CV Ahu with Reheat
Got a job that we are replacing a AHU. It's for a big salon. CV AHU that has reheats with old Barber Coleman stats. The AHU has an old XL50 that only controlled Occ and UnOcc. It has never worked right. The staff has just let the AHU run in occ all the time and they constantly adjust the DA spt. And in the summer shut down the Reheats.
So we are replacing the air handler. I'm going to throw a Sypder on it for some better control. I'm thinking a simple space sensor to control cooling and reset discharge air based on return temps. Setup electric heat to handle MWU and unocc heating.
Or should I just reset the discharge air based on OA.
The only time I have dealt with CV with reheat control is labs and OR's, so it's alot easier to control.
BTW- No VAV's
Why control off discharge air on a constant volume unit?
I would reset supply air based on return and keep the OA out of that part of the equation.
You stated the unit has electric reheat. Will it be staged or SCR?
I don't get it, why be concerned with resetting discharge air temp on a constant volume AHU? Shouldn't it be trying to increase cooling (stages of cooling or chill water flow, which is it anyway?) the further the space temp is from setpoint? Sounds like resetting supply air would be working against that. I would appreciate any enlightenment.
Sounds like a good plan to me too ... Cascade control works very well in large spaces.
Originally Posted by ascj
Assuming this AHU is CHW cooling I'd consider letting OAT control the Min DA sp too...
so DAsp is reset by RA but as OA goes high the minimum DAsp allowable can go lower. On a cool day maybe DAsp can go as low as say 16°C ...on a hotter day DAsp could go as low as something like 12°C. This way if there's a full cooling call on a cooler day ( maybe at startup or something) the system wont blast the occupants of the space with an super icy breeze.
for those askin the question WHY?
This is a 40ton DX CV AHU w/ staged electric heat. It feeds the 1st of a salon/spa. There are 10 zones controlled by electric reheat packages. No Vav's.
I've dealt with this setup alot in OR's and Pharma Labs. CV AHU's with electric reheats are used due to the required ACH's and/or room static pressures requirements. I reset DA based off RA or zone temps(if they are tied into the BMS). They normally have tight humidity control and keep the room setpoints the same, year round.
So now I have this setup at a salon. No humidity control and zone temps not tied into BMS. The problems I fore see are:
1) Employees setting room setpoints lower in the Summer and higher in the Winter. I run into this alot in our buildings, that don't have means of humidity control. This throws DA reset based off RA and/or space temps, out the window. I end up with DA with OA reset.
2) Looking at the space.....I foresee very different loads. There is locker rooms, saunas, nail studio, massage rooms, and lounge areas. And of course, I have no load calculations and history off what has been changed in the building over the years.
The existing AHU didn't supply proper comfort levels and the building engineer just dealt with it. Now there is new engineer and the existing AHU's remote condenser deteriorated.....so we installed a new AHU. The existing controls are off a XL50 that is beat and no database for.
So I'm thinking DA reset off OA during occupied periods and standard heat/cool based off single zone sensor during unoccupied periods?
Is there anyway to get better control of the reheats? If not, in a couple of years the system will probably be back in the same state as it is now. What you are proposing to do will work but, it's an energy hog. These type of systems are for critical controlled environments. It's a salon, somebody should to talk to whoever is paying the E-Bill and get them on the right track.
I got talking to maintenance.....they set the reheats for 72 degrees and leave them. They all have lock boxes. So I ended up programming occ heat/cool to run off return air(69/72) and unocc heat/cool to run off a space sensor.
You're playing the cards as they were dealt, good job and good luck.
Originally Posted by ascj