ASHRAE 135 - Heating/Cooling Changeover Delay

[mmcdonal143]

Does ASHRAE 135 (controls spec) make any provisions for a "change-over" delay when switching a VAV terminal unit from heating to cooling or vice versa?

This seems like a common sense practice, to prevent short-cycling, but I'm getting some resistence from the client and they're saying that it is not spec'd.

When a room VAV with a reheat coil and supplemental radiant ceiling panel heating is in heating mode:

1. VAV is at min CFM
2. Reheat & Radiant Heat valves are open

Switch to cooling mode -->

1. Reheat & Radiant heat valves close
2. 5 (or whatever number you choose) minute change-over delay.
3. VAV opens to max CFM

My questions are:

a. Does this time delay make sense?
b. Is this time delay prescribed somewhere in ASHRAE?

 

[Wareagl487]

I haven't run across a specified time for change-over. I would think that is as good a number as any.

What I would prescribe is the dead band between cooling and heating not be set to tight.

Andy W.

 

[cjw81]

it is a good idea to put a delay in the system to prevent "thermal shock" to people in the area. You don't want them getting sick, especially in a hospital ,long term care facility.