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25-04-2011, 15:44   #1
trav
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Immersion vs. boiler water heating

Hi,

Wasn't sure where to post this as it involves some techy stuff, but mostly related to cost so here goes.

I did a comparison on heating a tank of water with the electric immersion compared to a gas or oil boiler to see which was cheapest. Note, this is about saving money, not necessarily the planet. Think people assume that the electric immersion is the most expensive due to the hight unit cost of electricity. Boilers are not 100% efficient, though, and using a boiler to heat a remote tank through piping runs even less so.

Anyway, I have put the results up on a web-page, here:

http://www.randomprojects.info/immersion/

Hope it's of some use.
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25-04-2011, 15:47   #2
smokie2008
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So whats the cheapest? Don't make me go read it, far to hungover
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25-04-2011, 15:53   #3
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Yeah, sorry - asking a bit much given the day that's in it. Bottom line: immersion is cheaper (at the moment) unless you have a modern high-efficiency gas boiler. Oil is out of the question at current prices.

This is unless, of course, you are heating the house at the same time.
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25-04-2011, 16:20   #4
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Sweet, I always wondered which it was. I'm sure I'm not alone. I've asked loads of plumbers and none ever have a clue.

I did click on the link but FAR to scientific looking for a bank holiday Monday

Good work though!!
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25-04-2011, 22:07   #5
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Looks like a good piece of research, well done!

Are you just switching off all your radiators, or do you have an actuator valve to set the system to heat the hot water only?

Or can you drive the hot water through gravity feed (i.e, without the pump). Obviously not, because you mention having the pump running. If you don't know what I mean, see http://www.aquabrand.com/article/uk-...ms-and-boilers

What distance is there between your boiler and your hot tank? Are the pipes lagged? That could be where the loss is coming from, because there is definitely a pretty big loss there somewhere. If the electric is coming out cheaper despite the cost per unit being 4x more expensive, it means that around three-quarters of the gas energy is somehow being lost. Boiler losses don't come close to accounting for that.

I would be a little cautious about thinking that a boiler would deliver the full efficiency in those circumstances. The temperature differential between flow and return has to be in a certain range for the full efficiency features of the boiler to work. You would really need to check the boiler specs in detail to find out about this.

Have you multiplied out how many kWh it should take in theory to heat this much water?
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25-04-2011, 22:59   #6
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Thanks. I tried to avoid taking a theoretical approach to the problem as I have seen a few yes-but-no-but type discussions on the subjects as people try to quantify different gains/losses in the system. My take on it is what it actually does cost and think it represents a lot of similarly designed houses.

The gas boiler is sited in the kitchen downstairs and the hot-press is above and offset by a box-room, maybe 2.5m. I think the piping in between is uninsulated - there's no insulation showing in the hot-press anyway. There is no actuator, just a gate-valve to select rads-only or rads and h/w - all rads were closed during the test. Basically it is a typical 3-bed-semi setup of the early Celtic-Tiger era.

I agree that the boiler won't achieve close to full efficiency under the circumstances. I have heard that having an over-sized boiler for a given system will give very poor performance - this is effectively what the water-heating-only scenario is emulating. Also, I doubt if the efficiency figures are representative of the boiler being turned on, brought up to temperature and just turned off again - like you say there is a specified range of temperatures required for getting the reported efficiency levels.
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25-04-2011, 23:04   #7
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Well, gate valve is good as an actuator, providing, I suppose that no water is 'leaking' to other parts of the system.

I guess the pipes are copper rather than qualpex? Would make some difference.

When I was saying about being cautious - of course, I agree with you. But you would not really get much more efficiency out of a fancier modern condenser boiler because of the stuff mentioned. So having a newer boiler probably wouldn't make much difference in practice.

Would be interesting to compare the theoretical to the actual empirical figures, just for fun.

Last edited by antoinolachtnai; 25-04-2011 at 23:06.
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25-04-2011, 23:18   #8
Carlow52
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How do we know that the same volume of water is being heated in each case.

How far down does the stat go in the tank as opposed to the stat on the coil?

Your average element is a 2kw jobbie against a boiler which will be maybe 20-25kw so to say both need to be on for an hour....

The heat output from the coil will create much more turbulence in the tank than a pesky little element so I would think the increase in total hot water will be more for the coil.

re the following from the link
  • Most efficient gas boiler: 91.5%
  • Most efficient oil boiler: 97.2%
97.2%! where can I buy such a beast

From the planets perspective the elec is coming in at about 35% efficiency from the CCGT stations so....
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25-04-2011, 23:32   #9
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Would be interesting to see your raw readings/data and calculations too. Not doubting you, would just be nice to cast an eye over it.
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25-04-2011, 23:41   #10
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26-04-2011, 08:38   #11
trav
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Quote:
Originally Posted by antoinolachtnai View Post
Well, gate valve is good as an actuator, providing, I suppose that no water is 'leaking' to other parts of the system.

I guess the pipes are copper rather than qualpex? Would make some difference.
Yep, all copper.

