Man it's hot.

[LCHIEN]

I just got back from a 2 week cruise around the British Isles where it was 55 to 75 degrees all the time.
back home to Houston and it's sweltering 100 degrees every day. Besides being tired from the trip it's just too draining to go out and finish the last project. Maybe tonite when it cools to 90.

[capncarl]

Is it a dry heat? Isn’t that what mid west peoples say? It’s so dang hot here I think I’m going to melt! Along with 100 degree temperature the humidity has been bouncing around 100 percent most of the time. The condensate drain from my house AC has been pouring water like a bad leak! Storms that blew through the last couple of weeks left me cleaning up 5 16’x7’ trailer fulls of pine sticks, cones and straw and emptying at the landfill. Last week we talked to a tree service about removing 3 more large pine trees, the guilty trees for dumping the most cones and sticks. Maybe the other trees will take a hint and stop being litterbugs, or they will be next.

[LCHIEN]

No it's very humid here
Feels like temperatures are quoted in the 114-115 range
Although right now 7pm it's just 95, feels like 105 and 48% humidity.

[capncarl]

Photo of neighbors dog

[LCHIEN]

My downstairs Central AC just died.
Its been around 100° F every day for the last week...
You can see since Sunday its been running about 8 hours a day total to keep the temperature at 76.
Thursday it struggled and ran 12 hours and was hitting 79 at the end of the day,
Friday the AC ran all day and was only keeping it down to 83. And that is probably only because the Upstairs AC was running much harder.
Just had a A/C guy out and he said my system was fully charged, but the compressor pressure was low, probably failed.
Looking at a new Compressor now...

[cwsmith]

It's may not be the right thing to do, but we turn on our central air about 11:00 AM and just leave it running until about 8:30 in the early evening. no thermostat setting just 'on'. My wife with her ultra-sensitive hearing doesn't like to hear is cycle on and off, so we just leave it in the on position for that period.

The A/C was last repaired just before we bought the house, about 18 years ago and we haven't had any trouble since. I have it checked every couple of years and no problems so far. With the house being 2,400 sq. ft (not counting the walk up attic or basement) the place is relatively comfortable even when the outdoor temp is well into the 90's and a sunny day. There is about a eight degree difference between the floors, here in the basement it's 66, the main floor around 72, and the second floor about 80. Main thing for me is it keeps the humidity down.

I'm certainly no HVAC expert, but I wonder if just keeping the unit on is less stressful on the equipment running constantly, rather than cyciing on and off; although it would appear that with this old house it would pretty much run constantly anyway.

CWS

[LCHIEN]

AC's work at a constant heat removal rate, rated in BTUs.

Ideally, your AC will be rated to remove more than the worst case heat entry to your house, which is driven by the temperature differential and the amount of insulation and its effectiveness. A perfectly insulated house, once you get it to the right temperature will stay at that temperature if the insulation is perfect.

In practice, say your house on a hot day and you open the doors to go in and out, admits 7500 BTU per hour thru open doors, windows, wall and roof.
You have a A/C rated at 10,000 BTU. So, to balance the heat gain, it has to remove 7500 BTU so it will run 45 minutes every hour to keep your house at an even temperature, since it runs on-off-on and not proportionally, it will have an associated duty cycle.

Whether it runs longer each cycle or many short cycles depends on the hysteresis of your thermostat. Hysteresis is a dead band to keep it from switching on and off rapidly. So if you set it to 75 degrees, it will switch on the AC compressor at 76 but won't allow it to switch off until it lowers to 74 degrees, at which point it turns off the compressor.

As a result, 45 minutes operation per hour will look like maybe three or four operations of 10-15 minutes duration and the temperature swinging back and forth between 74 and 76.

Thermostats have hysteresis settings to adjust the dead band.
In practice you generally don't mess with those, you don't want a lot more than a 2 degree dead band for comfort. and too frequent switching is not good for the compressor.

Generally proper selection of the BTU capacity of the A/C compressor and system is needed.
If the BTU is too small the A/C compressor will run 100% continuously and the temperature will rise throughout the hot part of the day and not reach the set temperature.

If the BTU is too large, the duty cycle will be very low, among other problems, not enough dehumidification occurs.

