Room temperature flucation problems

[remp]

Hi have a major problem with a conference room that has its own dedicated air handling unit with heating and cooling coil
The room is only about 15m x 10m. It has 6 no. side wall supply air linear grilles (built into a bulkhead all around the room) with the blade tips facing down deflecting the air into the room and downwards towards the occupanct. The room sensor is on the wall under the bulkhead so there is no direct air flow onto the sensor.
The problem is im constatnly getting calls that it is too hot, too cold....too hot, really hot, reallly cold...I am been driven mad. The designers have cleared off!

From the BMS I can see that the room sensor is holding the room ( ie. the area around the sensor at 21-23 deg C which is OK but if i look at the supply air temp from the air handling unit it varies between 14 and 30 deg C in the matter of less than an hour.

I have a feeling that it is the draft caused by the side wall linear grille onto people that is causing the discomfort, especially if it varies from 14 -30 in the space of half an hour.

Should I change the grilles to ceiling mounted diffusers. I can do this easy, i have a T bar ceiling wiht lay in ceiling tiles. I feel this is my only option and would eliminate drafts. The air will not hit people directly and the swirl type have never caused problems for me. The BMS guys says he has tried every thing from slowing down to speeding up the loops and only looks at the room sensor and says its holding

Please help, my job depends on it at this stage the MD is going to get me if i dont sort it.! Its been going on 6 months since I built it.

Thanks!

 

[Compositepro]

A very common problem. One complication is that the same solution will not necessarily work for both heating and cooling modes. In cooling mode the grills should be turned-up to allow the air to mix before contacting people. The fans should be kept on all the time to keep the air mixed.

 

[tys90]

The issue with changing the air distribution is that it may or may not solve your problem. How is the unit controlled? Can you modulate the heating coil? If so, I would modulate the heating coil so you aren't overshooting the setpoint in the room. On the cooling side, modulate the airflow instead of the leaving air temperature.

 

[BM4BRANES]

I think it could be safely stated that a perforated t-bar supply that is properly sized will out perform a linear slotted diffuser in term of flow pattern, maybe not noise. You made no mention as to velocities, windows, or proximity. Return air location matters a lot, where is it in relation to sensor or glass. also, 2 degree throttling range seems a little tight, perhaps a greater span may settle a nervous signal? Are the air change rates correct? A smoke puffer may be very revealing, supply & return patterns, room currents, influence on stat, give it a try. Air patterns, volumes, grill layout, controls/signals, valves ALL matter.

 

[ChasBean1]

It's normal for the supply air to vary like that during temperature control, as long as the room is well mixed and controls work right. Remp, too many variables my friend. This requires some onsite analysis and troubleshooting.

 

[Zesti]

Maybe you can get the "room temperature" measurement in the return airduct instead of from a wall mounted sensor.

Do you get heating and cooling in the AHU from hot and chilled water? If so, are the control valves sized correctly? An incorrectly sized control valve can cause the temperatures to vary a lot during "control actions" by the valves. Is the maximum flow (at 100% control valve) set correctly?

You do not state the number of air changes per hour you have in the room but having supply grilles in the wall, blowing air towards the occupants sounds wrong to me no matter what (unless it is meant as low velocity displacement supply at low level).

As for you last line:
"Please help, my job depends on it at this stage the MD is going to get me if i dont sort it.!"

If this is normal practice at your company, I think there is something wrong with the ethics of the company. Apparently your MD does not have the capacity or knowledge to solve the problem himself but he IS judging you in this respect! Besides, how is firing you going to solve the problem?

But this is probably something that should be discussed in a separate topic on another part of this forum.

 

[SAK9]

Just try this simple measure.Bring down down the air flow by about 20% by setting the VSD down.The throw will be reduced and the problem may go away

 

[SAK9]

I must also add that the the system seems over designed as supply temperature is hovering overa wide range.So by reducing air volume you may get it to hold steady(assuming load is constant in a conference room)

 

[willard3]

It is difficult if not impossible to heat properly from overhead. The velocity has to be so high to heat the room with the warm air that occupants have adiabatic cooling from the drafts. This is true even with good controls.

Hot air rises, put your heat source at the floor.

 

[ChrisConley]

It is difficult if not impossible to heat properly from overhead.

Regardless, we do it everyday. Keep your discharge temperature below 90F and stratification is mainly avoided.

 

[marcoh]

Are the control calibrated? The BMS sensor might think it is 21/23oC but is it correct?

Also the unit is probably well and truly oversized (ie is designed for 100% population) for most days which makes control more difficult. If it is a chilled water AHU might need to check the authority of the control valve. Simple check is to check if the control valve is the same size as the pipework, if it is the valve is probably oversized and cannot turn down far enough at low load.

 

[remp]

Hi,, thanks for all your comments.... still checking everything

At low flow rate thro the coil say 0-35% valve open the coil is hot on top and cold on the bottom them around 40% open there is a sudden balst of heat and all the coil is hot , top and bottom....seems at low flow there is stratafacation at the bottom of the coil. I have checked and it is piped correctly....Is is a faulty coil,??

between 35 and 40% valve position it is more like ON/OFF control and I think this is the problem...
Any idea on coil stratafacation??

 

[kbs]

What type of valve is being used (ball, globe, etc)? They each have different flow characteristics. Your operating valve may be approaching 100% flow at 40% open.

Bruce

 

[willard3]

snip
Regardless, we do it everyday. Keep your discharge temperature below 90F and stratification is mainly avoided.
snip

Overhead air systems work badly for heating, especially in exterior zones where surface radiant temperatures are low.

We also have evidence from the original poster that it doesn't work well.