Traditionally a boiler is considered commercial or "non-domestic" once it is over 70kw (net) input or if the total gas supply volume is greater than 0.035m3 (or over 35mm diameter). Many manufacturers may also stipulate that their appliances are commercial if they are under 70kw but designed for that application rather than domestic. Viessmann
, for instance, have a 49kw Vitodens 200 gas boiler, a domestic engineer may feel that their qualifications legally cover them as it is within their 70kw limit however manufacturer instructions override generic legislation in all cases. Further to the 70kw limit per appliance, it is also the case that if the combined total of gas appliances exceed 70kw then it is no longer a domestic installation. For any installation where the total input of the gas appliances that share an installation space (room) or which share a wall for flues or even an adjacent wall then the installation is non-domestic.
So how can a property owner ascertain whether they need a commercial gas engineer?
Is there a single appliance rated at greater than 70kw net input? Is there any gas supply pipe greater than 35mm? Does the installation volume exceed 0.035m3? Is the combined total of gas appliance input greater than 70kw net input? * Are any of the appliances sold as and considered by the manufacturer as Non-domestic? Is the gas meter capacity greater than 6m3/h?
*(Where the flues are on the same or adjacent wall or where the appliances share an installation space).
There are many other instances where it should be clear that the installation is commercial heating, these outline more of the "grey" areas but as you can see it is not uncommon for many private homes to require a commercial gas engineer.