Vav with FCU 5

[saeedplc]

Dear experts

In some projects I have seen fan coil units are used along with vav for rooms and also halls but vav can cope with heating and cooling load so, what is purpose of using fan coils with vav to do the job?
Is there any benefit of using fan coils?
Thanks

 

[Viet Mai]

Hi Saeedplc,

I think it works in priciple though it is not a common practice. I have seen some projects using VAV with VRF FCU (c/w EC fan motor) in the perimeter zones and the designers said they want better control and energy saving for the FCU fan. Honestly, I'm not a fan of this idea because the fan power of FCU is small so it's not much energy saving.

 

[EnergyProfessional]

An FCU basically is a small AHU. So if you don't want to or can't have as many FCU as zones, you can split that up with VAV zones.

 

[saeedplc]

Thank you for the replies

Does it mean using fan coils is only for sake of energy saving?
In some halss i have seen more than 20 fan coils along with 20 VAVs are used.

 

[EnergyProfessional]

FCU normally don't save energy. Their smaller fans are less efficient and normally they have very crude control and often not even flow control. You could make them more efficient, but no one will pay for 20 sophisticated AHU controls and fans.
You also multiply equipment to be serviced and things that can go wrong. They often are also located in odd locations. So you add a lot of maintenance and repair cost compared to a central AHU.

 

[saeedplc]

So with all these interpretations
what is reason of designers to employ FCU along with VAV to control ventilation of a space?!!!
There is no reason?!!!

Really I have not still been abled to find a logical reason and your knowledge and experience can clarify it.

Thanks

 

[ProcessHVAC]

"logical reason" sometimes there isn't. It was likely well intentioned by the designer, but as others have pointed out, there are better ways to do whatever the designer was trying to achieve.

 

[EnergyProfessional]

first, don't assume all designers are engineers. wher eI'm many are just whatever adn get some higher level engineer to sign off on the code-related calcs. so not everything is based on good reasoning.

Second, many design decisions are made by managers. They somewhere heard FCU is cool, but also heard VAV is cool. so let's combine them. The engineer just goes along because they want to keep their job.

One good reason I can think of is if you have some zones that have different modes (e.g. core zone with all year cooling, server room) you could serve them by a separate AHU (FCU).

You really need to look at the whole package (building, use, climate) to make a decision on type of system. There isn't a "best system" for all situations.

 

[MedicineEng]

saeedplc:

I'm just guessing here, but could it be that the VAV is providing conditioned fresh air while the FCU is keeping the space temperature?
Or the FCU is only there to address peak events where VAV cannot cater for?

In both situations I could see a case for using a mix VAV/FCU solution

I'm not a fan of FCUs whenever VAVs can be used and a couple of weeks ago there was a thread discussing this that even if not directly connected to the question, can give some insight.

Link

 

[EnergyProfessional]

It would be good to clarify if the VAV here is a separate VAV system serving the same space as a FCU. I assumed there is a FCU that feeds into multiple VAV devices.

FCU=AHU: an AHU moves air, cools/heats it and has a filter. It "handles air". An FCU does the exact same thing.

Every once a while someone floats the idea of alternatives to VAV. FCU usually is done a s a cheap alternative and it basically is a constant volume setup serving multiple zones (to stay cheap). That is inefficient and uncomfortable (multiple rooms equal one zone). Then people suggest VRF and other options. I've done a few energy simulations, and every time the VAV comes out to be the more efficient system. sure, there can be reasons to select VRF (no space for ducts, very small office etc.) or FCU. but so far I haven't seen a convincing energy simulation that shows it saves energy. I'm talking about using a very sophisticated VAV with good dehumidification that doesn't rely on reheat.

We also had old cheap VAV with broken flow stations that basically turned into CV systems and are very inefficient. but a well designed and maintained VAV system is pretty good.

 

[GT-EGR]

I would guess that the FCU is recirculating in the space so provides heating and cooling but no ventilation. Then the VAV provides ventilation air to the space, but not at a quantity near enough to cool or heat the space.

I also see this a lot in labs with high cooling loads (FCU sized for it) but low air change requirements and or the need for pressurization (VAV provides the fresh air change rate and room positive pressure).

 

[Nidbaki]

Hi saeedplc,

Some FCU's are with EC Motors, where they can replace the VFD or play the role of VFD. Hence some consultants use VAV's with FCU's to provide individual control to particular spaces in the same zone, but I personally do not recommend this solution. It is very difficult to properly control fast response by VAV's thermostats.

 

[SwinnyGG]

I also see this a lot in labs with high cooling loads (FCU sized for it) but low air change requirements and or the need for pressurization (VAV provides the fresh air change rate and room positive pressure).

This is the scenario where I've seen this type of arrangement- a space which has high conditioning demand but requires flow conditions such that the volume of air provided by the VAV to meet pressure requirements would not provide sufficient cooling. Think medical/pharmaceutical lab spaces, medical red line areas (which can be very large, up to 6 figure sq footage), equipment rooms with very high heat loads, etc.

 

[dbill74]

@saeedplc without more specific information about what you are looking at, it is difficult to say.
In a typical office building I will use a traditional VAV system with central AHU and VAV terminal boxes throughout the office areas, then small FCU/split system(s) in select spaces. Most often these select spaces are server/computer rooms where the sensible/latent ratio is drastically different from an open office, but mostly there is a timing/occupied factor. Most offices/labs/classrooms are only occupied for a portion of the day whereas a computer/server needs cooling 24/7. It becomes more efficient to run the smaller FCU 24/7 while the big central AHU only operates for 10 hours of the day, then shuts off while everyone is home for the evening.

I have seen a number of buildings where the central AHU/VAV system provides cooling only, then uses perimeter heating.

There is no one "ideal" way to condition a building; and what works well in one part of the world does not work in another. If you want to know why an engineer designed a particular way and used certain equipment, the best thing to do is ask that engineer. I for one do not mind if a fellow engineer asks why I made particular decisions and don't think there are many who would.

 

[SAK9]

A FCU system is a replacement for a VAV AHU system. So a typical VAV AHU with say 15 VAV boxes will be replaced with a 15 FCU system. There should not be any VAV boxes attached to a FCU. In response to room thermostat reading FCU speed is to be varied to control airflow to suit room load. So a VAV box has no role to play here on a FCU system.