CH hot water question

[Spyder]

We have oil fired central heating. The boiler has a built in hot water tank to supply the needs of the house. We also have the usual electric water heater. In the summer we use the electric water heating as we shut the central heating down completely. What I am wondering now is should I continue to use the electric water heating even though the CH is on. In other words which method is cheapest bearing in mind the oil boiler heats up a tank of water even though we obviously don't use it if we plump for electric. Reading AnOthers recent post re cost of electric water heating it may well beat the cost of oil.

The other thing is related. At night we turn the thermostat right down but the CH still fires up to keep the tank of water at 60C. Does anyone know if it would be cheaper to turn off the power to the CH boiler then turn it back on in the morning? That would mean the tank of water would have to be reheated again but it may be cheaper than having the boiler fire up a couple of times during the night.

Informed comments would be appreciated

[AnOther]

I think there are too many variables to give any sort of meaningful answer, boiler design and efficiency and DHW capacity and daily requirements being examples, so experimentation may be the only way to determine what's cheapest for you.

For my own part the €10/mth my DHW costs me makes it just not worth the effort to try and improve on !

[BIG MAC]

I would bypass the cylinder for the oil system and just use an electric ballon on heures creuse We get enough hot water for a family of 7 from ours ...showers rather than baths though. save the oil for space heat and super insulate.

[AnOther]

That's exactly what I've done and all it entailed was the fitting of an isolator tap.

[Spyder]

[quote user="BIG MAC"]I would bypass the cylinder for the oil system and just use an electric ballon on heures creuse We get enough hot water for a family of 7 from ours ...showers rather than baths though. save the oil for space heat and super insulate.[/quote]

Yes we have fitted an isolator tap and just assumed using electric water heating on HC was cheaper than using oil but had not experimented. I think we would probably have to do records of a whole winter season to get any meaningful result. I agree with the "super insulate" and have done that.

[Théière]

I was thinking about fitting a 200 watt heater or thereabouts to my water tank and coupling that to a PV panel or two to heat the water and keep it topped up.  Reckon payback in a little over 6 months but certainly within a year.

[halfblind]

[quote user="Théière"]I was thinking about fitting a 200 watt heater or thereabouts to my water tank and coupling that to a PV panel or two to heat the water and keep it topped up.  Reckon payback in a little over 6 months but certainly within a year.

[/quote]

I wouldn't have thought that 200 watts would give you much, but I wait to be proved wrong. I know that PV panels are coming down in price due to Chinese dumping but would think the payback time would be quite long.

[Théière]

A 2000 watt heater as per normal, or there abouts is for quick heat up. using around 200w would be trickle heating but also consider with the normal time cycled heating as you use the hot it's replaced with incoming cold so lowering the overall temperature of the whole tank. constant trickle heating would keep it topped up at the safe temp.  Using low power the PV can easily acheive at least background heating.

I made an assumption of 2.2kw on for 3 hours per day over a 6 month only period. = roughly €150 which isn't far off the cost of the panel the heating element is only €25 and a bit for the welding style cables but not really expensive or long payback unless I have missed something?

[Quillan]

Looking back through the thread 'Another' seems to think it should be around €10 per month i.e. €60 for six months based on his bills.

[Théière]

Yes Q, Ernst, manages his very well on a timer, so illustrating how little electricity can do the job. Part of the reason I looked at PV for water, it may well be achievable on less and via a trickle of electricity from PV so even on Ernst's setup payback in 2 years.  The amount of hot water our guests get through is quite a bit more than us so hence the idea.

[Spyder]

What sort of price are you looking at for the panels? Have you got a link to where you are going to buy them? I was under the impression they were still pretty expensive.

[Théière]

Around £176 for a 200w monocrystaline panel on Ebay, price drops slightly for 10 panels.

[Quillan]

Can you get a 200W 12V heating element?

[Théière]

[quote user="Quillan"]Can you get a 200W 12V heating element?[/quote]

Ha ha, if you couldn't, I wouldn't have been looking into the idea [Www]

[halfblind]

I am sure someone will shoot me down in flames but if the average hot water use in a shower is 62 litres ( This figure comes from the website waterwise.org.uk for an 8 minute shower) then you have to reheat the cold water coming in to the tank to replace this hot water.

Lets say we are quicker in the shower and only use 40 lites of hot water at 65 degC and that the incoming cold water is at 15 degC, we then have to heat 40 litres of water by 50 degC to return the water to its original state. If 1 litre of water requires 4200 joules of energy to raise its temp 1 degC then we need a lot of joules and if 1 kwH = 3600 joules then thats a lot of kwH.

I think that a 200 watt heater is going to require a rather long time to replace that energy !

OK, take aim and fire at will !

[Théière]

I know it sounds crazy but simplicistically if the 2kw heater runs for just over an hour to heat the water normally then at 1/10 power it would be 10x longer so it's doubtfiull at any point other than the summer months there would be sufficient daylight but there are some assumptions you have made which are not so in practice.  The water (40 litres) is replcaced by cold water which cools the stored water so you are not heating it from cold only warm so less energy required I hope [:)]  In practice I would have to see how much a shower actually lowers the stored water temperature. This is only the very begining of looking into this so input is most welcome.

40ltrs x 50deg x 4.2 / 3600 = 2.3 kw

200 ltr x 5 deg x 4.2 / 3600 = 1.16 kw

200ltr x 10 deg x 4.2 /3600 = 2.3 kw

it's down to the cold water inrush to the tank and it's effect on the stored water temp.

[halfblind]

The amount of energy required to "return the water to its original condition" is unchanged whether the bulk of the 200 litres is heating by 10 or 20 degC.

