How to Model AHU with DOAS in DesignBuilder?

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When will v2025 be officially released? Unfortunately, our company does not allow installation of any beta software.

We expect to release v2025.1 by the end of this week, i.e. by 3 Oct. Please look out for the release on our download pages. We will publicly announce the new version sometime mid-October.

Hi Mr. mdedoes,

It is far more complicated than the thing you have mentioned. An AHU is serving multiple zones and a single DOAS is connected to multiple AHUs. Unfortunately I don't have a diagram to show the same.

Can you make the most closest representation of this scenario?

OK, that helps. To make it easiest on yourself, I would model the system as if each AHU had its own DOAS unit and include all components for both the DOAS and the AHU in the AHU object in DesignBuilder. Based on your comments to other users, you can put both a DX coil and a chilled water coil in the same AHU and control them using different setpoint managers. You'll just have to make sure that the setpoint manager for the DOAS coil is immediately downstream of the coil in the model.

Hope this helps!

Did you mean immediately downstream of DOAS coil and immediately upstream of the chilled water cooling coil? Also, do you recommend adding an additional fan too? I am not sure whether DB allows that

You may have to mess with the locations for the setpoint managers (I always have to do it a bit by trial and error), but I'm pretty sure you need one immediately downstream of the DOAS coil. The location for the setpoint manager for the second coil may be flexible.

And no, I don't think DB will let you put 2 supply fans in, so just enter the pressure in DB as the sum of the pressure of both of your fans.

As you have mentioned trial and error here to replicate the closest model, I also wanted to know whether you have tried the heat recovery option in AHU? The final I/O summary report provides the same energy consumption whether you add a heat recovery wheel or not. Any thoughts on why this happens?

Yes, I've put in a heat recovery wheel in the AHU successfully many times. Check that the heat recovery availability schedule is On. Check that the nominal supply airflow rate for the wheel is correct. Check that your effectiveness values are greater than zero. Check the heat recovery bypass control type to make sure you're not always bypassing the wheel.

Do you get any consumption in the heat recovery row in the I/O summary report?

In order to get consumption in the heat recovery row, you have to have a non-zero value entered for "Nominal electric power" in the heat recovery input. This input is meant to account for the power of the motor that spins the wheel. However, even if this is entered as zero, you should see your total energy consumption go down if heat recovery is active.

Can you kindly elaborate on the nominal supply air flow rate for the wheel( I generally provide the dehumidified air flow rate as calculated in the heat load summary sheet), effectiveness value( whether we have this as input in DB and how to use the option) and heat recovery bypass control type(again do we have this option and how to use the option)?

The nominal supply airflow rate will probably be how much outside air from the DOAS you expect to go to your AHU (I think you're saying the same thing with the dehumidified airflow rate). The effectiveness value would be determined from the data sheet for the DOAS or the wheel (sensible is usually around 0.7, latent is often closer to 0.6). The bypass control type will depend on your unit controls, but usually I've almost always used "1-Bypass when within economizer limits".

In India, we almost never use an economizer as the outdoor air is never cool enough to be utilised. May be I can try with other options available. But thankyou for the help. I have never tried these options before.

That first option should still work. If the simulation never goes into economizer, then it will never bypass the wheel.

Hi Mr.mdedoes, I was actually trying to incorporate the details given by you for heat recovery. There are effectiveness values provided for part air flows (75% & 100%) and by default sensible is 0.75 and latent is 0. Also, there is a frost control type which is by default 1-None but has options like Exhaust air recirculation , exhaust only and minimum exhaust temperature. Also, a threshold temperature of 1.7 deg C is provided. Can you elaborate the terminologies and how do we use it further for getting actual savings due to HR.

If you're not familiar with the DesignBuilder help website, this is the address: https://designbuilder.co.uk/helpv7.0/

If you search "AHU data", you should find the page with descriptions about all the AHU inputs.

Designbuilder does have a default template as DOAS,DX precool with Heat Recovery but that does not meet my requirements for AHU with DOAS. As the DOAS is serving to precool the air before it enters the AHU and gets distributed to the spaces, do you recommend using 2 cooling coils ( 1 DX coil and another Chilled Water Coil) to closely resemble the scenario?

2 cooling coils ( 1 DX coil and another Chilled Water Coil)

That's quite different from DOAS + AHUs, you know.

Answers are shown on this page. You can model with EnergyPlus, IES-VE, or DesignBuilder v2025 as Andy mentioned. I haven't tested DB v2025.

Before AirLoopHVAC:DedicatedOutdoorAirSystem was added to EnergyPlus, I used to model DOAS + AHUs with DOAS + Variable-speed FCUs in EnergyPlus. However, DesignBuilder v7 can't even model variable-speed FCUs. I'm not sure the reason you stick to DB. I recommend using EnergyPlus directly for your better understanding of energy simulation.

Our organization is using DB for almost a decade now I guess. Thats the reason we have still stick to DB and finding ways to mimick the closest representation to this. I am not sure how much will DOAS as AHU and AHU as an FCU inside the zone will closely replicate the situation as wrote by Mr. Mdedoes