I'm not a plumber, and would really appreciate some guidance if possible. I have a large unvented Megaflo cylinder (Heatrae Sadia CL250HE) for our 3-floor, 3-bathroom house. Always been fine since installation around 10 years ago. However, now I'm getting no hot water unless I use the immersion heater to heat the tank. Not a disaster, as I have that manual override available, but it's going to be costing me a small fortune in electricity. The boiler is functioning fine, it seems. The heating is acting as expected, and all indicators say that the hot water is, too. But when the hot water is on (I've even run it on constant for an extended period of time), the water in the Megaflo never gets hot. I've checked the hot water flow pipe, and it's very hot. The return pipe is slightly warm. I believe this is how it should be. But the top output pipe is lukewarm, at best. If I put on the immersion for a couple of hours - hey presto - enough hot water to last around 24 hours. I've had a plumber come to look, and he's suggesting replacing the zone valves. Does that sound reasonable? As far as I can tell, hot water is getting out of the boiler and making its way to the cylinder. I don't really see how replacing the zone valves will help, but I'm certainly not an expert. At around 70 quid per valve I'm not sure that's a sensible place to start. Any feedback or guidance would be much appreciated.