Upstairs AC unit can't keep up on these hot days

Does your upstairs have rooms with doors?

Our upstairs has a separate unit. My house Upstairs is an open hallway and two bedrooms, 1 on each end of the hallway. The hallway has the return and one vent. If both bedroom doors are closed, it is noticeably warmer in that hallway, but the bedrooms get cold. If one or both doors are open, it is a more even temperature distribution.
It also depends on what the downstairs unit is set on.
 
VortecJeep said:
Does your upstairs have rooms with doors?

Our upstairs has a separate unit. My house Upstairs is an open hallway and two bedrooms, 1 on each end of the hallway. The hallway has the return and one vent. If both bedroom doors are closed, it is noticeably warmer in that hallway, but the bedrooms get cold. If one or both doors are open, it is a more even temperature distribution.
It also depends on what the downstairs unit is set on.
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Two bedrooms and a bathroom with doors on the hall at the top of the stairs and then a bonus room with no door. Return is in the hallway between the bedrooms. Kids sleep with the door shut, but I haven't knowingly left the door closed all day to see how it would cool down. Worth a look to see how it compares.
 
My first house had the bare minimum tonnage for the square footage. Upstairs was always hot. It never kept up the entire time I owned the house. If I ever build or replace my current ones, I’m going overkill.
 
Wes said:
My first house had the bare minimum tonnage for the square footage. Upstairs was always hot. It never kept up the entire time I owned the house. If I ever build or replace my current ones, I’m going overkill.
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overkill is as bad an under. itll short cycle and burn up your unit
 
awheelterd said:
Semi serious question... Does it matter as much now with vfd compressors and fans?
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There's not enough control in a residential unit. In a commercial application with hot gas reheat you can cool AND heat to strip moisture. But residential units are either/or
 
CasterTroy said:
There's not enough control in a residential unit. In a commercial application with hot gas reheat you can cool AND heat to strip moisture. But residential units are either/or
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Yeah I have that on a unit at work.

My york unit at home has a variable speed fan and a variable speed compressor. I figured there was some setting where both the compressor and fan run slow to strip moisture
 
Took some temps yesterday with my @RatLabGuy recommended thermometer. If the temps are accurate…roof was about 130 degrees and the floor was around 120. Outside of the air handler measured around 111. With the unit running there was only about 5 degree difference between the air coming out and the air going back into the return.

Checked it this morning when the attic temp was 72 and the air handler was the same temp. There was a 15 degree difference between the output and the return…and I didn’t let it run very long before I checked.
 
jcramsey said:
Took some temps yesterday with my @RatLabGuy recommended thermometer. If the temps are accurate…roof was about 130 degrees and the floor was around 120. Outside of the air handler measured around 111. With the unit running there was only about 5 degree difference between the air coming out and the air going back into the return.

Checked it this morning when the attic temp was 72 and the air handler was the same temp. There was a 15 degree difference between the output and the return…and I didn’t let it run very long before I checked.
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whats the difference between the vent temps upstairs vs downstairs? Or most distant. from the condenser vs the closest?
When you say the air handler... do you mean the condenser is in the attic? Or is it someplace else and there is a distribution box up there?

EDIT: Evap coil, not condenser
 
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RatLabGuy said:
whats the difference between the vent temps upstairs vs downstairs? Or most distant. from the condenser vs the closest?
When you say the air handler... do you mean the condenser is in the attic? Or is it someplace else and there is a distribution box up there?
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The condenser is outside.
The air handler consist of a fan and an evaporator coil.

Essentially the cooling happens outside and carris the supercooled through copper tubing up to the air handler where there is essentailly a radiator that a fan blows air across
 
Ron said:
The condenser is outside.
The air handler consist of a fan and an evaporator coil.

Essentially the cooling happens outside and carris the supercooled through copper tubing up to the air handler where there is essentailly a radiator that a fan blows air across
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Facepalm. Yes I was thinking of the evap coil.
 
