Controlling HHW HX to delta T

[11t]

Hey everybody,

I got an interesting situation right now. I have a HHW system that is solely used on a pretty large dual duct dual fan AH. Because of this, the HD valve is locked @ 100% and we are doing all of our temperature control at the HX. The HHW VFD is controlled to a deltaP of 10 psi, but this just means it runs at 50%. Currently we have very limited room level feedback so demand control reset on the HHW TP is not really an option and we are using an OATP reset. We are consistently getting ~10 degF delta T but I have heard that 20 degF delta is optimal at the HX.

Without the HD valve position mattering since it is @ 100% I am curious if people think controlling the HHW VFD to increase delta T would be an acceptable control strategy, or what would be a good way to reset this so that it works better? Right now it seems extremely sub optimal but I am cautious about stratification etc...

Thank you,
11t

 

[ChasBean1]

What temperature is the hot deck coil designed for? That matters. I’d say have the pumps keep doing what they’re doing to maintain the 10 psid, if the balancer needs that DP for coil design flow.

Reset the hot deck outlet temperature between 75°F when it is nice and balmy outdoors to 100°F when it is miserably cold outdoors. I’d also reset the HX outlet temperature on OA conditions. So you may have:

OA HX Outlet Hot Deck DAT
20°F or less 180°F 100°F
70°F or more 120°F 75°F

 

[urgross]

Do you have a building management system? If so, does it include mixing box percent damper open? Not sure why having a hot deck implies running the HHW coil full open. Other information needed is how the economizers are operating. Using a hot deck economizer when return air and outside air can maintain setpoint is where free heating begins. If free heating is available, why run the HHW coil at all?

Attached article may be of interest

http://digitalcommons.unl.edu/dissertations/AAI3152614/

 

[11t]

Yes, we have a control system and I am the one writing the control code to operate the building systems. Currently the HHW_G pumps are set to maintain a deltaP of 10 over the supply/return but this just means the pumps (which are on a VFD) operate at 50% consistently.

The system is a modified dual duct air handler with 2 CD SFANs and 1 HD SFAN. It has been retrofitted into a dual duct dual fan air handler where the HD brings in 100% return air, and the economizer is only on the CD and it can bring in return air or fresh air depending on ventilation needs/economizer abilities etc...

The HHW_G system was originally designed to serve the HD and the PH coil on the fresh air. However, this building has a large amount of exhaust and the PH coil was "replaced" with an energy recovery system and ERC coil. The PH coil is still there but it no longer functions. When the PH coil was "removed" (it still exists but is an emergency backup in case the ER system fails) the HD valve was locked open at 100% and the goal was to control the HD temperature with the HHW_G system to reduce system noise from too many PI loops playing together.

We already have a temperature reset on the HHW_G system (which therefore is on the HD) but what we don't have is any sort of deltaP reset on the HHW_G system. I was thinking about attempting to control the deltaP set point to the delta T on the HHW_G system or possibly sending out a guy and doing an analysis of the coil to determine what the minimum pressure is that we have adequate flow through the coil to prevent stratification.

Also, thanks for the article urgross. I actually work for the University of Nebraska and have read the article. The reason we do 100% RA on the HD is that the summers here are very humid and we have dew point problems if we do not dehumidify fresh air during July/August.