Hybrid Ventilation System

From Green Garage Detroit
Jump to: navigation, search

return to Living Building Design Studio

What is It?


Hybrid ventilation is a building ventilation system that integrates natural (i.e. passive) and mechanical (i.e. active) ventilation components to create an high efficiency and healthy ventilation system for a building. An hybrid system can include:

The rationale for a hybrid type system is to allow the earth's natural systems (e.g. the ground or wind) to do as much of the ventilation work as possible and only when they can not meet the required ventilation levels to resort to high-efficiency mechanical systems to complete the job. We expect the assistance of mechanical system(s) would only be needed in more severe weather situations.

Highly efficient ventilation systems are an essential element for reaching net-zero energy goals in buildings, as evidenced by the work of the Passive Haus Institute and ZED Factory.

  • Also known as: hybrid ventilation, passive ventilation, natural ventilation, geo-heating.

Why is it Important?


A hybrid ventilation system is important to a building's sustainability because it:

  • Directly connects the building and its occupants to the earth's natural systems (e.g. natural ventilation.)
  • Demonstrates an "appropriate" use of technology (only after the natural systems are unable to meet the needs).
  • Includes renewable, high-efficiency, low-carbon components (e.g. earth air tubes).
  • Has been shown to result in healthier indoor environments when used properly.

When to Use It?


It is appropriate to use hybrid ventilation systems when:

  • The building has access to the earth's natural systems. (e.g. area of land large enough for placing earth tubes).
  • The building location has extreme weather conditions (i.e. heat and/or cold).
  • While easier to do in new construction, it is possible to do this in major renovations to existing buildings.

Green Garage Use of Hybrid Ventilation System


Sustainability Goals

The sustainability goals for the Hybrid Ventilation System are:

  • Fully integrated natural-mechanical system to meet the air exchange requirements with 50% of the required energy coming from natural sources and 50% from high efficiency mechanical sources.
  • Meet 20 cfm per person or 800 cfm for the building.
  • Maintain the indoor relative humidity at 45% +/- 15%.
  • Connect the building and the occupants to the natural systems.
  • Ensure healthy indoor environment.
  • Allow components of the system to be bypassed when they don't contribute to these goals.
  • The system should be simple to maintain, adapt and control.

Strategy and Conceptual Design

Hybrid Strategy

The major elements of our ventilation strategy were:

  • The hybrid ventilation system at the Green Garage has three main components:
    • Earth Tubes / ERU - Air Exchanger
    • Natural Ventilation
    • Moisture Control
  • Put the passive and active components in a series with the passive first in line. Only after the passive component cannot meet the needs does the active turn on and meet the "net" remaining requirement.
  • Reduction in size requirements for the active equipment. It also runs less frequently since it's second in line to the natural system. Both of these reduce energy usage.
  • Select the highest-efficiency active methods available.
  • Use demand controlled ventilation (DCV) adjusts ventilation rates based on actual occupancy at any given time instead of at a fixed rate for full occupancy.
  • Adopt a "Topping Off" strategy where we keep the indoor temperature in a tight range and the system is always running and very low levels and just topping off the heat when needed and cooling when needed. This basically eliminates any night time setbacks, because they lead to heating surges (sometimes followed by cooling surges when the sun comes out and people occupy the building.)
  • Make sure we are addressing moisture in every component (i.e. latent energy).


Overall Conceptual Design
Ventilation FINAL.png


Earth Tube / ERU - Air Exchanger Design

The Earth Tubes and Energy Recovery Unit are tightly integrated into the air exchange system. In general, the Earth Tubes and Energy Recovery Unit provide the year-round foundation for the building's ventilation. During a normal air exchange, the outdoor air will be brought in through the Earth Air tubes and will be pre-heated or cooled, then enter the Energy Recovery Unit where the outbound air will be used to heat/cool the inbound air, as well as dehumidify or humidify the air.

