Whole House Air Conditioning Ratings

Al

Joined
Jun 8, 2002
Messages
20,176
Location
Elizabethtown, Pa
Most houses with say 2,000 sq ft. in central PA have 2.5 to 3 Tons of AC installed. If I had whole house and needed a unit I would go no larger than 2 Ton. on a hot day (90F in PA) those compressors cycle mega times. It makes more sense to have a smaller unit that DEHUMIDIFIES way more than a larger unit.

I have a ranch type house and use window units.I have a 6,000 Btu unit in my addition that is 20x20 with a cathedral and 3 exposed walls. Now that room has a doorway and a "window opening" to the rest of the house. If it remains warm at night I sometimes run the unit 24/7 and the temp in the addition will be maybe 76.5+. If I closed the door it would be cooler. I do have AC window units in both my bedrooms and they are 5K Btu.

If I run all three the house is comfortable but right now (only running it16 hours/day its 77F....BUT... the humidity is 37%. It is honestly quite comfortable. I realize contractors over install by 30%bc they obviously don't want calol-backs.

My point of posting.

1. I enjoy arguments
2. It may get a person or two to rethink capacity if they are installing a new outdoor unit. The more it runs the more it dehumidifies. Even for a window unit. It is recommended 1/2T for 250 sq ft. And remember I have 400 sq ft and a cathedral ceiling and have a 1/2 T unit.
 
I was perfectly happy with a house full of window shakers.

We often didn't bother with actual AC in some areas but just a dehumidifier.
 
We had multiple days here over 100 and high humidity. My units run about 25-30% of the time when it’s that hot according to my Sensei thermostat app. I have them set to have longer cycle times for a bit more dehumidification. House is typically 45-50% humidity in the summer, I don’t think I could ever get down to 35%.
 
4600 square feet with one 3 ton unit and one 2 ton. Thermostats at 71 during the day and 66 or 67 at night. My power company has loved sending me bills this summer.
 
Depends on your thermostat setup also. I have a bigger unit and smaller house but have my temperature differentials setup accordingly along with humidity control. You can also control how much it cycles the unit. Pretty cool.
 
The more it runs the more it dehumidifies. Even for a window unit.
No, not true. It dehumidifies based on the amount of cooling it does, not whether it does it from continuous duty or on/off cycling. That cooling by itself causes the condensation from the air. I'd like to see a rebuttal to that if you have a source? I worked in HVAC a lifetime ago, still have a cousin that does.

Running constantly to reach some temperature is more efficient energy wise, but does not reduce humidity more, and after a certain point in the duty cycle will wear it out faster.

This is also true for the window units, less efficient and wear out faster than a (semi-modern) central unit, though you can again save energy by only cooling to a lower temp, the rooms you need to. Plus several window units are higher maintenance than one central unit, though the upside to them is if you aren't well versed in DIY HVAC, you may avoid labor costs by replacing your window units instead of calling in an HVAC company to replace central unit parts. These days just having an HVAC company make a house call, before parts+ costs, can cost about as much as a modest sized window unit.
 
You would get lots more with a 2 ton unit. What is your humidity in the house.
No, you would get zero more and probably less because with the blower running for a longer % of the time, it is evaporating more moisture back out of the evap coil drain pan and off the evap coil, back into the air steam put through the ducts.

It seems like there is some misunderstood info here. An A/C removing more humidity the more it runs, depends on it being the same tonnage, that it reduces the air temp more. Reduce the tonnage and it has to run longer just to attain the same humidity reduction, not more, at same resultant room temp.

IMO the best long term result is a thermostat with a wide temp margin, that you get a unit larger than the area needs, then you set say a 3 degree margin so that the A/C isn't cycling as often. If you are comfortable with even more than 3 degrees swing, so be it.

Whatever average temp that ends up being it will remove every bit as much humidity as a unit running 100% of the time to achieve the same avg temp, except you can't factor for 100% because the outside temperature varies and so will your A/C duty cycle.
 
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You would get lots more with a 2 ton unit. What is your humidity in the house.
2 ton with 2k sq ft in Florida would be miserable. Climate plays a big role. It desert SW we double the sq ft basically. 2000= 4 ton, 2ton would be a small 1000-1200 sq ft house. In the afternoon blazing heat a small unit will run non stop and maybe get below 80 humid or not, that’s not enjoyable
 
IMO the best long term result is a thermostat with a wide temp margin, that you get a unit larger than the area needs, then you set say a 3 degree margin so that the A/C isn't cycling as often. If you are comfortable with even more than 3 degrees swing, so be it.

3300sq foot house in Northern VA, 4-ton unit.

What I do is set the thermostat to 72F around 10pm. And it stays at that temp till 8am the next morning, when the programmable thermostat goes to the daytime setting of 80F.

Most days it takes till at least 6-7PM to reach 80F. So it takes nearly 12 hours for my house to gain 8F. The AC does not come on even once during that entire time.

