AC efficiency in regular weather

S-F · 08-12-12, 07:07 AM

I have been using AC in temperatures lower than I have ever before recently. The reason is that I have been working more hours in hotter weather than ever before. I recently finished an attic job that took two weeks of 12+ hour days. That attic was clocked at 130° most days before noon. We stopped looking at the thermometer after a while. There is always a disparity between what the thermostat says and how the air feels. When the thermostat says 70° in the winter the house doesn't feel like a 70° fall day. Here at night I have been keeping my AC at 71 and the outside temp has been hovering around the same. Opening all of the windows raises the humidity to the point that I start feeling ill with heat exhaustion. I'm wondering how efficient an AC is when the requested temperature is near to or slightly above the outdoor temp. You still have to run the compressor and fan and you have to remove the latent heat. What does this take? I've never seen information on this particular topic.

Xringer · 08-12-12, 08:52 AM

We have been using the Sanyos at night. Sometimes we run at 75, when it's 72 outdoors.

I knew it was going to be cool last night, so I set the Den Sanyo for dehumidify.
The living room Sanyo was left at 23C.
It keeps the house a little drier, (easier to sleep without sticking to the sheets)
and we only used 2.0 kWh since midnight. It was a mild night.

Earlier this morning, we shut off the Sanyos and opened the windows.
It's not real warm outside, but it's so dank! We didn't even last 2 hours.
Now the Sanyos are back on, set about 1deg C below outdoor temps. (23C)
It feels great now! And, it's not hard for them to keep up today.
It's mostly cloudy & 76F. The attic is only 81.9F

MN Renovator · 08-13-12, 03:02 AM

Anything above 55 degrees dew point and if you open the windows, you'll get discomfort inside and your basement will sweat. After running the dehumidification all night and then opening the windows, you pretty much wasted 2kwh.

MN Renovator · 08-13-12, 03:09 AM

Quote:

Originally Posted by S-F

.. I'm wondering how efficient an AC is when the requested temperature is near to or slightly above the outdoor temp. You still have to run the compressor and fan and you have to remove the latent heat. What does this take? I've never seen information on this particular topic.

If I run the A/C when it's 65 degrees out the full system(including indoor blower) uses 2.2kw. If it's 90 degrees out it will draw 2.5kw. This is with a 2 ton 8.5 SEER system from 1986. The temperature drop over the coils is higher when its cooler outside, so you don't just get lower power draw, you also get better latent heat removal/dehumidification and the house cools off much quicker, even more if the sun is down at the time. The extra capacity and dehumidification I get by running the system almost exclusively at night has made it comfortable enough to not run the system when it is 85 degrees outside. My mid-June to mid-July electric bill was 266kwh and I'm about to get my mid-July to mid-August bill and it will be lower than last month, which usually isn't the case but I've gotten used to the heat and have been running less a/c during the day and averaging a 2 hour steady run during the coldest part of the night and stay and sleep in the cooler lower levels of the house where it is usually 70-76 degrees depending on the day.

Xringer · 08-13-12, 07:11 AM

Quote:

Originally Posted by MN Renovator

Anything above 55 degrees dew point and if you open the windows, you'll get discomfort inside and your basement will sweat. After running the dehumidification all night and then opening the windows, you pretty much wasted 2kwh.

We wasted 0.8 kWh last night, because I just got up and opened the windows.
For some reason, I have not opened any basement windows yet this summer.

Sometimes, you just have to listen to the birds and smell mother nature..

MN Renovator · 08-13-12, 07:27 AM

If you opened your basement windows, you add moisture to the basement and not remove it. We've covered that before. Moisture migrates very well. I cool the top two floors of my house and don't cool the two lower floors. The basement dew point will drop to a similar dew point within a few degrees when running the air conditioning for two hours. If you dehumidify the basement, you are essentially dehumidifying the whole house. This is also why air sealing is so critical because the house temperature might not climb on a too much on a warm summer day but the humidity inside sure will affecting comfort significantly.
I read something on a building science type web site that said that on a hot humid summer day, even a hole in the wall the size of a baseball would be enough to cause a significant difference in moisture ingress even without wind to the point where not running the A/C for even a day could cause a dew point high enough to cause basement mold.

AC_Hacker · 08-13-12, 12:53 PM

Quote:

Originally Posted by MN Renovator

...I cool the top two floors of my house and don't cool the two lower floors...

I know that mountain goats move into higher, cooler elevations in the summer months (where it is cooler), and move down into the lower, warmer elevations in the winter months (where it is warmer).

So why don't you just move out of the top two floors in the summer (where it is too hot) and move into the bottom two floors (where it is cooler)?

Surely, mountain goats couldn't as smart as you are...

* * *

Also, if you sprayed a closed cell foam layer on the basement walls, and prevented the high-humidity air from coming in contact with the cold basement walls, wouldn't you stop your condensation problem forever?

-AC

S-F · 08-13-12, 06:24 PM

Quote:

Originally Posted by AC_Hacker

Also, if you sprayed a closed cell foam layer on the basement walls, and prevented the high-humidity air from coming in contact with the cold basement walls, wouldn't you stop your condensation problem forever?

-AC

A lot of moisture would still come through the slab.

AC_Hacker · 08-13-12, 08:31 PM

Quote:

Originally Posted by S-F

A lot of moisture would still come through the slab.

So, how about gluing some closed cell foam to the floor?

...as I recall, we were having this conversation before and I mentioned my revelation at how effective it was to wrap foam tape around cold pipe and how it instantly stopped dripping water.

As I recall you said you that it was an interesting idea and would get back to me on that...

Well, here we are again. My brain cells are still working...

-AC

S-F · 08-14-12, 05:06 AM

Could you please refresh my memory a little more?

08-12-12, 08:52 AM	#2
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	AC efficiency is good. We have been using the Sanyos at night. Sometimes we run at 75, when it's 72 outdoors. I knew it was going to be cool last night, so I set the Den Sanyo for dehumidify. The living room Sanyo was left at 23C. It keeps the house a little drier, (easier to sleep without sticking to the sheets) and we only used 2.0 kWh since midnight. It was a mild night. Earlier this morning, we shut off the Sanyos and opened the windows. It's not real warm outside, but it's so dank! We didn't even last 2 hours. Now the Sanyos are back on, set about 1deg C below outdoor temps. (23C) It feels great now! And, it's not hard for them to keep up today. It's mostly cloudy & 76F. The attic is only 81.9F Attached Thumbnails __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less.. Last edited by Xringer; 08-12-12 at 08:56 AM..

08-13-12, 03:02 AM	#3
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	Anything above 55 degrees dew point and if you open the windows, you'll get discomfort inside and your basement will sweat. After running the dehumidification all night and then opening the windows, you pretty much wasted 2kwh.

08-13-12, 07:27 AM	#6
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	If you opened your basement windows, you add moisture to the basement and not remove it. We've covered that before. Moisture migrates very well. I cool the top two floors of my house and don't cool the two lower floors. The basement dew point will drop to a similar dew point within a few degrees when running the air conditioning for two hours. If you dehumidify the basement, you are essentially dehumidifying the whole house. This is also why air sealing is so critical because the house temperature might not climb on a too much on a warm summer day but the humidity inside sure will affecting comfort significantly. I read something on a building science type web site that said that on a hot humid summer day, even a hole in the wall the size of a baseball would be enough to cause a significant difference in moisture ingress even without wind to the point where not running the A/C for even a day could cause a dew point high enough to cause basement mold.