I have a concept-level question regarding what happens to the OUTSIDE air with rooftop units. I am dealing with three rooftop HVAC units on a decent sized church that seem to be saturating the shingles on a pitched roof adjacent to the HVAC units. We have identified the HVAC units as being the source of the excess humidity (the humidity on the roof immediately downwind / upslope of the units is higher than anywhere else on the roof. Shingles on this patch of the roof are staying more water-saturated in this downwind area than anywhere else on the roof).
We are not looking for a problem with the HVAC units but rather questioning how an HVAC process might have a byproduct of increased outside humidity. The units are Lennox make, one is a 7200 CFM (serving a multipurpose room) unit and the other two are 3600 CFM units (serving offices and an open space). The actual exchange of air (inside for outside) is minimal on these units, so the excess humidity is taken to be a byproduct of one of these processes, not from internal humid air being released to the outside. There is no apparent malfunction with the units and condensate / coil drains appear to be working properly.
I have done some basic research but have only been able to find information on inside humidity levels and on collection of condensation from inside air being cooled, but not on the effect of HVAC on outside air.
From my basic understanding, there are three scenarios under which the HVAC units might be producing the excess humidity: 1) running the AC, 2) warming the building by means of a heat pump, and 3) heating the unit by means of firing the burners. This facility is located in Alaska, so temperatures rarely make it into the 90’s.
1) Running the AC. Focusing on the outside air, here is my understanding of what is going on: Outside ambient air is drawn across the unit’s external coils, is warmed (cooling the refrigerant), and blown back out the top of the unit into the outside ambient air. When warmed by the coil, this pocket of air decreases humidity / increases ability to hold moisture. When this warmed pocket of air is blown out the top of the unit, it comes in contact with the cooler outside air and cools back down to its original temperature. Technically this pocket of once-warm air decreases its ability to hold moisture from its previously warmed state when it is cooled. But does this actually increase its humidity to greater than the surrounding air? Or does it just return to previous levels, matching the outside ambient air? No more moisture was introduced to the air while it was warmed. Would air off the AC coils have the possibility of increasing the humidity downwind of the units? Enough to saturate roof shingles over time?
2) Warming the inside air by means of the heat pump. As I understand it, this is basically running the AC in reverse. Similar to the AC mode but with the opposite temperature effect, outside air is drawn across the external coils, cooled (refrigerant is warmed), and then blown out the top of the unit. This cooled pocket of air now has less ability to hold moisture / is at a higher humidity level. Might this pocket of more-humid air have a significant effect on the humidity downwind of the units, enough to keep shingles saturated? Or would the temperature of the cooled air return to match the outside air temp once again and humidity levels balance quickly?
3) Heating the inside air by means of the units’ burners. These units are natural gas, so exhaust that is produced when the burners are running is laden with moisture and is hot. When the exhaust exits the units, it is cooled by the outside air. Consequently, humidity goes up. Unlike the AC and the heat pump, burners running actually introduces an additional source of moisture rather than just altering the temperature one direction and back again.
Is there one that you would suspect over the others? Is there one of these three scenarios NOT likely to produce increased humidity immediately downwind of the units? The source of the excess humidity seems to be cumulative and we have not been able to distinguish which time of the year it mainly occurs during.
If you have an idea which of the three scenarios may be the main culprit for increased outside humidity, please weigh in!
Thanks!
We are not looking for a problem with the HVAC units but rather questioning how an HVAC process might have a byproduct of increased outside humidity. The units are Lennox make, one is a 7200 CFM (serving a multipurpose room) unit and the other two are 3600 CFM units (serving offices and an open space). The actual exchange of air (inside for outside) is minimal on these units, so the excess humidity is taken to be a byproduct of one of these processes, not from internal humid air being released to the outside. There is no apparent malfunction with the units and condensate / coil drains appear to be working properly.
I have done some basic research but have only been able to find information on inside humidity levels and on collection of condensation from inside air being cooled, but not on the effect of HVAC on outside air.
From my basic understanding, there are three scenarios under which the HVAC units might be producing the excess humidity: 1) running the AC, 2) warming the building by means of a heat pump, and 3) heating the unit by means of firing the burners. This facility is located in Alaska, so temperatures rarely make it into the 90’s.
1) Running the AC. Focusing on the outside air, here is my understanding of what is going on: Outside ambient air is drawn across the unit’s external coils, is warmed (cooling the refrigerant), and blown back out the top of the unit into the outside ambient air. When warmed by the coil, this pocket of air decreases humidity / increases ability to hold moisture. When this warmed pocket of air is blown out the top of the unit, it comes in contact with the cooler outside air and cools back down to its original temperature. Technically this pocket of once-warm air decreases its ability to hold moisture from its previously warmed state when it is cooled. But does this actually increase its humidity to greater than the surrounding air? Or does it just return to previous levels, matching the outside ambient air? No more moisture was introduced to the air while it was warmed. Would air off the AC coils have the possibility of increasing the humidity downwind of the units? Enough to saturate roof shingles over time?
2) Warming the inside air by means of the heat pump. As I understand it, this is basically running the AC in reverse. Similar to the AC mode but with the opposite temperature effect, outside air is drawn across the external coils, cooled (refrigerant is warmed), and then blown out the top of the unit. This cooled pocket of air now has less ability to hold moisture / is at a higher humidity level. Might this pocket of more-humid air have a significant effect on the humidity downwind of the units, enough to keep shingles saturated? Or would the temperature of the cooled air return to match the outside air temp once again and humidity levels balance quickly?
3) Heating the inside air by means of the units’ burners. These units are natural gas, so exhaust that is produced when the burners are running is laden with moisture and is hot. When the exhaust exits the units, it is cooled by the outside air. Consequently, humidity goes up. Unlike the AC and the heat pump, burners running actually introduces an additional source of moisture rather than just altering the temperature one direction and back again.
Is there one that you would suspect over the others? Is there one of these three scenarios NOT likely to produce increased humidity immediately downwind of the units? The source of the excess humidity seems to be cumulative and we have not been able to distinguish which time of the year it mainly occurs during.
If you have an idea which of the three scenarios may be the main culprit for increased outside humidity, please weigh in!
Thanks!
