Working Group

#609 AHU Standing WG

Jay Herron Fri 27 Apr 2018

This working group seeks to be a place of discussion and refinement for all future AHU proposals. I'm looking forward to working with everyone, and seeing all the creative ideas to extend the AHU structure.

Jay Herron Fri 27 Apr 2018

The first proposal I'd like to discuss is the Heat Recovery system defined here:

Jay Herron Thu 7 Mar 2019

I'd like to start a discussion about how we can improve the definition of AHUs. The current definition is:

equipment designed to heat or cool air.

which is extremely generic. One could argue VAVs with hot water coils are also designed to heat air, but we would not tag them with ahu.

Here are a few questions to get us started:

  • What is the defining feature of an AHU?
    • A fan?
    • Multiple chambers?
    • An outside air section?
    • Filters?
    • That air is delivered into ductwork?
    • Some combination of these?
  • Do we think the Wikipedia definition is better?

    A large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and dampers.

Scott Boehm Thu 7 Mar 2019

Yeah, definitely needs some work. As stated "equipment designed to heat or cool air" could be a radiator or a Yeti.

Don't much care for the wiki definition either.

"Fan" fits in the definition can't think of any AHUs that do not have a fan.

"Multiple chambers", do you mean chamber as equipment that has an air input and an air output chamber... that's the same as a toilet exhaust, but we're getting closer.

"An outside air section" perhaps fits, at least that would differentiate it from a toilet exhaust.

"Filters" are sometimes located remotely from the air handler unit so filters don't work.

"That air is delivered into ductwork" has merit. All AHUs that I'm familiar with have fans that deliver air into ductwork.

That leaves us with "Equipment with a fan that delivers outside air to a space via ductwork"

Still needs work, because my home has an AHU, but no outside air connection... of course that's not a commercial application.


James Donahue Fri 8 Mar 2019

I found a decent defintion from ANSI/AHRI Standard 430-2014 A central station AHU (CSAHU) is “a factory-made encased assembly consisting of a supply fan, or fans in parallel, which may also include other necessary equipment to perform one or more of the functions of circulating, cleaning, heating, cooling, humidifying, dehumidifying and mixing of air.”

I've seen commercial AHU's that do not incorporate outside air. Maybe this is just an uninformed mechanical engineer who is calling the unit an AHU when it should not be? But I would leave "outside air" out of the definition because I don't agree that all commercial AHU's have outdoor air sections.

I also agree with Scott that the delivery of air into ductwork is important, but I would add that the ductwork is typically recirculating air within a given space. This would differentiate an AHU from something like an exhaust fan or a supply fan.

John Petze Fri 8 Mar 2019

I think this additional work to extend the AHU tagging models is great and needed.

I also want to suggest that the AHU WG submit to get on the Haystack Connect 2019 schedule for a live meeting per our earlier invitation here

It will provide a great opportunity to discuss these topics in person and drive further work.

Just submit via the Call for Speakers link on the site

Scott Boehm Tue 12 Mar 2019

Good idea John. Jay as champion, can you coordinate this? I'm not a member but would be happy to participate.

Jay Herron Tue 12 Mar 2019

Thanks John, I've registered the AHU working group for a live meeting at Haystack Connect.

Great points Scott and James! My thoughts

  • It's interesting ANSI specifies "a factory-made encased assembly", because many AHUs are custom-built rather than factory assembled.
  • I agree with Scott about leaving the outside air section requirement out of the definition, as I have seen return-only equipment that otherwise seems identical to other AHUs.
  • I also like the fact that the Wikipedia and ANSI definitions call out things the AHU can contain or can do - I think that alone helps differentiate them from a simple supply fan.

Combining a few of the different ideas, I came up with the definition below. What do you guys think?

A large metal box with a fan that delivers air to a space via ductwork and performs one or more of the functions of cleaning, heating, cooling, humidifying, dehumidifying, or mixing the air.

Scott Boehm Tue 12 Mar 2019

That sound good to me. A bit concerned about the term "large metal box" perhaps "An enclosure with a fan..."

