The most common reason for heat exchangers to fail is a build of sludge and scale in the central heating system that has worked back into the boiler. Our home emergency policy only covers heat exchangers if they have not failed due to sludge and scale.
If the heat exchanger has failed for any other reason and falls within the length of insurance then a claim would be considered. The decision usually comes down to the reason for the heat exchanger failing rather than this being excluded under the terms and conditions. It will also only be covered if it results in complete loss of function of your boiler or warm air unit.