Natural ventilation, results, high increase of heatig values

Hi all

I have a question regarding the results when I set natural ventilation

First result without natural ventilation

when I add Natural Ventilation of 12C min_out_temp_ the heat consumption is way higher.
Also the infiltration is raised. That doesn’t make much sense to me.
Actually infiltration should drop or even disappear during ventilation, doesn’t it?

Also tried 21C temp to minimize ventilation which makes the graph look like without ventilation but it is not affecting heating very much.

single_family_energy_model_upd.gh (166.7 KB)

Maybe I’m missing something here or is there a way to avoid this model “reaction”?

Kia ora

The issue with ‘Natural Ventilation’ and ‘Infiltration’ I can only guess at, but I suspect that it is the same issue that you have found with the heating energy going up as you institute openable windows as a cooling strategy: as far as I can tell - and I want to have a grad student test this more thoroughly - what happens when the energyplus model is running is that if the indoor temperature is above your setpoint, then the window opens, and, depending on the ventilation model selected, the volume of outdoor air provided by the window opening size and wind pressure coefficient enters the room at say 12C! As EnergyPlus is an hour by hour calculation this could mean significantly cooling the room so it now needs heating.
What I want a grad student to check is the relationship of this to the E+ timestep. As I understand it, the energy balance being redone at, say, 6 times each hour (the default), will mean that the really cold air will only be brought into the room for 10 minutes if the indoor temperature drops too drastically. I would try your model with a timestep of say 30 times per hour (2 minute intervals) to assess the effect on cooling and heating…
Do tell me what you find

Hello @MichaelDonn
thank you for your thoughts on this topic. They correlate to what I already observed in several simulations.
During summer(warm)time something is triggering a strong heat demand when natural ventilation is used for cooling.

But before I follow your suggestion I have to clarify a mistake I made in the first post.
I just plugged in a single floor in the simulation without ventilation, therefore the result in the first chart was to low.
Here is the right one


meaning there is no distinct rise in infiltration.

First I went the opposite direction just because it is quick and to get a feeling where this might ends.
I set a timestep of 1


Looked to me like it basically supports what you think @MichaelDonn although the raise in peak power consumption is really high.

So I thought with a timestep of 30 the pendulum might swings as much in the other direction.


Strangely that’s not the case

And not really far from the timestep 6 result

So it looks like there is still something missing. I think what you described (partially cooling and reheating) should be part of the equation because it happens in reality and is for sure significant in spring and autumn conditions. But in the climate used in this simulation there is in reality for sure no (or almost no) heat demand in July.

I’m looking at the Berlin climate file in your GH script, and your heating demand in July makes perfect since once you plot the exterior drybulb temperature in relation to your setpoints.

You can see in the graph below that the July exterior drybulb (red line) goes far below the heating setpoint for your program of 21.7C (blue line), almost all of which you are ventilating indoors with your minimum outdoor ventilation setpoint of 12C (purple line). Even with the temperature increase from internal heat gain, that is going to trigger your heating system. For reference, I added the program cooling setpoint (green line) to illustrate where your target indoor temperatures need to be in order to avoid triggering the mechanical cooling and heating.

So all you need to do is fix your ventilation setpoints relative to your heating/cooling setpoints (i.e. bring the minimum outdoor temp to ~22C), and you’ll get the energy reduction you’re looking for.

@SaeranVasanthakumar I can understand there will be times when heating is required, even in the depths of a Berlin summer. However, I am puzzled by the recommendation that the minimum outdoor temperature should be ~22C. This is suggesting that only when the air outside is almost as warm as the air inside, will I open the windows to cool the interior? This does not seem sensible? What am I missing in your logic?

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You’re right, it should be lower, to whatever the actual balance point temperature is. For a modest residential building I suspect that balance point temperature is going to be close to the 21.7C heating setpoint, but I could be wrong.

@SaeranVasanthakumar thank you for your reply and ideas. Am I right you refer to this equation?
image
I calculated it and if I got it right the result is in this case roundabout 17C in July.


Still more or less the same.

I think this is a mayor point here. Usually you can be sure in a Berlin climate heatings are switched of anywhere latest from mid June until August/September. The massive buildings keep the heat gains easily until next morning. Usually there are no ventilation systems in use, cooling is done only by natural ventilation. But this doesn’t seem to work out same way in E+.

@Martin6

Yes, that’s one version of the balance point temperature, but keep in mind it’s just a guide as its still a summarization of a timeseries, so it’s not surprising it won’t give you zero heating/cooling for all of July.

I think it’s a lot clearer to simply plot the temperatures against the setpoints, and see where your zone air is activating heating. I’ve plotted temperatures for just July below to illustrate the problem. Green = Exterior air temperature, Purple = Zone air temperature, Light Green = Cooling setpoint at 24.4C, Red = Heating Setpoint at 21.7C. The separate graph below the temperatures shows the heating load (red line).

July - No Ventilation


Without ventilation (just ideal loads), you can see there’s definitly some influence of midday internal heat gain that is keeping the zone air higher then the exterior air temperature, but despite that your heating load in July is much larger then your cooling load. Which means that there’s not a lot of hot temperatures to cool down in the first place.

I didn’t look at your energy balance closely enough and missed this, but the main problem is really just the heating (system) dominated summer.

July - Ventilation Minimum Outdoor Temp = 12C


The too-low minimum ventilation setpoints are, unsuprisingly, just compounding the heating in July.
Here’s the ventiltaion with a 12C minimum ventilation temperature. The zone air is at the heating setpoint the vast majority of time. At colder exterior temperatures, the zone air actually goes lower then the zone heating setpoint. Caveat: this is with very minimal parameter tuning (shutting down some windows, reducing operation area, adding a room minimum temperature setpoint), but there’s so few cooling days I don’t think it matters.

Yes, that’s the first thing you need to fix then. It sounds like you may need to massively increase your internal heat gains, and insulation to reflect the building condition you are looking for, maybe even find a more representative EPW. One parameter that seems particularly unrealistic to me is the default heating setpoint of 21.7. That’s high for european residential occupants using natural ventilation, since I think they should tolerate a broader range of temperatures.

But I don’t think there’s anything unusual in the heating and natural ventilation loads as it is currently, that’s just the logical outcome of your current EPW and BEM combination.