Hire London boiler repair services now! Would they have to use the manhole to do the survey? i needed my drains to be checked, and they couldn't even find a manhole (they think it might have been tarmacked over by a previous owner. Are there any downfalls to combi boilers? Buying guides to help you make the right choice. Great hub! we have a couple drains running slow and haven't had a chance to buy draino yet, so these tips will be great to try! Many heating.