Raising the chilled water temperature is free efficiency until it isn't
6 °C to 10 °C lifts the coil surface with it. Past the room's dew point the coil stops dehumidifying entirely, and the room gets cold and clammy instead of cold.
Every energy report we read recommends raising the chilled water flow temperature. It is usually right. A chiller making 10 °C water instead of 6 °C runs at a better coefficient of performance, and on a system that is oversized anyway the saving is real and it is free.
Then somebody does it to a building with a kitchen in it.
What moves when the flow temperature moves
The coil surface temperature — the apparatus dew point — sits a few degrees above the mean water temperature. Lift the water and you lift the surface. That is fine for the sensible half of the duty; the coil goes on taking heat out, just with a smaller temperature difference and a bit more air-side resistance for the same result.
The latent half is not proportional. It is a threshold. Water leaves the air only where the surface is below the air’s dew point, and below that condition the coil dehumidifies at whatever rate the geometry allows. Above it the coil dehumidifies at exactly zero.
There is no gentle roll-off. A coil at 11 °C surface serving air with a 15 °C dew point is making about nineteen litres an hour. The same coil at 16 °C surface is making none. The room is still getting cooler and it is getting damper while it does.
The number to check first
Work out the dew point of the air actually arriving at the coil — not the outside air, and not the design condition on the drawing, the mixed condition on the day. If your new surface temperature is above it, you have converted a cooling coil into a sensible-cooling coil, and you should have wanted to.
Is there any moisture source in this space that the coil was quietly handling? Occupants, cooking, a wet process, an open dock door, an unhumidified fresh-air supply in August. If the answer is yes, the flow temperature is not free.
Where it is a straightforwardly good idea
Sealed switchrooms and comms rooms. Their air has nowhere to get moisture from, their dew points sit well under any sane surface temperature, and their coils were never dehumidifying to begin with. Raising the flow temperature on those costs nothing at all, and we have done it on several.
The distinction is not a matter of judgement. It is one comparison between two temperatures, and it can be made before anybody touches a setpoint.
Before you change the setpoint
We will measure the mixed-air condition at each coil and tell you which of them can take the change and which cannot.