Check out the case seal.

"Pete Cross" 1@2 wrote in message ...
The flue looks fine, it has two concentric tubes, the inner aluminium one
carries burnt gas out and the outer plastic one sucks outside air in, it's
fan assisted and the burner/fan/flue section is sealed from the room. Do
gas
valves leak eventually ?

--
Pete Cross

"IMM" wrote in message
...

"Pete Cross" 1@2 wrote in message ...
It is 6yrs old, when the main burner is lit there is a small smell of
gas
from the modulating gas valve but I can't see any leaks after brushing
detergent solution around anything that looks like a seal. There is a
flat
square plate on top with a visible gasket and a pipe comes off that up
to
the burners but other than that there doesn't look to be anywhere else
it
can escape. With the burner off and pilot lit there's no smell at all.

You may be confusing the smell of the product of combustion, which may
be
leaking into the room. Check the flue out.