FCUs vs. VAVs

[MedicineEng]

An interesting discussion came the other day when checking new design drawings and design assumptions for a potential new event center that we will hopefully build somewhere in the near future.
When evaluating HVAC options for BOH areas, 2 factions emerged: The FCU faction and the VAV faction.
I'm leaning more to the VAV faction for the following reasons:

-VAVs are typically less preventive maintenance intensive (no cleaning of drip trays, coils, etc.)
-The less chilled water pipes we have running across false ceilings, the less probabilities of leaks/condensations, and the like

So my approach would be to use VAVs whenever feasible and reduce FCUs to the special situations where VAVs couldn't be used (low height ceilings, lack of physical space to run ducts)

A point of contention was which system is the most energy efficient for the same duty and after a few good arguments, I don't think that we reached a final verdict

To which side of the discussion would you guys lean to ?

 

[EnergyProfessional]

I would avoid systems where you need to distribute many AHU (this is what an FCU is) throughout the building. They are noisy, bulky, have smaller less efficient fans and motors. And for ventilation you need ducts anyway. May as well upsize the duct a bit and make it VAV. You also still need an AHU that acts as DOAS to treat the OA.

And to save energy, you still need some sort of ventilation air control damper, metering etc.

Ask maintenance what they prefer. I probably depends on how many zones you need. 50 cheap motors vs. one commercial grade large motor etc.

 

[MedicineEng]

EnergyProfessional:

Thanks for your feedback.
I already asked the maintenance guy, which in my case is very easy because it's myself ).
I'm on the VAV field as my past experience mirrors many of the issues that you mention, but I'm not a fundamentalist on this.

But either way, I didn't want to close the door to any positive argument on FCU side as I'm sure that in some circumstances, these might have advantages in regards to VAVs.

 

[LittleInch]

It's one of those horse for courses issues.

For large open plan areas then VAV comes into its own.

For smaller spaces which may or may not be occupied point local cooling comes into its own and can be more easily controlled and turned on and off as required or adjusted more easily to suit either the load or the individuals.

that's also where efficiency is called into question and needs defining.

Why cool a large open space when only smaller areas are actually occupied?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EnergyProfessional]

I don't understand why size of space should matter in the selection. If anything, VAV devices can be had for smaller airflows than FCU. You can start and stop flow with VAV.

A VAV zone requires a modulating damper and a flow station. An FCU (mini AHU) requires the same but also needs a fan, fan control a cooling coil, a control valve, discharge sensor.

You also have to consider most FCU aren't built to AHU standards. They have really cheap crap installed. You couldn't afford a real AHU for each zone. So nothing in an FCU lasts long. there are better FCU, but the smaller size still comes with a shorter life than the large AHU and most projects don't use premium quality FCU.

Fan-powered VAV already are a pain to maintain. I wouldn't want to have a dirt-cheap AHU in each zone. Where we have those cheap FCU they often don't use stainless condensate pan and you know here the water ends up after the pan corroded....

I think 90% of the decision should be based on longevity and maintainability. Rest is efficiency and the tiny fans definitely don't win the efficiency race. Where do you actually locate those FCU where they are accessible for the lot of maintenance they require? Are you ready to flush the condensate lines in every office annually? How do you deal with noise? How do you replace them later? You know, after the sprinkler guy installed his pipes underneath the unit and it located above someone's desk?

There is more detail to a system than just saying VAV or FCU. Also depends on how long you keep the building and if it is yours. If you plan to tear it down next year, go with the cheapest.

 

[nuuvox000]

I know this isn't a "good" reason but I really like using fan coil units because they take up less space. I like designing and I hate coordinating ceilings/structure, it's really boring. So if it's ever a toss-up, I'll always choose fan coil units.

Here's a better reason though: VRF fan coils can be much cheaper than a VAV system a lot of the time, especially for smaller projects. I just did a small office building with VRF and DOAS and it would have been much more expensive to do a VAV system. With the VRF, we got rid of the boiler, coils, hydronic piping, balancing, controls, etc. that go along with it. Piping and pipe fitters' labor is very expensive. Nobody really does electric heating coils in VAV's around here because gas is so much cheaper than electric.

If you're talking hydronic VAV's vs FCU's, then I would prefer the VAVs because then you only have motors/filters on the roof and it's easier to access. It's also really nice because if you want to add a zone later, you don't need an electrician to bring power to it, we can just run our low voltage wiring to the actuator and we're done.

 

[EnergyProfessional]

Stating what climate, type of building and size for system (#zones, tonnage) would help. A 3 room office in Saudi Arabia will benefit from a different system than a skyscraper in Canada.
Also would be good if any other fixed conditions exist. Do you get free waste heat from a powerplant, or have natural gas (or none) or have extremely cheap or extremely expensive electricity?
What sophistication and ability does maintenance have and what parts and manufacturers are available? For VRF you deal with a lot of proprietary stuff that will be a black box from outside.

All this could make one recommend one system, or a completely different one. It is like the question what is the best vehicle - but not knowing if we do long haul trucking, or commuting in NYC.