Make-up Air Unit and Exhaust Fan

[trueman]

Hi Everyone,

I am working on a diesel vehicle storage building in Manitoba. The client wants to meet Green Globe Standard (another version of LEED). That's why we have a gas fired make-up air unit that comes with Supply Fan, Return Fan, Heat Recovery and High Efficient Furnace. This system is for ventilation only and can run up to 100% O/A.

In addition to MAU, we have also purge exhaust fans that are controlled by CO and NO2 in the vehicle storage.

My question is this: Do we really need those exhaust fans and why or why not? The part confusing me is that if we have a makeup air unit that can go up to 100% O/A, the return fan functions like exhaust fan as long as CO and NO2 dictate the MAU to run 100% O/A.

 

[MintJulep]

A simple schematic would be so helpful....

 

[trueman]

Hope this can explain...

 

[GT-EGR]

Multiple potential reasons:

Is your MAU relief fan the same CFM as the purge fan? If not, then the purge fan is selected for a higher value to handle the emergency condition.

Where are your intakes on the MAU return/relief? Carbon monoxide is barely lighter than air, so while your MAU is in the better spot for it, your EF may be located closer to the source, or have better ducting?

Is your MAU always on? How is it controlled? If it isn't setup to override to 100% outside air with relief to match, then it won't do the purge it would otherwise be capable for.

If you have limited emergency power, supplying it to an exhaust fan is a much lower load than a 100% outside air makeup air unit. So this could be the reason.

The purge fan could also just be there as some sort of redundancy if the MAU goes down, or if the MAU fails to reduce the concentrations.

 

[chicopee]

Do either a flow rate but from my standpoint a mass rate is preferable between E/A,S/A and EF. If mass rate of E/A plus S/A equals S/A then yes EF is needed. If E/A equals S/A then EF is not needed. I am assuming that the heating unit is not direct fired, otherwise, CO, CO2 and H20(vapor) could be a problem inside the structure. Also in the calculation you may want to include air infiltration.

 

[trueman]

Hi all,

I think I figured out after reading all the comments. My exhaust fans are totally redundant because my S/A = R/A and I don't think I need them as back up.

One question to @chicopee ; why would direct fired unit heater be a problem? Because of the condensation and gas emission?Yes, there will be filtration for sure.

Thanks!

 

[chicopee]

Direct fired unit will discharge products of combustion with the air flow into the area. If that's the case then you have to consider CO2 poisoning of the people and the carbonic acid (CO2 and H@O vapor mixture) long term effect on people and equipment.

 

[chicopee]

Rereading your post and I am thinking that diesel exhaust may be a problem particularly during winters within the work area and the EF was installed to dissipate the diesel fume and banking on air infiltration as the fresh air make up.

 

[ProcessHVAC]

If the system is balanced at the HRV the EFs would be normally off. Typically we would design with HRV running on normal operation and the exhaust fans to relief air if CO or NO2 are detected in the space, but then the EFs would have a separate intake louvre with an interlocked motorized damper..didn't see that in your sketch.