DIY Doctor UK DIY Forums

We recently replaced a radiator. We drained the system, fitted it then refilled. After refilling, there were 3 radiators not getting warm at all. We also didn’t have hot water. Have bled all rads and no air left in them but still no heat in the 3 rads or hot water.
Then tried rebalancing rads by turning off all apart from the cold ones. This got the 3 warm. We then released the others and seem to now have warm radiators throughout but still have no hot water.
The heating system is old. It’s an oil boiler and is not a combi. We have a hot water tank in an airing cupboard and a cold water tank in the loft. We are getting water into the hot tank as I also have an immersion back up and when using that we get hot water. Any idea why the water is not heating when only using the boiler? It’s driving us nuts!

Need to know the system, we have in general C, Y, W, and S plans, and the C comes in three options.

With C hot water is thermosyphon so slightest air lock can stop it working, depending on how set up, the pump can be designed so when running it staves the DHW of heat.

Y system is likely with oil boilers like the C the DHW cools the boiler as default is DHW, the W is similar to Y but DHW gets heated first not at same time. The S has a separate valve for CH and DHW and needs a by-pass in case valve does not open.

C often no motorised valves, some times one 2 port valve on DHW only.

Y and W three port valve.

S at least 2 x two port vales.

Try to explain which system then maybe can help, pictures always good.