ckf Posted July 17, 2019 Share Posted July 17, 2019 I was reading a thread on a grow forum about venting the heat/air from inside of a grow tent. A guy asked if he would be better off venting the air back in to the room and let the AC cool it, or to vent it outside.  My thought is that you would be better off venting the 120° air outside and deal with the 90° air from outside being brought in to replace the 200cfm of air that the vent is exhausting. Most everyone else says to vent it inside and let the AC cool it. Quote Link to comment Share on other sites More sharing options...
Woodtick Posted July 17, 2019 Share Posted July 17, 2019 Are you talking about using one of those portable ac units that uses a tube to vent the condenser air out a window? Most large green houses just cross ventilate with fans. The roof opens up on the fancy ones. If you are talking about cooling off a room in your house,you are better off using a traditional window unit. Those portable units with the exhaust tube,exhaust air out that you just conditioned. They were made for people that don’t want to put a window unit in. Quote Link to comment Share on other sites More sharing options...
XC.Morrison Posted July 17, 2019 Share Posted July 17, 2019 As far as energy usage and efficiency it comes down to enthalpy.  120 degree air at 10% humidity will have a certain BTU/lb enthalpy while 90 degree air at 40% humidity will have “x” enthalpy .  If the 120 deg air in the grow house has higher enthalpy than the 90 deg air enthalpy then it makes sense to exhaust.  I’ll have to dig around for a good formula to calculate enthalpy.  Quote Link to comment Share on other sites More sharing options...
ckf Posted July 17, 2019 Author Share Posted July 17, 2019 I'm talking about an enclosed tent with lights inside of a home. The heat from the lighting needs to be vented from the tent. In the winter I vent the it in to the room to take advantage of the heat from the light. (carbon filter removes any odors)Â During the summer I vent it outside. Quote Link to comment Share on other sites More sharing options...
ckf Posted July 17, 2019 Author Share Posted July 17, 2019 Just now, ckf said: I'm talking about an enclosed tent with lights inside of a home. The heat from the lighting needs to be vented from the tent. In the winter I vent the it in to the room to take advantage of the heat from the light. (carbon filter removes any odors)Â During the summer I vent it outside. The guys on the forum are saying that you are better off to vent the heat back in to the room and let the AC cool it. I think it's better to vent the heat outside even though you are going to create negative pressure and pull the equivalent air from outside to replace what you vented. Quote Link to comment Share on other sites More sharing options...
XC.Morrison Posted July 17, 2019 Share Posted July 17, 2019 1 hour ago, ckf said: The guys on the forum are saying that you are better off to vent the heat back in to the room and let the AC cool it. I think it's better to vent the heat outside even though you are going to create negative pressure and pull the equivalent air from outside to replace what you vented. More than likely, during the summer, you're doing the right thing by venting to outdoors and allowing an equivalent cfm of outdoor air to be drawn into the home by negative pressure. I've attached an excel spreadsheet that you can plug the temperature and humidity of the air inside the tent and the outdoor air values if you know them to determine the enthalpy. Note that in the couple of values I've plugged in, even with 105 degree air inside the tent @ 25% RH, that's still 38 BTU/lb of enthalpy (energy). That's still higher than even 90 deg outside air @ 40% RH, which would be approx. 35 BTU/lb - so that would make sense to vent in that situation. You can replace my example values with your own to see the results as I've got the formula built into columns C and F.  Enthalpy_Excel.xlsx 2 Quote Link to comment Share on other sites More sharing options...
ckf Posted July 17, 2019 Author Share Posted July 17, 2019 11 minutes ago, XC.Morrison said: More than likely, during the summer, you're doing the right thing by venting to outdoors and allowing an equivalent cfm of outdoor air to be drawn into the home by negative pressure. I've attached an excel spreadsheet that you can plug the temperature and humidity of the air inside the tent and the outdoor air values if you know them to determine the enthalpy. Note that in the couple of values I've plugged in, even with 105 degree air inside the tent @ 25% RH, that's still 38 BTU/lb of enthalpy (energy). That's still higher than even 90 deg outside air @ 40% RH, which would be approx. 35 BTU/lb - so that would make sense to vent in that situation. You can replace my example values with your own to see the results as I've got the formula built into columns C and F.  Enthalpy_Excel.xlsx Thank you very much. It doesn't let me plug in new values because it opened in read only If you get a chance could you plugin these #'s ? tent temp 110 humidity 50%  outside temp 90 humidity 65% Quote Link to comment Share on other sites More sharing options...
