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

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

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

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

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

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

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

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

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

Jay - I think that definition looks great!

Brian Frank Tue 12 Mar

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

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

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


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..."

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