Nick A wrote: "I've been asked to install a third tap solar thermal hot water system. It apparently consists of a panel, cylinder and dedicated pipework to a second hot tap at ususal hot outlets. It is entirely independent of the existing hot water system and only has the solar heat source. The customer has a combi and prefers this option to either 'decombificaiton' or pre-heating the incoming cold to the combi.
Is anyone familiar with this system?"

Yes. A few years back the Solar Clubs handbook had a TTS as a case study. As far as I am aware the householder has not contracted any nasties, but it would almost certainly not be able to meet current regs.

"What are the advantages and disadvantages over the other options?"

It avoids "de-combification" or similar, and in certain situations (educational places for example) can educate people as to what solar can (and can't) do. You want hot water? Turn on the Sun Tap. ONLY if it does not deliver water as hot as you want should you turn on the 'real' hot tap.

"Could I use a regular single coil cylinder (proposed open-vented) as I only have one heat source?"

Yes, minor losses compared to a 'solar' coil, but still OK.

"How can I prevent having stored water at a tepid temperature which contravenes water regs and is potential breeding ground for nasties?"

You can't!!

I did such an installation some years ago and advised the users as follows:

1. Ensure all headers etc are fully covered and regularly cleaned to prevent growth of slime and nutrirnts.
2. Drain down and clean the cylinder at regular intervals. Copper is in any case a natural biocide, and will tend to militate against the growth of 'nasties'.
3. Above all, DO A LEGIONELLA RISK ASS'T. IF your assessment shows that, after your control measures, your risk is low (and if you manage to do this w/out a secondary heat source), you may be OK.

Try me via www.syec.co.uk if you need more.

Nick