The manufacturer's guarantee refers solely to the appliance supplied. For example: If you have had a new boiler added to your existing central heating system and it is leaking causing regular introduction of fresh water or be contaminated with sludge due to improper installation (lack of inhibitor or proper flushing out), the operation of the boiler will be affected. In such an instance the central heating system may detrimentally affect the performance of the boiler and therefore invalidate the boiler guarantee.