Sealed Heating system

Hi
A question for anyone who knows about plumbing. I have recently bought a house with a sealed heating system. This has to be topped up regularly as water seems to be leaking out of the discharge pipe, maybe the pressure release valve is faulty. My main problem is I cannot see an expansion vessel anywhere in the house and I have looked. Is it possible this could be part of the boiler or is there any other explanation...
Thanks

missing expansion vessel

Modern boilers have an expansion vessel build in. Take the casing of and have a look.Your heating system does not seem to be "sealed",otherwise it would not loose water.I installed a new heating system in my home and never topped it up since I put it in ,about 18 month ago,the pressure constantly at around 1.1 bar. There is certainly something wrong with yours,get a plumber in.If you contact the maker of your boiler (via the web?)than they will send you a manual which will tell you about maintenance ,pressure,etc..Every boiler should be serviced at least once per year by a serviceman/woman that is recommended by the maker,many plumbers are simply not able to service a modern boiler.No measure instruments,not trained to to write a report etc..

Re: missing expansion vessel

Thanks for that , I looked around for the manual in the house and found one for an oil boiler , trouble is we have gas??
Anyway I have booked a plumber as the leak is a bit constant. I just wanted a bit extra knowledge before he called. I don't mind looking stupid but I don't want to have to pay for the privelege.