Heating of a Ceiling Plenum

[bsrd]

I am in the process of hiring design professionals for overdue maintenance to about a 15,000 sqft office building that was built in 1980. After doing some investigation I realized that the ceiling plenum is being heated by two or three RTUs and then a large air handler brings in the heated air and distributes this through a duct system. The air handler is also used for cooling. I am a professional engineer (primarily structural and civil)and don't pretend to be an expert in HVAC but this just doesn't seem right. Is something like this still allowed by code. I have never seen anything like this before.

Thanks for your comments in advance.

Boyd

 

[EnergyProfessional]

You mean the RTU dum the heated/cooled air into the plenume and from there separate AHU move that air into the spaces via ducts?
Do those AHU heat or cool in addition, or are just AHU with fans (no coils)

the only explanation would be the RTU are DOAS for outside air and the AHU operate to condition the space. Even then, it wouldn't be good practice to dump air into the plenum.

Obviously we don't see what is going on, but what you describe doesn't seem to be good practice. I don't know if at the time of construction there was a code in your jurisdiction that would have prohibited a bad practice. the code cares about safety, health and in some cases energy efficiency. The latter would be a problem with such odd design. I hope the HVAC professional you are about to hire can give you better advice.

 

[willard3]

Tell you boss you are not competent to solve this problem and hire an HVAC Engineer.

 

[chicopee]

When you say plenum, is that the space between the suspended ceiling tiles and the roof? If so, that is a strange setup up to cut down on the cost of ducts unless there are sprinkler heads and related piping exposed to freezing temperatures. Another possibility is that the suspended ceiling if there is one may have been added later on for cosmetic purposes.

 

[bsrd]

Thanks for your responses, I have spoken to two other HVAC designers and they have never seen anything like this either. The plenum is between the roof deck and suspended ceiling and there is adequate roof insulation to protect from freezing sprinkler pipr. The AHU has coils in for chilling only and the RTU heat only. There are actually (4) RTUs and three of these dump into the plenum and all are duct to either an exterior door corridor or supply 4 office vents on outside walls. Other exterior walls have electric baseboard heaters, more open office concept, in addition to the AHU supplying additional supply vents that VAVs. The thermostats for the RTUs are in the plenum. Based on the temperature in the plenum seems like a waste of energy and very inefficient. The building was constructed in 1980 and all equipment is original.

 

[chicopee]

All indications seems to point to the installer/contractor who either did not figure out his budget properly or wanted to remain competitive and consequently took a shortcut. So how is the air returned into the RTU? Also a bad place for the thermostats within the plenum.

 

[Gene M.]

It sounds like the plenum is being used as a mixing box for some reason. I would never design something like this, and it sounds like a crime against energy efficiency, but it exists so it has to be dealt with.

You should consult with a local engineer regarding code compliance, since codes can vary from location to location. The international energy conservation code has a clause that permits the continued use and maintenance of existing systems that were lawfully installed (read: code compliant) at the time, so chances are your local code has similar wording. This means you *probably* won't be forced to upgrade anything, depending on the scope of any alterations you're doing.

If that equipment really is getting to 40 years old, you might want to replace it anyway if there's budget for it. I'm a tad bias but I lean heavily towards VRF systems with ERVs, which give efficiency, flexibility and relatively low labor costs to install. A VAV system might also be appropriate depending on how it's zoned or will be zoned. What you have now sounds like a half-hearted attempt at a VAV system anyway!