imac fan

Hi I have a 3 year old imac which has worked fine until now (just after the end of my extended warranty!!) I have never heard the fan come on since I've had the machine.
Now I have switched on and the fan is on constantly, this is after it had been off all night and believe me it's not that hot in the UK any thoughts?

Since an iMac is a challenge to open and have a look inside, all I can suggest is to check the intake vents at the bottom of the display to make certain they're not clogged. Also check the exhaust vents at the top.

And reset the System Management Controller (SMC). Do the following:

Shut down your iMac and remove the AC cord from the mains. Leave it set for awhile (around 5 to 10 minutes). Plug it back in. If the fan(s) have not quieted down, there may be something else causing them to spin up.

There are three fans in your iMac: CPU, HDD, Optical Drive. Download the free iStat Pro widget and use it to monitor the fans speeds and temperatures. Let us know what they are. Here is the download LINK.

Thanks for your reply HD is 36 and cpu is 28. but the fan comes on all the time even after being switched off all night so it should all be cool

Those temperatures are normal. The three fans in your iMac will be spinning at all times, however, the rate of rotation will vary with the ambient temperatures.

My three fans as of right now (temperature outside is currently 106 F) are spinning at approximately 1000 to 1200 RPM. (My home has central air conditioning.)

Hi Many thanks for your help. I vacuumed the vents top and bottom switched it on and no fans so hopefully it's all sorted thanks