What's the deal with Central Air?

[mikea]

Jann's condo has a Bryant Furnace with Carrier central air conditioning.

I replaced her first thermostat, an old Honeywell that was corroding away with a Ritetemp (which I later discovered comes from Home Depot) programmable a co-worker gave her. When A/C season came she couldn't get cold air. She was about to call for service when I decided to crack open the thermostat. I see a sticker inside that says "FIRST push the reset button." The reset button is a pinhole that you need to put a paper clip in. I pushed it and suddenly we had A/C.

Life is good until a few weeks later we have no cooling again and we're boiling in the middle of the night. I groggily go over and push the reset button. It works again.

So we decide to replace the piece o' crap thermostat.

I got a Honeywell 5-2 which I put in this morning. I notice on the way that the Rh-Rc jumper wire in the old stat was loose even though I had tightened down all of the screws when we had trouble. *sigh* That could have been it.

I carefully note wire locations and installed the new one. It works fine and looks to be much easier to program for Jann.

But it doesn't seem to be getting cool. It's warm. The fan is running full tme. The air coming out doesn't seem to be cold and blowing much.

I double, triple, quadruple checked the wiring.

The temperature at the stat gets to 77 degrees and stays there.

I opened up the furnace and spent over an hour making sense of how the furnace side was wired and whether the thermostat side as I understood it made sense. It did. I even checked the wiring diagram inside the cover of the furnace which mostly showed the internal wiring, not the external.

I found a web site that explained how things should be wired. http://xtronics.com/reference/thermostat_wires.html It turned out that my understanding was exactly correct. The colors of the wires in the cable don't match but what they are and where they go does.

Finally, knowing that if I short Rc/Rh to G and Y; I should be calling for full cool. I do that. The A/C works the same as it did when the thermostat was connected.

I vaguely recall that we had the same problem the first time I replaced the stat and when the reset was needed. It seems like when the A/C is cooking the fan motor runs at a faster speed and the heavy cooled air noticibly pours out.

I noticed that the furnace has a multispeed fan. I can't imagine how it decides to change the fan speed. Would it make it faster for cooling vs. heating? It could know that based on the G wire which AFAIK isn't used for heating.

We finally gave up and I shut it down with the power switch. As I remember the last time, after we let it rest for several hours it came to life. Could it be that the compressor gets air in the lines due to the excessive power cycles while I'm messing around?

Anybody have any ideas?

We're about to call in the $$$ troops. Worse case they can charge the refrigerant (I hope).

 

[Dakota Duce]

Here's a good all around A/C - Heating web site you might want to look at.

http://www.toad.net/~jsmeenen/

Hope this helps you out.

Dakota Duce

"May All Your Flights Be Of Good Weather!"

 

[JRitt]

Try this link and it may answer your question about wiring the thermostat.
http://www.askmehelpdesk.com/advice/t-8317.html

As far as the multipule speeds on the furnace fan the unit only uses one of the speeds. The speed is set by what pins go where on the plug to the fan and the selection is based on the BTU of the furnace and the AC.

If the link does not help post the wire colors availible to the thermostat and I can get you the correct wiring diagram for it.

 

[mikea]

Try this link and it may answer your question about wiring the thermostat.
http://www.askmehelpdesk.com/advice/t-8317.html

As far as the multipule speeds on the furnace fan the unit only uses one of the speeds. The speed is set by what pins go where on the plug to the fan and the selection is based on the BTU of the furnace and the AC.

If the link does not help post the wire colors availible to the thermostat and I can get you the correct wiring diagram for it.

I'm CERTAIN it's wired right. It just doesn't always work right.

The fan switch works. ON turns the fan on. In Auto, the fan turns on when it calls for cool. We are getting some, sorta cool air.

The wiring is right. I had puzzled it out by the time I found that diagram and explanation site above.

It's something in the mechanics or the compressor that's weird.

I didn't mention that she has a lot of windows facing south. I blamed the heat load from the solar heating but the A/C has overcome that before. In the winter we can turn off the heat on sunny days and the place stays warm.

 

[Dakota Duce]

As far as the multipule speeds on the furnace fan the unit only uses one of the speeds. The speed is set by what pins go where on the plug to the fan and the selection is based on the BTU of the furnace and the AC.

Actually if the unit is wired correctly the fan (blower motor) will run on two different speeds. On a three speed fan, you usually wire the fan for slow speed in "heating" mode.. and high speed in "cooling" mode.

The reason for this is: in heating mode it takes less fan speed to move the heated air which is lighter and easier to flow along the duct work. The A/C cooled air is heavier, (in relationship to the heated air), which takes a more forcefull air flow to move it the same distance.

That's why you find, in most cases, your supply duct work in the floor if you're in a cold climate because you do mostly heating (be it gas, oil, or electric). Because hot air naturally rises, the fan is set to slow speed.

Down South and out West you mostly A/C cool. That's why you mostly find the supply's in the ceiling where the cold air (being heavier) will fall, thus the high speed fan. It's better to set your fan limit on a furnace to 103*F on shutting down/off.. and somewhere in the area of 135*F to *138F for blower fan on. A lot of techs set the fan off to about 98*F, but with older Sr. citizens or real young children in the home the 98*F seems too cold for them.

If you were to run high fan speed in heating, the air actually has a tendency to reduce the actual amount of heat being supplied to the supply registers.. especially at burner start up and shut down.

Just the opposite is true with with A/C. Depending on the btu size of the evaporator, to SLOW of a fan speed has a tendency to cause the evap coil to freeze up and/or freeze over. This is readily apparent when one has a very dirty return air filter. If, as a example, the evaporator is putting out 100 units of cooling and the circulating fan speed is too slow.. (and/or the very dirty return air filter) allowing 50 units of air to pass through the evap coil.. then the coil will put out 50 more units of cooling than be absorbed by the return air, thus causing the coil to freeze.

