Central A/C not cooling all rooms?

Reply

  #1  
Old 07-30-20, 06:01 AM
O
Member
Thread Starter
Join Date: Dec 2014
Location: USA
Posts: 222
Received 2 Votes on 2 Posts
Central A/C not cooling all rooms?

Hi All,

We have two separate central a/c units, one for upstairs and one for downstairs. The downstairs one is new and works well (except for the separate dehumidification control, but that's a subject for another post). The upstairs one is at least 9 years old (from when we moved in). It puts out the correct temperature cold air, but it never seems to cool a couple of rooms all that well. I've tried keeping the fan on the "on" position (instead of auto) so that it would hopefully circulate the air everywhere, but doesn't help. Our guest-bedroom-turned-office (where I currently spend most of my day) gets up to about 80 degrees, even when the thermostat is set to 75 or 76. The thermostat itself is in the central hallway, and it does get down to whatever temperature we set it at. I can't make it lower than 75 or 76, because then my kids and wife start freezing.

Anyone have any idea what (if anything) is wrong and/or what I can do?

Thanks!
 
Sponsored Links
  #2  
Old 07-30-20, 06:23 AM
M
Member
Join Date: Sep 2019
Location: Canada
Posts: 1,456
Received 151 Votes on 138 Posts
Need more info of the ducting.

Where is/are the air returns?
Where are the AC units?
What size is the ducting?
Other than leaving the fan on have you tried anything else. Like opening the registers full in the warn rooms and partially closing them in the OK rooms.
Are the door to these rooms left open or are they usually closed?
 
  #3  
Old 07-30-20, 06:38 AM
O
Member
Thread Starter
Join Date: Dec 2014
Location: USA
Posts: 222
Received 2 Votes on 2 Posts
Hi,

The air returns for both upstairs and downstairs are right next to each other, at the top of the stairs (basically, at the end of the central hallway upstairs).

The air handlers/fans are in the attic, and the compressors are outside.

Not sure what you mean about the size of the ducting - how do I measure that? It is the same size as what we have downstairs when the new one was installed (and pros other than the installers have also said the downstairs is the correct sizing).

I have tried closing the ducts and the door to our bonus room (at the other end of the central hallway from the air returns), in the hope that doing that would help. It made a very, very tiny difference. I have not thought of closing ducts in cold rooms and leaving them open in hot rooms. I'll try that next! Thanks for the idea!

Thanks!
 
  #4  
Old 07-30-20, 06:51 AM
T
Member
Join Date: Jun 2012
Posts: 976
Received 12 Votes on 10 Posts
You might want to check the main trunk in the attic for dampers. Slightly close off rooms that are cooling better to direct more airflow to the room(s) that aren't. I find the dampers on the main trunk are more effective in controlling air flow then closing registers.
 
  #5  
Old 07-31-20, 02:58 AM
F
Member
Join Date: Jul 2020
Location: United States
Posts: 78
Received 10 Votes on 10 Posts
The room with the thermostat will be the most comfortable room on a system, in this case your second floor hall. You might consider moving the thermostat into a heavily-used room. That way, the stat will "see" room temperature, not some combination of first and second floor temperatures ("air returns for both upstairs and downstairs are right next to each other, at the top of the stairs"), and will not be subject to warm air rising from the first floor when that system isn't running.

Then, follow T's recommendation to balance at branch duct dampers - not at ceiling registers (whose dampers can't be locked, and may create air noise when throttled). And you might also look at the branch ducts supplying the rooms in question, to see if there are any duct restrictions.
 
  #6  
Old 07-31-20, 04:47 AM
B
Member
Join Date: Oct 2015
Location: Ct.,USA
Posts: 1,649
Received 75 Votes on 66 Posts
Not sure I understand your statement about the 2 returns being right next to each other. Returns are normally located at the top of the walls in that zone. Can you help me understand?
 
  #7  
Old 07-31-20, 05:35 AM
F
Member
Join Date: Jul 2020
Location: United States
Posts: 78
Received 10 Votes on 10 Posts
I wondered about that, then I reread the IP and realized both units are in the attic. A "DUH" moment for me.
 
  #8  
Old 07-31-20, 01:25 PM
B
Member
Join Date: Oct 2015
Location: Ct.,USA
Posts: 1,649
Received 75 Votes on 66 Posts
Are the return ducts or registers right next to each other. I am still confused.
 
  #9  
Old 08-02-20, 08:12 AM
F
Member
Join Date: Jul 2020
Location: United States
Posts: 78
Received 10 Votes on 10 Posts
"The air returns for both upstairs and downstairs are right next to each other, at the top of the stairs (basically, at the end of the central hallway upstairs)" would seem to indicate they are "right next to each other".
 
