I have a heat only Worcester Bosch 30kw boiler and a 300L Ideal unvented cylinder. There are 3 zone valves, one for ground floor heating, another for 1st floor/loft heating and the last is for hot water. There is only one return pipe for the radiators. I had this and completely new pipework done for hot, cold water, secondary hot water return as well as all new radiator pipes in the summer. The radiators weren't used at the time because the weather was hot and it was briefly tested. Shortly after the install unvented cylinder seemingly imploded and I got it replaced by the manufacturer Ideal only recently. Now that it's winter and I'm using the heating properly I found that when I turn on the ground floor heating with the thermostat (I use Hive) that some of the radiators on the upper floors heat up and vice versa. My understanding is that the first radiator pipe to get hot is the flow pipe. When I turn on the ground floor heating and check which pipe gets hot, the opposite pipe on the same radiators would get hot when the ground floor heating is off and the upper floor heating is on. I have tried to contact the fitters who haven't been too responsive other then to say that they think Ideal caused the problem when they changed the cylinder (even though they did a straight swap out). I wanted to see if anyone has any theories about what the issue could be. I'm hoping that it's zone valve related and can be fixed in the boiler cabinet rather than any underlying pipework which is now buried under flooring and behind decorated walls. The radiator pipes getting hot on one side for 1 zone and the other side for the other zone have me worried as it would be a nightmare if each zones flow connects to each side of the radiator.