some radiators heat up in H.W. only mode

[thumbsup]

I have had a Vaillant EcoTec system boiler installed recently.There are two 2-port motorized valves controlling C.H. and H.W. fitted onto the 'flow' pipe from the boiler. The new system has replaced a Potterton Puma combination boiler. The new system has been connected to the originally fitted C.H. radiators, to the new radiators fitted in the newly built extension to the house and to hot water supplied to bath/shower rooms & kitchen. From the day one, the radiators originally fitted to bedrooms get heated up by hot water fed from the common 'return' pipe returning from the H.W. cylinder coil in the H.W. only mode. Hot water flows towards radiators as well as returning to the boiler. There is no problem in 'C.H. only' mode and in 'C.H. & H.W both' mode.
The flow from boiler was connected to bottom side of the H.W. coil to the cylinder by the installer initially which has been connected to top side of the coil. The installer suspected either a 'One pipe' old installation to the affected radiators or a 'by-pass' fitted around them but the said radiators are definitely connected to a common 'return' pipe & 'flow' pipe from C.H. motorized valve and there is no by-pass fitted. He has spent a considerable time to investigate and rectify the problem without any joy. He managed to stop flow of hot water by a 'one-way' valve but it produced an excessive hissing and whistling noises which has been removed.
The installer is unable to cure the problem and has lost his interest.
I have called a number of plumbers/engineers but unfortunately none of them has managed to resolve the problem. I have called Vaillant engineer who has simply concluded as an 'installation' fault and have not identify the cause of this mysterious problem.
Any opinion, suggestion or help from any fellow members of he forum would be appreciated.

 

[Billy Bob Bob]

last tee is not going to the cylinder return rads teed of before this,thats why some warm up in dhw mode

 

[WHPES]

Need to trace the circuit and identify the connections whic are incorrect and reposition. Impossible to tell without seeing.

 

[moony]

reverse circulation.... would have to lift all boards over pipes and check it all out...

havent followed the 3 tee rule somewhere down the line lol

 

[thumbsup]

Thanks 'gas man' for your sincere help and guidance.
You have identified 'root' of the problem correctly. Yes, the radiators which were getting hot in 'Hot Water only' mode were teed off from the return pipe BEFORE returning to boiler.
The installation has been altered by connecting the affected radiators to the common 'return' after it returned to boiler.
The problem has been resolved successfully.

Thanks to all of the fellow members of the forum who have responded and tried to help me to resolve the problem.