Jay Herron Tue 12 Mar 2019

Haha yeah, that was the part I was least comfortable with Scott. Revised to:

An enclosure with a fan that delivers air to a space via ductwork and performs one or more of the functions of cleaning, heating, cooling, humidifying, dehumidifying, or mixing the air.

James Donahue Tue 12 Mar 2019

Jay - I think that definition looks great!

Brian Frank Tue 12 Mar 2019

While you guys are working on this, can you please also consider the definitions for unitary equipment:

  • fan coils
  • heat pumps
  • unit ventilators

And maybe re-examine terminal units such as VAV. And what others are we missing?

For example, does that definition for AHU you propose make a clear distinction b/w an AHU and FCU?

We are also introducing the notion of subtyping in Haystack 4.0, so we also need domain experts to define the taxonomy/tree structure of these equipments. For example should we create the concept of a terminalUnit and/or unitaryEquip that groups more specific equipment?

Jay Herron Wed 13 Mar 2019

That's a great lead-in Brian, because how AHUs relate to fan coils and unit ventilators was what I was actually interested in with defining AHUs. Let's attack FCUs first.


The current haystack definition:

Fan coil units are unitary heating/cooling assets which use the zone itself for supply air.


I'd like to qualify that I'm less technically familiar with FCUs than AHUs, so please forgive me if I ask stupid questions.

  • Can FCUs have input airsteams in addition to zone air? Perhaps discharge air from an upstream AHU?
  • Is an FCU a type of AHU, or are they mutually exclusive? That is, could we use the fcu and ahu tags simultaneously on a piece of equipment?
  • If they are mutually exclusive, what is the distinguishing feature between them?
    • Do AHUs deliver air into ductwork, whereas FCUs deliver it directly into a zone?
    • FCUs are unitary, and AHUs are not? How do we reconcile that with the rooftop tag?
    • FCUs are sourced by zone air? Are there not return-air-only AHUs?
    • Are FCUs always located within the zone, whereas AHUs are remote?
  • If they are not mutually exclusive, what makes an AHU an FCU?

Dana Miller Tue 26 Mar 2019

Hi - I've been following Project Haystack's work with interest.

If you are not already familiar, it may also be helpful to know that there is a glossary of HVAC-related terms maintained by ASHRAE through TC 1.6 (Terminology), that includes terms like AHU, heat pump, fan coil, and can be slightly different than the ANSI definitions above.

I've seen cases where it looks like a definition needs to be updated or made more precise (eg radiant is defined as radiating rays of light), but it's an existing public glossary of over 3,000 terms that will presumably be maintained over time.

For what it's worth, their AHU definition has:

"assembly consisting of sections containing a fan or fans and other necessary equipment to perform one or more of the following functions: circulating, filtration, heating, cooling, heat recovery, humidifying, dehumidifying, and mixing of air. Is usually connected to an air-distribution system."

Scott Boehm Thu 28 Mar 2019


Thanks for pointing that out. Think that we got pretty close to the ASHRAE definition with a few less words. I'm in favor of less verbose definitions as long as the context is equal or improved.

So for FCU our ASHRAE brethren have:

FCU - fan coil uni, factory-made assembly that provides the functions of air circulation, cooling, heating, or cooling and heating.

That definition works for me. Seems that the distinguishing factor is the lack of air distribution system.

heat pump ASHRAE has the following for heat pump - thermodynamic heating/refrigerating system to transfer heat. The condenser and evaporator may change roles to transfer heat in either direction. By receiving the flow of air or other fluid, a heat pump is used to cool or heat. Heat pumps may be the air source with heat transfer between the indoor air stream to outdoor air or water source with heat transfer between the indoor air stream and a hydronic source (ground loop, evaporative cooler, cooling tower, or domestic water).

Probably gonna have to whittle on this one a bit :-)

Then we have...

unit ventilator - fan coil unit package devised for applications in which the use of outdoor and return air mixing is intended to satisfy tempering requirements and ventilation needs.

That's agreeable as well, however a bit verbose. Think that we could perhaps delete "...devised for applications..."

Alan Jones Sun 28 Apr 2019

Could you explain to me, why a rooftop AHU is different to any other AHU. Here in the UK we do not distinguish between an outside AHU and an inside unit. After all the AHU might be on the ground rather than the roof. I appreciate it will need to be weather tight but that’s the only difference. Alan Jones BMS consultant.

