DIY Doctor UK DIY Forums

We have a Lennox Warm Air central heating system (Furnace) - no radiators - which has worked efficiently and is still working great but the local Gas Safe people keep telling me they are not familiar with the system and won't service it. It is the simplest of systems to check out and so reliable and efficient. Our water heater is also a Lennox stand alone gas-fired system. Any comments or answers to this?
Thank you.
MRH