Solar DHW help please...


Postby cmhicks » Tue Aug 11, 2009 3:49 pm

Hi All,

I have recently bought a house in which there is a 25-year-old solar water heating system in need of some TLC. I've reverse-engineered most of how the system is supposed to work, I think, but I could do with some help in a few specific areas.

The system is a standard vented setup fed from a cold water tank in the loft, but it has two copper hot water tanks instead of the usual one. These are connected in series, so that the hot outlet from tank 1 is connected to the cold inlet of tank 2. The heat exchanger coil of tank 1 is connected to the solar panels, and the heat exchanger coil in tank 2 is connected to the boiler. Thus, the solar panels pre-heat the water in tank 1 so the boiler has less work to do in tank 2.

There's a pump, pressure gauge and expansion vessel in the solar panel circuit. This circuit certainly needs refilling/pressurising/bleeding etc, but before I do so I need to find out about suitable anti-freeze and corrosion inhibitor (that's my first question) and what safety precautions there should be against the water in this circuit from boiling (there seems to be just the pressure release valve built into the pressure gauge at present). Incidentally, said PRV seems to vent all over the nice clean laundry in the airing cupboard; presumably it could have been installed better :-)

Secondly, there's a pipe connecting the hot outlets (ie the top tank connection) of the two hot water tanks together, with a manual valve in it. This valve is closed at present (so the tanks are in series as described above) and I am unsure as to the purpose of this connection, or why I would ever want to open this valve. Opening it would result in the outlet from the solar tank being connected to both the inlet and the outlet of the second tank, which seems daft. Any ideas?

Thirdly, I'm quite keen not only to get the system working, but to install some extra monitoring hardware to measure how much solar power is actually being captured by the system. To do this I need a flowmeter that can measure fractions of a litre in the cold inlet to tank 1. Any problems with doing this, and any suggestions as to the type of meter?

Thanks,

Christopher Hicks
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Postby plumbbob » Sat Aug 15, 2009 10:56 pm

I have to admit to not being an expert with solar heating but I may be able to offer a few pointers to help you along the way.

Solar heating has changed dramatically in the last decade or so and the double cylinder set up you mention is extremely inefficient which probably means the panels are too. In fact, I can say the two tank system didn't work. You may have a tank full of solar heated water and no way of drawing it off which could be why that valve you mention was fitted.

Modern water tanks have double coils - one for the boiler and the other for the solar panels so the boiler only cuts in if the solar can't keep up.

Modern panels will out perform your panels in every respect too. Not just when the sun is overhead, but they retain their efficiency from dawn to dusk as the sun tracks round a feature that was unheard of 10 years ago.

It seems reasonable to suggest a normal heating inhibitor will suffice, but you must add an antifreeze too.

It is possible the panels you have do not have the capacity to heat the water to boiling point which makes the danger minimal.
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Postby cmhicks » Mon Aug 17, 2009 3:58 pm

Thanks. What you have said is very helpful. Matters are complicated further by the fact that it is a listed building... Since the solar panels are part of the "external appearance" I do not want to open the can of worms involved with replacing them yet. However, I will investigate changing over to a double-coil tank.

Christopher Hicks
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Postby Fixter » Mon Aug 31, 2009 10:32 pm

Suggerst you try Tyfocor L inhibitor/antifreeze/heat transfer agent. Can be sourced from Barilla Solar ( www ). Agree with plumbob regards a new twin coil cylinder.
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