My plumber has installed a Vaillant ecotec plus 418 boiler. The vertical flue used does not utilise a cowl over the exhaust outlet, without a cowl over the top it would seem to allow rain or dust to fall into the pipe when the boiler is not on. This flue appears to be a standard Vaillant flu kit but is the arrangement ok? All other vertical flue's I have seen fitted appear to have a cowl fitted on top of the outlet pipe presumably to stop ingress from the atmosphere. Can an expert please advise, am I worrying unnecessarily about this? [Can't get access to makers technical team for advice]