Winter Example:
Outdoor Air Temp: 0 F
Outdoor Air Relative Humidity: ??
Ground Temp: 45 F
Indoor Temp: 68 F
Earth Tubes - Air Temp Coming In: 0 F
Earth Tubes - Air Temp Coming Out: 23 F (Rule of thumb: air temp gain equals 1/2 of the difference between the air and ground temperatures with 100 ft of tubing.)
ERU - Air Temp Coming In: 23 F
ERU - Air Temp Coming Out: 55 F (assumes 70% efficient)
Summer Example:
Outdoor Air Temp: 90 F
Outdoor Air Relative Humidity: ??
Ground Temp: 57 F
Indoor Temp: 78 F
Earth Tubes - Air Temp Coming In: 95 F
Earth Tubes - Air Temp Coming Out: 86 F (Rule of thumb: air temp gain equals 1/2 of the difference between the air and ground temperatures with 100 ft of tubing.)
ERU - Air Temp Coming In: 86 F
ERU - Air Temp Coming Out: 80 F (assumes 70% efficient)

The powerful aspect of this design is the contributions of the Earth Tubes and Energy Recovery Unit components naturally increasing as as the building's demand for energy increases due to the very hot or cold weather. This is because as the difference between the ambient temperature (e.g. outdoor air temp) and the ground or indoor temperature increases, the energy contributions of the Earth Tubes and Energy Recovery Unit increase naturally.

Another benefit of placing the earth tubes in front of the ERU is that the the ERU is susceptible to freezing if the air temperature drops below 23F for more than two days in a row. It would be very unusual for this to happen with the earth tube pre-heating the air before it enters the ERU.

The attached chart shows the relative contributions of each component.


Natural Ventilation Design

Natural ventilation is expected to be able to assist in about 50 - 90 days per year during the spring and fall months. Clearly in the months of extreme weather, natural ventilation would make no contribution. Natural ventilation is provided by opening the building's windows. More information is available on our Natural Ventilation pattern page.


Moisture Control Design

We have attempted to control moisture in and through every component of the ventilation system. The two main areas are the dew point in the envelope systems and the control of humidity, especially in the summer. We are planning on using a pressure-based strategy to control the humidity. It is an approach that has come from the in-depth experience of one of the professionals who has contributed extensively to our Net-Zero Energy design. For more on moisture control please see our Moisture Control pattern page.


Integration and Controls Design

Overview

Integrating all these components does require significant thought. Some of the controls will be manual (even behavioral) and some will be automated. The key integration areas are:

  • Shown in Sequence of Operations Matrix - Ventilation ERV.
  • There are three major subsystems in the ventilation system:
    • Energy Recovery Ventilator - which is responsible for bringing fresh air in and favorably exchanging heat and humidity from the exhaust air into the fresh air to prep it for entry into the space. The ERV is controlled by CO2 sensor (CO-1) and the time-of-day / day-of-week building occupancy schedule.
    • Dehumidification Coil - allows the humidity of the incoming supply air from the ERV to be lowered using cool water from the Altherma. The dehumidification coil is controlled by two humidistats HS-1 and HS-2. The fan will be programmed to continue running after the dehumidification cycle is ended. This will serve to dry out any residual moisture in the coil.
    • Air Mixer - supplying two capabilities:
      • mixing indoor air with the cool air exiting the dehumidification coil to prevent condensation on the supply distribution ducts. The Air Mixer is enabled by dampers and activated by the dehumidification coil operations.
      • with the ERV off, it allows the indoor air to be recirculated through the dehumidification coil to draw down the humidity of the indoor air, without introducing a new supply of outdoor air. The Air Mixer is activated by the two humidistats HS-1 and HS-2 and the time-of-day / day-of-week building occupancy schedule.
  • The ERV will:
    • be controlled by CO2 sensors in the main and annex buildings when these call for make up air the ERV operates at 800cfm.
    • must run at 450cfm when the building is occupied (controlled by schedule) to exhaust the bathrooms and enclosed offices.
    • when the outdoor temperature is above 55F (adj) and less than 80 (adj) and the outdoor relative humidity is less than 50%, the desiccant wheel is deactivated when the ERV fans F-1 and F-2 are operating. This is an economizer mode.
    • Exhaust air is drawn from the space by F-2, through the ERV, and then exhausted through the exchange roof vent to atmosphere.
    • Variable speed exhaust is controlled by the building static pressure sensor SP-1 so that the building pressure relative to outdoor pressure is kept at a slight positive pressure (+0.02" w.c.) in the summer and a slight negative pressure (-0.02" w.c.)in the winter.
  • Sensors Design
    • Outside air temperature, humidity and static pressure are monitored by a weather station on the exterior of the building. It includes temperature sensor WST-1 and humidity sensor WSH-1.
    • Indoor space conditions are monitored and controlled by space thermostats TS-1, TS-2, and TS-3 in the historic building and TS4 in the annex building
    • Indoor space humidistats HS-1 in the historic building and HS-2 in the annex building.
    • Indoor CO2 sensor (CO-1) With CO-1 in the ventilation return duct.
    • Building differential pressure sensor DP-1 is located in the historic area.
  • Assisted natural ventilation
    • Allows natural ventilation where the windows can be opened when the ambient conditions meet the indoor setpoints (temp and humidity) for a sustained period of time.
    • the ERV will need to run at 450cfm (adj) at all times...it thereby "assists" the natural ventilation in drawing air through the building.
  • Dampers and grills
    • the return grills in the bathrooms and enclosed offices should be adjustable so the whole return system can be properly balanced.
Default Settings

