VAV Reheat and convector control

[sunsentinel]

Hi, we have vav boxes with reheat and convectors at perimeter zones. Convectors are sized to offset envelope loss. Vav reheat valves are not commanded open before room temperature drops below set point (vav boxes set to min flow), and we have reheat valves 100% open when room temperature is 3 degrees below set point.

In this scenario,
1)when should the convectors kick in? When outside temp. is below, say 40F? (AHU SA set point is 55F)
2)How should they be operating? In unison with reheat? How do we control the convector valve? (single space thermostat)

Thanks for suggestions.

 

[imok2]

Nothing is written in stone with environmental control You need to use common sense If the perimeter rooms are cold set to turn on the convectors at perimeter zones and if that doesn't take care of the problem then reset the vav reheat valves for more heat. Look at Air Conditioning like a wife going through the change and expect to make adjustments >)

 

[mauricestoker]

IMOK,

That wouldn't be funny except it's true.

 

[ChasBean1]

I like the idea of the radiators first before reheat. If not, reheat could come on and make the thermostat happy while it’s still uncomfortable near the perimeter wall.

With just a single thermostat at a perimeter zone, trying to keep things simple (straight proportional control), a possibility would be:

+2°F to 0°F of set point, no heat, just VAV full flow to min flow

0°F to -1°F (and OAT <45°F), perimeter heat 0% to 100% open

-1°F to -2°F, reheat 0% to 100% open.

I’d probably also consider boosting up that 55°F set point a bit during winter if the loads allow. There would be a little more ventilation and fan power but less reheat needed. Energy should be roughly equal but could improve comfort and IAQ (overall more ventilation, generally better throw from diffusers, etc.).