Why is the heater in my Ford Fiesta not working?

My Ford Fiesta doesn't give out hot air until I hit a pothole or speed bump. Any idea what the problem could be?

Asked on 31 December 2019 by Lewis Remon

Answered by Honest John
Most likely to be sludge in the bottom of the heater matrix impeding the flow of engine coolant though it. When you hit the speedhump or pothole, that disturbs the sludge, allowing the coolant to flow.
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