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HVAC Experts I got a question

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(Matt) #1

I have two rooms upstairs and there is no central HVAC yet so I have redneck engineered a way to get it cool. In the crawlspaces I have installed a couple of 10" ducts with a 760CFM hydroponics fan to draw the air and 14" vents in the walls. Now as you can see in th drawing I have one pulling air from the room with no AC to the cooled room and another pulling from the cooled room to the noncooled room. Each room has a temp sensor and I use a Harmony hub to control the AC based on temp in both rooms. currently I set it to run if the temp is above 73.5 and turn off when both are at 72.5 but the cooled room cools off fast (naturally) so I set a stop point of 71 degrees so the AC turns off no matter what the temp in the non cooled room is.

Now the question. is the flow of air I have ideal or would I be better moving all of the hotter air from the room without the AC to the room with and letting the cool air flow to the non cooled room thru the doorway or should I pull cold air from the cool room to the hot room? Or should I leave it as I have it? I can keep it within about 1.5-2 degrees as is but I know in the summer it will be harder to control the temp.

Thanks for any advice


(Bernie H) #2

First thing is the A/C unit up to cooling both rooms? How big are the rooms and what BTUs is the unit? If the unit isn’t big enough then your just fighting a losing battle.


(Jason "The Enabler" as deemed so by @Smart) #3

If it’s at all possible, have you liked at a window unit?

I was in a rental and had the mastery bedroom in a converts attic, in Alabama. There was a/c to the room, but it was bad, real bad. The unit was over 20 years old, it was undersized by half at least. The guy had built on to the house converting a 1000 Sq ft attic and adding a 1000 Sq ft sunroom.

He left the original house a/c…

I installed 2 high efficiency window units that I got at home dept for 85.00 each. Digital with timers. I put them in each of the upstairs rooms and they worked amazingly well.

They would cool the room and turn off, then wake up every 30 minutes to test the temp or just go off of the built in thermostats.

The two units raised my electric bill less than 20.00 total for the month. And my rooms were comfortable. I closed them off from the main a/c and that helped with the rest of the house.

Just an idea.


(Matt) #4

The unit it 15k btu the rooms are about 750 total sq ft but they are right on the roofline. It’s enough to cool them it’s just a matter of getting enough air from one room to the other. If there was a way i would just do two units but the second room doesn’t have a window.


(Bernie H) #5

15k may be just shy if right on the roof line and sunny hot days. You don’t need a window to mount a second A/C That’s what Sawsalls are for. :slight_smile:

Otherwise airflow will be the key to keeping them both cool. the two fans should help move the air.


(Matt) #6

Thanks. The questions is what’s the best direction for the fans to blow?


(William Brooks) #7

If you push or pull 1500 CFM through your doorway you may have issues with door slamming. I would leave it circulating as you have it. It also depends on which room control is more important. If you are more concerned with non ac unit room then it may be better to pull from the cool room into the warm room. That way your “return” will come in the door to the ac unit’s intake. But the big thing is, if your fine with 2 degrees off set point then don’t change it. Mainly because efficiency wise it is in the better configuration now. That’s my two cents.


(Matt) #8

thanks for the reply. the doorway has no door so thats not an issue. I guess i will wait and see how it does when it hits 100 outside