[EnergyPlus_Support] HVAC modelling - Key Temperatures

[Ana Oliveira <anarui.oliveira@xxxxxxxxx>]

Hi again.

In my (Compact HVAC) Fan Coil model, when I edit the expanded idf files, there seem to be several user-defined Design Temperature variables that can be independently set, apparently with posible inconsistencies, although no errors or warnings are issued regarding that aspect (for simplicity, I did not define any internal gains or infiltration, and outside air rate is set to zero):

- I realize many of the following quastions would probably be answered if I could decode the relationships between the various nodes in the model, but I find it reather obscure to draw that from the expanded file...

1. I identified 3 sets of design temperatures, under, respectively - Design objects (Plant sizing), Boiler (water oulet temperature), Coils - but it seems to be that the algorithm ignores all inconsistencies (inlcuding, for instance, Boiler outlet Temperature being lower than Coil water temperature) and that the Temperatures under Design Objects override all other user-defined design temperatures; is that it?

2. In my model, the water temperature at the oulet of the boiler is always equal to the outlet temperature of the hot water plant loop; is that equal by definition for all models, or only in this simpler model?

3. The return Temperature of the hot water plant loop, however, shows a constant difference to the inlet temperature to the boiler (0.055ºC). Why? In auto-sizing mode, is this temperature driven by the design loop Delta T in Design Objects?

4. The inlet temperature to the heating coil is, by definition, the same as the outlet temperature from the hot water plant loop, right?

5.. Does it result from the above conditions that the eventual thermal losses between the boiler and the zone units are not modelled? can they be implicitly modelled by diferent user-defined temperatures (boiler outlet and coil inlet temperatures?)

6. The DESIGN Temperatures are only relevant when auto-sizing? (flow rates "take over" when user-defined?)

7. If I set the outside air rate to ZERO, what might be making the "Zone/Sys Sensible Heat Rate" have a non-zero value when the boiler (and chiller) are off (this happens in night hours in both winter and summer week, when the thermostat schedules are set to such temperatures so as to require no heating or cooling)?

8. In auto-size mode, it sets the Air inlet Temperature to the Cooling Coil equal to the Cooling setpoint temperature of the zone (24ºC).. Is that reasonable?

Thanks again for any enlightenment.

Regards,
Ana Rui.


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