Hi there and welcome
It was not clear sorry but is the Mac still functioning correctly apart from the Fan spinning at excess speed?
Please let me know.
It is not clear if the Mac is still functioning correctly apart from the Fan issue, this is what I need to know please.
I am sorry if you are fed up with me.... but its a question I need answered.
The most common cause is over heating due to the vents blocked up or dust.
Can you vacuum via the vents on the iMac to try to get all the dust out (make sure it is powered off and the power cable is removed first)
Then connect the power, turn it on and tell me if the Fan operates correctly please.
ok thank you.
It appears its overheating, its possible they did not put the heatsink in properly, or the cabling is not laid correctly and its covering the vents, so making the computer work much harder in order to cool off. But it really points to an overheating problem.
You will need to have someone physically look at it, and I would advise to just return there with it and demand they do this. The work should be covered by a warranty.
Let me know how you go please.