The default settings for the fans and dampers in the ventilation system are shown in the "Default" column of the Sequence of Operations Matrix - Ventilation ERV.

Heating (Winter) Modes
Hybrid Ventilation System - Shown in Heating and Humidifying Mode (01/22/10)
  • Occupied - Windows Closed with ERV @ 800cfm (max), Dehumidification @ Off and Outdoor Temp > 0F.
    • See "ERV @ 800cfm / Dehumid OFF / Outdoor Temp > 0F" column in the Sequence of Operations Matrix - Ventilation ERV for a summary of the fan and damper settings.
    • During Occupied - Windows closed hours when the CO-2 sensor (CO-1) calls for makeup air and the indoor humidity HS-1 and HS-2 are less than 55%RH adj, the energy recovery unit (ERV) supply fan, F-1, and exhaust fan, F-2, are activated and dampers D-1 and D-5 (adj 250 cfm) are open. Outside air enters through the outside damper and passes into the ERV. The ERV runs the desiccant wheel if it meets the operational conditions (see instructions in Overview above) to bring the make up air in. Variable speed fan F-3 is modulated to provide 800cfm supply through the air distribution system.
    • When the CO-2 sensor (CO-1) indicates that desired CO2 levels are met, the ERV fans F-1 and F-2 are modulated down to 450cfm and distribution fan F-3 is modulated to 450cfm.
  • Occupied - Windows Closed with ERV @ 800cfm (max), Dehumidification @ Off and Outdoor Temp = or < 0F.
    • See "ERV @ 800cfm / Dehumid OFF" / Outdoor Temp < 0F" column in the Sequence of Operations Matrix - Ventilation ERV for a summary of the fan and damper settings.
    • During Occupied - Windows closed hours when the CO2 sensor (CO-1)calls for makeup air, the indoor humidity is less than 55%RH adj, and the outdoor temp WST-1 is less than or equal to 0F, the energy recovery unit (ERV) supply fan, F-1, and exhaust fan, F-2, are activated to 800cfm and dampers D-1, D-3, D-4 and D-5 (adj to 250cfm) are open. The distribution fan F-3 is activated to supply 1,400cfm. Outside air enters through the outside damper and passes into the ERV. The ERV runs the desiccant wheel located in the ERV, if it meets the operational conditions (see instructions in Overview above), to bring in tempered fresh air. Variable speed fan F-3 is modulated to 1400cfm to distribute the air throughout the supply system until the CO-1 sensor indicates that desired CO2 levels are met and the humidity remains below 55% RH (adj).
  • Occupied - Windows Closed with ERV @ 800cfm (max) and Dehumidification @ On
    • See "ERV @ 800cfm / Dehumid ON" column in the Sequence of Operations Matrix - Ventilation ERV for a summary of the fan and damper settings.
    • During Occupied - Windows closed hours when the the CO2 sensor (CO-1) calls for makeup air and the indoor humidity is greater than 55%RH adj, the energy recovery unit (ERV) supply fan, F-1, and exhaust fan, F-2, are activated to 800cfm and dampers D-1, D-3, D-4 and D-5 (adj to 250cfm) are open. The distribution fan F-3 is activated to supply 1,400cfm. Altherma is activated to send cool water to the dehumidification coil. Outside air enters through the outside damper and passes into the ERV. The ERV runs the desiccant wheel located in the ERV, if it meets the operational conditions (see instructions in Overview above), to bring in tempered fresh air. Variable speed fan F-3 is modulated to 1400cfm to distribute the air throughout the supply system until the OS-1 and OS-2 sensors indicate that desired CO2 levels are met and the humidity is below 55% RH (adj).
    • If the desired CO2 levels are reached first, the ERV fans F-1 and F-2 modulate down to 450cfm, the dehumidification coil remains activated and the distribution fan F-3 is modulated down to 800cfm, adj. If the desired humidity levels are met first, the Altherma is deactivated from sending cool water to the dehumidification coil, dampers D-3 and D-4 are closed and the ERV fans F-1 and F-2 continue to run at 800cfm and F-3 is modulated down to supplying 800cfm.
  • Occupied - Windows Closed with ERV @ 450cfm, Dehumidification @ Off and Outdoor Temp > 0F...Most Common
    • See "ERV @ 450cfm / Dehumid OFF / Outdoor Temp > 0F" column in the Sequence of Operations Matrix - Ventilation ERV for a summary of the fan and damper settings.
    • During Occupied - Windows closed hours when the CO2 sensor (CO-1) does not call for makeup air, the indoor humidity HS-1 and HS-2 are less than 55%RH adj and the outdoor temperature WST-1 is greater than 0F, the energy recovery unit (ERV) supply fan, F-1, and exhaust fan, F-2, are activated to 450cfm and damper D-1 is open. Outside air enters through the outside damper and passes into the ERV. The ERV runs the desiccant wheel, if it meets the operational conditions (see instructions in Overview above), to bring the make up air in. Variable speed fan F-3 is modulated to supply 450cfm to the distribution supply duct system.


