Hot Water/Central Heating return pipes

[pjc]

My central heating system consists of a baxi condensing boiler, which has one flow and one return pipe, a pump, a 3-way motorised valve, a programmer and a cylinder thermostat. All these components appear to be working fine.
However, the pipework has been altered recently and something strange is happening. Basically, when the system is switched to hot water only, the return flow from the cylinder not only goes back to the boiler, but also flows up the return pipe to the central heating radiators. The effect of course is that the radiators heat up.
I have checked the motorised valve and this is working fine. When the system is switched to hot water only, the flow pipe from the 3-way valve to the radiators remains cold, which is what you would expect of course.
I suspect that this is a pipework problem. Basically, when this type of system is working correctly and hot water only is selected, normally what stops the water from flowing back up the return pipe to the radiators, at the point where the return from the cylinder and the radiators join? As I understand it, there isn't a valve at this point.
If I can understand this, hopefully I will be able to figure out what is wrong with the plumbing in my system.
Any advice will be gratefully received.

 

[ollski]

What work have you done?, have you tapped in a new radiator?

 

[pjc]

Basically, we had a new boiler fitted. However, the old boiler had two separate systems for the hot water and the central heating system, whereas the new one makes use of a 3-way motorised valve. Consequently, the return pipe from the cylinder and from the radiators had to be joined. We did in fact add one more radiator downstairs.
I was wondering if the position of the junction between the return from the cylinder and the return from the radiators was crucial. I have read somewhere on the internet that this must be the last junction before returning to the bolier. Currently, this is not the case in my system. Could that be causing the problem?

 

[doitall]

Yes very crucial, you have created a short circuit and the heating side is reversing back through the return.

No T should be connected after the cylinder and heating connect to the common return.

In other words all the returns must be connected before they join the common return

 

[ChrisR]

Standard plumbing error - look up "reversed circulation".
It can be cheaper to correct using a one-way valve of the right type in the pipe rather than alter the pipework. Needs a gravity check valve.

 

[pjc]

Thanks for all the advice.

I should be able to alter the pipework without too much trouble. I will ensure that the return pipe from the cylinder joins the return from the radiators just before the boiler, making sure that it is the last junction. Hopefully, this should cure the problem.