XC.Morrison Posted July 17, 2019 Share Posted July 17, 2019 3 minutes ago, ckf said: Thank you very much. It doesn't let me plug in new values because it opened in read only If you get a chance could you plugin these #'s ? tent temp 110 humidity 50%  outside temp 90 humidity 65% Under those conditions, the tent enthalpy would be 57.7 BTU/lb vs. 43.4 BTU/lb for the outside air. Outside air wins! I'm not sure why it came up as read only but you might be able to copy and paste those cells into a new spreadsheet and save that. 1 1 Quote Link to comment Share on other sites More sharing options...
Platinum Contributing Member steve from amherst Posted July 17, 2019 Platinum Contributing Member Share Posted July 17, 2019 8 hours ago, ckf said: I was reading a thread on a grow forum about venting the heat/air from inside of a grow tent. A guy asked if he would be better off venting the air back in to the room and let the AC cool it, or to vent it outside.  My thought is that you would be better off venting the 120° air outside and deal with the 90° air from outside being brought in to replace the 200cfm of air that the vent is exhausting. Most everyone else says to vent it inside and let the AC cool it. Can you vent hot to outside and draw cool back in it from inside? Quote Link to comment Share on other sites More sharing options...
ckf Posted July 17, 2019 Author Share Posted July 17, 2019 51 minutes ago, steve from amherst said: Can you vent hot to outside and draw cool back in it from inside? Not easily. It's not that big of a deal for me as we don't need the AC that much and have the windows open most of the time. I was just surprised to read that most everyone on the forum said not to vent outside because it would be easier on the AC to wcool the hot air from the tent than the warmer outside air. From what @XC.Morrison showed me today it can vary depending on your specific situation. Always good to learn something new Quote Link to comment Share on other sites More sharing options...
Platinum Contributing Member steve from amherst Posted July 17, 2019 Platinum Contributing Member Share Posted July 17, 2019 1 hour ago, ckf said: Not easily. It's not that big of a deal for me as we don't need the AC that much and have the windows open most of the time. I was just surprised to read that most everyone on the forum said not to vent outside because it would be easier on the AC to wcool the hot air from the tent than the warmer outside air. From what @XC.Morrison showed me today it can vary depending on your specific situation. Always good to learn something new Just curious, when others responded , did they know where you are located and that the climate is a bit cooler then most of the country? Quote Link to comment Share on other sites More sharing options...
ckf Posted July 17, 2019 Author Share Posted July 17, 2019 1 minute ago, steve from amherst said: Just curious, when others responded , did they know where you are located and that the climate is a bit cooler then most of the country? It was someone else's thread that I replied to. None of the replies gave any temp differences. They were just telling him not to vent outside. Â Quote Link to comment Share on other sites More sharing options...
ArcticCrusher Posted July 17, 2019 Share Posted July 17, 2019 8 hours ago, ckf said: Thank you very much. It doesn't let me plug in new values because it opened in read only If you get a chance could you plugin these #'s ? tent temp 110 humidity 50%  outside temp 90 humidity 65% Save As?  1 Quote Link to comment Share on other sites More sharing options...
ckf Posted July 17, 2019 Author Share Posted July 17, 2019 2 minutes ago, ArcticCrusher said: Save As?  I never thought of that. I have to do the 'save as' thing when I open a photo from in email in photoshop.  I'll give that a try.  Thanks Quote Link to comment Share on other sites More sharing options...
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