  #10  
Old 08-02-20, 08:21 AM
B
Member
Join Date: Oct 2015
Location: Ct.,USA
Posts: 1,649
Received 75 Votes on 66 Posts
Please explain how the first floor A/C air gets recirculated if the first floor return register is at the second floor ceiling?
 
  #11  
Old 08-02-20, 07:34 PM
O
Member
Thread Starter
Join Date: Dec 2014
Location: USA
Posts: 222
Received 2 Votes on 2 Posts
Hi All,

Sorry for the confusion about the returns. I stand by what I said, but maybe I didn't state it accurately.

Our house is basically an exact rectangle. The staircase from the first to second floor is at one end of the rectangle, with a landing in it so that the end of the rectangle is two stories tall. I'm posting a picture while standing on the landing, with the two red circles being the returns (left is for downstairs, right is for upstairs) - I could only fit the corner of each in a single picture, but the rest of the returns are there! To avoid confusion, I also circled two other things so that people know what they are - the blue circle is for the whole house fan, and the green circle is for an a/c vent (i.e., cold air comes out of it). These are at 90 degree angle from the floor and face out (on the other side is the attic, so its the wall of the attic that they are in). As for how the first floor air a/c air gets recirculated - no idea; I guess the system just sort of does the best it can? If air is blowing into the first floor, I guess it sort of pressurizes that area very slightly, forcing air up to where the return is (which is actively sucking the air in)? Not sure, but I'm assuming that's how it works. Probably not anywhere near the best design, but its how the house was when we bought it (the house was a single level, until the people before us added a second level in 1999 or 2000; we purchased in 2011).

Hope this helps, but let me know if any other info is needed. I finally got a chance to shut 4 of the vents in the middle rooms (which are colder) tonight, so I'll see what happens over the next few days.

Not sure how to easily move the thermostat, unless I get an electrician involved (thermostat is on the long hallway running across the house that you can sort of see on the right side in the picture).



 
  #12  
Old 08-03-20, 08:35 AM
B
Member
Join Date: Oct 2015
Location: Ct.,USA
Posts: 1,649
Received 75 Votes on 66 Posts
A picture is worthy a thousand words. I can see the 2nd floor thermostat being satisfied when it calls for cooling and the first floor temp is less than the 2nd floor thermostat setting, but not the other way around. The open space from the first floor to the roof at the staircase is an additional large volume to cool when the 2nd floor thermostat calls for cooling.
 
  #13  
Old 08-03-20, 08:43 AM
O
Member
Thread Starter
Join Date: Dec 2014
Location: USA
Posts: 222
Received 2 Votes on 2 Posts
Funnily enough (or maybe not, as this type of physical dynamics is a bit beyond me), the whole staircase area is always very cool and comfortable. Its the rooms at the other end of the hallway (from where the picture is taken) that stay too hot. There is an attic above the whole house except at the ends of the rectangle (one end being where the picture is taken, the other end being in our bonus room which is above the garage (and which does not have its own attic space).

The 1st floor is always at a good temperature and has no problem getting there. Its also the newer unit. I'm wondering if the unit for upstairs is mis-sized from the people before us (who took MANY shortcuts and did things the very wrong way for certain things).
 
  #14  
Old 08-04-20, 12:38 AM
F
Member
Join Date: Jul 2020
Location: United States
Posts: 78
Received 10 Votes on 10 Posts
If you're pulling every CFM of room temperature air up the stairs and through the balcony, perhaps the wall register also supplying conditioned air to the area might tend to cool it below room temperature. Overcooling wastes energy - in this case in a seldom-occupied area where pinpoint comfort is not necessary - and I'm wondering why that register is there.

Your HVAC service person can move the stat, no big deal. You'll need to patch/paint holes in the hall wall.

And do check the ducts supplying the two rooms for obstructions.
 
  #15  
Old 08-06-20, 08:21 AM
R
Member
Join Date: Nov 2005
Location: US
Posts: 15
Received 0 Votes on 0 Posts
Rather than moving the thermostat, you could replace your existing thermostat with a model that has sensors that can be placed in different places around the house and then set it to take those sensors into account when determining when to run the A/C and when to shut it off. We have a spacepak system in our house, so it's somewhat similar to the OP's situation in that our unit is in the attic and we only have one return on the second floor. We use an Ecobee thermostat, which has sensors. I have two sensors on the first floor, and I have it set to only use data from those sensors during the day (or the downstairs would get too hot) and then only the thermostat upstairs works as a sensor at night. Without that thermostat set-up it would be a lot harder to keep our house close to balanced.
 

Last edited by ReidJH; 08-06-20 at 08:21 AM. Reason: clarification
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: