Immersion vs. boiler water heating

trav

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.

smokie2008

So whats the cheapest? Don't make me go read it, far to hungover

trav

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.

smokie2008

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:o

Good work though!!

antoinolachtnai

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-heating-systems-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?

trav

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.

antoinolachtnai

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.

Carlow52

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....

antoinolachtnai

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.

dudara

Moved to Home & Garden > Plumbing & Heating

dudara

trav

antoinolachtnai wrote: »

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.

antoinolachtnai wrote: »

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.

antoinolachtnai wrote: »

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.

trav

Carlow52 wrote: »

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.

Carlow52 wrote: »

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.

Carlow52 wrote: »

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.

Carlow52 wrote: »

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.

Carlow52 wrote: »

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

antoinolachtnai

One other thing - is there 3/4 inch piping to the hot tank?

heinbloed

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

Here the EU project COMBISOL:

http://www.combisol.eu/servlet/KBaseShow?sort=-1&cid=21722&m=3&catid=21723
Interesting reading promised!

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

http://www.solarthermalworld.org/files/CombiSol%20Publishable%20Report.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.

Carlow52

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/thermal-conductivity-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/specific-heat-capacity-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

trav

heinbloed wrote: »

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

Correct, in a comparison between electric immersion and boiler - see thread title - solar thermal is not considered, nor is nuclear, nor physical-activity.

heinbloed wrote: »

Here the EU project COMBISOL:

http://www.combisol.eu/servlet/KBaseShow?sort=-1&cid=21722&m=3&catid=21723
Interesting reading promised!

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

http://www.solarthermalworld.org/files/CombiSol%20Publishable%20Report.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.

There is no requirement to retrofit existing homes, the vast majority of which have an immersion/boiler combination. Hands up all those buying/building a new home from 1st June?

heinbloed wrote: »

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.

That's great, and I'm not advocating the system I have in any way - in fact I'll hold my hand up and say it's totally crap. Would love to have one of the alternatives you are detailing. I am simply trying to find out and present to those in my situation what the cheapest option is - this group includes:

• Households with pre-existing immersion and gas/oil boiler - most Irish households.
• Houses with no modern/energy-efficient alternative - most again.
• Those who can't currently afford an upgrade - do I need to comment?

So to say that this is useless is true if you exclude home owners with the most common heating configuration in the country. Not the way it should be, just the way it is I'm afraid.

trav

antoinolachtnai wrote: »

One other thing - is there 3/4 inch piping to the hot tank?

Yep, 3/4 copper throughout.

trav

Carlow52 wrote: »

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

Granted - I had my concepts garbled there.

Carlow52 wrote: »

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

I don't. Have a more standard size - maybe 80-100 litres.

antoinolachtnai

But a good point. Have you tried having a 'race' to see how long it takes each of the two systems to reach your target temperature?

It may be that the gas heating is more efficient at heating up the water, but less efficient at keeping it warm.

heinbloed

Trav wrote:

There is no requirement to retrofit existing homes, the vast majority of which have an immersion/boiler combination. Hands up all those buying/building a new home from 1st June?

Wrong.

Once a change to the existing supply of DHW is made (after 31st of May 2011) the EU demands the inclusion of 'renewable' energy. As well as the labelling/energy rating of the DHW system. To be precise: of the energy consumption per liter of DHW tapped.

A defect electric heating element can't be replaced anymore without following these rules.
Similar to the old boilers, if a F-rated boiler fails you're not allowed to replace it with another F-rated boiler. Or electric heating element.

They're in the pipeline since many years and have been agreed-on unilateral last year.
1st of June that is, don't forget.

A plumber/installer ignoring these rules will be made responsible, for a life time there will be a come-back by the consumer if an illegal heating system had been installed. Basic consumer law that is.



Here the link:

http://www.europarl.europa.eu/sides/getDoc.do?pubRef=-//EP//TEXT+TA+P6-TA-2009-0278+0+DOC+XML+V0//EN

trav

antoinolachtnai wrote: »

But a good point. Have you tried having a 'race' to see how long it takes each of the two systems to reach your target temperature?