You should know, working with air compressors. that the tank has a on-off point that is separated by a large hysteresis (maybe 120-150 PSI) to keep the compressor motor from starting too frequently.

[leehljp]

6 inch walls with full insulation plus a good wrap before siding really helps. I can tell the difference when going into a house/structure/church - with that kind of construction.

And I really liked the ductless - split-units they had in Japan years ago. Very efficient. Duct work really puts a restriction on air flow as compared to ductless. I helped build a church about 10 years ago and suggestd the church to look at ductless/split units. They did but it was hard getting someone to install it. It has worked great until this year. Both split units quit working. Upon inspection, mice and a couple of frogs got inside the outside unit and fried the electronics. The church is rural with fields all around it! Learned a lesson.

[LCHIEN]

Its like this

[twistsol]

Meanwhile at home in Minnesota, AC is off, windows are open and the high today is forecast to be 75 degrees Fahrenheit. Pretty much the same here in South Dakota where I'm on an emergency grandpa mission this week ... Day care is closed.

[LCHIEN]

I'll remember you when its 65 here in Mid January and I have to run the A/C in the car because it got too hot in the sun.

[twistsol]

There are only about two weeks a year Minnesotans can brag about the weather.Mid July isn't normally one of those times.

[capncarl]

We replaced our 17 yr old heat pump early this year. Its Freon is phased out and some tax breaks made it the best time before it dies one hot day. I replaced a single speed split unit with a variable speed condenser and evaporator units. So far this summer I haven’t noticed the condenser operating on high, and it’s been 100 degrees, the old unit would have been running in overdrive for months by now!

Concerning insulation, I’ve worked under the assumption that more insulation doesn’t keep something from freezing or heating up, it just delays the time it takes to freeze or heat up.

[leehljp]

"Concerning insulation, I’ve worked under the assumption that more insulation doesn’t keep something from freezing or heating up, it just delays the time it takes to freeze or heat up."

True, but to a practical point - that is only true if no heating or air conditioning is used. When evenings and early mornings are in the mid 60's and mid day has jumped into the upper 90's, a well insulated room or even church will be in the mid 70°s at 11 o'clock (without AC turned on) vs 90° or more in a shed or uninsulated room. An air conditioning unit can keep the large sanctuary or house cooled and with less energy used - in a well insulated house - much more so than with little insulation.

IOW: GOOD insulation "delays" will allow a/c & heating to manage heat swings well and with much less energy.

[capncarl]

My 750 sf detached garage shop had blown in insulation in the attic. A 14,000 btu window heat pump ac cooled the shop nicely. Hurricane Michael came through as a cat 3, blowing down trees, destroying roofs, and it sucked the ceiling out of the carport of my shop and sucked about half of the insulation out of the shop and induced moisture in the remaining insulation. The 14,000 btu ac would no longer keep up wit( the heat load. I called in a spray foam contractor with intentions of them removing all of the blown in insulation and spray foaming the attic ceiling over the shop and carport. They didn’t do that, they vacuumed the blown in insulation and spray foamed 4” on the roof deck. By this time the 14,000 btu ac worked itself to death, and Covid made it nearly impossible to buy anything air conditioning, especially window heat pumps! HD online could get me an Amana 24,000 btu window heat pump in 3 weeks so I did what everyone else was doing, I threw wads of cash at it and got it in 4 weeks. My new drastically oversized AC never cooled the shop as well as the older smaller unit did. I expected the spray foam insulation to make my shop like a refrigerator. When my house AC was recently replaced their ac expert speculated that my heated/cooled square footage was drastically increased when we added my shop attic to the heating/cooling load. If I had known it would turn out like this I would have simply replaced the blown in insulation with fresh, dry batts. Maybe the old ac would not have been killed in the process.

[leehljp]

I'm finding (from comments from different ones such as yours) that newer AC units may be more efficient in electrical use but less powerful in cooling. It seems that companies are overstating the BTU cooling, or the standards of measurements have changed. It should be a pure scientific measurement, IMO, but in two window units my family has purchased for detached rooms similar to yours - those units do not cool as quickly as the older replaced predecessors even though they are rated the same. YOU got me to thinking and I am going to look it up - but I always thought sprayed in foam would be cooler than blown in insulation.

