My son & partner have just moved into a house with a Vaillant 3257 NG Internal HW system in the laundry. After noticing a smell of gas in the laundry occasionally, upon further investigation it appears that when using a certain hot-water tap that has low water flow (a newly-installed laundry tap with narrow piping!), the HW system “kicks in” and gas is emitted to the burner, but the burner does not ignite. This does not seem very safe – replacing the piping is probably the ultimate solution, but is/should there not be a mechanism in the system that shuts off the gas if the burner does not ignite?
The burner ignites ok when (all) other hot-water taps with higher water flow are opened.
Clearly what is happening here is that due to the extremely low water flow the heater is allowing a very small amount of gas to weep through part of the main burner but is insufficient to light up from the pilot burner. The result is an annoying smell, but I do not believe it is a dangerous situation if this is the case. Get the tapware that is causing the problem upgraded or replaced to avoid this fault developing into something worse.
The water heater relies upon water flow to lift the diaphram and open the gas valve proportionally if the water flow is too low then no gas will flow – they were never designed to operate on any water flow of less than 4 litres of pure hot water per minute flowing through it.