Plumbing - switching between electric heater and old boiler

[HoneySuckleDreams]

Hello

We had Solar panels fitted which has an electric heater as part of the water heater. Our old (fuel) boiler is still connected (for central heating) but it can still create hot water and is still plumbed in (albeit with a gate valve turned to the off position).

Normally the hot water is generated from the new electric system. However, we have lots of people coming to stay over the next week and the solar system takes ages to heat the water. I would like to be able (in an emergency) to use the old fuel water heater. I know it's more expensive but you can get 2-3 showers out of 5 minutes heating with fuel.

I asked the bloke who put the system in and he said not a problem...open the old valve, close the water input valve on the new system and it will work.

It doesn't.

When you open a hot water tap, the water is pulled directly from the mains supply straight into the hot water supply via (see photo) a valve on the exit to the electric system. I think this is a mixer to regulate the temperature of the output hot water (why they fit them I'll never know as all the taps in the house are mixer taps). My guess is that as the mains pressure is 6-7bar and the output of the old system is reduced to about 1.5, then the new higher pressure will win everytime, which is why i'm getting freezing cold water. But I'm no heating engineer, which is why I'm asking for advice.

(sorry I can't get the photo to appear properly, the IMG tag is missing, and I can't copy and paste either!)

[IMG]http://i492.photobucket.com/albums/rr283/greeninfrance/DSCF1387.jpg[/IMG]

Question: (finally) If I fit a gate valve to the cold input on the new system, would this effectively isolate it...without damaging it all. I reckon it will but I'm not a heating engineer.

The input side is all 22mm so a simple gate valve should suffice. And this is a gripe about costs....as I don't have an oxyacetylene torch it has to be compression fittings. The local plumbers merchant have the valve for 9euro. You can't buy it all together, ie. simple nut and olive, you have to be the raccords as well...15euro each. So what would cost about £4 in B&Q costs close to 40euro here...which i find astounding! Plumber bloke told me to get a man in to solder the cheaper copper fittings, but that will probably cost more.