Ventilation: Compact vs. simple

espenlok · Posts: 21

I have a question regarding the difference in output values between the compact and simple modes for modelling the ventilation system.

When using the simple mode, I get a output value called "External Mechanical Ventilation", which I assume illustrates the amount of energy "lost" in the ventilation. If using compact mode, there are no such value, and I cannot see that this value is reflected anywhere else among the "fabric and ventilation" data. That is very confusing for me, since the ventilation system normally is one of the major energy losses in the systems, and really should be reflected somewhere.

I also wonder how much energy is used to heat the ventilation air compared to how much energy is used for space heating. This is necessary to know according to the new Norwegian standard, which I think is based on a European standard. My idea is that 'zone heating' is what I call 'space heating', and then I micht calculate the 'ventilation heating' as 'Heat Generation'-'Zone Heating'. However, this is just possible in the compact mode, as 'zone heating' does not exist in simple mode.

I am also confused because both in compact and simple mode, it is possible to choose heat recovery for the ventilation equipment. However, in simple mode, it does not seem to be possible to define how many percent heat recovery there are.

Espen

Laurent_Support · Posts: 292 Location: Vincennes

Hi Espen,

Outputs data are linked to what is done in Simple and Compact mode. Let's me try to explain what do each ones in few words.

Simple mode : add hot/cool air to the zone (Purchased Air in EnergyPlus language). No HVAC system is simulated so no option concerning the system as recovery is available.
The consequence is that natural, mechanical ventilation are directly displayed with thermal zone balance. Heating/cooling is one of the thermal data in this balance.

Compact mode : a HVAC system is modeled (unitary or multiple). Many options are then available.
Outputs are displayed differently because the HVAC system heat external air and then supply it to the zones. Concurrently natural ventilation, infiltrations will come directly into the zone without passing through the HVAC equipment.

This is to explain why you could see external air entry in the zone thermal balance and not external air coming from the HVAC system.
The energy needed by the system to heat the outside air to the zone setpoint is included in the graph below (as Zone Heating if I well remember its name).

Of course this data is the total of fresh air heating and reheat necessary to fight heat loses through surfaces.

Please have a look to the help for more details.

Regards,
Laurent

espenlok · Posts: 21

Thank you for the clear-up regarding the difference between the comapct and simple modes. This difference has not been clear to me when reading in the help system. I would claim that it is confusing that we can choose different types of HVAC systems (here I talk about the HVAV templates) (incl. heat recovery and VAV systems) in the simple mode, when these are not modelled in this mode. Maybe this should be specified somewhere in the simple-mode HVAC screen?

However; my main question in the last post was why there are no value for 'External Mechanical Ventilation' in the heat balance in the compact mode. This should definetly be one of the main energy losses in the building, so I think it should be included among the other heat losses (also if it is included somewhere else in the output).

Although I now understand that the HVAC system is not modelled in detail in the simple mode, I would think that the results should be more or less the same in simple and compact mode when using the template values without any modifications. However, this does not seem to be true.

To show an example: I have used the simple, rectangular building as I attached in my other thread (about glassing). When comparing the results from the simple and compact modes, there are a few significant differences.
1. In simple mode, about 9,5 kWh more energy is needed per m2 for heat generation (gas) than in compact mode (yearly simulation).
2. At the same time, more energy for the chiller (electricity) is necessary in compact mode.
3. In simple mode, there is a loss from 'external mechanical ventilation' at about 11,5 kWh/m2.
4. There is also a difference in 'Zone/Sys sensible heating' and 'Z/S sens cooling', (which follows from the above differences).
5. For the rest of the output values, there are just small differences.

One possible explanation for the above listed differences is that the heat losses from the ventilation air is not included in the analysis in the compact mode. Can that be possible???

(BTW, the background for these questions and the other questions regarding glassing is that I am trying to compare the results from DesignBuilder with the results from another simpler model (and energy demands based on experience), and I try to understand why there are some huge differences here. The main 'problem' is that the energy needed for heating in the DB simulations are much, much, much lower than expected, and as long as I am not able to understand why is that, I cannot use DB for any useful purpose.)

Laurent_Support · Posts: 292 Location: Vincennes

Hello,

In Compact mode, zone thermal balance and system energy consumption are displayed separately.
The zone thermal balance is focused on gains/losses and not take into account system heating/cooling energy because some system are unitary AHU for a whole building. In this last case you can't break down AHU consumption between the zones.

About your model, may I suggest to check that :
- plant efficiency are the same for Simple and Compact simulation (take care of distribution losses you can enter in Compact mode)
- in v1.4, you can choose the area used to divide energy value : all m2 or only those in occupied zone

Well, this is my first ideas. I hope it will help.

Regards,
Laurent

espenlok · Posts: 21

Andy Tindale · Posts: 2432 Location: Stroud

One of the weaknesses of the Simple HVAC modelling mechanism is that it does not take into account heat recovery. For this reason heat recovery data is not displayed on the HVAC tab in simple HVAC mode.

I take your point that when you load an HVAC template called 'VAV with HR + outside air reset' you would expect both heat recovery and outside air reset to be modelled even if you are running in simple HVAC mode. As you have probably figured out by now this does not occur.

DesignBuilder could possibly give an approximate modelling of heat recovery systems in simple HVAC mode by reducing the mechanical ventilation air flow rate consistent with the percentage sensible heat recovery requested.

I will look into whether this is practical.

In the meantime, I would recommend that you use compact HVAC if you wish to model heat recovery systems.

Regards,

Andy

espenlok · Posts: 21

Hi Andy!

Thank you for your response.

Yes, I would like to use compact modelling as this give me a lot of extra opportunities. However, in the same time, there seems to be this problem that heat lost through the ventilation system is not accounted for in the simulation.

I have attached a file from the heating design calculation for my building. This shows that the main energy loss is through the mechanical ventilation. I have attached another file showing a screen shot from the simulation screen for the same building. Here you can see that the heat loss from the ventilation system has mysteriously disappeared. I think this may give a large mistake in the calculations.

Note also that the total amount used for heating this building is 3,5 kWh/m2, which is less than for DHW. That does not make any sense. The average temperature outside is 6 degrees (and down to -15) and we have 22 degrees inside (19 during nights and weekends). This needs a lot of energy, in particular because of the ventilation system that takes out all the warm air from the room. I would expect more than 23.5 kWh/m2 (based on calculations from another program).

espenlok · Posts: 21

Hi again!

I have now made a lot of simulations in my litlle shoebox, and my theory that heating of ventilation air is not included in the calculation is now skipped. It seems like the problem is somewhere else, and I'll try to find it.

However, I still would like the value to turn up in the heat balance, and it would be great if space heating and ventilation heating was separated in the energy budget, because that is necessary when setting up an energy budget according to the Norwegain stanard (which we think is also a EU standard).

Espen

Andy Tindale · Posts: 2432 Location: Stroud

Hi Espen,

You can generate extra data on ventilation loads by including some extra reports in your simulation. The process is:

1. Download the attached Reports.idf file and save to the EnergyPlus folder on your PC

2. Select this file from the Simulation options dialog (as shown in the screenshot below) or from the Simulation tab of the program options dialog.

3. Run the simulation. This includes the IDF reports in the file and generates an extra report in html format in a file called eplustbl.htm in the EnergyPlus folder.

4. When the simulation is finished, view the file eplustbl.htm in a web browser. I think the data you need is called ZONE MECHANICAL VENTILATION HEATING LOAD INCREASE [J]

The process is quite easy and you can tailor the report as you need with reference to the E+ documentation.

I hope this helps.

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