Thread: Commercial Question

There were some large gas furnace / AC units on the roof at a commercial property. I had not seen this type of unit before, where there seems to be an opening to bring outside air into the system.

It seems like there are some motorized louvers that would open up and then pull air from the roof into the system. Would that be at scheduled intervals, or could you set the controls to pull outside air when the outside temperature would make that an advantage? I wasn't able to operate the units very well, so I was not able to see them in action.

Originally Posted by Jim Robinson

There were some large gas furnace / AC units on the roof at a commercial property. I had not seen this type of unit before, where there seems to be an opening to bring outside air into the system.

It seems like there are some motorized louvers that would open up and then pull air from the roof into the system. Would that be at scheduled intervals, or could you set the controls to pull outside air when the outside temperature would make that an advantage? I wasn't able to operate the units very well, so I was not able to see them in action.

It is has an air exchange system in it. This document should help.
http://www.docs.hvacpartners.com/idc...848-169-25.pdf

Thank you Scott.

"The standard economizer uses an outdoor-air temperature
sensor to sense outdoor-air temperature. When the outdoor-air
temperature is between two configurable ambient temperatures,
the economizer is allowed to cool. These temperature configura-
tions are the Economizer High Temperature Lockout (ECL.H)
and the Economizer Low Temperature Lockout (ECL.L). The
economizer will provide cooling when the outdoor temperature
is suitable as defined above by the lockout settings and if there is
a cooling demand. In addition, if an outdoor enthalpy sensor
accessory has been installed, then the enthalpy reading must
also be “low” before economizer cooling can occur.
When free cooling is available, the economizer sequences
free cooling with up to three stages of mechanical cooling to
maintain comfort in the space. When free cooling is not avail-
able, the economizer modulates to an adjustable minimum
position to maintain a supply of fresh air entering the building.
Optional barometric relief dampers provide natural building
pressurization control. An optional power exhaust system is
available for jobs requiring greater relief."

http://www.google.com/url?sa=t&rct=j...58187178,d.cWc

Standard economizer set-up.
If you are going to look at RTU's for commercial buildings you should consider looking into what your local Code requirements are for make-up air for various occupancies.
Not so much a big deal in warehouses, etc.; but pretty huge issue in bars and restaurants. Especially if your guy is buying a closed establishment and will need new licensing.
Just a couple thoughts,

Originally Posted by Jim Robinson

There were some large gas furnace / AC units on the roof at a commercial property. I had not seen this type of unit before, where there seems to be an opening to bring outside air into the system.

It seems like there are some motorized louvers that would open up and then pull air from the roof into the system. Would that be at scheduled intervals, or could you set the controls to pull outside air when the outside temperature would make that an advantage? I wasn't able to operate the units very well, so I was not able to see them in action.

I know this was not your question but in the first photo it looks like there is an exhaust fan located directly adjacent to the air intake (shown by the red arrow in the picture). There are separation requirements to assure that air from an exhaust system is not taken right back in by the HVAC system and subsequently delivered back into the condition space.

Originally Posted by Scott Patterson

It is has an air exchange system in it. This document should help.
http://www.docs.hvacpartners.com/idc...848-169-25.pdf

I have a customer that has a pair of these - One problem they ran into which you should be aware of - when the units were installed (side by side) the exhaust from the heating unit was less than 5 feet from the fresh air intake of the other unit so it was possible to suck in the CO from the unit next to it. The other issue I have run into is where they are sucking in from the vent pipe of the sewer system - Brilliant placement [not] - let's just kill the occupants or make them sick. So when checking these not only check to see if they work (and maybe they have been disabled - as I did with the case of the paired units) but also look for any exhaust or venting near by

My 2 cents: I agree with others that this is an outdoor air economizer hood that is intended to provide free cooling when outdoor air conditions permit. Let me add that typically the economizer on a packaged unit would be an enthalpy based economizer so that if the outdoor air is at or below cooling set-point temp but higher relative humidity it would not necessarily bring in 100% outdoor air to cool with. A high humidity indicates more heat capacity in the air which is undesireable when cooling.