It may be that the gas heating is more efficient at heating up the water, but less efficient at keeping it warm.

I was actually thinking of doing that as I have some temperature sensors which I can attach to the tank in different locations. Would give a more accurate result

trav

heinbloed wrote: »

Here the link:

http://www.europarl.europa.eu/sides/getDoc.do?pubRef=-//EP//TEXT+TA+P6-TA-2009-0278+0+DOC+XML+V0//EN

I don't see much in that link which backs up what you are saying, and nothing concerning the dates May 31/June 1. Mostly fairly vague EU-speak. The closest I can find is the statement:

'Member States shall take the necessary measures to ensure that when buildings undergo major renovation or building components and technical building systems or parts thereof are retrofitted or replaced , their energy performance is upgraded in order to meet at least minimum energy performance requirements in so far as this is technically, functionally and economically feasible.'

In other words, if your boiler fails, get a higher rated one - the F rated boilers will be effectively obsolete anyway. This does not indicate that you call the electrician to change an immersion heater element and he is required to get you to pay €2k+ for a solar panel on top - not "economically feasible".

Do you have another link which details the requirements this June 1st renewables bonanza you were referring to?

heinbloed

@ trav:

here the directive's proposal

http://ec.europa.eu/energy/strategies/2008/doc/2008_11_ser2/buildings_directive_proposal.pdf

and the date under 5.1

5.1. Need to be met in the short or long term
The Directive requires the Commission to establish and implement a methodology
for the calculation of the cost-optimal level of minimum energy performance
requirements for buildings. This methodology needs to be operational by 31
December 2010 at the latest. The Commission also needs to lay down the principles
for defining low or zero energy and carbon buildings.
The Directive requires Member States to use the abovementioned methodology and
to report on its results every three years, starting on 30 June 2011.

I have no idea how far this directive is actually translated into Irish law. But as far as I can see it is done " directly by the comission", see point 5.4

Meaning the Irish taxpayer would have to pay penalties, subsidies to the ignorants who have blocked the implementation.

I've tried to find something on EUR.lex but wasn't sucessfull with the search facility. Any luck?

trav

So an as yet undisclosed methodology has to be reported on by Ireland in relation to the calculation of energy performance of buildings by June 30th - doubt if that will be an occasion I'll need to remember somehow

@antoinolachtnai, I have posted the original raw data at the bottom of the web-page as it was easier than trying to format it on the forum:

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

antoinolachtnai

OK, I am looking at the graph now after looking at the raw data.

You are using cumulative figures for cost in the graph. Some things are being hidden by this.

Could you redraw it so that we can see the cost per minute or the energy consumed per minute? This would then indicate how long the water is taking to heat to the target temperature with each of the systems. It could give us a better indication what is going on.

SparKing

With all due respect Heinbloed you are way off-topic and are drawing the attention away from a bit of practical research which I personally have always sought and never found.
The fact is that a €30 immersion will always be replaced with another €30 in most cases for the foreseeable future.
What does "subsidies to the ignorants who have blocked the implementation" mean?

trav

Here is an example of a graph with instantaneous cost on the Y-axis - really just a derivative of the cumulative one so the same data presented differently. Don't think it's that intuitive myself.

antoinolachtnai

What it suggests to me is that the boiler is pretty effective at the beginning, and manages to mostly heat the tank within 15 minutes or so. It is then totally ineffective at keeping the tank hot and wastes a lot of energy and money.

The immersion is slower at heating the water, but is very efficient at keeping it warm. And you get much faster hot water. You only need to turn on the tank 15 or 20 minutes before you take a bath or shower or whatever.

That conclusion depends on whether the boiler is really getting the tank to the target temperature in that short time.

heinbloed

@ Sparking who asks:

With all due respect Heinbloed you are way off-topic and are drawing the attention away from a bit of practical research which I personally have always sought and never found.
The fact is that a €30 immersion will always be replaced with another €30 in most cases for the foreseeable future.
What does "subsidies to the ignorants who have blocked the implementation" mean?

