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CAV air system with constant temperature

asked 2015-12-23 07:39:36 -0500

BasP gravatar image

updated 2015-12-24 16:18:05 -0500

I'm struggling for quite a while now with my simulation model in EnergyPlus. I hope someone can give me some advise how to solve this problem.

A short description of my case: I'm modelling a one zone model with one window. The HVAC system consists of: - a climate ceiling with a constant temperature and a variable flow to provide both heating and cooling - a Constant Air Volume (CAV) ventilation with a constant volume of 60 m3/h with a constant temperature of 18oC with humidification and dehumidification

The problem: The climate ceiling works well and provides the appropriate air temperature in the room. But when want to include the CAV ventilation this doesn't work out the way I wanted. I tried several options: IdealLoadAirSytem, model the system myself in different ways. But I didn't manage it to get it right. I think the problem is that the Air system wants to react on a certain thermostat. But in my case the airflow and temperature has to stay constant in a certain humidity range (43-54%) and don't have to react on the room temperature.

In the graph below you can see that the dehumidification does not work. By setting a 'temperatureandhumidityratio' in the coil controller I tried to dehumidify the air but it seems not to work this way.

image description

Here is a link to the IDF-file.

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@BasP Could you elaborate on what the system is doing that's not working the way you wanted?

Eric Ringold gravatar image Eric Ringold  ( 2015-12-23 10:18:25 -0500 )edit

I want the air system to work constantly on 18oC with 60m3/h and a relative humidity range between 43-54%.

BasP gravatar image BasP  ( 2015-12-24 05:49:44 -0500 )edit

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answered 2015-12-23 10:01:41 -0500

updated 2015-12-23 10:04:48 -0500

Someone has asked a similar question here.

Your CAV air system with constant supply air temperature (SAT) isn't able to modulate cooling or heating provided to the space, since it's always at the same flow and temperature. This will make it very hard and inefficient to control your zone within specific temperature ranges. I would change the system configuration, unless you're trying to match an existing system.

With that said, the CAV air system does "turn on" and provide air when the zone's temperature is above the cooling setpoint or below the heating setpoint specified by the Zone Thermostat object. You CANNOT assign multiple thermostats to one zone in EnergyPlus (one to control the CAV and another to control the climate ceiling). To override the thermostat signal used by the CAV air system, you may have to use EMS Controls, which are complicated but allow you to do pretty much anything in EnergyPlus.

As a side note, using Ideal Load Air System is similar to connecting a zone to a black box that will meet heating and cooling loads with 100% efficiency. The object should really be used for sizing calculations so that you can assign better capacity values to zone HVAC components and avoid unmet hours.

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Note that if the climate ceiling is modeled with ZoneHVAC:LowTemperatureRadiant:VariableFlow objects, temperature control for those systems is based on the object fields for 'Heating/Cooling Temperature Schedule', which can be different than the space zone thermostat. So in effect you do have 'multiple thermostats' when using those systems as well as an air loop.

Eric Ringold gravatar image Eric Ringold  ( 2015-12-23 10:14:42 -0500 )edit

Thanks @Eric Ringold, great point

Aaron Boranian gravatar image Aaron Boranian  ( 2015-12-23 10:45:27 -0500 )edit

@Eric Ringold, That's true. The climate ceiling has its own thermostat. However, the temperature in the zone is already above 18oC. So when my thermostat for the air temperature is set at 18 degrees, the air system wants to cool down the air.

BasP gravatar image BasP  ( 2015-12-23 11:20:13 -0500 )edit

@Aaron Boranian, thank you for your comment. However, I want to model an existing HVAC system. That's why I want to model it this way. Since this is not a standard solution, it is hard to model it in EnergyPlus. Do you know a way to handle this?

BasP gravatar image BasP  ( 2015-12-23 11:24:24 -0500 )edit

Lack of dehumidification means the cooling coil capacity isn't large enough. After looking at the IDF, the air system has SAT of 18C from both the heating and cooling coils. If so, you don't need a reheat coil (notice that your reheat coil electric use is 0). Focus on allowing SAT less than 18C for the cooling coil to allow adequate dehumidification, then have the terminal reheat coil bring the SAT up to 18C before supplying to the zone.

PS: You're missing a EnergyManagementSystem:ProgramCallingManager object.

Aaron Boranian gravatar image Aaron Boranian  ( 2015-12-28 14:44:57 -0500 )edit

answered 2015-12-24 09:46:20 -0500

updated 2016-01-05 13:08:44 -0500

You need to drill down into the results to see what is happening with the controls. Add node temperature, setpoint temperature, humidity ratio, and humidity ratio setpoint max (see rdd file using Output:VariableDictionary, IDF; in your input file) for the coil outlet node. If the setpoint is -999, a SetpointManager is needed. You might also include the coil inlet node temp and humrat to help identify when the coil should be on. Plot these node conditions. If using a water coil, include the coil water node flow rate, or for DX the part load ratio, to see when the coil is maxed out and cannot provide additional capacity.

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Asked: 2015-12-23 07:27:35 -0500

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Last updated: Jan 05 '16