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Make-Up Air FAQs

 

Answers to your most Frequently Asked Questions about Greenheck make-up air units.

 

Q. Are equipment supports the same height as curbs?
A: Not always. Combination curbs (FPEs) and evaporative cooling can cause these to change. Greenheck's CAPS selection program will provide the proper heights.
Q. Are face & bypass dampers available for steam and hot water coils in make-up air units?
A: Yes. They can be selected in Greenheck's CAPS program with discharge air control.
Q. Can a direct fired unit be controlled by a building management system?
A: Yes, the burner can be set up for an external 2-10vdc or 4-20ma signal to modulate the heat output. The fan and burner circuit can be energized through the BMS with dry contacts.
Q. Can evaporative cooling be added to an existing unit in the field?
A: It may be possible to add evaporative cooling to an existing unit. If the airflow is in the low range for the housing size, the chances are good that evaporative cooling can be added in the field. Each case needs to be evaluated individually. Please consult the factory with this unit serial number.
Q. Can I get an indirect fired unit in a vertical configuration?
A: No, the indirect fired unit is available in a horizontal or down blast configuration only.
Q. Can make-up air units that have direct gas heat be mounted indoors?
A: Yes. Structural support requirements are by others. Note - indoor mounting of units with evaporative cooling is not recommended.
Q. Can make-up air units that have indirect gas heat be mounted indoors?
A: Yes. Indoor installation is selectable in CAPS®. With that selection you will need to specify the type of combustion gas venting required. See page 16 of Greenheck’s Indirect Gas-Fired Technical Guide for specifics. However, structural support requirements are from others outside of Greenheck. Note - all Greenheck indirect gas units are power vented. Indoor mounting of units with evaporative cooling is not recommended.
Q. Can you add a variable frequency drive (VFD) to the make-up air units? Which units?
A: All make-up air units can have a VFD added. The direct fired units require a burner by-pass damper to keep the air velocity constant across the burner at all times. This option is selectable in 3.10 CAPS. Airflow turndown can be down to 50% and greater on some units depending on the start point on the fan curve. The CAPS program will tell you how much you can reduce air flow when making the unit selection.
Q. How does freeze protection work?
A: Freeze protection consists of an on/off duct stat and timer that prevents the discharge of cold air into the building when the burner is not providing adequate tempering. If the discharge temperature is below the set point, the make-up air unit is shut off.
Q. Is freeze protection available for steam and hot water coils?
A: Yes. However, it is very different from freeze protection for a gas heat unit. If the coil face temperature drops below the set point, the blower will shut down until the coil temperature rises.
Q. Is it possible to supply a remote control panel for an existing unit in the field?
A: Yes. Please consult the factory with the unit serial number and the requested remote control panel options.
Q. What does an inlet air sensor do?
A: An inlet air sensor consists of an on/off duct stat that automatically de-energizes the burner when outside air temperature is near or equal to the desired discharge air temperature. Typically, this option is used to prevent the burner from firing on mild days.
Q. What does ETL stand for?
A: Intertek Listed, formally known as Edison Testing Laboratories (ETL). The ETL mark is an alternative to American Gas Association (AGA). ETL is a listed mark and is accepted and recognized by jurisdictions across the US and Canada. ETL is nationally recognized by the major building code agencies: Building Officials and Code Administrators International (BOCA-ES), International Conference of Building Officials (ICBO-ES), and Southern Building Code Congress International Inc. (SBCCI PST & ESI). These agencies establish building codes and standards which most municipalities and states adopt as their own. The ETL label is provided on direct fired units where Greenheck supplies the control center.
Q. What is included in a control center and where is it located?
A: Control centers include a factory pre-wired disconnect switch, terminal blocks, transformers, fuses, overloads, and starters. A Greenheck control center provides the benefit of a single point wiring connection in the field. Greenheck requires a control center for all gas-fired make-up air units.
Q. What is the difference between Maxitrol 14 and Maxitrol 44 direct gas-fired controls?
A: Maxitrol 14 uses a discharge air sensor and a discharge temperature selector to control air temperature at the point of discharge for direct gas-fired units only. A room override thermostat is available for the Maxitrol 14, which boosts the discharge temperature when the room temperature is below set point. Maxitrol 14 is commonly found in kitchens, restaurants, paint spray booths and other industrial applications. A Maxitrol 44 controls space temperature with a remote mounted thermostat for direct gas-fired units only. This control creates an even air temperature for the total space similar to heating a room in a home. Common applications include factories and warehouses.
Q. What is the recommended minimum service clearance for make-up air units?
A: The recommended minimum for housing 32 and less is 42 inches on the control's side of the unit. On housing 35 and larger the recommended minimum is 48 inches on the controls side of the unit. Note - clearances for component removal may be greater than the service clearances listed.
Q. What is turndown ratio and is it different for direct vs. indirect gas heaters?
A: Turndown ratio compares the maximum to minimum heat output. Turndown ratio for direct gas burners is 25:1, which means that the burner can modulate from 4% to 100% of full fire. Indirect gas turndown ratio changes based on the controls. Example based on a single furnace unit (< 400 MBH): 4:1 electric modulation limits the heat output from 25% to 100% of full fire. 8:1 stage limits the heat output from 12.5% to 100% of full fire. See the Indirect Gas-Fired Technical Guide for more information.
Q. What side of the unit are the controls located on? Can this be changed?
A: When you are looking into the intake of the unit with the air hitting you in the back of the head, the controls and connections will be on the right side of the unit. Yes this can be changed, but requires a special design request (SDR). If you need the controls and connections on the opposite side, consult the factory.
Q. Where is direct gas fired heat not permitted?
A: According to National Fire Protection Association (NFPA) 54, direct gas fired heat cannot be used in sleeping quarter areas. Please be aware that local authorities may have different interpretations or further restrictions. If you are not sure, please contact your local authorities for interpretation.