Solar water heating


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Because it is "foolproof" and will "always" work, no matter what pump, or extra panels you insert later. Just putting the bombproof method forwards, nothing else.

As the end of the pipe is open to atmosphere in the tank at "all times", it will not cause a vacuum and lock in the water. When the pump is on, it will pump some way up the pipe, how much depends on pump friction, etc. Once the pump stops, the water in the pipe will drop back and as it is a open vented pipe, the water will just fall back.
The last thing you want is water dropping from a small hole inthe wtaer in the tank, as this aerates the water. Not what you want.
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snipped-for-privacy@ece.villanova.edu wrote:

yes, thats what I had in mind. Perhaps I should have said drainback rather than draindown, though I'm not sure about that one.
Derek Broughton wrote:

If a system design depends for its survival on a component that will fail, then yes it is incompetent enginering.

If the collector gets cold enough to freeze in a draindown system, that collector will have been empty for hours.

why? (thats not exactly what I said anyway).
NT
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On 9 Mar 2006 14:05:11 -0500, snipped-for-privacy@ece.villanova.edu wrote:

Yes this was my assumption, the water header being sufficient cross section, and within the insulated area of the house, that once the pump stopped it just filled up a bit. The only drawback I could see was that the pump would always work against the head from the header tank up to the panels. In a sealed system the pump only circulates against the friction in the system.
Most of UK is a temperate marine climate with few excursions below freezing in recent years.
AJH
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On Thu, 09 Mar 2006 22:22:35 +0000 someone who may be AJH

http://www.imaginationsolar.com/PDFs/newPDFs/ISL%20Datasheets%20in%20Marketing%20Section/datasheet_drainback_v1.pdf shows the unit promoted by one supplier. In addition to this box there is just the panel, cylinder and some piping/wiring.
The drainback system does depend on the valve working, just like human life depends on valves in the heart working. I suspect that the valve in the unit is chosen to have a very large number of working cycles, as it will be operating a minimum of once a day and so the possibility of it failing and the panel to freezing is remote enough to be placed with the other remote possibilities, like the panel being hit by a meteorite.
--
David Hansen, Edinburgh
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On Fri, 10 Mar 2006 08:45:34 +0000, David Hansen

Thanks I was considering a more simple open vented system. In general I have a preference for sealed systems (only just becoming common in UK and then almost never for wood burning). Once the option for sealed system is taken I can see little advantage in drainback over a glycol or food grade antifreeze system.
AJH
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On Fri, 10 Mar 2006 09:54:57 +0000 someone who may be AJH

In general I have a preference for open vented systems, in small installations. In such installations the advantages of a "sealed" system largely fall to the installer and maintainer. The occupier gains little with such systems, in many/most cases. Large installations (I have been responsible for some very large heating systems) are a different matter.
However, for solar water heating the disadvantages of an open vented system include the trouble of placing the header tank high enough, then maintaining it and the inability to heat water above 100C. In this case I think the advantage lies with a "sealed" system.

The system at http://www.imaginationsolar.com/system.htm has a number of advantages. One of these is that it does not use mains electricity, which can exterminate 20% of the savings gained by using solar hot water heating [1]. The control system is very simple, if the sun is shining it pumps water into the panel and starts warming up the cylinder. If the sun is not shining, or there is snow on the PV panel, then the pump is not running and the panel is drained of water.
The system at http://www.navitron.org.uk/solar_collector_panel.htm is more sophisticated and so will extract more heat from the sun. On the other hand it needs mains electricity 24 hours a day, because the controller might need to turn on the pump occasionally to warm up the panel header a bit on cold nights (assuming it is run without anti-freeze). Mains failure means possible freezing in winter and overheating in summer, though these are rare in most of the UK.
The system at http://www.solartwin.com/easy_to_plumb_in.htm involves much less plumbing, so is easy to fit and has simple PV powered controls. Provided that the existing hot water cylinder is well insulated and of an adequate size this has great advantages.
These are just three possibilities, but they do show a range of possible solutions in one small part of the world.
[1] - "Side by Side Testing of Eight Solar Water Heating Systems" DTI/Pub URN 01/1292 http://www.dti.gov.uk/energy/renewables/publications/pubs_solar.shtml
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David Hansen, Edinburgh
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wrote this:-

?
That's the one we have, with which we are so well pleased.
You'll also note at the bottom of the page, "Solartwin offers minimum disruption when installing, and then is as equally easy to un-install and load in a van when moving house.
It'sguaranteed for up to (I think) three moves. So no potential buyer of the house need be put off and the happy owner won't be deprived.> :-) Mary
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On Fri, 10 Mar 2006 15:35:59 -0000 someone who may be "Mary Fisher"

