Vaillant Turbomax Pro 28e slow to heat and hard to balance

We have a Vaillant Turbomax Pro 28e which is about 8 years old. Last year we had our house extended so that we now have 14 rads totalling approximately 28kw. The house is double glazed and well insulated but without cavity wall insulation.The recent cold spell has highlighted some problems, mainly that it takes ages for the house to warm up. Sometimes it is only just getting comfortable by 10pm when the heating has been on since 5pm. I realise that you are probably going to tell me that the boiler output is too small for all those rads which I am happy to accept but I don't think we are getting maximum output from the boiler. Could it be modulating unnecessarily? I know when we switch on the CH the boiler is working constantly until the roomstat reaches temp hours later. The CH water temp switch is set to max. The other problem is that the system is very difficult to balance - its hard to get all the rads at the same temp top to bottom and if I turn one up even slightly another one suffers. DHW is fine. Sorry for the long post and thanks for any advice.

HI

boiler not big enough.
14 rads x average 2 kw (with heat loss etc )=28 kw
boiler has to be atleast 32 kw .so you can not balance the rads.your boiler is working very hard to supply all the rads .



FED UP OF COWBOYS ? TRY INDIANS .

Hi Lunat
Thanks for that, I suspected that would be the answer. The bit I don't get is the difficulty balancing - if the boiler is too small then that would explain the slow warm up but why would it make it difficult to balance?