How about killing off FanControl and install this: TG Pro it will offer you a better view on whats happening inside your system. Post a screen capture so we can see.
Given the systems age I would recommend cleaning the dust buildup inside. See if that helps.
Are you sure you have the original drive? This model only came with a 1 TB drive. If you have a 2 TB drive it's not the original drive. Please review this IFIXIT guide: iMac Intel 21.5" EMC 2308 Hard Drive Replacement Jump down to Step 10, if this is a replacement drive the sensor my not be installed correctly or be the wrong unit which could also explain why you are having problems and are using software over-ride instead of allowing the system to self manage its temp. I strongly recommend not using software over-rides as it shortens the lifespan of your system and will wear out your fans prematurely. If the drive was replaced you may need a new sensor this is what I would get: OWC In-line Digital Thermal Sensor for iMac 2009-2010 Hard Drive Upgrade. Unlike Apples sensor setup which is dependent on the internal sensor of the HD this unit adds an external sensor so it will work with any HD or SSD. The apple sensor cable is designed to fit a given vendors HD so if you had a Seagate drive you would need to put in as a replacement a Seagate drive, you would need to switch out the sensor cable if you put in a Western Digital drive. The only rub here is the many of the newest generations of drives don't offer the same hookups for the internal sensor Apple's cable was designed for so you'll need the OWC cable instead.
Last issue here is your system is only a SATA II (3.0 Gb/s) so the drive you put in must be able to run at this SATA speed. Many of the drives today are fixed SATA III (6.0 Gb/s) so you'll need to find one that offers either a jumper or auto senses the SATA ports speed and matches it. Also make sure your systems firmware is up to date, follow this Apple T/N to check it: About EFI and SMC firmware updates for Intel-based Mac computers, mare sure your at the level listed or newer.