Provided it is under enough pressure water can be heated to above 100C. It will not turn into steam, unless the pressure is reduced. Such systems are used in big heating systems.
However, this is very much something for those who know what they are doing. It is best to think of such high temperature hot water systems as liquid steam, because if it gets out the results are very nasty. My staff once had to recover the bodies of two men who had been killed by such a system and it haunted them.
The application to solar heaters is more complicated than I wanted to explain, but I will now. In any heating system one wants an adequate margin between what happens normally and what might happen in a strange situation. If an open vented solar water heating system was running with the water coming out of the collector at 95C then there is little margin between hot water coming out and the hot water turning into steam. There is little pressure because the header tank is likely to only just be above a roof mounted collector and thus the water will turn into steam at 100C. This isn't so much of a problem with flat plate collectors, which are unlikely to reach this temperature, but it will be with evacuated tube collectors.
With a pressurised system there is more margin, so that in odd conditions the collector could get to say 120C without too much risk of a problem. This could happen if the pump is not working for some reason, such as a power cut. With the Solartwin and Imagination systems the collector will never get to this sort of temperature anyway and the pump is solar powered and thus fewer things can go wrong with its electricity supply.

That is one of the advantages of the system. I think that if they could reduce the price of the unit they could corner a large slice of the market. When they started selling it some years ago ISTR a price of 1500, rather then the current 2000. However, reducing the price means increasing the volume sold and that depends on reducing the price...
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David Hansen, Edinburgh
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David Hansen <> wrote:

Not if the pump turns off and the water drains down at 95C. For long life, EPDM-lined tanks should be less than 77C.
Nick
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Mary Fisher wrote:

Does seem nice and simple but 2000 for the DIY kit buys a lot of gas heated hot water. Certainly our house would be very suitable for solar water heating (rear of house faces due south, v. little shade).
Tim
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On Fri, 10 Mar 2006 17:02:26 -0000 someone who may be "Tim Downie"

If the price of gas were to remain the same the simple payback period would be around 20-25 years. However, unlike the price of sunshine, the price of gas is unlikely to remain the same.
There are also other reasons for installing such things, for example reducing greenhouse gas emissions and reducing the maintenance on/extending the life of the boiler (which can usually be turned off for several months a year).
--
David Hansen, Edinburgh
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<sigh>
b) it might at present
a) Is that your sole criterion for the value of solar water heating?
> Certainly our house would be very suitable for solar water

Then do it. save money and emissions in some other imaginative way.

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David Hansen wrote:

When you choose sealed you're choosing not drainback, so the above doesnt seem to make sense, unless you know of a sealed drainback design?

I cant see how this is a problem. The loft is normally fine, sitting on the usual dividing wall. The tank is below the panels for drainbacks, it only needs to be above them for thermosyphons, which are not the usual choice.

again I dont see any downside with vented system maintenance. Sealed system maintenance does introduce extra issues, and for CH seems to result in more maintenance need in practice, as well as being less easy to work on.

That is one of the plusses for vented systems, that they will never go into that dangerous region of pressurised steam. Any sealed system that does requires extra certification, which isnt free, and must be designed to withstand much higher pressures. If it is not pressure safe then its just dangerous. Vented eliminates any such risk. Worst case for vented is steam coming out the overflow pipe, a non issue, or in the highly unlikely event of both a system boiling and a blocked overflow pipe, the steam simply comes out from around the tank lid. Safely.

Imho this is more the hype than the reality. The one advantage of a solar panel pump supply is simplicity, but thats all. The electricity created by the solar panel is the opposite of a worthwhile investment for 99% of end users. The control system is a bit crude, and will result in the system actively cooling your HW some of the time instead of heating it. In short it isnt worth it, unless you're just not skilled to do anything better.
NT
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wrote this:-

Sealed and pressurised are two differnt things. You can have a sealed system with an air gap in the top of a cyldiner, yet it is not pressurised - only when heated. When cold it will be atmospheric.

It isn't a problem an open vented drainback system does not requires a header tank at all, just space in a tank or cylinder at a lower level for the panels to drain into.

