Consider that the smell might not be from the boiler at all.
If the smell you detect has an ammonia or fishy pong then check the electrical fittings in the room - it's not unusual for the plastic of electrical fittings - ones that get warm like lamp holders for instance - it's not unusual for the chemicals in the plastic to break down over time and give off a strong smell.
Boiler cases are sealed units - so check the exhaust air outside the property - give it a good sniff. If you can detect the smell then it's a boiler issue - if you can't smell anything then it's something else that's causing it.
Good luck...
April 2015
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