HVAC Return Air Fan Coil Units

I'm not clear on the actual circuiting of the air. From where does each fan draw air, where does it blow the air, and then where does the air go after that?

The air for each apartment probably should not be mixed with the air from other apartments, because then cooking and smoking odors go to the wrong places.

The ducted fan coil unit mounted above the false ceiling of each room supplies air through ceiling supply diffusers and the return is drawn from the return air diffusers also mounted in the same ceiling. However the return air is not ducted and it is drawn from the open plenum above the false ceiling. The plenum above the false ceiling in some rooms is totally partitioned from other adjacent spaces by walls and in some rooms it is not partitioned. Besides the infiltration of smell is there any other problem of temperature control, etc., due to the possibility of drawing return air from adjacent areas due to the absence of walls?

That sounds odd to me, but I am not a domestic/commercial HVAC guy. If several apartments were running warmer than others, the mixed return air would not help the apartments that were running cooler. This effect would probably be rather small; the odor carryover would be the greater concern, I suspect.

Hi 786verycool

My experience of installations such as the one you describe is that the actual mixing of return air between the various fan coil units is not a major problem as the RA will be taking the path of least resistance. If the temperature of RA to the one unit is being affected by some air being taken from the adjacent unit's then, assuming that the temperature of the different RA not to vary more than a degree or so then the 3 way control valve should be able to respond and compensate for the slightly increased/decreased load.

As far as the walls being built up to full height this would more than likely be done from a privacy/security consideration. Yes, you would have closer control but not that much better than the ones drawing from the ceiling void.

Hope this helps.

The problem actually stems from the very fact that you state viz. a viz., the RA will take the path of least resistance. The air above the false ceiling will be easier to draw than pull the return air from the room through return ceiling diffusers. Therefore, the bulk of the air that will be go through the FCU will not be the return air from the concerned room but from the air that is sitting above the plenum regardless of which space it belongs to.

Hi 786verycool,

You are correct in your belief about the origin of the FCU's on coil air in that there is no way that, drawing from the ceiling plenum, you will be able to ensure that any of the FCU's are actually getting the bulk of their on-coil air from the same area that they are serving. However, unless these are areas which require very close control and are not being air-conditioned purely on a comfort control basis then the partial mixture of return airs should not be a major problem. If it is then you would probably have to consider separating the conditioned areas by sealing off the voids above the ceilings to achieve this.

Places where I have come across this type of installation is in hotel guest rooms (RA drawn from that area only) and open offices with the fan coil units located in bulkheads down both sides of the office (common return air drawn from the complete office).

Things that may increase the problem you may have are a) the location of the FCU's (all together and ducted to conditioned areas?) or a ceiling void that is not sealed or picking up heat load from the roofing structure.

You may be able to check the room temperature at the return air grill and compare it with the on-coil temperature at the FCU serving that room. In the old days we used to observe air flow by use of a smoke generator which used to reveal some interesting results especially when looking at discharge patterns from diffuser I don't know if these tests are still used these days?

Hope this helps you address the problem.

I am a commercial HVAC contractor. I don't see a problem. Most of the walls you refer to are probably fire rated and part of the overall design . What exactly is your question? Are you having a problem now or just trying to understand an open plenum system?

Thanks for your feedback. The problem that we are having is that out of a total of 35 rooms in the building 5 have walls only till the false ceiling and above the false ceiling these rooms are open to each other and the corridor. given the scenario that some rooms might have the windows open or the FCU is OFF in some rooms, it could impose a greater load on the coils of the 5 rooms in question, more than what the coil could handle.

I hope this clarifies the problem at hand because the Consultant is insisting that the comfort conditon might not be achievable without the walls.

A little more information would be helpful; FCU sizing for the common plenium units, room sizes, which rooms have openable windows, etc... But with the information at hand, I do not see any reason for concern.

Hi 786verycool,

I think Fredski's assumption that the full height walls may be 'Fire Walls' is possibly the reason why not all the rooms are closed off.

One question is left begging here though - you mentioned a 'consultant'.

Was this an existing design/installation, was it designed/installed by you or designed by the consultant? If the latter is true then the consultant should be aware of the advantages and disadvantages of using FCU's in conjunction with an open plenum. The problem of open windows plus FCU's that may not be switched on is incidental and should have been allowed for in the original calculations/design.

Regards, Keith

Actually some of the rooms that are without walls all the way to the ceiling are offices. If the walls were completed then the plenum above the false ceiling of rooms would become pressurized because the 100 CFM of fresh air that we are supplying to each of the fan coil unit would have no way of exiting from the room, given that these offices do not have attached toilets. Had the toilets been there, the 100 CFM of excess air would have been drawn into the toilet by the roof exhauster. So the pressurized plenum would then interfere with the return air diffuser ???

A short duct between the return air grille in the room ceiling and the fan coil unit will solve your concern. The remaining concern will be noise transmission and cross talk with an open plenum but, depending on the usage of the space, that may not be a problem. However every piece of material in the plenum will have to be fire and smoke rated. I would vote for that inexpensive piece of sheet metal duct.

Lou Bindner