It is true a solar powered pumps may reverse the panel/cylinder and dissipate heat. But it is its overall performance that matters over a year.
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On 10 Mar 2006 09:48:43 -0800 someone who may be snipped-for-privacy@care2.com wrote this:-

All people are not alike. Some want something simple. There are many reasons for this approach. Lack of skill is just one reason, another reason is less to go wrong. If people want that sort of thing then I see no reason to denigrate them. The Solartwin system has not got any lights or things like that, though they do supply a digital thermometer, it just works away in the background and lets people get on with more important things. The Imagination system has a few lights.
At the other end of the spectrum is something controlled by say http://www.resol.de/en/Produkte/Solarregler/Solarregler.html the Resol BS Pro. This may wring every last bit of heat out of the panel. Whether it is worth having or is just a boy's toy is debatable. I say each to their own.
--
David Hansen, Edinburgh
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David Hansen <> wrote:
http://www.imaginationsolar.com/PDFs/newPDFs/ISL%20Datasheets%20in%20Marketing%20Section/datasheet_drainback_v1.pdf shows the unit promoted by one supplier...

Sounds like a drainDOWN system. DrainBACK systems have no valves.
Nick
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AJH wrote:

I realised this isnt so. Once the panel circuit fills with water, the pump is only pumping against the running friction, and not the head. The panel output piping is filled with water after a second or 2, so gravity has the same effect on both sides, so no extra pumping resistance.
NT
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wrote:

It is pumping against the head (pressure) always. Gravity asserts pressure. A 3 floor British house with attic. Inc' space between floor it could be 10 foot per floor. A radiator in the attic space and the pump at the bottom of the ground floor. The pump has to overcome 1 bar (~30 foot), one atmosphere, to get to the top, even in a sealed system. OK is assisted a little when the water is dropping from 30 foot on the return, but it still has to overcome one atmosphere plus system friction. Gravity doesn't care if it is open vented or sealed.
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Wrong. Eric Hawkins at Powertech Ltd (UK) has been done this for years in lots of systems with no loss of prime (the vacuum in the Apricus evac tube header loop above the unpressurized tank) after initial priming. Then again, they don't shut off the pump at some max water temp. So a "slow draindown system" with a small hole in the return pipe above the tank water line should work fine, with a lower flow and higher head to start with and higher flow and lower head after the pump has pushed all the bubbles out of the underwater return pipe.
Nick
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wrote in message

I was on about a sealed system, any system whether hearting or whatever. Hawkins vacuum is not new.
<http://powertech-solar.com/article.php?newsID >
Eric Hawkins is pushing product that is clear. In his house:
- He stripped out the copper pipe in his house and installed plastic. Most new homes are fitted out in plastic these days, with copper where pipes are seen. What he thought he would gain with plastic is beyond me,
- He fitted solar panels fitted to a thermal store in his loft/attic. I have a thermal store in my loft too. Nothing special there.
- He insulated pipes in the loft. All new homes require that exposed pipes are insulated. Nothing new there.
- He fitted solar panels to heat his thermal store. Nothing new there.
- Now! He stripped out his NG boiler and fitted an air to water heat pump in the loft. In the UK gas is approx 4 times cheaper than electricity. And the condensing boilers are very small in physical size and highly efficient. So a heat pump that "averages" COP 4 would equal a NG boiler in running cost. NG condensing boilers are cheap (some 26kW version were available from B&Q for 300 [same as your Home Depot]), heat pumps are not cheap at all. NO GAIN THERE at all, except a very cold loft, as that is where he is moving the heat from. No wonder he needs to insulate the hell out his pipes up there.
- He fitted low temperature underfloor heating in his house. One hell of an upheaval in an existing house. But nothing new there. Low temperature UFH is ideal for solar panels.
What he has done is have a thermal store heated by solar panels that heats wet underfloor heating. For backup he has an air to water heat pump.
He would have been wiser to keep, or update to, a NG condensing boiler. Much cheaper and guaranteed to give heat when needed, which the heat pump is not, and then insulate the hell out of his house. Cavity wall insulation is cheap enough. We have cavities in the UK and this is now very popular with recent energy price hypes, with government grants too. It is pumped into the cavity. So a wrap round blanket of insulation. He then could have had 400mm of Warmcell celluloid insulation sprayed on the floor of the loft (this seals up the ceiling/loft too). Then did some air tight measures around the house. That would have dropped his heating bills further than his 180 per ann, with the heat pump and wind turbine. Service for NG boilers is comprehensive, not so for heat pumps.
The rainwater harvesting is off the shelf stuff. Mandatory in the BenNeLux countries in new builds. It is worth it I believe with a 7 year payback. Nothing special there.
PV panels are not new and so far not worth it. Micro turbines in a built up area are difficult to get permission to erect. They also have long payback periods, but fine in off-the-grid locations.
His point is: Has the Traditional Boiler Had Its Day? A long time to go yet. Heat pumps are fine for off the gas grid systems. Even then you have to do your arithmetic well to justify one. The capital cost of them is horrendous in the UK. Solar panels a thermal store and low temperature UFH could be worth it. Unless the air to water heat pump is the same price as a NG boiler, I would think very hard.
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