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Creating a MVHR system for residential application

asked 2023-05-07 18:56:05 -0500

matbla's avatar

updated 2023-05-13 22:35:41 -0500

Hi there,

I have a residential project that has mechanical ventilation with heat recovery. I am using designbuilder detailed HVAC to model this system, and am aiming to provide summer bypass function (economiser).

System design: Typical residential MVHR system with supply and extract fan inside the MVHR unit, including summer boost bypass of heat recovery. The design controls for summer bypass based on a thermostat installed in living room. When summer bypass is in operation, the other two supply zones (2 bedrooms) recieve boost air, as dependent on the demand for boosts at living room thermostat.

The control logic is summer boost bypass ON

IF Troom > 22C, AND Troom > TOA, AND TOA > 12C

TOA = Temp outside air

Troom = Temperature living room thermostat

System in designbuilder: I have modelled the system as a VAV central AHU with VAV No Reheat air distribution units serving three zones (living/kitchen and 2 bedrooms). Reciculation is used to enable use of the economiser which emulates the boost summer bypass. The AHU OA fraction is set to always 1.0, to prevent any recirculation and provide 100% fresh air. I have put a setpoint manager on supply side inlet (after AHU), controlling for 'single zone cooling', with living room as selected zone for cooling control.

The problem: I have been successful creating the above control logic. However, the problem I have is that the zone airflows under boost mode are independent based on each zone's unique thermostat, rather than based on just the living room thermostat. Instead, I would like each zone's airflow to boost dependent on the living room thermostat. Each zone airflow should follow the same varying profile albeit different airflow based on each zone max airflow rate. But even with single zone cooling setpoint manager, and living room selected, the zones still vary their airflow independently.

One thing I have tried was to edit the idf text creating a single thermostat, and applying a ZoneList of all three zones to the one thermostat object. However, this didnt change the result. Each zone airflow was still modulating independent.

Has anyone created a similar system before, and does anyone know how each zone airflow can be dependent on a single thermostat control?

I have uploaded the system idf here


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If you need help, please illustrate the air-side schematic of your HVAC system so that others can easily understand.

Keigo's avatar Keigo  ( 2023-05-15 03:50:55 -0500 )edit

Do you really have a residence with a central ERV (MVHR) system that has VAV boxes? Or do you have zone ERVs (MVHR). Either way, I think you have to write a script for a logical schedule at the zone ADU.

Greg Estep's avatar Greg Estep  ( 2023-05-15 12:20:45 -0500 )edit

Hi Keigo, thanks for your response - I will add a schematic in the coming days.

matbla's avatar matbla  ( 2023-05-16 04:48:28 -0500 )edit

Hi Greg, thanks for your response. The designed system does not have VAV terminal boxes. Without yet having EMS scripting skills, I have tried to build a system within the limits of the available features. The VAV boxes are used to vary zone airflow, to which the central AHU airflow is a sum of the zone OA.

After more reading of IO reference doc, I think AirTerminal:SingleDuct:Uncontrolled may be possible with VAV AHU, along with EMS script to force the zone OA to vary with actuator on zone thermostat sensor? I am currently in the process of learning EMS to try solve it.

matbla's avatar matbla  ( 2023-05-16 04:51:20 -0500 )edit

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answered 2023-05-23 00:10:47 -0500

matbla's avatar

This query is now solved using a custom EMS program. The HVAC system used was a VAV central AHU, combined with CAV no reheat ADU terminal units. This combination allowed control using mass air flow rate actuators on the CAV no reheat ADUs to vary the flow to each zone based on the required logic. I have uploaded the EMS program here for anyone that may find this a useful example. Note, there is some minor short cycling which I am attempting to iron out. However, for the most part the system performs as expected.

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Asked: 2023-05-07 18:56:05 -0500

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Last updated: May 23