Air Changes an Hour Confusion

[emalsyd]

We are doing some work at a small private school. They've asked us to go through the school and determine the ACH in each classroom and if they are getting the required 4-6 ACH as suggested by the CDC based on the new Covid Requirements for schools.

The document they gave us lays out the standard ACH formula. As an example they use a room 6883 cu/ft.
760 cfm of supply air. 20% outdoor air =152 cfm OA so 1.3 ACH which doesn't meet the 4-6 ACH.

Then they mention adding supplemental ventilation or filtration as options to increase the ACH. As their example, they can't add more ventilation but can use MERV 13 filters. They then use this calculation.

760 cfm
20% OA 152 cfm
recirculated air 760-152=608 cfm
Clean recirculated air through MERV 13 608*.8 efficiency = 486 cfm
486 recirc air + 152 cfm OA= 638 cfm

ACH (638cfm * 60/hr)/ 6883cu/ft=5.6 ACH

I've never seen the portion where they use the recirculated air through the filters for the ACH calculation.
If the school currently has MERV 8 filters, can I use 35% in the formula?

Whenever calculating ACH in the past we've always just used the supply air being supplied to the room in the calculation. Should I be doing it using this formula they are using?

 

[EnergyProfessional]

IMHO, ACH is the air that changes through the room regardless of if that is outdoor or recirculated air.
So if the diffusers deliver 760 cfm, this is what you use.

 

[MintJulep]

Is this really 6 air changes per hour?

 

[LittleInch]

That's an interesting definition of ACH.

What exactly does CDC say about the air being used? That is is fresh outside air or what? The CDC issues such a document for hospitals etc but I can't see one for schools -

https://www.cdc.gov/infectioncontrol/guidelines/environmental/appendix/air.html#tableb1

I'm having difficulty in believing that partially removed potentially virus ridden air can be pumped back into the room it came from or spread around the school / building from one room to another.

From everything I've read simple recirculating A/C systems are not great at removing potentially COVID filled air from a room because most is re-circulated.

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

 

[EnergyProfessional]

ACH definition is not necessarily based on virus removal (or any healthcare requirements). Is is a requirement to exchange air in a room. there is diversity in a building and recirculated air is not 100% used up (i.e. not at 3000 ppm of CO2 etc.). Imagine a building with 10 rooms, even if one room has some higher levels of CO2 or other noxious gas, the recircualted air is diluted. Room with production of noxious gases (bathrooms, cleaning supplies etc.) typically have an exhaust requirement and are not allowed to recirculate.

What CDC or other authorities make with this, is another story. In case of virus that potentially could be in the recirculate air, they may focus requirements on OA requirement, or filtration or UV treatment of recirculated air.

 

[LittleInch]

And I never knew schools had A/C... they don't this side of the pond for sure.

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

 

[3DDave]

This is a tough problem to resolve. The desire for energy efficiency has certainly to be at odds with ejecting potential pathogens present in the air out of the school. The knowledge at this point of the particles per volume below which infection cannot spread are not well established but reaching zero is unrealistic with any recirculation.

Were it mine I'd include electrostatic precipitation and UV exposure**, but there still remains the problem that there is no quantization that is meaningfully measured.

As important is to provide a supply-return path that carries air to and then from individual students so no one is downwind, so to speak. Airflow control has been seen as a factor in hospital settings where changing the direction of airflow can halt disease spread.

On a related topic - due to the decreased use of large facilities, such as schools, certain bacteria have been having unopposed growth. Particularly Legionella bacteria aren't being flushed with chlorine and have been found in drinking water in re-opened schools. I expect similar organisms and fungi have been enjoying the lack of air circulation and are ready for a restart.

**and of course that can create ozone. If it's not one thing it's another.

 

[emalsyd]

I will add, we installed bi-polar ionizers in all the rooftops units. Looking on the ionizer manufacturers site, they have an article and an excel sheet explaining and calculating ASHRAE 62 Ventilation Rate Proceduce and Indoor Air Quality Procedure.

Reading through the article, it appears the ventilation rate procedure VRP is somewhat similar to the calculation used by the CDC above.
Example was a classroom. Outside air is 10cfm per person & .12cfm per sq ft. If ventilation effectivness is .8 (which I discovered is an ASHRAE 62 Table 6.2 Zone Air Distribution Effectivness) the outside air required to the space would be as follows for 30 students and an 800 sq ft space

(10x30+.12x800)/.8= 495 cfm of outside air

The IAQP states the outdoor air can be reduced because the air in the building is being cleaner through the ionizer and so more air can be re-circulated. I need to play with the excel spreadsheet to see how it works.

 

[AdrienneGC]

ACH does not equal the ventilation rate. It is just the measurement of how frequently all the air is exchanged in the room. The ventilation rate can also be given as ACH or cfm/occupant... They are two different requirements and both need to be met. Here is a calculator for the air change rate

Adrienne Gould-Choquette, P.E.

http://www.adicotengineering.com

 

[emalsyd]

Thanks for the clarification and calculator.

 

[ProcessHVAC]

The definition of whether ACH is 100% outside air or just air movement depends on the standard. Some NFPA standards for petroleum facilities and wastewater treatment facilities define ACH as air changes per hour of 100% outside air. It is also the same for some healthcare standards. Most other guidelines for less hazardous buildings use ACH simply for air movement in spaces and ventilation to define outside air. I would check and clarify whatever standard you are following, most school standards I have seen use it for air movement but since now it is being use for pathogen control (i.e. healthcare) it might not be the case anymore.

 

[GT-EGR]

Haven’t read that standard, but air changes mean different things for different space types. For a chemical lab it wants to quantify the amount of outside/fresh chemical free air is being introduced. For a clean room it is indifferent to outside air and wants to see how much HEPA filtered air is being moved through the room.

If I had to guess, the CDC is saying that air through a merv 13 is basically 80% cleaned of all virus, hence the 0.8 factor. It counts outside air as 100% cleaned of all virus, hence there is no factor (it’s 1.0).

You probably need the whole standard and if it’s a good/real one, one that is worth following it had to explain itself.