With the introduction of fanned flued boilers and no-compartment ventilation the places where boilers can actually go have massively widened and the question of “where is the best boiler position?” is pretty much difficult to answer.
Every job is different and can actually be governed by the services, exhaust and customer more than where technically the boiler can go. So there are a few things to consider to get the best place for the boiler, and it can be also governed by budget and actual works in the property. There is definitely no one size fits all.
If you are replacing a boiler then you may want to consider the gas, water and if it is pre-condensing then you will need a drain. If it is fanned flued then the customer may want to re-use hole and not have the budget or space for a new hole.
Boiler changes can also be instigated by work within the house so if it is a relocation and new boiler then you have to consider the services and how much it is going to cost to get them to the new location.
If it is a new install then again it can be governed by the services and client but in this circumstance if you have an outside wall that is close to a drain, good services and is in an ideal place for servicing and out of the way then that is the perfect scenario.
Boilers are definitely quieter, smaller and more versatile today but they do need a good gas supply, condense drain and definitely a filter and lime scale protector, so although the flue positioning is not restrictive there are other things to consider.
Gas supply is a major factor and homeowners do not understand that it takes a lot more work to run a larger gas supply into the loft to service a 40kW boiler.
So in summation “what is the best boiler position in the home?”, well the answer is wherever the customer wants it and wherever you can get it and they are happy. Good luck and happy plumbing.