VAV vs FCU system

[mabed]

I'm working on a ten floors office building, and the A/C system is chilled water air handling units in first and roof floors with VAVs (about 35 VAV per floor). I am checking if it will save money and installation time to change to chilled water fan coil units since my chilled water riser already going to roof. I just want to know if anyone did this exercise and what they got.
And one more thing in general isn't using FCUs instead of VAV better to save space, material and easier for control.

 

[willard3]

Fan-coil units are more expensive to install than VAV systems.

 

[DRWeig]

How will you ventilate the spaces?

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[mabed]

I have fresh air handling unit and ducts connected to each fan coil.

True, but you save in ducting, dampers and AHUs

 

[Drazen]

mabed, it seems that you will have to rephrase your question for us to understand it.

how do you save in ducting if you yourself said that you already have ahu and ducting that will supply fresh air to fcu's?

from what you described you are replacing vav's with fcu's, and, using the same duct system, that means you will have fcu's, that are more expensive than vav's plus the whole hydronic system to supply fcu's. in such circumstances, fcu's are much more expensive than vav's and i am failing to find different view of it if you do not provide more clarifications.

 

[mabed]

Fresh air duct is relatively small compared to supply/Return air duct. I need 10 Tons FA duct vs. 35 Tons supply and return ducts. With VAV I need 14 AHU; with FCU I only need 2 for fresh air. As for chilled water network like I said before it is already going to roof so I'll replace the duct network in each floor with chilled water network which is cheaper to install and need less labor hours.

 

[cry22]

mabed

I take it that these FCU's will be in the ceiling space, think of maintenance and noise, not to mention water leaks in occupied space.
Consider security also, you do not want maintenance dudes all over your offices.
AHU per floor, no disturbance to occupied space during maintenace, can take place during working hours.
Think of access panels over hard ceilings, low ceiling space, existing obstructions, relocation of electrical conduits and fire protection piping, structrural beams in the way, etc.. AND... the change orders associated with it.
You cannot use DCV with FCU system as opposed to VAV where you can have VAV system on an ERU supplying the AHU's.
FCU's will require multiple control valves to install, program, Commission, and maintain.
Multiple fans to power, each with dedicated circuit breaker from a control panel.
Imagine the number of filters you have to change every three months. speaking of filters, with FCU's you get swiss cheese filters (20% efficient) as opposed to 85% plus filtration with AHU's. Better IAQ with AHU's and less sick people.
FCU's over 2000 CFM will need smoke detectors and connections to FACP.
Your have a large building, look up ASHRAE 90.1, must use VAV system over a certain size.

I HATE CEILING MOUNTED FCU'S.

 

[cdxx139]

You can make an argument either way.

Noise - you try to place FCU in common or corridor spaces
Water Leaks - Dont exterior VAV boxes need reheat? Those can leak too.
Security - You cant think the maintenance people are going to steal stuff (what about the cleaning crew?)
AHU per floor - just try taking up floor space needed from the architect or developer
Access - office building will probably have mostly drop ceilings
DCV - Of course you can, add a VAV damper on conference room FCU (yes more money)
Control Valves - You will have 2 control valves vs just 1 (see water leak comment)
Multiple circuts - true
Filter change - true, maintenance concern.
Smoke detector - true, but I max my FCU selection at 2k to keep noise down.
90.1 - if its a DOAS system you cant run in VAV or you will drop below minimum ventilation (although a VFD drive is a good ideas, if not just for balancing)

If the existing VAV duct system can be reused, I cant believe changing to 4 pipe FCU will save you money. IF the maintenance of the building is lacking, then stick with VAV.

If its a long time client, then yes, comfort is a big factor and you can suggest 4-pipe FCU, if they have a good maintenance crew. And understand they are getting a better more comfortable system, but more upfront and maintenanxce costs.

Im not challenging anyone, just trying to show their are arguemnts either way and the OP needs to review them all. Go throuh 2012 ASHRAE Systems Ch1 with client to help with the decision.




I guess there are many wqays to skin a cat.

knowledge is power

 

[317069]

Mabed
- you said in your question that you work on a project( seems existing project), but what are you doing? redesign forexample.
- did owner want to change whole his A/C system or what?
- if the owner want to replace whol his system then simply prepare tow option for him and let his contractor tell him which one is less expensive and you can consult manufucturer for equipments performance and operation cost for both systems too.

 

[urgross]

Sounds a lot like an induction system as opposed to a fan coil system. Are there any requirements with minimum air changes, or meeting heating/cooling loads with only the OA?

 

[cry22]

Sorry CDX, I disagree

Noise - you try to place FCU in common or corridor spaces
Often times, FCU's in corridor ceilings result into obstructed valves, motors. AND YES, them FCU's are very noisy and there is nothing you can do about it, and when a tenant complains, Mr. Engineer is nowhere to be found.

Water Leaks - Dont exterior VAV boxes need reheat? Those can leak too.
Most water leaks come from condensation at the condensate drain pans, traps, piping, not in pressurized piping.

Security - You cant think the maintenance people are going to steal stuff (what about the cleaning crew?)
Yup, security is an issue in some spaces.

AHU per floor - just try taking up floor space needed from the architect or developer
We do this as standard, never had problems. Most engineers do not educate the architect, they just assume that the architect will complain. In my experience, I have never run into an arhitect refusing to give 600 to 1000 SF of mechaqnical space for AHU. Mechanical rooms are rentable space prorated per tenant similar to toilets and corridors.

Access - office building will probably have mostly drop ceilings
Try to service or replace a ceiling mounted FCU with or without drop ceiling and let us know of the downtime.

DCV - Of course you can, add a VAV damper on conference room FCU (yes more money)
And what will slow down the ERU Fan? without a VAV box throughout. Air not going to the Conference room will go elsewhere.

Control Valves - You will have 2 control valves vs just 1 (see water leak comment)
do your math - you're looking at 20 CV per floor on average.

90.1 - if its a DOAS system you cant run in VAV or you will drop below minimum ventilation (although a VFD drive is a good ideas, if not just for balancing
With AHU, you can have a VAV box at each AHU for DCV as opposed to DOAS.

Save for Hotels, Ceiling Mounted FCU should be banned from regular office space. see ASHRAE 90.1 Appendix G3.1.1.A - over 25000 SF, you need VAV system, not CV.
In can buy into floor Mounted FCU along the perimeter windows on the FLOOR, but in ceiling, nahh

 

[cdxx139]

cry, you make some vaild points. You seem to have more experience in the installation/maintenance of FCU and issues that come up.

Either way, there are PROS and CONS to each system, and the OP needs to take them all in a count, in his decision.

knowledge is power

 

[317069]

- What the difference between AHU and fan coil unit?
if we have an existing AHU with duct system, can we remove this unit and install a fan coil unit that gives the same parameters such as cfm, static pressure ..Etc,
- ASHRAE 90.1: when we say 25000 sq.ft, do we mean the area of one floor or area of one floor times number of floors? And do we mean this area is covered by on system or more than one system, for example:
- if we have a 5 floors building, 5000 sq.ft/floor, do we
consider it as a 5000 sq.ft or 25000 sq.ft ?.(Forget about
Numbers of floors mentioned in 90.1 for now)
- if we have a 25000 sq.ft floor, and this floor is served by
10 CV RTU's each one has its own duct system and is
Separate from others, do we consider it as a one system or
10 systems?.