Dibs-h
Established Member
Mike Garnham":1ncukbso said:
I recall something similar - the -10 is to cater for the winter sun (IIRC).
PV and other alternative energy sources.
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I recall something similar - the -10 is to cater for the winter sun (IIRC).
dedee
Established Member
Is the expansion tank deemed necessary because the solar water heated water would just get hotter and hotter and needs somewhere to "expand" to?
If one had a valve that prevented the warmed fluid in the collector and exchanger from circulating over a certain temperature would the expansion tank still be required?
Cheers
Andy
If one had a valve that prevented the warmed fluid in the collector and exchanger from circulating over a certain temperature would the expansion tank still be required?
Cheers
Andy
MikeG.
Established Member
Yes Andy, because hot water takes more room than cold water, and if there is a closed loop there is no allowance for expansion. However, it is really easy........like a header tank on a hot water system.
Here http://www.solarfriend.co.uk/ is a simple diagram. The first system is the type of thing I have described.
Mike
Here http://www.solarfriend.co.uk/ is a simple diagram. The first system is the type of thing I have described.
Mike
Header tank or pressure vessel on a sealed system Andy.
But to go into a little theory Andy.
Firstly, the water in the domestic hot water tank can never be hotter than the temp of the water delivered to the tank's heat exchanger.
If you were to purchase a heat exchanger from a supplier it would have stated with it that it was capable of raising a given quantity of water through a given temp range in a given time.
It is the heat exchanger therefore that determines how long it takes to heat the tank full.
Roy.
But to go into a little theory Andy.
Firstly, the water in the domestic hot water tank can never be hotter than the temp of the water delivered to the tank's heat exchanger.
If you were to purchase a heat exchanger from a supplier it would have stated with it that it was capable of raising a given quantity of water through a given temp range in a given time.
It is the heat exchanger therefore that determines how long it takes to heat the tank full.
Roy.
dedee
Established Member
Mike thanks for the link.
I knew there would be complications.
The 1st plan on the page you liked to is doing what I will call the usual solar water heating system
Using the heat the exchanger filled with antifreeze to preheat mains water before entering a water heater is a wee bit different. Bit I get the flow of the diagram.
As regards the expansion tank where does it go on my rather embarrassing diagram but I think it captures what I know so far.
cheers
Andy
I knew there would be complications.
The 1st plan on the page you liked to is doing what I will call the usual solar water heating system
Using the heat the exchanger filled with antifreeze to preheat mains water before entering a water heater is a wee bit different. Bit I get the flow of the diagram.
As regards the expansion tank where does it go on my rather embarrassing diagram but I think it captures what I know so far.
cheers
Andy
RogerS
Established Member
Mike
I have asked this question (and which you answered) in another thread but I can't find it.
If you replace the immersion heater with that clever 'doofa' then how do you heat up the water in winter or in summer when the sun isn't shining?
I have asked this question (and which you answered) in another thread but I can't find it.
If you replace the immersion heater with that clever 'doofa' then how do you heat up the water in winter or in summer when the sun isn't shining?
mickthetree
Established Member
what about a solar air heater? / pop / beer can heater? to reduce the requirement for central heating.
I found thistoday. Not seen one before, but it looks great as there is no water involved. Just air!
I have a flat roof at the back of my house which faces south/south/east which would would be a potential location.
aaaaaaaaaaaaaaaaand you have to drink 105 cans of beer to make it a valid project.
I found thistoday. Not seen one before, but it looks great as there is no water involved. Just air!
I have a flat roof at the back of my house which faces south/south/east which would would be a potential location.
aaaaaaaaaaaaaaaaand you have to drink 105 cans of beer to make it a valid project.
MikeG.
Established Member
Mick,
great little demonstration.....but not much use to us over here. We have low, weak sun and lots of cloud at the time of year when we need central heating. This simply won't work in cloudy Britain. You'll have noticed how clear the sky was in the demonstration........and people who travel from Britain are usually amazed at how bright and sunny it can be in other people's winters.