Scott Boehm Mon 29 Apr 2019

As I read the "rooftop" tag it is used in conjunction with ahu to further define the equipment being described. As you state the difference between the two is that one is a weatherized version, which I feel is appropriate. Indeed there are cases that an RTU is not mounted on the roof, but it is nonetheless a weatherized package. Does it make sense to drop the notion of "rooftop, ahu" and replace with "weatherized, ahu"? For novices this maybe a reasonable description and term, but the majority of us would think it odd and use the term rooftop.

I'm of the opinion that the "rooftop" tag is a bit shortsighted. There's a lot of equipment that we can find on a "rooftop" that are not associated with "ahu". I looked at the "geo..." series of tags which are related to lat/long but not level. Tag "level" doesn't fit either as it's associated with fluids.

I can't seem to find a series of tags that describes the floors and/or areas of a building, which to me seems to be a necessary tag.

Stuart Longland Mon 29 Apr 2019

How about outdoor? Also indoor… so something mounted on the roof or otherwise exposed to the weather is tagged outdoor, and something that is largely sheltered from the weather is indoor.

Could work for lots of things (e.g. outdoor temp).

Ross Schwalm Tue 30 Apr 2019


Check out the public review site for Haystack 4.0 and look at the space def ( There will be a series of defs (tags) that describe the floors and areas of a building in Haystack 4.0.

Jay Herron Thu 9 May 2019

Hey, sorry for being MIA for the past few weeks. There's been some good discussion here, and I've reworked a few of the suggestions into the proposed definitions below:

  • ahu: An enclosure with a fan that delivers air to a space via ductwork and performs one or more of the functions of cleaning, heating, cooling, humidifying, dehumidifying, or circulating the air.
  • fcu: A unitary, factory-made enclosure with a fan that delivers air directly to a space and performs one or more of the functions of cleaning, heating, cooling, humidifying, dehumidifying, or circulating the air.
  • uv: A unitary, factory-made enclosure with a fan that uses outdoor and return air mixing to satisfy tempering requirements and ventilation needs.

Most of my questions revolve around the unit ventilator definition:

  • How do we differentiate between ahu and uv, or is one a sub-category of another?
    • Is it that uv is unitary and ahu is not?
    • Is it that ahu has a downstream ductwork system and uv does not?
  • How do we differentiate between fcu and uv, or is one a sub-category of another?
    • Is it that uv has outside air and fcu does not?
    • Or that uv lacks heating/cooling coils?

Finally, defining a general heatPump tag seems wider than the scope of this discussion, since the term "heat pump" spans airside and waterside systems.

Brian Frank Fri 10 May 2019

Please see airHandlingEquip taxonomy for what the current proposal entails.

There needs to be a general term for the "base class" of these things which seems to be a very elusive term. I picked airHandlingEquip for now as a place holder.

The definitions need to correspond to how the taxonomy is organized (which is also just a proposal). For example what is common to all of them? What specifically makes a unitVent different from a FCU? Note based on current feedback I've make unitVent a subtype of fcu.

Joseph Firrantello Fri 10 May 2019

(With apologies, as I doubt I have the bandwidth to maintain consistent engagement in this discussion, but felt the need to chime in. Tried to scan the thread to see if I was repeating anything.)

  1. Either fcu or unitVent could be subsets of the other. It might be worth considering that some energy modeling software tends to put both under "unitary equipment", which I admit isn't the strictest definition itself.
  2. Unit ventilators may have coils for conditioning air, and (news to me) looks like energy recovery now too? Example of a unit ventilator. (Full disclosure: I have no association with the manufacturer, this is just what popped up.)
  3. I would offer that the tag unitVent is preferable to uv. With uv, I see future confusion with ultraviolet germicidal irradiation (UVGI) equipment that is used in certain AHU applications.
  4. I like ASHRAE's AHU definition, but my first thought was of recirculating filtration units used in some healthcare spaces. I think they can be ducted (don't quote me), but they are a fan and a filter with no conditioning. One could make the argument that it "handles air", but there's part of me that would cock an eyebrow at using the phrase "air handling" while describing them. Then again, that's more based on what I'm used to from industry than anything else.
  5. Agreed that heatPump is a separate item. Heat pump applications can vary in size from unitary to full AHUS (whatever that means) to water-to-water. It's more of a cooling/heating conditioning option in the same lane as defining whether something is served by DX, gas heat, hot water, chilled water, etc.

