Vailliant eco boiler-constant top-ups. Tips on leak-finding?

[bungeehamster]

greetings all
I can see that many people are having probs with having to top up their Vaillant boilers. Mine is 18months old & didn't need topping up for a year - since then, top-ups have been required more & more frequently - to the point where I've topped it up twice in 3 days as it has totally emptied - cdoe 22.
There are no signs of any obvious leaks from the rads, valves or first floor pipes (I'm assuming this much water would be dripping through the floor?). The overflow outside is bone dry. The house was built in the 1980's and has concrete ground floors - any ideas how to find a leak in the pipes underground ( and does it really need finding & fixing or should I let the plumber stick some rad-weld stuff in the system?)
Any other thoughts/solutions?
Sorry if there is an obvious answer - I've not had to deal with a problem like this before.
thanks in anticipation

 

[Agile]

The auto air vent in the boiler needs checking and the PRV vent outside needs a bottle or bag tying on to see if it leaks overnight.

Leaks in pipes embedded in concrete flors are a nightmare!

Any significant leak needs to be found as constantly repressurising will cause rust in the rads.

Tony

 

[bungeehamster]

The auto air vent in the boiler needs checking and the PRV vent outside needs a bottle or bag tying on to see if it leaks overnight.

Leaks in pipes embedded in concrete flors are a nightmare!

Any significant leak needs to be found as constantly repressurising will cause rust in the rads.

Tony

I've topped up the boiler this morning at 6am and tied a bag over the PRV vent outside- having re-filled the boiler again at 2.30pm (it was totally empty) the bag is completely dry

I'll ask the plumber to check the auto air vent.
Thanks for your help!