hence you're using up more energy.  

Assumptions: 
The same amount of energy required to heat it from 20 to 30 degrees, or from 50 to 60 - which appears to be true (and makes sense to me).
Newtons law of cooling applies to our hot water tank - again can't see why not.  

Result: Fit a timer + insulate to minimise losses


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## aircobra19 (8 Aug 2008)

DavyJones said:


> Because its not hot enough to scold , but it is hot enough to kill off any bacteria and germs that may want to live in your cylinder.
> Ofcourse you can have any temp you are comfortable with. In public buildings it should be no higher than 60C.


 
I thought there might be an official guideline or something.


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## dem_syhp (8 Aug 2008)

DavyJones said:


> Where are you getting the 22.22 degrees F from, please?
> 
> Oh and by the way I meant 125L not that it matters.
> The best way to use your immersion imo is to turn it on 30 mins before you need it. who needs a full cylinder of hot water on stand by, heat as much as is needed.



Sorry, I was basing it on 20 degrees C to 60 C which is the same figures as other poster.  This is a difference of 40C.  
To convert to F is: 
40 x 5 / 9 = 22.22


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## DavyJones (8 Aug 2008)

aircobra19 said:


> I thought there might be an official guideline or something.



There probably is, I was only speaking to a colleage today about it as some pump manufactors state that water no hotter then 55C should enter pump from cylinder. The public building one is in the regs.


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## DavyJones (8 Aug 2008)

dem_syhp said:


> Sorry, I was basing it on 20 degrees C to 60 C which is the same figures as other poster.  This is a difference of 40C.
> To convert to F is:
> 40 x 5 / 9 = 22.22



Is 40/1 x 5/9 not 72 or if 20C = 68F and 60C = 140F differance is 72F?


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## j26 (8 Aug 2008)

dem_syhp said:


> Sorry, I was basing it on 20 degrees C to 60 C which is the same figures as other poster.  This is a difference of 40C.
> To convert to F is:
> 40 x 5 / 9 = 22.22



Nope it's 40 x 9 / 5 = 72 

My calculation is based on
3.412 watts per lb per deg F
7.52 watts per kg per deg F (3.412/.454)
1014 watts per 135kg tank per deg F (7.52 x 135) (I ignored the 1.01 litre/kg)
1826 watts per tank per deg C (1014 x 9/5)
73049 watts per tank for 40 deg C (1826 x 40)

which is 73.05 Kw

It doesn't sound right to me, but the maths add up

Edit:   This lad works it out using joules etc, so its probably more accurate - 7.5kw for a 150 litre tank


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## dem_syhp (8 Aug 2008)

Oops, sorry - why not to do sums late at night.  

I'm not going to look at it again till fresh eyes!


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## DavyJones (8 Aug 2008)

This is fun

(1 BTU/lb °F)(300 lb)(140°F - 68 °F) = 21,600 BTU

1 BTU = 3.412
so 
21600BTU = 6314 watts

= 6.31 KW


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## z109 (8 Aug 2008)

DavyJones said:


> This is fun
> 
> (1 BTU/lb °F)(300 lb)(140°F - 68 °F) = 21,600 BTU
> 
> ...


If 1 BTU = 3.412w
then
21,600 BTU must equal 63,000and something watts?
= 63kw which is the same are of nonsense sounding answer j26 came up with!

(Not saying it is nonsense, just that it sounds mad). 

Somebody somewhere is missing a carry one!


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## DavyJones (8 Aug 2008)

yoganmahew said:


> If 1 BTU = 3.412w
> then
> 21,600 BTU must equal 63,000and something watts?
> = 63kw which is the same are of nonsense sounding answer j26 came up with!
> ...




Sorry, My mistake. 3.421 BTU's in 1 watt.


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## Leo (8 Aug 2008)

You need to factor time into these calculations if you're talking costs, so one unit of electricity is 1 kWh, ... so 1 BTU is 2.931 ×10-4 kWh, which correcting the above means 1 watt is 3.412 BTU/h 

Which all goes to remind me why I didn't attempt this in the first place 
Leo


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## DavyJones (8 Aug 2008)

Thats what I meant 3.412 BTU = 1 watt. If I never see a watt again I'll be happy!


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## z109 (8 Aug 2008)

Thank you for your hard work DJ!


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## dem_syhp (8 Aug 2008)

Phew - I'm tired just looking over it - thanks!  Of course doesn't really answer question of most economical way of heating water. Just validates what the wee spec on the tank should have told us if any of us had looked it up


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