Before you apply please make sure you have the following available. Drain cleaner contains lye lye is an amazing chemical, but it's extremely dangerous. If your boiler is too old or uneconomical to repair then our engineer will advise you straight away on the options to replace with a suitable new vaillant boiler if you are a reapir & protect customer you may receive a contribution towards a brand new one. Want to check a few things before you call us? Local plumbers.