Calculated Natural Ventilation and Window Operation Schedule

jteasterling · Posts: 13

Hi,

It seems to me that turning on calculated natural ventilation AND having a window operation schedule will result in conflicting signals. My questions are as follows:

1. Does calculated natural ventilation "open" and "close" windows and doors in your model based on the Natural Ventilation Setpoint (activity tab) and the Natural Ventilation Operation Schedule (HVAC tab)? Then Based on these openings it calculates infiltration based on area open, wind, pressure differences, etc?

2. If that is true does DB ignore your Window Operation %Area and Schedule and the Door %Area, %Time and Operation Schedule (Openings tab)?

In the model I am running it seems that my Openings Schedule is running the show.

Thanks,

John

AJ Lester · Posts: 90 Location: Auckland

Hi John,

My understanding (and I am more than happy to be corrected) is that the operation schedule as defined under the Openings tab establishes the availability of the windows (& doors where fitted) to open. When you change this schedule and inspect the .idf file, this is the schedule listed under 'availability schedule' for all of the windows (& doors).

That said, with the opening control mode set to 'temperature' in the idf, the temperature limits will still govern the operation of the windows.

It is, in essence an IF, AND logic. i.e.

IF operation schedule permits opening

AND Tinside > Toutside

AND Tinside > Tsetpoint

ventilation will occur. If any of the conditions above are not met, ventilation cannot occur.

No allowance is made for the operation schedule under the HVAC tab when running a simulation.

This operation schedule (HVAC tab) is used only for heating and cooling design where calculated HVAC does not apply, and the ACH value immediately above the operation schedule (HVAC tab) defines the level of outside air being introduced.

Hope this all helps you to understand the different places you can enter natural ventilation details!

If you have any questions, feel free to ask.

All the best,

Andrew Lester

AJ Lester · Posts: 90 Location: Auckland

Hi again John,

A quick clarification: I assumed you were running simulations and finding this issue, so my first response (where operation schedule on the openings tab defines availability) relate to simulations (as opposed to heating/cooling designs).

The last third of my post details the situation when running heating/cooling calcs.

Again, I hope it helps.

Best regards,

Andrew Lester

jteasterling · Posts: 13

Thanks for the reply Andrew.

That definitely clears things up. One final issue I seem to be seeing is that the Natural Ventilation Operation Schedule does not seem to be affecting the model.

I have the following:

ACTIVITY TAB
Nat. Vent. Set Point = 64 F
Delta T = -50 F

OPENINGS TAB
% Glazing Open = 70%
Glazing Operation = "on"

HVAC TAB
Nat. Vent. = checked on
Nat. Vent. Schedule = "ON may - sept, OFF Oct - Apr"

With this configuration I get year round ventilation which leads me to believe that when calculated natural ventilation is used the ventilation schedule does not run the show, the openings operation schedule does.

Thoughts?

Thanks,

John

AJ Lester · Posts: 90 Location: Auckland

Hi again John,

Sounds like I may not have been very clear in my initial post. I thought that might have been the case Embarassed

.

So, to clarify:

There are two situations to consider: Simulation mode (annual/sub-annual simulation) and Heating/Cooling Design mode (design day calculations only).

The EnergyPlus object used by DesignBuilder is the same in each case.

When in Simulation mode, DesignBuilder uses the schedule on the Openings tab to define the availability of the natural ventilation, which is then controlled by temperatures. The schedule on the HVAC tab is not used at all in simulation mode.

When in Heating/Cooling Design mode, DesignBuilder uses the schedule on the HVAC tab to define the availability of the natural ventilation. The schedule on the Openings tab is not used at all in Heating/Cooling Design mode.

This explains why your Openings tab schedule is the one running the show when you perform an annual simulation. If you perform a Heating/Cooling Design calculation, you should see the schedule defined on the HVAC tab come into play.

If you are planning on using both Heating/Cooling Design and Simulation in the same model, you should set the two schedules to be the same.

Hope that I've cleared up any misunderstanding from my earlier posts.

Best regards,

Andrew

P.S. To the DB team, perhaps this point would be clearer if the

jteasterling · Posts: 13

Yeah, that clears it up totally.

The only thing left to be answered is: why? I cannot foresee a scenario in which I would need one schedule for my annual simulations and another for my heat/cooling design. Maybe the DB guys could give one of the schedules the axe?

Thanks for your help,

John

AJ Lester · Posts: 90 Location: Auckland

Hi John,

Firstly, I made a bit of an error initially when I said the two EnergyPlus objects were the same: they aren't. As such, the heating/cooling design object doesn't make use of temperature control in the same way as the simulation object does. Apologies for my oversight.

One thought that might justify the use of two schedules, though it does get a bit tricky to find one:

If you were designing a building using mixed-mode ventilation, but wanted to check a number of different parameters for their effect on approximate energy consumption you might want to run quick cooling design calcs to check those parameters.

When you were making the accurate design model you would use the temperature controlled, sophisticated object provided by the simulation mode. You would schedule the windows to be always able to open, but then close them if temperatures exceeded a certain value. This would allow you to ensure that you made as much use of free cooling as possible, but then allowed mechanical cooling to take over when temperatures got to high.

In order to test the various parameters, without totally losing the effect of the natural or the mechanical cooling, you would want to run cooling design calcs. If you scheduled the natural ventilation (from the HVAC tab) always on, ventilation would always operate, no matter what was happening outside. This would potentially lead to problems with your mechanical cooling. Instead, using the HVAC tab schedule you could then schedule approximate operation times for natural vent and mechanical vent so that they didn't clash.

The use of a seperate schedule is not absolutely necessary for this situation (you could just reference the openings tab schedule) but it might help avoid confusion if you were trying to schedule the two methods in this way, and wanted to be able to go from one to the other without losing the schedule you used for each.

I admit it's a little long-winded and awkward, but it might be one reason for having both.

Anyway, glad I was able to help you a bit.

Best of luck with the modelling,

Andrew Lester

Andy Tindale · Posts: 1235 Location: Stroud

On the 2 schedules, actually there are more than 2!

For 'Scheduled' nat vent, the schedule on the HVAC tab is used, and all is clear.

For 'Calculated' nat vent you can enter different operation schedules for the various types of opening (internal and exeternal window, vents and doors). Do this on the Openings tab. These different schedules allow external and internal vents, windows and doors to operate at different times. e.g. you might be able to open vents at night but not windows because of security issues.

The design nat vent operation schedule and ac/h to be used for heating/cooling design can still be entered as well when using Calculated nat vent.

Hopefully this clarifies the reason for all those schedules when using Calculated nat vent!

Andy

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