Were there an element of thermal mass about this.............say, it heated up a big rock store, for instance, then it could make a difference.
Roger,
the "doofa" would only replace the electric element if it was the secondary (back-up) form of water-heating. Most people heat water with gas or oil boilers, and have an immersion simply as a back up. If you consider the solar as a sort of back-up for your gas boiler then you can see how it could work.
If you rely on electricity as your primary source of water heating, then you will need to fit a second tank to take the hot water from the solar panel, but it will still work fine.
Mike
great little demonstration.....but not much use to us over here. We have low, weak sun and lots of cloud at the time of year when we need central heating. This simply won't work in cloudy Britain. You'll have noticed how clear the sky was in the demonstration........and people who travel from Britain are usually amazed at how bright and sunny it can be in other people's winters.
Were there an element of thermal mass about this.............say, it heated up a big rock store, for instance, then it could make a difference.
Roger,
the "doofa" would only replace the electric element if it was the secondary (back-up) form of water-heating. Most people heat water with gas or oil boilers, and have an immersion simply as a back up. If you consider the solar as a sort of back-up for your gas boiler then you can see how it could work.
If you rely on electricity as your primary source of water heating, then you will need to fit a second tank to take the hot water from the solar panel, but it will still work fine.
Mike
mickthetree
Established Member
well I'm going to drink the beer, regardless. 8)
Cheers for the explanation though Mike.
Cheers for the explanation though Mike.
Another excellent site for all things solar is Navitron. This is the outfit that Dick Strawbridge used for the solar installation in his house in Cornwall, and televised a couple of years back as It's Not Easy being Green. There is also a solar forum, which whilst a bit "wacky" at times also contains some pearls ( a bit like W/S UK really!) The Navitron site in particular is aimed at provision of materials and advice for the diy installer - I am very tempted.
One of our neighbours has just had a combined solar h/w and p/v system fitted by these people. According to him (and he's a pretty clued up guy) every watt produced is fed into the grid, and you're paid 36p per unit for everything you produce - including what you draw off yourself, so there is no storage problem. The system generates about 2kw on a sunny day, and in the event that you generate more than you use, you get an additional 10p per unit for the surplus. Could be useful if you spend a long time away from home - it'd be generating an income for you in your absence. This compares with a cost of about 12p per unit from your domestic supplier.
This is not a cheap system - about £12K I believe, but he reckons that at current fuel prices it gives a return on capital of about 8% - equivalent to 10% gross - so much better than cash in the bank so far as investment return is concerned, and implies a payback period of about 12 years at current electricity prices. And of course we all know what fuel costs are likely to do over a 12 year period - if they only double we'll have got away lightly.
The main drawback I can see is that the economics rely heavily on continuing to get the inflated subsidy from the govt for the power you produce because it involves trusting politicians not to withdraw the scheme once you've invested in the system, and anything that involves trusting politicians to keep their word, or to do the right thing, is fraught with problems. Just IMHO of course.
The solar H/W system he's installed seems to feed into a heat exchanger in a very small tank that sits beside the main h/w tank and which in turn feeds its h/w into the top of the domestic h/w tank - just where it is needed, and where it doesn't get diluted by feeding it in to the bottom of the tank. Cunning. The secondary h/w comes from glass tube collectors on the roof, and apparantly with these a pv powered circulation pump is not sufficient to circulate the water fast enough to stop the water boiling in the tubes on a sunny day, so must use a mains powered c/h style pump. However, the power drawn by the pump is more than compensated for by the extra efficiency of the glass tube collectors.
I think you also need to think a little about how you use your hot water. For instance most washing machines and dishwashers are cold fill and then use electricity to heat their water. Modern machines often allow hot fill, but then you'd need some system to revert to cold fill when there is no solar h/w or else you'd be using expensively heated h/w for the rinse cycle which is usually done in cold water. So in practice, you'll only be using solar heated h/w at the sink or for bathing/showering.
As always, the devil is in the detail, which Mike alluded to in an earlier post. Interesting thread!
