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[EnergyPlus_Support] Re: air to air heat exchanger

The equations for sensible and total heat recovery in the 
Engineering Document are incorrect (i.e., the temperature difference 
is not shown correctly, page 248). In the program, the sensible heat 
transfer is being calculated as:

(mdot,supply) * (Cp,supply) * (Tsupairout - Tsupairin)

Similar for total heat transfer: (mdot,supply) * (Hsupairout - 

In winter the supply air exiting the heat exchanger is warmer than
the cold outdoor air, so the sensible heat exchange is positive. In 
summer, you generally get negative sensible heat exchange (when the 
outdoor dry-bulb temp is higher than the relief air temperature).

Your point is well taken that the "sign" of the heat transfer should 
always be positive. We'll address this issue in the next release of 
the program (possibly add separate output variables for when the 
outdoor ventilation air is heated and when the air is cooled).

As stated in the documentation, the current air-to-air heat exchanger 
object models heat transfer between the outdoor (supply) and relief 
(exhaust) air streams whenever it is scheduled to operate 
(availability schedule) and when air flow is present. Additional 
control options will be available in the next version of EnergyPlus 
(e.g., suspend heat exchange based on outdoor air or relief air 
temperature, enthalpy, etc. to prevent overheating the supply air 
when zone cooling is required ---> economize).

You state that "When the relief air enthalpy is greater than outside 
air enthalpy, the heat exchanger should be turned off."  This is true 
if zone cooling is required (e.g., summer), but is not true when zone 
heating is required (e.g., winter).

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "zhaicq98" <chaoqinz@a...> 
> Hi,
> I have an air to air heat exchanger, which transfer sensible and 
> latent heat between the outside air and building relief air. When I 
> output its heat recovery energy, lots of nagative numbers happen in 
> summer, while in winter, most are positive numbers.
> In my opinion, there should not be any nagative number. When the 
> relief air enthalpy is greater than outside air enthalpy, the heat 
> exchanger should be turned off. Is it correct?

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