Quote:
Originally Posted by antoinolachtnai View Post
When I was saying about being cautious - of course, I agree with you. But you would not really get much more efficiency out of a fancier modern condenser boiler because of the stuff mentioned. So having a newer boiler probably wouldn't make much difference in practice.
Yeah, have to agree with this - all you can say is that each boiler will approach the efficiency level, but I had no other point of comparison available. Maybe someone with a snazzy boiler can try the test.

Quote:
Originally Posted by antoinolachtnai View Post
Would be interesting to compare the theoretical to the actual empirical figures, just for fun.
Should be quite easy without including losses:
Assume 50 litres is heated by 50 degrees. It takes 4.2 kJ to heat 1kg of water by 1K, so around 10.5MJ for the tank of hot water.
1 Joule = 1 Watt/second so
10.5MJ =~ 2.9kWh
Which conveniently matches the immersion heater output

Also, I have the original data in a spreadsheet - will post it this evening when I'm home.
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26-04-2011, 09:37   #12
trav
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Quote:
Originally Posted by Carlow52 View Post
How do we know that the same volume of water is being heated in each case.

How far down does the stat go in the tank as opposed to the stat on the coil?
The method for comparing the two was described on the page - running the gas and comparing the trip-point on the electric stat or vice-versa. The stat for the coil is a little below half way down the tank. Also, I have switched over between the two quite a bit in the process and the thermostatic shower valve position is the same regardless, also the amount of time the water will last.

Quote:
Originally Posted by Carlow52 View Post
Your average element is a 2kw jobbie against a boiler which will be maybe 20-25kw so to say both need to be on for an hour....
This is exactly the point - the boiler may be able to output 20kW with 8 radiators and the immersion in the loop, but it can not transfer this amount of energy through the immersion coil. This means the burner is cycling on/off yielding low efficiency. 20kW applied directly to the water tank would heat it in less than 9 minutes - never comes close in practise.

Quote:
Originally Posted by Carlow52 View Post
The heat output from the coil will create much more turbulence in the tank than a pesky little element so I would think the increase in total hot water will be more for the coil.
Heat-flow in water is quite rapid due to its high specific heat capacity - as long as the element extends deep enough into the tank, its shape has little to do with it over the period of time we're talking about.

Quote:
Originally Posted by Carlow52 View Post
re the following from the link
  • Most efficient gas boiler: 91.5%
  • Most efficient oil boiler: 97.2%
97.2%! where can I buy such a beast
The beast in question is a Grant Vortex 26-36 condensing boilerhouse model apparently - no idea where you can buy one though.

Quote:
Originally Posted by Carlow52 View Post
From the planets perspective the elec is coming in at about 35% efficiency from the CCGT stations so....
Like I said in the original post - about saving money, not the planet
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26-04-2011, 09:51   #13
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One other thing - is there 3/4 inch piping to the hot tank?
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26-04-2011, 10:42   #14
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re.

The cheapest option-solar thermal- is not even considered?

Here the EU project COMBISOL:

http://www.combisol.eu/servlet/KBase...=3&catid=21723
Interesting reading promised!

Here some samples of hydraulic arrangements at page 25, all tried and tested:

http://www.solarthermalworld.org/fil...t.pdf?download

Since all homes must include a certain 'renewable' coverage of their thermal energy demand trav's research is not very usefull. Rather useless, no insult intended!

According to EU legislation ALL DHW systems must be rated from the 1st of June this year, i.e. the primary energy input must be compared to the output. At the tap that is.
Shown in clear numbers to the consumer (home owner).

And the combination of clear energy labeling/rating with the fact of the renewable energy mandate leaves only 1 option to be most cost effective.
And that is the usage of ST energy.

B.t.w.:

Modern (combi-) boilers modulate, are fully suitable to use pre-warmed water topping up only what is necessary at the point of draw.
From ca. 1.5 kW upwards. So using a 26 kW boiler to provide 3-9 liters of warm water per minute (sink or shower)is technical and economical non-sense, insult intended (smiley).

If using electricity in the DHW system then for the solar pump. 4-12 Watts per hour of sunshine, not more. So called 'A-rated circulation pumps'.
These A-rated circulation pumps are soon to become mandatory anyhow (January 2013?).

And if still insisting on electric resistance then a direct flow module. No storage loss. And of course: solar compatible, ready to use pre-warmed water, see above.
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26-04-2011, 11:03   #15
Carlow52
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Quote:
Heat-flow in water is quite rapid due to its high specific heat capacity

The property in question here is thermal conductivity and not specific heat capacity.

http://www.engineeringtoolbox.com/th...ity-d_429.html

Its the property that allow wetsuits to work


The Specific Heat Capacity is the amount of heat required to change a unit mass of a substance by one degree in temperature.

http://www.engineeringtoolbox.com/sp...ity-d_339.html

Its the property that makes water v good as a thermal store, as well as being non toxic and cheap


The coils in the more modern cylinders are available in capacities to match the boiler.

I have a 300 litre cylinder and from cold to 70 in 17 minutes
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