A/C questions

[rob-the-oil-nut]

I have a few A/C questions I was hoping a few people could help me with.
With the A/C on my car, maybe it's just me doing a "lucky charm" sort of thing but I shut it off before shutting the engine off, and have the engine on before hitting the button to bring the compressor online. Is this smart? Needless?

Home unit: IIRC we have a 3.5 ton that works fine as far as I know. Can it not be on for 48, even 72 hours straight? What causes some AC's to ice up and break if set to 60F? I like to set mine that low, not because I believe it will ever hit 60 F but because I don't want it cutting in & out. The opinions I have heard & that I have grasped include: That it is mainly a giant de-humidifier, should be on all the time, and for goodness sake do not shut it off and open windows at night. It may go down to 68 but there's all that humidity you spent the whole day getting rid of.
I also believe, as a basement-dweller, that if you feel it's getting too cool, open the door to the basement, and maybe check in an hour if you are still too cool. But leave the A/C cranked.
I was also told, at night just turn it up. Switching the mode from cool to off is a BAD thing.
Are these things correct??

When it gets this hot (OK all you Southerners, it's not 100 F but we still have Humidex advisories), I have a towel over my car seat so the sweat doesn't go from me into the cloth seat. Shirt off, AC cranked (full cold, max fan) and the passenger side vents are closed, and the passenger says it's too cold.... I disagree

Still crazy after all these years

Rob the oil nut

 

[kenw]

I have a few A/C questions I was hoping a few people could help me with.
With the A/C on my car, maybe it's just me doing a "lucky charm" sort of thing but I shut it off before shutting the engine off, and have the engine on before hitting the button to bring the compressor online. Is this smart? Needless?

>>>>>>>>
pretty smart. if you shut it down (leave fan on, compressor off)a few minutes before turning of the engine, you will help dry the evaporator (inside) fan area and prevent or reduce mildew buildup. That may not be an issue in Manitoba, Rob, but it sure is in Houston!

*********
Home unit: IIRC we have a 3.5 ton that works fine as far as I know. Can it not be on for 48, even 72 hours straight? What causes some AC's to ice up and break if set to 60F? I like to set mine that low, not because I believe it will ever hit 60 F but because I don't want it cutting in & out. The opinions I have heard & that I have grasped include: That it is mainly a giant de-humidifier, should be on all the time, and for goodness sake do not shut it off and open windows at night. It may go down to 68 but there's all that humidity you spent the whole day getting rid of.
I also believe, as a basement-dweller, that if you feel it's getting too cool, open the door to the basement, and maybe check in an hour if you are still too cool. But leave the A/C cranked.
I was also told, at night just turn it up. Switching the mode from cool to off is a BAD thing.
Are these things correct??

>>>>>>>
your compressor cycles based on the t-stat setting, and if it really is on for 48/72 hours you should be able to hang meat in the den and write your name on the inside of the windows.....

Setting the tstat to 60 is misleading as it doens't realy have much control at that range. Are you sure the compressor is really running all that time? Yes, it will hit 60 and keep dropping. You may have you fan set to ON, yet the compressor will still cycle. On most tstats, there are 2 switches, one for the inside fan (ON-AUTO) and system (COOL-OFF-HEAT). They are pretty much independent, if you set the fan to auto, it only runs when system starts cooling or heating. Set the fan to ON and it blows regardless of where you set the system switch.


Turning from COOL to OFF is not a problem. it's exactly what the tstat does when it reaches the preset temp.

If you wish to turn it off at night, it's perfectly fine to do so. Don't know how damp your nights are there, but if i did that in Houston i would be green in the morning....it's the hunmidity that counts. We do turn ours up (programmable t stat) at night.

we don't have basements here in Houston, Rob, so I can't help there. But for the most part, the giant dehumidifier idea is right.

 

[Gary Allan]

quote:

---

I like to set mine that low, not because I believe it will ever hit 60 F but because I don't want it cutting in & out. The opinions I have heard & that I have grasped include: That it is mainly a giant de-humidifier, should be on all the time, and for goodness sake do not shut it off and open windows at night. It may go down to 68 but there's all that humidity you spent the whole day getting rid of.

---

There are formulas or specs for proper loading of an ac system for a house. Most do the unwise move of getting a bigger unit than needed for some "bigger is better" type feeling. The problem is that the duty cycle is so short that you don't dehumidify enough cubic feet of air ..or at least as much as a smaller unit would. You tend to over shoot your setpoint. The same thing happens with too big a boiler/furnace.

The driest environment will be achieved with the fan constantly on and the compressor on as much as possible. Naturally if the compressor is too big the house will get very cold.

There are contemporary HVAC techs on the board that I'm sure will chime in here with the "fat" on this.