Jay Herron Tue 28 May 2019

Alright, thanks for the input guys! It was awesome to discuss these topics with all of you that came to Haystack Connect 2019.

I've finalized a proposal, and listed the proposed definitions below:

  • ahu: Air Handler Unit: An enclosure with a fan that delivers air to a space via ductwork and performs one or more of the functions of cleaning, heating, cooling, humidifying, dehumidifying, ventilating or circulating the air.
    • mau: Makeup Air Unit: An AHU that brings in 100% outside air and no recirculated air.
    • rtu: Roof Top Unit: A unitary AHU that is weatherized for outdoor use.
  • fcu: Fan Coil Unit: A unitary, factory-made enclosure with a fan and coil that recirculates air in a space and performs one or more of the functions of cleaning, heating, cooling, humidifying, dehumidifying, or ventilating the air. Unit heaters are examples of FCUs.
    • unitVent: Unit Ventilator: An FCU that has direct access to outside air to provide the space with ventilation.
    • crac: Computer Room Air Conditioner: An FCU designed to condition a computer room.

These definitions appear to align closely with the ASHRAE Terminology and most community interpretations.

If you have strong feelings or issues with these definitions, please speak up. Otherwise, I think we can call this working group complete.

Jacqueline Walpole Wed 29 May 2019

Just a small (perhaps slightly pedantic) suggestion: in English, AHU was originally an abbreviation for Air-Handling Unit often known simply as an Air Handler but not generally Air Handler Unit which is a mashup of the two.

Richard Seaman Wed 29 May 2019

Hi All,

I want to echo some of the concerns raised already around the use of rtu:

rtu: Roof Top Unit: A unitary AHU that is weatherized for outdoor use.

What it's really trying to capture is if the unit can be exposed to the elements. Perhaps this was discussed at the meeting, but why use Roof Top when it could apply to any exposed location?

Scott Boehm Wed 29 May 2019

Yes Richard we did discuss it. I believe that we resolved that location could be determined by tags related to specific floors of a building (ref: Haystack 4). It was pointed out that some applications have "RTUs" on the ground level. We decided that rtu was appropriate because it is an understood type of equipment that is being described.

Jay Herron Thu 30 May 2019


AHU was originally an abbreviation for Air-Handling Unit

Great point! Thanks. This will be changed.

@Richard & @Scott

Scott is exactly right - thanks for speaking up. Since "rooftop unit" is so widely used in the industry and typically denotes the unit design more than the physical placement, we determined that it should be reflected here.

There have been some concerns about using the rtu abbreviation, as it may be confused with a "remote terminal unit" from the networking world. I find the rooftopUnit tag name to be a decent alternative, but I'm interested in others' thoughts.

Richard Seaman Thu 30 May 2019

@Scott @Jay, thanks for the clarification. Completely agree on location being determined by other means.

Using "rooftop unit" to describe an AHU that isn't on the roof just seems odd to me. To be honest I wouldn't really use the term at all, I'd just call it an AHU (maybe prefixing with "external" or "outdoor"). It's probably down to the fact that the normal industry terms may change depending on where you are in the world. My experience is in Ireland and the UK. I note that Alan Jones, from the UK, also questioned it above.

Apologies for not inputting sooner, but when I saw it included in the finalised proposal I wanted to flag it. I appreciate that a wider audience has already discussed and accepted it, so will not push it further.

Cory Mosiman Fri 31 May 2019

Hey Jay - would the rooftopUnit be camel case or utilize the new conjuncts? rooftop-unit might be problematic because of the unit def...?

Jay Herron Fri 27 Sep 2019

Hey Admins, I believe this working-group's work was completed with Brian Frank shortly after Haystack Connect this spring. Can we close it? Thanks!

Login or Signup to reply.