[capncarl]

Older appliances were built much better than their new replacements! The Westinghouse 14,000 btu heat pump heated and cooled my shop nicely. Its cold air output was always around 30 degrees lower than the room temperature. It was uncomfortable working in the room with it on. The new Amana 24,000 btu heat pump cold air output is never as cold. The Westinghouse unit cycled on and off like I am use to, while I can’t get the “SMART” energy star Amana heat pump to cycle on and off. It just goes to a slower fan speed and varies the temperature output. Smart doesn’t seem that smart when it runs 24/7. Our local appliance repair shop tells me that they can seldom repair the newer heat pump ACs like this one.

twistsol’s post reminded me of the conversation I had with the foam installer. Garage attics are not require to have any insulation, nor are the walls! When the shop was built I insulated the walls and had whatever the house attic had blown in. The attic had about 8’x24’ of floored walkspace that did reduce the thickness of part of the shop attic insulation. It was decided to install 6” of open cell spray foam. Closed cell was significantly more expensive.

[twistsol]

There are two types of spray foam. First is open cell which from an R-value is about the same as fiberglass batts and allows air and moisture to flow through it. Closed cell is about twice the R value as fiberglass and open cell as well as an air tight / moisture barrier.

My HVAC guy told me that undersizing and oversizing are equally bad situations for a system. If it is undersized, it runs continuously and works itself to death. Oversized systems start and stop too frequently and that causes other issues.

[LCHIEN]

Here is an engineer’s attempt to describe insulation and cooling (or heating for that matter)

I’d like to set up an analogy that you can get a visual handle on.

Your house is like a bucket, you want to keep cool air in it, cool air is like the level of the water in a bucket, the higher the level of the water the cooler the temperature. We measure heat or cool energy (works the same, just the sign changes) in BTUs.

Insulation is the tiny holes in the bottom of the bucket. Perfect insulation is like a completely waterproof, non leaky bucket, whatever you put in the bucket stays in the bucket. The worse the insulation the larger the holes and the faster your cool leaks out and the level drops.
One thing to note is that the leak rate increases as the bucket is fuller – this is the differential pressure or temperature in the analogy. If the desired temperature is the same level as the bottom, you need not add any cool to the bucket. The higher the bucket is filled above the bottom, the faster the leak.
To summarize, the loss of cooling, the leak, is measured in BTU and a function of the effectiveness of the insulation, the total area (walls, doors, windows), and the temperature differential.

To keep the bucket at a constant level or temperature, the set point, you have to pour in water at the top to match the rate in BTU its leaking from the bottom. So the BTU has to match. Smaller AC units are sold by the BTU, and larger ones by tons, but one ton is 12,000 BTU. ACs run either on or off for the most part. Their on-off duty cycle percentage determines the actual BTU delivered. A 12,000 BTU A/C that runs 50% of the time will average 6,000 BTU. 75% duty cycle will deliver 8000 BTU.

So its clear that you need a BTU rating that’s higher than the worst BTU leakage of your house and that improved insulation can mean your A/C will run less time saving your proportionally more money. As Twistsol mentioned too small a BTU rating will make it run 100% on hot days and never reaching setpoint. But too small will cause it to run such a short time and infrequently it will cause other problems, a big one being inadequate dehumidification.

Ultimately the thermostat controls the duty cycle switching off then the temperature is cooled below the set point and switching on when its above. Thermostats have a deadband of a degree or two so that it won’t be switching on and off too quickly; between that and the amount of air that needs to be cooled or filled in the analogy, it will run for 10-20 minutes at a time to achieve this, limiting excessive start and stop cycles with too small a dead band. A proper thermostat will allow your larger than necessary A/C compressor to match the average BTUs added to the house/bucket to a dynamically changing loss of BTUs flowing out through leaky insulation, opened doors, changing outdoor temperatures, as well as internal heat sources subtracting BTUs like lights, cooking, bodies, and electronics.

[dbhost]

My electric use went WAY down this summer since I added a dehumidifier into the kitchen. I could probably use one in the guest bath and the master suite as well, but overall, getting the RH in the house down to 40% and all of the sudden 75-77 deg F inside is comfortable...