I like it cold at night, but I'm also fine with it being warmer during the day.

It also helps that the outdoor temps are lower during the night, so the unit not only uses less electricity (about 1kW less) but moves more BTUs so the discharge air temp is lower.

My most recent electric bill was $114. House is all electric except for a gas (propane) cooktop, and some of that cost is to charge a 2013 Chevy Volt.
 
Most houses with say 2,000 sq ft. in central PA have 2.5 to 3 Tons of AC installed. If I had whole house and needed a unit I would go no larger than 2 Ton. on a hot day (90F in PA) those compressors cycle mega times. It makes more sense to have a smaller unit that DEHUMIDIFIES way more than a larger unit.

I have a ranch type house and use window units.I have a 6,000 Btu unit in my addition that is 20x20 with a cathedral and 3 exposed walls. Now that room has a doorway and a "window opening" to the rest of the house. If it remains warm at night I sometimes run the unit 24/7 and the temp in the addition will be maybe 76.5+. If I closed the door it would be cooler. I do have AC window units in both my bedrooms and they are 5K Btu.

If I run all three the house is comfortable but right now (only running it16 hours/day its 77F....BUT... the humidity is 37%. It is honestly quite comfortable. I realize contractors over install by 30%bc they obviously don't want calol-backs.

My point of posting.

1. I enjoy arguments
2. It may get a person or two to rethink capacity if they are installing a new outdoor unit. The more it runs the more it dehumidifies. Even for a window unit. It is recommended 1/2T for 250 sq ft. And remember I have 400 sq ft and a cathedral ceiling and have a 1/2 T unit.
A manual-j calculation is probably a good starting point for properly sizing a unit, but small factors can also change the outcome. It's a tool, but you gotta be careful about how you use it.

For example....if you do a manual-j based on the heat load of 2 people, but you regularly have 15 person parties, that will be a problem. Or if you replace the windows with more efficient ones, now you're suddenly oversized.

For your use you should really consider some mini-splits.
 
My house is 2900 square feet, three floors, and I have a three-ton unit. It doesn't get crazy hot here usually but we do have 90+ days consistently every summer and my house has lots of south facing windows. I keep the temp at 75-76 with no problems.
 
My parents had a Rheem variable AC installed a few years ago. Big difference in feel from the traditional setup. You can set the temp higher and the compressor will run at a lower output 24/7 in the summer pulling out more humidity than a cycling single speed can.

So it actually feels nice inside at higher temps that both my mom and dad both can handle.

With the old system, my mom wanted the thermostat set higher than my dad which made the house muggy.
 
My parents had a Rheem variable AC installed a few years ago. Big difference in feel from the traditional setup. You can set the temp higher and the compressor will run at a lower output 24/7 in the summer pulling out more humidity than a cycling single speed can.

So it actually feels nice inside at higher temps that both my mom and dad both can handle.

With the old system, my mom wanted the thermostat set higher than my dad which made the house muggy.
They're great when they work. But the parts for them are ungodly expensive.
 
My parents had a Rheem variable AC installed a few years ago. Big difference in feel from the traditional setup. You can set the temp higher and the compressor will run at a lower output 24/7 in the summer pulling out more humidity than a cycling single speed can.

My trane air handler has an input for a humidistat, which slows down the fan speed if the indoor humidity is too high, for better dehumidification. I bought a Honeywell Humidipro to use with it, and I installed it in the basement supplied with 24V power from the air handler, but I never got around to wiring it to control the fan speed. Not sure I really need to, humidity seems to be kept under control the way it runs now.
 
No, not true.
You are responding to a situation that does not exist in my case.

Assume:
1. Large airtight box with specific amount of air and hence WATER
2. The boundry allows a certain amount to pass
3. Outside temp is 85 and inside air is 78 (heat transfer coefficients [conduction and convection of inside surfaces] are not required all that is required is #2 above.
4. The cooling unit is very low compared to most houses.

As the cooling unit operates its inlet temp is 78F and outlet temp is 50F. In the process water is removed.. The "cooled" air immediately is quickly heated by the bulk of the rest of the box air and negligible change in occurs to the bulk air (remember heat transfer is still occurring through the walls.

After a period of hours the amount of moisture in the air is removed but the temperature has not changed bc heat has moved through the walls at equilibrium conditions (outside 85F and inside 78F). At some point the air humidify will have dropped to a value where more significant cooling will occur (energy is used increasingly to cool air vs lowering the humidity. (ie. Enthalpy decreases). If the air tries to get cooler bc of more cooling, more heat will enter the house (but no moisture-in the ideal case). The result will not change a lot at the cooling unit bc there will be an EQUILIBRIUN if heat through the walls and heat rejected at the condenser. The humitity will also stop dropping.

Thats why my humidify is 37% and larger units in the same Box will have higher humidity but lower temps.

Hope that was clear. I know its correct.
 
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