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# Heat Exchanger with autosized flow rate generates warning about flow rate

I have a HeatExchanger:AirToAir:SensibleAndLatent object that is generating this error.

** Warning ** HeatExchanger:AirToAir:SensibleAndLatent "DOAS HEAT RECOVERY"
**   ~~~   ** Average air volume flow rate is <50% or >130% of the nominal HX supply air volume flow rate.
**   ~~~   ** Air volume flow rate ratio = 0.446. Environment=MIAMI INTL AP ANN HTG 99.6% CONDNS DB, at Simulation time=01/21 00:00 - 00:15

*************  ** Warning ** HeatExchanger:AirToAir:SensibleAndLatent "DOAS HEAT RECOVERY":  Average air volume flow rate is <50% or >130% warning continues. Air flow rate ratio statistics follow.
*************  **   ~~~   **   This error occurred 830 total times;
*************  **   ~~~   **   during Warmup 0 times;
*************  **   ~~~   **   during Sizing 0 times.
*************  **   ~~~   **   Max=0.499836  Min=0.438976


Looking in the engineering reference,

but the Nominal Supply Air Flow Rate field is autosized.

Is there a recommended way to fix the error?

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When the HX is in the outdoor air system and the air loop operates at a flow rate less than the autosized outdoor air flow rate then the amount of outdoor air is reduced to match the air loop flow rate (since the OA can't be higher than the air loop flow rate). At this time the amount of air through the HX can be less than the nominal HX flow rate when autosized.

You could either manually size the nominal HX flow rate to something less than what is used now so that the average is within 50% and 130%, or you could adjust the outdoor air controller setting to not allow the minimum outdoor air to go lower than some value.

Another option is to disregard the warnings since they only mean that the effectiveness values entered in the HX may not be accurate at those conditions. It is unlikely that the effectiveness with change dramatically at flow rates outside the range of 50% and 130% but beware that it is possible to have some change in effectiveness.

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