Combi boiler AND header tank in loft??

[InfAddict]

Hi, I've just moved into a new house which has a combi boiler for CH and DHW. I'm no expert, but the boiler doesn't appear to have a top up valve on the cold water inlet pipe (to allow me to re-pressurise the system).

Also, I have found a header tank in the loft which seems to contain cold water, a ball-cock and inlet pipe. I tried draining a radiator and sure enough, the inlet pipe starts feeding in water to the tank. I have not found any hot water cylinder or similar anywhere in the house.

Can anyone explain why I might have a combi boiler AND a header tank in the loft? My understanding was that a combi removed the need for any header tanks etc.

Thanks in advance.

 

[PEDANTICVINDICTIVEMAN]

Worcester 24 and 28Cdi combi's can be fitted with a feed/expansion cistern using the Worcester open vent kit, fitted loads no problem, if it's a different make ask the manufacturer.

 

[InfAddict]

Thanks for the reply... it's a Vaillant boiler (not sure of model, will check it out when I get home).

After reading further on the internet, it sounds like I have a "vented low pressure system", rather than a unvented pressurised system. Some internet articles say these are unusual but can be used where pipework/radiators are old and may not withstand a pressurised system. There are other reasons such as building regs/certification etc.

Does this sound correct and if that's what I have, how exactly does it work and what is the purpose of the loft tank? Does the incoming mains cold water go into the tank rather than the combi I wonder? How does the combi know to kick in when a tap is opened (if there's no pressure)?

 

[InfAddict]

Not sure of exact model of Boiler but the instruction lists the following codes on it :

BCW 6B 242 EB
VCW 6B 282 EB

Any advice is much appreciated.

 

[dmccormick]

I'll chance my arm here:

The tank in the loft feeds water into the primary circuit. The water in the primary circuit flows around your radiators and through the boiler when the pump is on - when you drained a radiator, water from the tank replaced what was lost and the ball valve opened to refill the tank.

The hot water is completely separate set of pipes. Mains water is piped through the boiler's heat exchanger. When you open a tap a flow switch senses this flow and tells the boiler to supply heat for the hot water.

So the hot water coming out of your tap never mixes with the water circulating in the radiators.

 

[PEDANTICVINDICTIVEMAN]

Mr dmccormick has it sussed. I dont have any experience with Vailliants so would need to see installtion instructions to know if it fitted correctly.

 

[InfAddict]

yep, sounds like dmccomick is spot on with his explanation. Thanks a lot for all your help guys .