| Canada Blower Co. produces various types of air | | | | minimum of heat added to the building. |
| makeup units for industrial building ventilation. In | | | | Return Air Units are widely used for direct fire |
| industrial plants where there is a high density of | | | | space heating applications. They can be supplied |
| employees and many manual operations involved, | | | | with a maximum 80% return / 20% fresh air, |
| an improvement in the environment can produce | | | | per ANSI standards. Other options include a 50/50 |
| substantial benefits in terms of increased | | | | arrangement. “80/20” Units are configured |
| production, reduced errors, and a decline in | | | | to modulate from the 80% return/20% fresh air |
| complaints and absenteeism among employees. | | | | to up to 100% outside or fresh air. 50/50 units |
| A Direct Gas-Fired Air-Handling Unit AHU may be | | | | are generally designed to switch from 50% return |
| used as a primary or secondary source to heat a | | | | / 50% fresh to 100% fresh, with no modulation |
| building space. When used for this application, a | | | | between. |
| room thermostat controls unit outlet air | | | | Direct-fired units utilize air that will be sent to the |
| temperature. | | | | heated space for combustion without use of a |
| On a call for heat, a space-heating unit will provide | | | | heat exchanger. The products of combustion are |
| air at the desired room temperature. An airhandler | | | | mixed directly with large volumes of outdoor air. |
| may also be designed to increase outlet | | | | Such mixing is considered safe because of the |
| temperature when room temperature is low. A | | | | high dilution ratio. |
| duct stat or probe at the outlet of the unit | | | | In addition, thorough burning of the natural gas |
| prevents outlet temperatures from exceeding the | | | | takes place so that no harmful products of |
| maximum allowable value. | | | | combustion enter the airstream. One product of |
| When a unit blower runs continuously, room air is | | | | combustion is water vapor, which can be |
| monitored and the outlet air temperature is | | | | problematic with very tight building construction |
| modulated to maintain the temperature. | | | | due to the potential for condensation in colder |
| Systems are usually designed for either Space | | | | climates. For tight buildings, it is best to consider |
| Heating or for Make-Up Air use. However, | | | | the use of indirect-fired equipment. There are |
| standard Make-up Air Units may be provided with | | | | several common configurations of direct-fired |
| a room thermostat that monitors indoor minimum | | | | units. |
| set point, a signal is sent to the unit to override | | | | All return air should be brought in downstream of |
| outlet temperature set point and turn the unit to | | | | the burner and not recirculated through the |
| full fire. When minimum room temperature is | | | | burner. This is to prevent buildup of waste |
| satisfied, unit returns to outlet air temperature | | | | products from the combustion process within the |
| control mode. | | | | building. It also prevents contaminants originating |
| Return Air Units are a design that includes a | | | | within the building environment from being passed |
| damper to bring room air back into the unit. | | | | through the direct fire burners. The combustion of |
| These re-circulate indoor air through a unit with a | | | | unknown contaminants can be a serious Safety |
| minimum of outside air added. This type of unit is | | | | or Indoor Air Quality problem, and so must be |
| able to provide air movement for indoor comfort | | | | avoided. |
| and reduced temperature stratification, but with a | | | | |