staged setback for AC

tekmar

Thanks for the thought. I am pretty sure I asked a Tekmar rep who was doing a powerpoint demonstration on their new multithermostat multi output control systems this past fall if they had an AC solution and he said no, but it never hurts to try, given that I can't remember some stuff I did yesterday, I may be misremembering my discussion with him.

As to the Trane unit running wide open, there are actually two 'trane units' involved. The chiller is a trane unit. Don't know much about its relative efficiency. It is about 15 to 18 years old. It currently runs with a very simple analog lead/lag control that kicks in the second compressor if the difference between the inlet water temp and the setpoint is 5 degrees. I put in a 3 way swith that I switch every season to alternate which is lead and which is lag.
the setpoint itself has no adjustment mechanism other than physical changing. In that sense we leave the setpoint at around 38 degrees and it runs 'wide open' to make this setpoint throughout air conditioning season. There is no compensation for outdoor temperature (although obviously it will run less to make the setpoint when outdoor temperatures are cooler).

The fan coil units inside are also Trane units and they are designed (typically, I can't blame Trane for the basic idea) so that the fan runs all the time and there is a zone valve that admits the conditioned water when the room air temp calls the valve through a thermostat integrated in the unit. The thermostat is a volatile fluid probe placed in the path of room air returning to the unit along the floor just inside the air filter. There is an electrical comparator that senses the water temperature offered by the circulating water, presumably for shoulder seasons when one room might want cooling but the heat is actually on so opening the zone valve would be counterproductive.

These fan coil units are older than the AC chiller (which has been replaced once during the life of this system - maybe it is due for replacement again if efficiencies have been markedly improved maybe by two smaller units more like stage boilers.) Anyway,the fan coils are about 35 years old and there is not one that has a working zone valve on it anymore. There were no unions or replacability built into the design of the unit and there is very little spare pipe providing for installing any kind of update. It is remotely conceivable that you could remove the zone valve workings from the valve body and retrofit some carefully organized replacement but service access is miserable and the wiring is substandard (old mtw, not thhn, I consistently get units where the wire coatings give inside flexible metal conduits and short out.) The addition of operational voltage (all 120 AC, including fan motor and zone valve) water temperature sensors and wiring to the zone valve is just another nightmare. Generally, once a unit melts down I just cut the zone valve out, eliminate the wiring harnes and pipe straight over (not easy to do either because the piping employed on the unit side of the zone valve is like copper foil. Far thinner than even 'm' tubing and subject to damage from cutting and attempts to swage or otherwise make it possible to attach regular copper fittings)

These have to be some of the worst designed things I've ever dealt with, but I know it was pretty much par for the course in those days and Trane was the 'cutting edge'so I'm not out to suggest that any other company would have been better since I do not know.

Brian