calculated ventilation when Tin<Tout

Donnie · Posts: 8

Hi all,

The building I am modelling works with diurnal, summer ventilation, even though the outside temperature is higher than the inside one (this, for humidity and air velocity concerns).
On the other hand, I need to use the calculated ventilation option, since I don't know much about the ventilation rates.

I couldn't find any way to do this with DB. I tried almost everything I could think of, with no success.
So my question is:
Is it possible to do it with DB?
If not, is it possible to do it with EnergyPlus?
If not, does anyone have any idea about how to deal with that somehow?

Thanks alot in advance.
(I'm using version 1.5.0.076.)

Donnie

Laurent_Support · Posts: 292 Location: Vincennes

Hi Donnie,

If you're using calculated natural ventilation and don't see any air exchange, you should check :
- activity tab (ventilation setpoint)
- advance tab from Model Options (openings modulation rules)
- openings schedule and opened area %

On natural ventilation, DesignBuilder offers all features E+ has. So the problem is more How to model what you want.

I hope this will help, regards

Donnie · Posts: 8

Hi Laurent,

You seem to mean that, when calculated ventilation is checked, DB should model the ventilation even if T_in is smaller than T_out. Right?
A colleague of mine was telling me the same, so I guess I'm doing something wrong.

Below is an example:
Case one: let's take a simple building. The natural ventilation is set to "calculated", and works all the time (schedule "on"); openings are 50% open all the time as well; wind factor is set to 1 and opening modulation rules are unchecked. The setpoint for "cooling by natural ventilation is set to 0.
(Results are attached)
As you can see, it ventilates all the day long. That's OK.

Case two: te same, but i just add a large shading component block over the building, so that the inside temperature falls below the outside one (results are attached).
Now, each time T_in is lower than T_out, the natural ventilation stops. I just don't understand why! It seems that E+ only allows ventilation for cooling.

Is that a limitation from E+?
If yes, how can I overcome it?
If not, what did I do wrong?

Please find my DB file attached.

Thanks
Donnie.

Donnie · Posts: 8

Oops.
It seems I can't attach more than two files.
So, here are the results for case 2.

Regards,
Donnie

Laurent_Support · Posts: 292 Location: Vincennes

Hi,

lklawrie · Posts: 76

I don't know if you can control what you want within DB but EnergyPlus allows a lot of flexibility in bringing in natural ventilation.

Donnie · Posts: 8

Hi,

Laurent: I've already set the temperature setpoint at 0 °C. But this is not sufficient, since, as you say, this is for cooling. In my case, the aim of natural ventilation is not to cool down the room (and, actually, it tends to heat it instead).

Iklawrie: Yes, actually, I would be highly surprised if E+ couldn't model this kind of ventilation. But i can't find a way to make it do it.

What I've figured out so far is that calculated natural ventilation works when:
schedule for ventilation allows it AND schedule for opening allows it AND T_in>T_setpoint AND T_in > T_out.

The first three conditions are OK, since you can easily define what you want. But the last one sounds arbitrary.
How can I get rid of this condition?

(The more it goes, the more I feel like I've done a simple, stupid error somewhere when defining my model; something like checking or unchecking a box unintentionally.)

Donnie

lklawrie · Posts: 76

See if you can set the lower temp limit to -100C (that's the minimum for EnergyPlus).

If you need to run EnergyPlus directly, you will need to export your file and run from there.

bhandari · Posts: 89

Donnie,
I did some investigation to find out the answer to your question, Is that a limitation from E+?

The answer is - Yes, the current ventilation setting in E+ is for cooling only. It is in the wish list for heating.

Even though DesignBuilder only allows the control based on ( T_in > T_out) in the current version (Ver 2.0 of DB will include additional choices) ,there are several choices in E+. You can find these choices in the field Ventilation Control Mode in the object AirflowNetwork:Multizone:Surface. Please look at the section of AirflowNetwork:Multizone:Surface in IO Reference (pp 668).

Mahabir

lklawrie · Posts: 76

And to clarify, so you don't think I am totally off base.

I was talking about the VENTILATION object in EnergyPlus whereas DB/Natural Ventilation triggers AirFlow Network simulations.

Donnie · Posts: 8

Hi,
Sorry for the late reply, I was working on another, time consuming, project.

Anyway, thanks for your replies.
And, particularly, thanks, bhandari, for the hints you gave.

BUT
I haven't solved the problem.
It's getting better, though, but i still don't get what i want.

If I set the control parameter to "constant", then it doesn't consider the outside temperature to define the openings' area (which is good), but I can't define the schedules i want (which is not good).
That is, when defining a (fraction) schedule : 0 means that the opening is closed, but 0.05 or 0.5 or whatever else than 0 means that the opening is fully open (to the value I set in the openings tab - opening's area %).

To sum up, and make it clear:

Donnie · Posts: 8

Hi,

Did I say something wrong? Did I offend all the designbuilder/energyplus team by something I said?

So, since I didn't get any answer, I guess that what I want to do is not possible. But could someone tell me why there is such a restriction in the energyplus code?
I can't see where the problem comes from:
-Schedules are correctly handled when the temperature is lower outside than inside
-Calculation works OK (even when the outdoor is warmer than the indoor) when the "constant" control option is set.
So why isn't it possible to combine both features?

I really would like to know, since I spent a lot of time on this and would not like just giving up without understanding.

Thanks,
Donnie.

AJ Lester · Posts: 86 Location: Auckland

Hi Donnie,

I am no expert when it comes to these things, but from a bit of a read of your problem and the E+ handbook I have found the following things that may be of interest:

The fact that you get no modulation in your control may be related to the fact that the AirflowNetwork that is used by DB relies on temperature modulation parameters to define the extent to which air is allowed to enter windows at any given time. These parameters are set up using the advanced tab in model options which then passes these variables to the airflow network components. What I found by exporting a trial idf file is that these variables are not passed to the airflownetwork:zone object. The zone object suggests that it controls the modulation of all windows attached to that zone and the default parameters are set to no modulation.

This, as I say, is only explaining one possible reason why there is no modulation in the control of your windows. I would suggest trying to modify the parameters in the idf file for the airflownetwork:multizone:zone object. What may work for you is setting up temperature parameters there that allow modulation.

However, another suggestion is this. This may not work at all and I may have completely mis-interpreted the functionality of the various parameters, but you could try the following:

In the airflownetwork:multizone:zone objuct, try setting control to constant and then define the schedule to control your openings at 10/50% etc.

Then, in each of the airflownetwork:multizone:surface objects, mirror the same parameters (constant and the same schedule name).

This could be what you're looking for if you aren't interested in modulating to maintain control, but rather in having constantly open windows at a certain level.

Where this perhaps differs from what you may have done so far is that you also have to change the zone object.

No idea if this will work or not, but give it a go. Export the idf file, change the parameters as necessary, and if it all works out ok, copy the relevant sections and include them in the 'include this idf code' section in DB.

Apologies if you've tried all this before, I'm no expert on these matters.

As always, all the best with the modelling!

Andrew Lester

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