If it is the CT-30 thermostat that they installed, the Red light is for when it cannot connect to the Access Point for Comcast Home.
I use the same thermostat, but with a different company to allow me to control it through their service. It has been a while since I have actually seen the Red LED light up on mine.
Try taking out the batteries on the thermostat, turn off the furnace either at the breaker panel, or at the switch on the side if there is one. Then unplug the Comcast Home Access Point. Plug back in the Access Point and after it syncs with Comcast's network, then turn the furnace back on, then put the batteries back into the thermostat.
It should hopefully go to a Green LED. If not, try pressing the Menu button on the right side, then press and hold under the Antenna icon on the touch screen, with the word "Radio" by it, the "Mate" icon. You may have to press it again after you hold it for a bit. It should then hopefully sync with the Access Point.
If it does not, then you will need to contact Comcast home, due to one of two things may have happened. The thermostat somehow lost the PIN to sync it with the Comcast Home system, or the radio unit or U-Snap module as it is called, has gone bad, which I have seen happen with them.
Follow up on this, as to what the problem was, and mark it as solved, once Comcast home resolves the issue. That way if anyone else has the same issue, they can find this topic and know exactly what steps to go through for troubleshooting.