I have an air handler that has a three-way valve feeding heated water to the coil. The actuator is very slow, that works well with keeping temperature steady once it settles down. At start up the unit is warm so it is likely calling for zero % heat. It quickly cools below setpoint and starts to call for heat. The three-way valve does not supply water until it is roughly 33% open. By the time I have hot water in the coil the temperature is way below setpoint and might even trip the freeze protection. The controller is now calling for 100%. The temperature will now far exceed setpoint causing the controller to lower the signal to zero %. As the actuator responds slowly I have a roller coaster. Slowing the control signal only makes the initial warm up that much worse when added to the slow actuator. Making it quicker only gets the signal into wild swings while the actuator is still slow.The three-way valve keeps hot water available at the unit and keeps the pipes from freezing even if the unit is not calling for heat. Ideas other than changing the actuator?
Randyman
What kind of actuator/valve body is it? Is this a recent problem, or has it always been like that?
We used to have a heat only unit. As it brought in hot air off the roof during the summer, we would have to shut the unit down all summer long as the assorted fan coil units would be working in panic mode to keep rooms cool. We replaced the unit with one that has cooling and heating. What I found was that when Trane installed the new unit they disabled the outdoor reset for the heated water supplying the unit. They spent months getting the unit to work, somewhat poorly, without tripping the freeze stat. Now I know why they had so much trouble. The poor control is what led me to tear out their controls to begin with. We did not have a BAS at the time so it had the controls Trane put in it. I never did like the fact that only Trane could make changes with their proprietary software and I had to pay them to do it. It is now controlled by our BAS. I have re enabled the reset so the swings shouldn't be so dramatic and it seems like it will be okay. Unfortunately I won't know until next heating season.