 

[brianl703]

The one thing I've heard is that you should never set the fan to continously run because water that has condensed on the evaporator coil will evaporate off instead of draining out, raising the humidity levels in the house.

I have a programmable thermostat and I set mine to 83F during the day when I'm not home and 79F when I return. The AC runs for a good hour or two to cool the house when I get home, but it also removes a lot of humidity in that time too. It dumps the condensate water into the sump pump pit and I always hear the pump run at least once when the AC is on..

 

[kenw]

re: bigger ain't always better": good point Gary. The irony is that humidity is what determines comfort (for the most part), yet a typical t-stat only controls temperature.

However, running the fan constantly will not aid in drying the air any further since the drying is done only when the compressor is running. Unless the unit is actively removing moisture, the moisture that collects in the coil pan will be re-absorbed by the air blowing thru there, thereby just adding it back into the house. It's probably not much but it is some.

But running the fan 24/7 WILL reduce hot/cold spots, stratification in multilevel homes and filter the air better.

We set ours at a relatively high temp, the fan to auto and use ceiling fans in every occupied room 24x7. But we are in Houston, and preferences will definitely vary.

 

[brianl703]

There's a device called the AirCycler which automatically runs the fan for a programmable interval and duty cycle, but is smart enough to wait to run the fan if the AC unit has been on (so that any condensate can drip off the condensor).

It costs $80 last time I checked, but it would seem to save money over running the fan 24/7.

http://www.aircycler.com/

 

[oilyriser]

I've always wanted to make a wet-bulb thermostat for the a/c here, so it could compansate for humidity.

 

[kenw]

quote:

---

Originally posted by oilyriser:
I've always wanted to make a wet-bulb thermostat for the a/c here, so it could compansate for humidity.

---

they are available, only ones I ever saw were commercial types and probably not cheap.

We called them Humidistats, whether or not that was the "correct" term, a proprietary name or just jargon, i dunno. try Grainger's to start.

 

[kenw]

hey, while we're on the subject, if you live in very dry areas, there is another device that you can home-brew that can have major impact on your AC energy usage and efficiency.

In a nutshell, an air filter pad that is sprayed with water (mist) placed at the air intake area of the outside unit. The water evaporates on the pad, cooling the air going thru it before it gets to the coil. Cooler air going to the coil allows more heat shedding by the unit, and therefore lower current draw. I built an experimental one years ago just to see, the current draw on the compressor dropped a couple of amps.

don't spray water directly on the coil, the evaporation will lead to mineral deposits on the coil and make the situation worse. You will need to replace the pad when it gets crusty or the airflow gets blocked.

The dryer the air, the more evaporation occurs, therefore the more cooling takes place. In humid areas, you won't have enough evaporation to make it work. If swamp coolers (evaporative coolers) are popular in your area, it will probably work. Unfortuantely for me they don't do to well in Houston.....

 

[tom slick]

I have read that it is much better to have a slightly undersized AC system than a slightly oversized system. with the smaller system running more it will mix the air more thoroughly and run longer to dehumidify the air. an oversized system will putt out a big puff of cold air quickly and then shut off.

if you live in an area that is really dry (desert) a evap cooler works much better than a ac unit.

 

[Matt_S]

Regarding AC units icing up, that will happen if the freon level gets a little too low. When that happens, the evaporator gets too cold, causing the moisture in the air to condense on the evaporator, then freeze. A properly charged system should be able to run all day and night without freezing up.

 

[kanling]

With straight air conditioning (not heat pump), ice on the unit is usually caused by low refrigerant. (Or really by the leak!) Setting the thermostat low shouldn't cause it. And, by the way... too much refrigerant causes reduced efficiency and capacity.

There is a really good AC forum out there too:
http://www.hvac-talk.com/vbb/

 

[kenw]

Too too much will damage the compressor by sending liquid into the compressor's intake side, also known as "slugging". The internal check/reed valves are pretty wimpy and get bent by the liquid. Once bent, they no longer seal, the compressor no longer compresses, etc. It's also why you add gas, not liquid when adding since you are adding on the compressor inlet/suction line.

Remember, liquid isn't compressible, compressors compress gas into liquid.

We used a rule of thumb of looking at the frost on the evap coil outlet tube to get a quick indication of proper freon loading. Frost appearing on about 1' or less of tubing past the evap coil was our clue.

If there was no frost on the outlet tube at all it was undercharged and all the freon was evaporating before it reached the end of the coil run; more than a foot of frost meant that it was still evaporating past the coil and therefore still liquid, and overcharged. If that liquid reaches the compressor, it's really bad.

Obviously this isn't the scientific way, but i do still use it as a quick visual to see if low/high freon is possibly causing the problem before breaking out the gages.