I have a bunch of HeatExchanger:AirToAir:SensibleAndLatent in my model that are placed on DOAS.
I'm getting a lot of combination of a single warning and single severe:
** Warning ** HeatExchanger:AirToAir:SensibleAndLatent "HEAT EXCHANGER AIR TO AIR SENSIBLE AND LATENT 26"
** ~~~ ** Average air volume flow rate is <50% or >130% of the nominal HX supply air volume flow rate.
** ~~~ ** Air volume flow rate ratio = 12301.169. Environment=RunPeriod, at Simulation time=01/02 07:30 - 07:40
** Severe ** HeatExchanger:AirToAir:SensibleAndLatent: "HEAT EXCHANGER AIR TO AIR SENSIBLE AND LATENT 26" unbalanced air volume flow ratio through the heat exchanger is greater than 2:1.
** ~~~ ** ...HX Supply air to Exhaust air flow ratio = 3.04734.
** ~~~ ** Environment=RunPeriod, at Simulation time=01/02 07:30 - 07:40
In this specific case, a lot of my zones have zonal exhaust (fume hoods, chemical cabinets, etc), so I do expect that at time (always in some zones) I would definitely have less or significantly less relief air than outside air, since it's actually how it was designed[1].
The Engineering reference does mention the following:
The operating volumetric air flow rate through the heat exchanger (i.e., the average of the actual supply and exhaust air flow rates for the simulation time step) should be between 50% and 130% of the nominal supply air flow rate specified for the heat exchanger. Operating air flow rates outside this range result in a warning message and a recommendation to adjust air flow rates to within the appropriate range.
[...]
Air flows through the heat exchanger may be unbalanced (supply greater than exhaust, or vice versa), but an unbalanced air flow ratio greater than 2:1 is not recommended (beyond this range a warning message is issued).
For the second, I'm actually getting a Severe rather than a Warning, something I will have to explain to my reviewers.
Is this only to warn users that weren't careful when setting up their model? Can I safely ignore this? How can I justify it does not negatively affect the result of the model?
[1] whether that makes sense from a mechanical standpoint for a chem lab to not have a dedicated make up air supply for the fume hoods is another question that need not be addressed here