  • Occupied - Windows Closed with ERV @ 450cfm and Dehumidification @ On
    • See "ERV @ 450cfm / Dehumid ON" column in the Sequence of Operations Matrix - Ventilation ERV for a summary of the fan and damper settings.
    • During Occupied - Windows closed hours when the occupancy sensor (CO2) OS-1 and OS-2 do not call for makeup air and the indoor humidity sensor HS-1 or HS-2 is greater than 55%RH adj, the energy recovery unit (ERV) supply fan, F-1, and exhaust fan, F-2, are activated to 450cfm and dampers D-1 and D-3 are open. The distribution fan F-3 is activated to 800cfm (= 450cfm * dewpoint factor). Outside air enters through the outside damper and passes into the ERV. The ERV runs the desiccant wheel located in the ERV, if it meets the operational conditions (see instructions in Overview above), to bring in tempered fresh air. Variable speed fan F-3 is modulated to supply 800cfm to the air distribution supply system until the indoor humidity HS-1 and H-2 are below 55% RH (adj).
    • When the desired humidity levels are met, the Altherma is deactivated from sending cool water to the dehumidification coil, damper D-3 is closed and the ERV fans F-1 and F-2 continue to run at 450cfm and F-3 is modulated down to supplying 450cfm. This is the same as "Occupied - Windows Closed with ERV @ 450cfm and Dehumidification @ Off" above.
  • Occupied - Windows Open....assisted natural ventilation
    • Signal is sent to occupants that windows and designated doors may be opened for natural ventilation. When conditions for natural ventilation are no longer valid, or at the end of the work day, occupants are sent a signal to close all windows and doors.
    • The ventilation system runs under the mode: "Occupied - Windows Closed with ERV @ 450cfm and Dehumidification @ Off" above to provide the ventilation needed for the bathrooms and enclosed offices.
  • Unoccupied (The unoccupied hours are determined by the building time-of-day, day-of-week schedule, adj.)
    • Dehumidification @ Off
      • The unoccupied hours are determined by the building time of day/day of week schedule, adj.
      • System is in the default (deactivated) state. See "Default" column in the Sequence of Operations Matrix - Ventilation ERV for a summary of the fan and damper settings.
    • Dehumidification @ On
      • See "ERV @ 0cfm / Dehumid @ ON" column in the Sequence of Operations Matrix - Ventilation ERV for a summary of the fan and damper settings.
      • Dampers D-2, D-3 (adj 600cfm), and D-4 are open. The distribution fan F-3 is activated to supply 1400cfm. The Altherma is activated to send cool water to the dehumidification coil. (note: The ERV is deactivated.) The variable speed fan F-3 is modulated to 1400cfm to draw air from the mezzanine return and bring 800cfm through the dehumidification coil and 600cfm bypassing the coil and distributes a combined 1,400cfm of lower humidity air through the supply system until the indoor humidity HS-1 and H-2 are below 55% RH (adj).
  • Emergency
    • Upon system failure, the system should be able to be set and held in "Emergency" mode as shown in the Emergency column Sequence of Operations Matrix - Ventilation ERV until the emergency is resolved.
    • Capability to shutdown each of subsystems manually including, ERV, Dehumidification Coil, Air Mixer and Distribution Fan F-3 is needed.
    • ERV has defrost cycle. Refer to manufacturer's operation and maintenance information for emergency information.
    • Refer to manufacturer's operation and maintenance information for emergency information on packaged controls.



