Hi have fitted last July solar pv with a solar iboost which diverts surplus electric to the unvented cylinders emersion heater. It's a tribune 180 Ltr. The emersion is set at a high temperature if there isn't enough solar to heat the water then my water is heated by my system boiler if required at a lower tempeture. What I would like to do is install another cylinder 100/180ltr with just has an emersion element in it so through the sunnier days the iboost can heat two cylinders. Cylinder 1 then cylinder 2 after which the iboost periodically switches back to first tank just incase it might need reheating. I don't won't to hook this cylinder up to the boil it's just making best use of FREE electric Pre warming. Can I plumb in series ie:- cold water into the second cylinder first which then feed the first cylinder it doesn't matter if the second tank is heated or not it just feeds the first tank with what ever temp water is in it. I can't see why I Would need a second expansion vessel or pressure relief and tundish as all this is installed on first cylinder and pressure would be equal across both cylinder.
Expansion vessels are sized by the manufacture for the cylinder ,so yes you would need another vessel as you are adding more water volume to the system.
This is from Mc Donald Engineers.... Yes our Unvented cylinders can be used as a pre-heat. We would normally make them with a solar coil to suit the amount of panel area. Generally it would be one expansion vessel per cylinder.
So if he adds another cylinder, by your own statement he would need another expansion vessel. Pretty obvious really as your adding a large amount of additional fluid and this needs to expand somewhere, given that the existing has been sized by the manufacturers for the cylinder it is supplied with, plus a little extra for the system. But if you want to double that capacity you need to add more room for expansion.
One expansion vessel would still work. If I was diying it for myself I'd only fit one. Less hassle servicing!
You will have to service the combination valve and safetys on the second cylinder it takes only a few minuets to check and expansion vessel so that a wasted argument. You still havent explained where the excess expansion will be taken up apart from a constantly running PRV.