solar hot water with single coil cylinder

[adamabcd]

Hi,
I've got a 210 litre mains pressure indirect (single coil) hot water cylinder.
The water in the coil is heated by a gas boiler and is on a closed (pressurised) system along with my central heating.
I was wondering if it would be possible to connect a solar panel with pump and valve into my existing system so that the coil can be fed from either source?
Apart from the increased compexity of the control system, the only disadvantage I can think of is that the two systems will share their water, and so I may need to anti-freeze the whole system.
Anybody got any thoughts?

Adam

[Ivan]

Hi Adam.

Yes you can share the coil with the boiler. It will have less surface area than a proper solar coil, so it will not work as efficiently, but the savings in terms of not having to replace your pressurised cylinder (and the work that changing a cylinder involves) makes it a good proposition. I would recommend a couple of solenoid valves or a three way valve to isolate the different feeding circuits depending on which source is producing power. It is not that complex, but it is imperative that the normal boiler loop is pressurised (as yours is) - just in case other people are considering following your example. If it was not, you would need to get rid of the header tank and convert to a pressurised boiler/solar loop. This could be done with both vented and unvented cylinders.

A slightly neater alternative is to fit the retrofit coil in place of the immersion heater (assuming you have a top mounting immersion - side mounting immersions are too short to be useful in this application). This allows both circuits to be kept separate.


Ivan

[Matt300]

Hi,

I have exactly the same system and have been thinking of fitting a retrofit coil, unfortuntely I have a Megaflow cylinder which has a side immersion but it is near the bottom so I assume it may be better than at the top.  Can I ask what you did in the end and how well it works?

Can anyone tell me if I need to worry about overheating, I assume the safety devices on the Megaflow will deal with any increase in pressure etc???  I am installing on a flat roof over a garage and therefore can cover the panels while I am away on holiday!!!

Matt

[Ivan]

There are three disadvantages I can think of:

1. Ideally antifreeze required for the whole volume - as you pointed out

2. You will need to operate the combined solar loop and boiler loop as a pressurised system - so if you have a header tank feeding the boiler loop, you will have to replace it with the pressurised system

3. The coil will be much lower surface area than the solar coil (probably around 0.6m2, compared to 1.6m2), so it will run less efficiently.

Having said that, if the cost or thought of replacing the cylinder makes solar unattractive, it is far better to have a compromised solar system than none at all. You can always upgrade it at a later stage if you find you wish to.


Ivan

[tongo]

I am thinking of adding a solar panel to my oil fired boiler and oso 170 litre unvented cylinder, I have read some of the comments regarding this and have become a bit puzzled. One proposed solution is to use the cylinder coil being fed from the oil fired boilerand by fitting furter electrically operated valves be able to introduce the solar heat source.
As far as I am aware the connection of a potentially uncontrolled source of energy (the sun) to an unvented storage cylinder would be considered a potential explosion waiting to happen, the contents of an unvented cylidermust not exceed 85 deg c. If the cylinder temp. exceeds this how can the excess heat be dissapated ?
Is there any fail safe way of achieving this without breaking the law and complying with building regs and water by laws ?

[Ivan]

solar water heating systems are rarely considered 'uncontrolled' - therefore they would not contravene regulations.

There are various methods which limit the maximum cylinder temperature and provide heat dump capability within the controller. The controller will NOT allow the tank to get up to boiling temperature under any circumstances, and will shut down the pump when the temperature gets too high.

The only example I can think of where the solar would be uncontrolled, is if you had a gravity -feed solar system - this would not be able to shut down at high temperatures. So you would NOT use gravity-circulation with a pressurised cylinder!
ivan