iMac Getting Hot in TDM as second monitor

I am running my old iMac as a second monitor for my new iMac. Everything works fine but the old iMac is running hot, the fans are running all the time and it is hot.

I first put the iMac to sleep and then plug in the cable to go into TDM, so you would think the imac would just continue sleeping.

Anybody have any ideas on how to make the TDM iMac just sleep and stay cool?

I just don't like the idea of it pulling much power when it is not necessary.

Thanks a bunch.