Cooling (Summer) Modes
Hybrid Ventilation System - Shown in Heating and Humidifying Mode (01/22/10)
  • Occupied - Closed
    • Same as heating mode.
  • Occupied - Open
    • Same as heating mode.
  • Unoccupied
    • Same as heating mode.
  • Emergency
    • Same as heating mode.



Shoulder (Spring - Fall) Modes
Hybrid Ventilation System - Shown in Heating and Humidifying Mode (01/22/10)
  • Occupied - Closed
    • Same as heating mode.
  • Occupied - Open
    • Same as heating mode.
  • Unoccupied
    • Same as heating mode.
  • Emergency
    • Same as heating mode.



Controls - Open Design Points
  1. Confirm the 450cfm dewpoint calculation...currently 800cfm.
  2. Is intent to have active building pressure control or just to do it by balancing supply/exhaust for summer/winter conditions to maintain slight positive and slight negative? Written for active control currently.
  3. Do we need flow sensors in the duct to make sure the flows are right...see D-5 and D-3 adj settings.


Supporting Science / Experience

Months Natural Ventilation would be Most Likely in RED BOX

The detailed thermal calculations are shown in pages included here. We thank Laurie Catey for her great contributions to our understanding of how to work with natural systems through a better understanding of the science that describes them. You need to click on the link on this page and the next page to get the pdf to display.

  • File:EAT Pressure Humidity.pdf ... relationship between air pressure and humidity in Earth Tubes or in any system for that matter. The point is the the pressure gets higher if lowers the dew point and reduces the amount of moisture in the air.


Proposed Materials / Suppliers

  • The material for the earth tubes can be found on the Earth Tubes page.
  • Manufacturers of the Energy Recovery Unit that we considered during this phase was SEMCO FV Series ERV.
  • The natural ventilation materials can be found on the Natural Ventilation page.
  • The moisture control materials can be found on the Moisture Control page.


Development Story

The Hybrid Ventilation System - Development Story page contains many images and videos documenting the process used at the Green Garage to design, build and operate our Hybrid Ventilation System.

Related Internal Links

Resources


To Do's

  • Laurie's data
  • Resources
  • In Earth Tubes section, get numbers for relative humidity levels.
  • In Earth Tubes - last point - where is attached chart?
  • Upload images onto Development Story page
  • Image for top of page - Kevin

Peggy has reviewed this page  ;)

Gg.jpg