The grant schemes do, unfortunately, create a problematic situation exactly as you describe. About 15 years ago I discovered my conventional open vented hot water cylinder was corroding around the bottom seam. It seemed sensible to take the opportunity to use the inevitable replacement exercise to change to a taller, twin coil cylinder and provide a heat input from a wet solar panel. One of my neighbours had had a system installed by a company from the midlands for close on 5 thousand pounds. It was continually losing primary pressure and spent several days each time standing idle waiting for attention.
They kindly allowed me to study it in detail. I realized that the installers had fitted an automatic air vent at the highest point of the sealed primary system which was atop the evacuated tube roof panel. On sunny days with low hot water usage their cylinder heated up to scalding temperature so the transfer pump was inhibited, the liquid in the panel turned into vapour and the auto air vent released the vapour. Consequently this loss meant when the sun went in and the panel cooled down there had been a loss of primary fluid leading to loss of pressure and the system locked out until it was recharged.
Needless to say I didn't engage that company's services. I bought a "kit" from eBay which comprised tall twin coil prelagged vented cylinder, 1.2m x 1.2m evacuated tube panel assembly complete, electronic controller with sensors, flow controls and indicators, primary pump and corrosion inhibitor/antifreeze. All for the grand total of 650 pounds delivered. I added copper pipe and installed it myself on the South gable wall (with a manually operated vent above in the loft). On dull days it takes the chill off incoming cold water thus reducing heat input required from other sources, on sunny days in summer it provides all the domestic hot water we need. Between the two extremes it contributes "some" of the total daily energy required. Of course being diy I couldn't get grant towards that job but I still ended up in pocket. I had to vent the primary loop a couple of times during the first week of operation until all the dissolved air in the fill water cleared the system. I'm happy to report it has not required any maintenance expenditure during the intervening years.