Distance from air intake and exhaust needs to be ten feet minimum.

Minimum outdoor air varies but is usually around 5 CFM per person unless it is a polluted environment. Then the outdoor air needs rise to around 20 CFM per person. I say "around" because the number is so dynamic. The numberof people at any given time and the in-ability to balance a system to within 5 CFM of the desired setting is such an unknown.

HTH.

Originally Posted by Rod Butler

My 2 cents: I agree with others that this is an outdoor air economizer hood that is intended to provide free cooling when outdoor air conditions permit. Let me add that typically the economizer on a packaged unit would be an enthalpy based economizer so that if the outdoor air is at or below cooling set-point temp but higher relative humidity it would not necessarily bring in 100% outdoor air to cool with. A high humidity indicates more heat capacity in the air which is undesireable when cooling.

Distance from air intake and exhaust needs to be ten feet minimum.

Minimum outdoor air varies but is usually around 5 CFM per person unless it is a polluted environment. Then the outdoor air needs rise to around 20 CFM per person. I say "around" because the number is so dynamic. The numberof people at any given time and the in-ability to balance a system to within 5 CFM of the desired setting is such an unknown.

HTH.

Rod , do you have any reference for the 10 ft distance ( just curious as to a source for this distance )

Originally Posted by Dwight Doane

Rod , do you have any reference for the 10 ft distance ( just curious as to a source for this distance )

From the 2012 IMC: (bold and underlining are mine)
- 501.3.1 Location of exhaust outlets.
- - The termination point of exhaust outlets and ducts discharging to the outdoors shall be located with the following minimum distances:
- - - 1. For ducts conveying explosive or flammable vapors, fumes or dusts: 30 feet (9144 mm) from property lines; 10 feet (3048 mm) from operable openings into buildings; 6 feet (1829 mm) from exterior walls and roofs; 30 feet (9144 mm) from combustible walls and operable openings into buildings which are in the direction of the exhaust discharge; 10 feet (3048 mm) above adjoining grade.
- - - 2. For other product-conveying outlets: 10 feet (3048 mm) from the property lines; 3 feet (914 mm) from exterior walls and roofs; 10 feet (3048 mm) from operable openings into buildings; 10 feet (3048 mm) above adjoining grade.
- - - 3. For all environmental air exhaust: 3 feet (914 mm) from property lines; 3 feet (914 mm) from operable openings into buildings for all occupancies other than Group U, and 10 feet (3048 mm) from mechanical air intakes. Such exhaust shall not be considered hazardous or noxious.
- - - 4. Exhaust outlets serving structures in flood hazard areas shall be installed at or above the elevation required by Section 1612 of the International Building Code for utilities and attendant equipment.
- - - 5. For specific systems see the following sections:
- - - - 5.1. Clothes dryer exhaust, Section 504.4.
- - - - 5.2. Kitchen hoods and other kitchen exhaust equipment, Sections 506.3.13, 506.4 and 506.5.
- - - - 5.3. Dust stock and refuse conveying systems, Section 511.2.
- - - - 5.4. Subslab soil exhaust systems, Section 512.4.
- - - - 5.5. Smoke control systems, Section 513.10.3.
- - - - 5.6. Refrigerant discharge, Section 1105.7.
- - - - 5.7. Machinery room discharge, Section 1105.6.1.

What's interesting about that is the unit as delivered has less than ten feet from the exhaust to the intake. It's probably about six feet from the vent to the back of the unit where the intake is located.

Originally Posted by Jim Robinson

It's probably about six feet from the vent to the back of the unit where the intake is located.

6 ft?

I was talking about the unit on the far right. The exhaust is about six feet from the economizer panel. There may be some interlock system in place.