We had Vokera Unica HE36 fitted 3 weeks ago and have had problems with radiators ever since. Upstairs are very hot downstairs are cold. The heating installer has been back 4 times and it only lasts 2 days before going off again. He told us it was the pipework and so has changed 3 lots of pipework to downstairs radiators and the manifold. This has not solved the problem and he is now saying we need new radiators. Previous boiler was a vailent combi and worked fine for 12 years but heat exchanger went so decided to replace.