I have a chromotherm 3 that displays the 'repl batt' constantly. I replace the batteries and the display does not change. Periodically the total display is blinking meaning I have to reprogram. I would like to replace th 3 with a 4 for use with a heat pump. Are there any 'gotchas' that I should be on the lookout for, or is it pretty straight forward? thanks . . tim manville

Well let me start from the beginning I've had the hvac company out here twice and now for a third time for a problem that keeps occuring and now seems to have changed. Last thursday they came out because my low pressure switch was open. I check the states of the switches before I called them out and the low read open. When they got out here they found a pin hole leak in one of the 1/8 lines that feeds the coils in the fins on the outdoor unit. The tech said the lines weren't spaced right at the factory and two were rubbing together for the last two years since the unit was installed. He repaired the line by just soldering it, it looked like. He then purged the system and recharged the line, he checked for a leak with a micron gauge. The system temps coming out of the vents were fine and flow was fine, he left. I came home from work thursday and the system was not working again, and the low pressure switch was open again. I called them on friday and they couldn't get a tech to me til monday morning. Monday, the same tech came out, and appologized that his repair didn't hold, the leak was in the same place. He patched again, looked the same to me, and repeated the same process. It ran for 24 hours and now its doing something thats baffling me. I have it in cool mode, the out door unit turns on for roughly 30 seconds and cuts out. The low pressure switch and the high pressure switch are in closed states. If I put my meter on the 24vac coil on the contactor when the unit turns on it gets 24vac to the side of the contactor and starts, 30 seconds later it loses power to the contactor and shuts off. I still read 24vac when I meter from the Red power wire to the Yellow stage 1 cooling wire that should energize the contactor. Are there any other steps that can prevent the unit from turning on other than the high and low pressure switches? And does anyone have a clue as to whats going on. I've been run through the ringer already and fought to have the second bill wiped, I'd like to be armed with some info when they come back. Thanks in advance.