I have a Tempstar split system multi stage heat pump with a Honeywell RTH7400D108 thermostat. My problem is intermittent and is that the AHU fan continues to run after the t'stat reaches the desired temperature. The outdoor unit will cycle back on when called for or I can open the breaker to the AHU and shut the fan down. Problem happens both heating and cooling. Any advice would be greatly appreciated.

Could be a sticking fan relay OR a short in the thermostat wiring.

I appreciate your reply to my question. If the fan relay is sticking why doesn't the fan restart after turning the power back on?

Good question but this was not actually mentioned in your first post. Something is holding the fan on after the thermostat is satisfied. It could be an electronic glich in the system circuit board. Also make sure yout thermostat configuration settings are correct for your system.

http://customer.honeywell.com/techlit/pdf/PackedLit/69-1726.pdf

Thanks so much for your time. I had checked the t'stat settings prior to posting and am pretty sure they are correct but I will check again to be absolutely sure.
Where is the circuit board you mentioned? Is it in the outdoor unit or the indoor unit. Another note is when this happens I can pull the t'stat off the base and the fan will continue to run.

The seems like a dime store novel. More info leaks out on every post. LOl It is a good possibility your indoor unit has heat sequencers and one of those controls the indoor fan in the heat mode. They can stick from time to time and the one controlling the fan will need to be replaced if your unit has them. There are a few different types but here is a pic of the most common.

Sorry for the
dime store novel approach. I checked my t'stat (wiring & setup) and all appears good. I located the sequencer in the unit and this is what I have. My electrical drawing doesn't show this device or the aux heaters. If it is not too expensive think I will change it out and see what happens, otherwise I am about ready to call a service tech. You have been very helpful. Thanks so much for your input . Be Blessed, Tim

Many times the AUX heaters are a package added to the unit. This way the contractor can pick the size best suited for your home. The sequencers are not to expensive just make sure you get the same one or its replacement. Look in store area at bottom

sequencer, great deals on HVAC on eBay! (http://business.shop.ebay.com/i.html?_nkw=sequencer&_sacat=1266&_trksid=p3286.m270.l1313&_odkw=&_osacat=1266)

Easy test: next time the fan is running beyond normal cycle duration, remove the front panel from the unit & carefully bump the top of the switch stack with the butt of a screwdriver. If the fan stops, the sequencer is sticking.

Lightly tapped the sequencer as suggested and the fan shut down. I will replace it as soon as I can find a new one. Thanks guys, y'all have been a big help. Be Blessed.

Thanks guys, y'all have been a big help. Be Blessed.Likewise. And, good luck with finding something to do with the $$$ you saved vs. calling in a pro. Let us know if you need help... :D