The Navitron site in particular is aimed at provision of materials and advice for the diy installer - I am very tempted.One of our neighbours has just had a combined solar h/w and p/v system fitted by these people. According to him (and he's a pretty clued up guy) every watt produced is fed into the grid, and you're paid 36p per unit for everything you produce - including what you draw off yourself, so there is no storage problem. The system generates about 2kw on a sunny day, and in the event that you generate more than you use, you get an additional 10p per unit for the surplus. Could be useful if you spend a long time away from home - it'd be generating an income for you in your absence. This compares with a cost of about 12p per unit from your domestic supplier.
This is not a cheap system - about £12K I believe, but he reckons that at current fuel prices it gives a return on capital of about 8% - equivalent to 10% gross - so much better than cash in the bank so far as investment return is concerned, and implies a payback period of about 12 years at current electricity prices. And of course we all know what fuel costs are likely to do over a 12 year period - if they only double we'll have got away lightly.
The main drawback I can see is that the economics rely heavily on continuing to get the inflated subsidy from the govt for the power you produce because it involves trusting politicians not to withdraw the scheme once you've invested in the system, and anything that involves trusting politicians to keep their word, or to do the right thing, is fraught with problems. Just IMHO of course.
The solar H/W system he's installed seems to feed into a heat exchanger in a very small tank that sits beside the main h/w tank and which in turn feeds its h/w into the top of the domestic h/w tank - just where it is needed, and where it doesn't get diluted by feeding it in to the bottom of the tank. Cunning. The secondary h/w comes from glass tube collectors on the roof, and apparantly with these a pv powered circulation pump is not sufficient to circulate the water fast enough to stop the water boiling in the tubes on a sunny day, so must use a mains powered c/h style pump. However, the power drawn by the pump is more than compensated for by the extra efficiency of the glass tube collectors.
I think you also need to think a little about how you use your hot water. For instance most washing machines and dishwashers are cold fill and then use electricity to heat their water. Modern machines often allow hot fill, but then you'd need some system to revert to cold fill when there is no solar h/w or else you'd be using expensively heated h/w for the rinse cycle which is usually done in cold water. So in practice, you'll only be using solar heated h/w at the sink or for bathing/showering.
As always, the devil is in the detail, which Mike alluded to in an earlier post. Interesting thread!
mickthetree
Established Member
he gets paid for using electricity? Really?
I think the efficiency of solar panels degrades over time too, so after 5 years he may not be getting as much output as when first installed.
I think the efficiency of solar panels degrades over time too, so after 5 years he may not be getting as much output as when first installed.
dedee
Established Member
I still don't see where to place the expansion tank on my proposed system.
The existing electric water heater vents out a litre or so of water most nights.
Does my proposed preheated water tank require an expansion tank, if so as this tank is filled with water at mains pressure what is the device caused that prevents this tank from emptying into the expansion tank?
Does the solar heat collector/heat exchanger loop require some sort of expansion tank/vent as well?
cheers
Andy
The existing electric water heater vents out a litre or so of water most nights.
Does my proposed preheated water tank require an expansion tank, if so as this tank is filled with water at mains pressure what is the device caused that prevents this tank from emptying into the expansion tank?
Does the solar heat collector/heat exchanger loop require some sort of expansion tank/vent as well?
cheers
Andy
mickthetree":25gd9jsc said:
Thats what they claim here.
Agreed - but maybe offset by the rise in fuel costs?
FWIW, I think in most cases, an economy Solar HW system makes good economic sense, particularly if you can DIY for around £800 - £1200, when you can also service it yourself because you know how it works. Not sure that a commercial £6k installation will ever pay for itself though.
So far as PVs are concerned, I think the money will be better spent on low energy lighting and improved insulation first.
dedee
Established Member
Roy,
The reasons for the separate tank are as follows.
I have not seen that type of immersion heat exchanger in France. If I buy one in the UK it will not be easy to fit any tank bought over here either. If I put a system together as per my sketch with UK sourced parts these will not meet with french standards and could cause me a problem with insurance and when we come to sell the house.
By adding a separate preheat tank I can isolate the whole system and make it easy to remove if necessary and even by pass it if it does not work.
