ROOM ACH RATES, VAV

[Rayohray]

ASHRAE 170-2008 (medical facility) requires exam rooms to have an ach rate of 6.0 minimum. With VAV system the VAVs will adjust damper position and CFM by temp, and load. Does the ASHRAE standards declare that the minimum amount ACH in each exam room be no less than 6.0 at all times? Also, are air balancing and controls commissioning checking this? If buildings and systems are older and cannot achieve this rate, what exactly happens?

 

[willard3]

I don't know where you are, but I would check your local hospital HVAC code for this information

 

[immsk]

I'm not familiar with the ASHRAE standard referenced but do have experience evaluating ACH rates. You should clarify if the ACH refers to ventilation rates or simply ACH (total supply air to the space).

With regards to VAV's ramping down in response to load - the engineer will need to provide minimum and maximum air flows for the VAV boxe. These settings will be programmed into the VAV controller. You'll need to confirm if the VAV min flow rate will not lead to over cooling or over heating of the space. You may need to specify reheat coils or do some type of supply air temp reset to keep things comfortable during low load conditions.

In my experience, balancers don't typically verify ACH's they simply verify if the VAVs are delivering the max specified airflows. You can also ask them to confirm if the VAV's are able to measure airflows at both the high and low end of the range. The controls contractor has some factors they can adjust on the VAV controllers to improve the accuracy of measurements based on feedback from the balancing contractor.

I work mostly in retrofits and most ASHRAE standards have specific sections on what applies and what doesn't apply when trying to work within the constraints of an existing mechanical system.

The client should be informed upfront what can be done to bring the system in line with the standards and what the costs will be.

 

[Tumpis]

Rayohray you've touched on a common problem. If the controls contractor is paying attention they will lock set the minimum damper position of the VAV to maintain the flow required for the 6 ACH. That is when the room is occupied. Hospital requirements are really difficult to meet. Even your average HVAC person often doesn't grasp how quickly that volume of air adds up and how large your ductwork must become, and then the fan to push that amount of air.

For your final question about what happens if you cannot achieve the rate, the answer varies based on how on top of things the facility engineering team is and how nice of a hospital you are at. I'm willing to bet even high quality hospital systems have rooms that don't meet the code. Most exam rooms don't get tested after initial setup and we all know mechanical systems are always changing. Although if you know you have a problem you have a liability you should certainly address.

Sometimes you can fix things by rebalancing the whole area. Something else to consider is the exam room only needs 2 of those 6 changes to be outside air. Meaning you might be able to find a way to recirculate air within the space depending on how your air handler is setup.

 

[cdxx139]

Air Balancers and controls contractors are not checking the design, they are executing the design.

Last time I did an exam room, we used an AIA guide, and it was 2 ACH of ventilation air and 6 ACH of total air, with no VAV during occupied hours.

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