Subsidised are those who blocked the fast implementation of the EPBD. They block it with excuses like " we can't, we haven't heard off, we don't have the control mechanisms, can't afford...we're to greedy for the fast buck! "

We look at

" 'Member States shall take the necessary measures to ensure that when buildings undergo major renovation or building components and technical building systems or parts thereof are retrofitted or replaced"

(from trav's post)

and then see your post

The fact is that a €30 immersion will always be replaced with another €30 in most cases for the foreseeable future.

This is simply not true anymore.

These statements- from the EU comission and from you- are contradicting, are contra-poductive. Your "future" won't be lasting for long anymore (smiley).

Fact is that trav has NOT taken into account the monetarian and energetic losses caused by storing and distributing.
And you went the easiest path of thinking: forget about the consequences. Nice that someone else does the thinking.

Fact is that the immersion can't be replaced anymore once the EPBD (Energy Performance of Buildings Directive) comes into effect.
I have undelined the reasons, see above.
Since DHW (domestic hot water) is now treated separatly from the rest of the building's energy demand the thermal source counts on its own.

And an F-banger for water heating (the storage cistern heated with an electric resistance heating element ) is outdated, not allowed anymore. Once it is defect it has to be replaced with a better performing method.
Similar to the F-rated CH boiler.

The reasons for this are the enormous losses caused by a.) oversupply of energy since the volume of hot water is exagerated and b.) the storage/transport losses.
Both energetic losses -storage AND transport- have now taken into acount.
That's the main issue with the EBPD concerning hot domestic water.
And this makes the immersion-in-the-cistern illegal to replace since it is the worst case scenario.
One simply can't use more energy when preparing DHW.

Direct electric heating would be more ecomical, would be an alternative to the immersion heating.
The better choice.
So when insisting on electric warm water preparation means for the not-so-far-away future pre-warming the water in the cistern as usual (via CH or solar thermal source or whatever) and then topping it up at the point of demand.

But certainly not preparing it with the immersion element, this will be banned very soon.
First of June this year as far as I know.

There is already plenty of choice of direct electric heating elements for water. Since many decades.
Electric showers and taps are available from every DIY shop.

heinbloed

@ antoinolachtnai:

A propper insulated and installed(plumbed) storage tank won't loose much heat.
1 kWh of energetic loss per 24 hours for a 300 liter tank can be achieved.
1 kwh of gas costs? Compared to electricity?

Electricity is the most expensive method to heat water.

Transport losses are to be added of course.
But most installations are crap, won't make this. And here the electricity will be wasted. " Keeping it warm" as you've said....

SparKing

heinbloed wrote: »

This is simply not true anymore.

While I don't disagree with your understanding of European Directives with regard to energy efficient heating etc., What I referred to when I said that the immersion element will be replaced in kind for the foreseeable future was the likelihood that the implementaion of those directives will be lax and that for most people the thought of having to upgrade from an immersion to a preheated Solar Panel setup, which may cost at least 30 times or more the cost of an immersion element, is unthinkable at the moment.

Also this is still off topic as it doesn't refer to the comparison of cost of heating a cylinder of water by gas or immersion (with existing equipment only )

trav

heinbloed wrote: »

These statements- from the EU comission and from you- are contradicting, are contra-poductive. Your "future" won't be lasting for long anymore (smiley).

Yeah, good point Heinbloed.

Oh, looky here, there are some people on this thread talking about buying old Ford Fiestas, Corollas, etc.:

http://www.boards.ie/vbulletin/showthread.php?p=71624443

No one has even suggested investing in a new Prius!! Think you might have to go over there and set them straight.

heinbloed

I totally agree, my posts on this thread are off-topic.

Trav described his observations concerning monetarian profits/expenditures in an existing DHW system, without changes in the technological set-up or consumer behaviour.

And my postings lead to a discussion what's feasible for the future.