Thanks for the replies chaps.even the largest of bungalows would only require in the region of 18kw as the 418 is capable of.
The range rating of the 418 will allow it to run a lower min rate than a 428.
This means that the boiler will be able to run for longer periods and not cycle as much.
Best to get your installer back. If not ring Vaillant and get them to send an engineer out.
basically more or less what he said Also, running an over rated boiler on a small system in worst case can give you noise, overheating and locking out issues and your boiler will be very unhappy with its flow and return temps :/
Don't bother phoning vaillant, they will not be interested in your circulation problems.
i do understand what you said about getting an overated boiler,like being overkill for the job.
the plumber is coming up tomorrow morning.
he did put a bypass for the water to the tank,and also another pipe underneath the pump,said something about it allows the heat out of the boiler in between it going off and coming on again?
I didnt have those 2 alterations on my potterton netaheat.
the storage tank is underneath the boiler in the airing cupboard.
he said something like because the storage tank is so close to the boiler,i would only need about 10,000 btu,so i dont know if thats correct.