In practice in a normal home or holiday home situation its not just one shower taken but several people will take showers over a short period, like just before going out to the restaurant in the evening, and the water will be reheated by the 2Kw heater on nightime rates.

If 5 people take showers over a short period of time then possibly 200 litres of hot water is used ( in effect its more like 50% less because the shower temperature is controlled by either a thermostatic mixer or by hand manipulation of the mixer taps) and for a solar powered 200w heater to operate overnight is asking a lot !

I think you will find that most showers are taken at night and therefore the heating requirements are taken care of by the overnight cheap rate electricity.

We have strayed somewhat from the OP which was about Central Heating, but an interesting discussion.

[Théière]

[quote user="halfblind"]

We have strayed somewhat from the OP which was about Central Heating, but an interesting discussion.

[/quote]

I thought the topic was done so just borrowed the thread.  Going on from what you said about evening use, if that is the case then a decent battery charged during the day could be used in the evening to run the heater element. 90 amp/hour battery would give 5+ hours but capacitors will soon take over from batteries (super capacitors). 

[Quillan]

The battery needs to be a 'deep cycle' battery not like the ones you use in cars. For that you will need a 'deep cycle charger' often referred to as a four stage charger because you need to charge these batteries differently to a car battery. The price of it all soon mounts up I am afraid. You might be better off going the solar heating panel route just using the immersion to top it up at night when it needs it.

[Théière]

You could well be correct on that Q, maybe wait until super capacitors are more common.  I would use solar but not being in full time residence then there is the issue of it all melting if something goes wrong. I cant take much as the roof gets to such a high temp in summer and quite warm the rest of the time.  heat pumps are not great for hot water prefering mid temperature.  BTW have you seen they are putting heat pumps into tumble dyers now.

http://www.sust-it.net/energy-saving.php?id=41

[Quillan]

Well I know the bit about batteries and PV panels is correct because we use them on our boat which has solar panels built in to it. A more common term in the UK for his type of battery is 'leisure battery' and the gel ones are by far the best.

Using solar heating should not be a problem in the hottest of summers as they use 'antifreeze' and work in very high temperatures. Heat Pumps just give a boost like a pre heater if you want but they do reduce the cost of heating water and of course power radiators as well. Expensive to retrofit which is the real problem.

[Théière]

Thanks Q, A pro installer recommended LiPo battery bank as faster charging and heavier tolerance of high drain devices as in this case.  The super capacitors are almost instant charge and don't dissipate like cells so the obvious solution albeit in a little while as they are used in hybrid vehicles capture the charge in things like F1 where it's very very quick indeed.

Having had a chance to reflect on this, all solar water heaters would suffer the same as Half blind stated for evening usage but would still go some way to help with morning showers and after the evening ones there wouldn't be much call for more hot water.  Therefore providing a sufficient surplus to bathing was available it would make it through until the next daylight session. A lot less complicated than a wet system was the attraction, it would be handy to arrange a hot water supply to feed the washing machine as guests seem to do so much washing.

[Quillan]

There are plenty of solutions out there for what you want to do and many of them are very good but at the end of the day it all comes down to price and how long it takes to get your money back. Supercapacitors, when I looked, seem cheap enough for small electronics but when you get to the sort of power levels you require they cost out at about £150 to £200 per 2.7V cell then thee is the specialised charger that goes with them.

I was looking to see how much combi hot water cylinders cost (electric and solar heated in one tank) and found the following article which you may find interesting especially when you consider it is for the UK which will be cooler perhaps than where you live.

http://www.plumbcenter.co.uk/en/info/cylinders

I was rather drawn to the bit about 'buffer tanks' and the chart that explains what you need depending on number of people, baths and showers.

With the washing machine I am not so sure going hot and cold fill. What you save in one area you take from another i.e. cheaper to run the washing machine but then you run out of hot water for baths.

I think the 'wet' system might be the best way to got. Heats the water in the day, gets used at night and also topped up at night (on cheap rate electricity) by an immersion for those showering in the morning.

If I was going down the PV route I would go with proven technology that I know works. Solar panels and a wind generator feeding a battery bank. A it is 'on shore' I would go for second hand submarine or telephone exchange batteries. I would place a Mastervolt inverter/charger on the side which would keep the batteries fully charged and supply 220V at night. To cut down on usage I could systematically replace all lighting with LED's. I would also install solar heating panels with thermal stores for hot water.

It is an interesting subject and the technology is changing all the time. When I retire I intend (or would certainly like) to build a house with the primary view of making it as cheap to run as possible. After all energy will only get more and more expensive and you don't want to be giving all your pension away just to keep warm and the lights on.

[Théière]

Tried and tested is ok for consumers but mad scientists need their fix too! [geek]  I had a mate who bought a milk float cheap for the batteries when milk rounds were disapearing fast.

The other route is solar thermaldynamic

<iframe width="560" height="315" src="//www.youtube.com/embed/HF2A7hBX8AY" frameborder="0" allowfullscreen></iframe>

http://www.youtube.com/watch?v=HF2A7hBX8AY

One of the links should work.

It's just an air source heat pump (air to water) but with a massive collector panel so it can also work at night.  I came across this a few years back but as the company was from Essex I was careful to avoid them, others sell similar but the costs via some are pretty high and the swimming pool version was just eye watering!

[Quillan]

YOu sound like my mate and his re-engineered ac unit for heat reclamation, mad as a hatter but it does actually work. I think you may find that Brico Depot sell those units. I am sure I saw one the other week when I was buying some paint.