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.
Similar questions
The blower is working on my Ford Fiesta, but it's not warming up. Any idea what the fault could be?
My son bought a car on Sunday from a secondhand dealership. On the test drive, the air con was blowing but didn't seem very cold so he assumed maybe it hadn't had a chance to cool (he knows this shouldn't...
The air conditioning temperature control doesn't work in my Citroen. When it's at the minimum it's okay, but you change temperature it starts heating?
 

Value my car