Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page

# E+: Unbalanced air volume flow ratio through the heat exchanger is greater than 2:1

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 "HX AirToAir 1"
**   ~~~   ** 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: "HX AirToAir 1" 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

edit retag close merge delete

Sort by » oldest newest most voted

My understanding is that the <50% or >130% warning is because the HeatExchanger:AirToAir:SensibleAndLatent only provides inputs for heat exchanger effectiveness at 75% and 100% airflow. At higher and lower airflows, the effectiveness may vary significantly from that given. (wouldn't it be nice to have a curve, instead of a few inputs?) If your heat exchanger has a similar effectiveness at low and high airflows, this warning can be safely ignored.

The severe warning is more serious. Like high and low airflows, different types of heat exchangers will have different effectiveness with unbalanced airflows.

I would obtain the actual effectiveness of your heat exchanger at various airflow rates, and look at simulation detailed outputs:

Output:Variable,*,Heat Exchanger Sensible Effectiveness,hourly; !- HVAC Average Output:Variable,*,Heat Exchanger Latent Effectiveness,hourly; !- HVAC Average

more

Some ERV/HRV manufacturers offer calculators to estimate the effectiveness of their equipment at different conditions: RenewAire This is a web based tool. Thermotech has a stand alone software for their equipment.

more

You won't be able to put them in really: http://bigladdersoftware.com/epx/docs...

( 2016-05-10 10:45:06 -0600 )edit