Ron said:
The condenser is outside.
The air handler consist of a fan and an evaporator coil.

Essentially the cooling happens outside and carris the supercooled through copper tubing up to the air handler where there is essentailly a radiator that a fan blows air across
Click to expand...

Technically cooling happens at the inside unit. The cold refrigerant vapor returns to the compressor outside which compresses the refrigerant into a hot gas then the condenser makes it a hot liquid. The liquid goes back to the indoor unit and hits an orifice at the evaporator (today its mostly expansion valves) and the pressure change creates the cold liquid/vapor that feeds the evaporator coil. Repeat.
 
This thread might as well be written about my house :shaking: 1920s four square house with a slate roof that sits in the afternoon sun. AC system runs from 4pm until after midnight most summer nights just to hold 75 degrees. Heat wave days the thermostat will register 78 upstairs. However, my upstairs unit thermostat is placed in a bad spot and my stairs don’t have a door to separate the zones. The bedrooms are all comfortable but none of that cool air reaches the thermostat.
 
Ron said:
The condenser is outside.
The air handler consist of a fan and an evaporator coil.

Essentially the cooling happens outside and carris the supercooled through copper tubing up to the air handler where there is essentailly a radiator that a fan blows air across
Click to expand...
Lankford said:
Technically cooling happens at the inside unit. The cold refrigerant vapor returns to the compressor outside which compresses the refrigerant into a hot gas then the condenser makes it a hot liquid. The liquid goes back to the indoor unit and hits an orifice at the evaporator (today its mostly expansion valves) and the pressure change creates the cold liquid/vapor that feeds the evaporator coil. Repeat.
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Well really its a net loss system with a byproduct of heat and waste energy, so while your house is cool, the outside air is even hotter because of the byproduct, and it's not 100% efficient, so really cooling your house is heating the world and causing global warming.
 
NickMaul said:
This thread might as well be written about my house :shaking: 1920s four square house with a slate roof that sits in the afternoon sun. AC system runs from 4pm until after midnight most summer nights just to hold 75 degrees. Heat wave days the thermostat will register 78 upstairs. However, my upstairs unit thermostat is placed in a bad spot and my stairs don’t have a door to separate the zones. The bedrooms are all comfortable but none of that cool air reaches the thermostat.
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Some smart thermostats now have remote sensors you can use to control it instead.
 
My first house had a split system. One outside unit, but two thermostats with a damper that was supposed to regulate both floors. Biggest piece of shit system ever. Had nothing but problems and the upstairs was never right. Do not recommend.
 
Wes said:
My first house had a split system. One outside unit, but two thermostats with a damper that was supposed to regulate both floors. Biggest piece of shit system ever. Had nothing but problems and the upstairs was never right. Do not recommend.
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They've come a long way. My downstairs has 1 unit and 3 zones. It works well. I've had to replace 2 damper motors but they're easy and available on Amazon.
 
I was talking to a guy today that reminded me of these things - register booster fans. Can be useful in select circumstances, like when there's a pressure drop getting up to upstairs vents or at the end of a long run. Its just stealing a little big of volume from elsewhere so can be helpful to balance things.

If you have access to the ducts there are similar things you can add in-line by cutting the duct and slipping it in.
 
without all the data, my initial bet is on minimal insulation on ductwork and air handler in unconditioned attic, on a system that was likely not pressure tested and or actually sealed.

Meaning so much loss thru the ducts themselves makes the cooling ineffective upstairs. That loss being from air leakage and minimal insulation.

TLDR: don’t put air handler and ductwork in unconditioned attic space. Envelope of the living space or in conditioned portion of attic space.
 
Mac5005 said:
TLDR: don’t put air handler and ductwork in unconditioned attic space. Envelope of the living space or in conditioned portion of attic space.
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...but thats more work for the designer / HVAC guy and after the house is sold its not his problem
 
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