If I buy a DIY type kit over here using conventional solar heating panel, new tank, etc etc the cost would be be close to euro 1600-2000.
I think I could put together my proposed system for about £800-900 gbp
However I am stretching both my knowledge and abilities so the concerns raised about expansion tanks concern me somewhat.
Andy
The reasons for the separate tank are as follows.
I have not seen that type of immersion heat exchanger in France. If I buy one in the UK it will not be easy to fit any tank bought over here either. If I put a system together as per my sketch with UK sourced parts these will not meet with french standards and could cause me a problem with insurance and when we come to sell the house.
By adding a separate preheat tank I can isolate the whole system and make it easy to remove if necessary and even by pass it if it does not work.
If I buy a DIY type kit over here using conventional solar heating panel, new tank, etc etc the cost would be be close to euro 1600-2000.
I think I could put together my proposed system for about £800-900 gbp
However I am stretching both my knowledge and abilities so the concerns raised about expansion tanks concern me somewhat.
Andy
wobblycogs
Established Member
dedee, I'm not a plumber but I am friends with one and he has taught me a thing or two about sealed systems which is what you diagram seems to propose. An incorrectly built sealed system can be very dangerous, think explosion and boiling water flying all over the place http://www.youtube.com/watch?v=JmJoyuUJj2Q.
At it's most basic level there is only one thing to think about when building a sealed system: where is the water going to expand to when it gets hot? Your solar heating loop - the green arrows and coil - are completely sealed in your diagram so you need an expansion vessel somewhere in that loop.
Sealed systems should also be fitted with an emergency pressure relief valve in case something goes wrong - this valve will blow and should vent all the hot water to a safe location. In the UK there are strict rules about what is considered a safe location but typically near the ground away from where people generally go (a drain is not necessarily a good choice it could crack or melt) - remember if the valve blows it will be venting boiling hot water under pressure.
A kit like this http://www.plumbworld.co.uk/8-litre-sealed-system-kit-584-704?CAWELAID=205025542 would probably be what you are looking to fit.
The alternative option is open vented which is the most common system installed in the UK http://www.hashheating.co.uk/Central-heating-Systems.html. This system has a header tank open to the air so the heating loops are always maintained at atmospheric pressure. There are two downsides with open vented systems: it lets air into the heating circuit which corrodes radiators over time (small amounts of atmospheric oxygen dissolve in the heating water) and it requires a tank in the loft. This second downside could be a problem if you want to use this design for solar heating, the expansion tank needs to be the highest point in the solar loop which could be hard to achieve if you have the panels installed on the roof.
Don't forget - I'm not a plumber. Plumbers have a special set of exams they have to pass before they are allowed to commission sealed systems
At it's most basic level there is only one thing to think about when building a sealed system: where is the water going to expand to when it gets hot? Your solar heating loop - the green arrows and coil - are completely sealed in your diagram so you need an expansion vessel somewhere in that loop.
Sealed systems should also be fitted with an emergency pressure relief valve in case something goes wrong - this valve will blow and should vent all the hot water to a safe location. In the UK there are strict rules about what is considered a safe location but typically near the ground away from where people generally go (a drain is not necessarily a good choice it could crack or melt) - remember if the valve blows it will be venting boiling hot water under pressure.
A kit like this http://www.plumbworld.co.uk/8-litre-sealed-system-kit-584-704?CAWELAID=205025542 would probably be what you are looking to fit.
The alternative option is open vented which is the most common system installed in the UK http://www.hashheating.co.uk/Central-heating-Systems.html. This system has a header tank open to the air so the heating loops are always maintained at atmospheric pressure. There are two downsides with open vented systems: it lets air into the heating circuit which corrodes radiators over time (small amounts of atmospheric oxygen dissolve in the heating water) and it requires a tank in the loft. This second downside could be a problem if you want to use this design for solar heating, the expansion tank needs to be the highest point in the solar loop which could be hard to achieve if you have the panels installed on the roof.
Don't forget - I'm not a plumber. Plumbers have a special set of exams they have to pass before they are allowed to commission sealed systems
Right Andy, point taken. Expansion tanks are also the supply tank. Water entry is via a 'ball valve' and the expansion pipe returns any excess water to the top of the tank. The water level when cold must leave sufficient volume to accommodate the 'expanded' water without causing any overflow.
HTH.
Roy.
HTH.
Roy.
dedee
Established Member
Wobblycogs/Roy
thanks for you thoughts. Open vented systems (ie with an open header tank) I am sure are not allowed here in France. So the sealed expansion vessel with relief valve would seem to be the answer?
All of my planned system would be located in the basement/garage (except the solar panels of course).
I have re drawn my plan:-
[
Does the preheated water tank also need an expansion vessel?
Cheers
Andy
thanks for you thoughts. Open vented systems (ie with an open header tank) I am sure are not allowed here in France. So the sealed expansion vessel with relief valve would seem to be the answer?
All of my planned system would be located in the basement/garage (except the solar panels of course).
I have re drawn my plan:-
[
Does the preheated water tank also need an expansion vessel?
Cheers
Andy
wobblycogs
Established Member
I don't think open vented is allowed for hot water tanks in France but it might be allowed for the solar heating loop. One of the benefits of a sealed hot water system is that the hot water should always be potable whereas in an open vented system it might not be.
As for whether you would need an expansion vessel for the pre-heat tank that's hard to say. If it is going to be a proper sealed system hot water tank maybe not, if it's just a bog standard copper tank then probably and a fairly large one at that. As the water in that tank heats up it will obviously expand, that expansion would probably be dealt with by whatever expansion relief system your existing hot water tank is using but personally I wouldn't count on that.
Interestingly though I think you said that your hot water tank vents about a litre of water each night which would concern me a little. Modern sealed system hot water tanks have a bladder in the top or sometimes just an air gap which takes up the expansion of the water so that you don't need a external expansion vessel for the tank. If your tank is of this design and it is venting water it might be that the air gap needs re-establishing. There is normally a valve on the side to do this but it varies from model to model and you would have to look it up. It could be that the system is designed to vent through a relief valve and is therefore working perfectly but it's worth checking.
I think you also will probably want to consider what will happen if you manage to boil the water in your solar heating loop. An expansion vessel will deal with the fairly modest expansion as the water heats up it can't deal with water turning into steam. Your safety valve would deal with this situation but it would leave you with a system that had insufficient water in it which could damage the pump (and it's a bit last resort for my liking). This is the benefit of an open vented system it will / can automatically top it self up if necessary.
As for whether you would need an expansion vessel for the pre-heat tank that's hard to say. If it is going to be a proper sealed system hot water tank maybe not, if it's just a bog standard copper tank then probably and a fairly large one at that. As the water in that tank heats up it will obviously expand, that expansion would probably be dealt with by whatever expansion relief system your existing hot water tank is using but personally I wouldn't count on that.
Interestingly though I think you said that your hot water tank vents about a litre of water each night which would concern me a little. Modern sealed system hot water tanks have a bladder in the top or sometimes just an air gap which takes up the expansion of the water so that you don't need a external expansion vessel for the tank. If your tank is of this design and it is venting water it might be that the air gap needs re-establishing. There is normally a valve on the side to do this but it varies from model to model and you would have to look it up. It could be that the system is designed to vent through a relief valve and is therefore working perfectly but it's worth checking.
I think you also will probably want to consider what will happen if you manage to boil the water in your solar heating loop. An expansion vessel will deal with the fairly modest expansion as the water heats up it can't deal with water turning into steam. Your safety valve would deal with this situation but it would leave you with a system that had insufficient water in it which could damage the pump (and it's a bit last resort for my liking). This is the benefit of an open vented system it will / can automatically top it self up if necessary.
To be honest Andy I cannot see how that can work at all. Your current hot water cylinder is heated electrically? Does it have more that one heater boss or any plugged connections? Is it copper or SS, and is it lagged with sprayed on foam?
Roy.
Roy.
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