Mechanical Extract - No impact on internal room temp?

[NKhan]

Hi All,

Hoping someone may be able to shed light on the issue below;

I'm currently modelling a Classroom, which was initially intended to be 100% naturally ventilated via openable windows. In order to test this, I have run the following simulations;

1 - Openable windows with no mechanical ventilation, Result = slight overheating.
2 - Openable windows with mechanical extract only, Result = EXACT SAME AS ABOVE.

I don't quite understand why there is no impact by the presence of mechanical extract ventilation.

Is it correct to now, add in an "infiltration" in the form of natural ventilation matching the rate of extract, to simulate the air movement? When this is added in, the temperature drops to below permitted levels.

So my question is, is this the correct thing to do? Or "fudging" the results!!

Any info will be much appreciated.

[PCully]

Hi,

It's not so much fudging the results as using the software to model what is going on in your building, knowing what inputs to use, what results you are looking at etc any software relies on what you put in...

When you say you specified mech extract I assume you defined this on the system tab of room data by ticking the box and specifying the SFP. This doesn't explicitly model the air into the space, only the energy usage to reflect that.

The most appropriate way to cover this in conjunction with the MacroFlo nat vent would be using ApacheHVAC but there are many users not at the level (and it is a massive step up) to properly use this so your other option would be defining the extra air as an Air Exchange and then we have the effects of the extra air coming into the space and you can define the flow rate. Only note of caution here is that the air exchange isn't modelling the actual air coming into the space, only the effect it has on say room temperatures. What I mean is it isn't going to have any impact on the MacroFlo air which is being calculated based on conditions inside and outside the room and the openings you have defined. Just so that is clear I always find it worth mentioning.

Phil

[NKhan]

Thanks for your quick response Phil, that makes sense... Unfortuanatley, I am not yet competant at using ApacheHVAC, nor do we have the facility so will have to go down the "infiltration" route.

Much appreciated.