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Frequently Asked Questions
What reaches the building?
Heated outside air and nothing else.
Combustion happens inside a sealed exchanger.
Products of combustion are flued to atmosphere.
Supply air only passes over the outside.
The two never mix.
That separation is the whole point.
When is indirect the default?
Wherever supply air reaches general occupied space.
Offices, retail and assembly areas.
Anywhere direct-fired restrictions apply.
Anywhere the argument is not worth having.
It is the conservative choice.
Direct firing needs justification.
What does the separation cost?
Efficiency and equipment.
Heat must cross a metal wall.
The flue carries away part of the fuel energy.
The unit is larger and more complex.
A flue route is needed as well as gas.
Capital and running cost both rise.
What should be asked about the heat exchanger?
Warranty length and inspection access.
It cycles between cold air and combustion products.
Stainless construction is common for that reason.
A crack puts combustion products into supply air.
That defeats the unit's entire purpose.
It matters more than headline efficiency.
What does a condensing version add?
Recovery of more flue energy.
Efficiency rises meaningfully.
It produces acidic condensate.
That needs a drain and often neutralisation.
The drain is a real installation constraint.
It narrows the gap with direct firing.
How is the exchanger inspected?
Visually, through access panels.
Cracks develop at stress points over years.
Thermal cycling drives the damage.
Some units allow better access than others.
Ask how inspection is carried out.
Build it into planned maintenance.
What does the flue require?
A route, a termination and clearances.
Termination must avoid the air intake.
Recirculating flue gas into supply is a real risk.
Prevailing wind affects that.
Manufacturers specify separation distances.
Settle it at roof layout stage.