Well I guess that's all for "Heating & A/C 101" for today.

Dakota Duce

"May All Your Flights Be Of Good Weather!"

 

[ScottM]

I'm CERTAIN it's wired right. It just doesn't always work right.

The fan switch works. ON turns the fan on. In Auto, the fan turns on when it calls for cool. We are getting some, sorta cool air.

The wiring is right. I had puzzled it out by the time I found that diagram and explanation site above.

It's something in the mechanics or the compressor that's weird.

I didn't mention that she has a lot of windows facing south. I blamed the heat load from the solar heating but the A/C has overcome that before. In the winter we can turn off the heat on sunny days and the place stays warm.

Had a similiar problem two years ago. It turned out to the the controller which is in the furnace. I also needed a new gas plenum and the cost was around $350. It was just as the purchase of the high efficieny furnace was going to give me a ROI over a standard one. Dealyed that for one more year, sigh!

 

[JRitt]

It sounds like the main contactor (relay) in the condensing unit (outdoor) is going bad. You can check it real easy you need a multimeter) by removing the small access panel (remove power first)where the AC power goes into the condensing unit). There will be a large 30+ amp contactor. reapply power and turn on the air conditioner (at the thermostat) and the relay should close and the compressor and fan should turn on. If the relay closes and the compressor and fan do not turn on the contactor is probably the cause. You can check it with the multimeter. 24vac on the coil closes the contactor. you chould get 240vac on the incoming side and 240vac on the outgoing side if the coil is energized. These go out between 5-10 yrs. You can get a replacement at any heating and air conditioner place

 

[mikea]

It sounds like the main contactor (relay) in the condensing unit (outdoor) is going bad. You can check it real easy you need a multimeter) by removing the small access panel (remove power first)where the AC power goes into the condensing unit). There will be a large 30+ amp contactor. reapply power and turn on the air conditioner (at the thermostat) and the relay should close and the compressor and fan should turn on. If the relay closes and the compressor and fan do not turn on the contactor is probably the cause. You can check it with the multimeter. 24vac on the coil closes the contactor. you chould get 240vac on the incoming side and 240vac on the outgoing side if the coil is energized. These go out between 5-10 yrs. You can get a replacement at any heating and air conditioner place

Thanks! That makes sense. That would lead to more adventure because AFAIK now each unit has it's own condensor on the roof. It's a 4 story building.

As before, after I let the system rest for several hours, I threw the power switch and all is well. It was COLD this morning.

I still wondered if the compressor cavitates or something when it cycles quickly too many times. The funny thing is the new thermostat has a built-in several minute delay to avoid cycling the A/C too often. That wouldn't have mattered as I was messing with the wiring while I was troubleshooting.

I want back the precious weekend time I wasted on this! I had the thermostat replaced in 20 minutes. Then the fun began.

Jann is going to call in a pro to check it out. I HATE the idea of having to pay $100s for something I might have been able to do, but on the other hand, I can't do a recharge and my time is worth something, too.

Can they still recharge a home R-12 system without retrofitting it?

 

[Dakota Duce]

"mikea" said in part..

..on the other hand I can't do a recharge.. and my time is worth something too. Can they still recharge a home R-12 system without retrofitting it?

If it's a split system like you described, then it's most likely R-22.. and not R-12. You've got a bigger problem if it needs to be re-charged. In a sealed system your freon doesn't leak out just for the heck of it.. nor is the equipment manufactured to do so.

If the tech tells you they need to re-charge/add freon then obviously there's a leak somewhere.. and to add freon is just a waste of $$$! Have them look for and find the leak first.. then depending on where it's at is when you decide to either repair.. or replace. The best way to decide is to use the 50% method. If it's going to cost you 50% or "more" to repair, in comparison to replace, then you're better off replacing. You'll end up saving more money in the long run.

Keep us posted as to your situation.. and what you've decided. Things like this is what keeps this board interesting.

Dakota Duce

"May All Your Flights Be Of Good Weather!"

P.S. End of lesson #2 in "Heating & A/C 101".

 

[gismo]

Thanks! That makes sense. That would lead to more adventure because AFAIK now each unit has it's own condensor on the roof. It's a 4 story building.

As before, after I let the system rest for several hours, I threw the power switch and all is well. It was COLD this morning.

I still wondered if the compressor cavitates or something when it cycles quickly too many times. The funny thing is the new thermostat has a built-in several minute delay to avoid cycling the A/C too often. That wouldn't have mattered as I was messing with the wiring while I was troubleshooting.

I want back the precious weekend time I wasted on this! I had the thermostat replaced in 20 minutes. Then the fun began.

Jann is going to call in a pro to check it out. I HATE the idea of having to pay $100s for something I might have been able to do, but on the other hand, I can't do a recharge and my time is worth something, too.

Can they still recharge a home R-12 system without retrofitting it?

Mike, something you ought to know about this. Almost any residential AC compressor will fail to start if it has recently run and shut down. It can take several minutes for the high pressure to bleed off and until it does, the compressor motor will remain stalled when you turn it on. The usual result of that is for a the motor to overheat until the thermal overload protection kicks in removing power from the motor. Recovery from this can take a long time (5-15 minutes) especially if the compressor sits where it's really hot to start with (like on a roof next to a bunch of other units).

And FWIW the typical residential combination furnace AC blower has two operational speeds, one for heat and one for AC. One or both of those may also be "adjustable" by changing pulley dimensions or selecting taps on the blower motor windings. Normally the fastest speed is used for AC and something much slower runs with the heat. A combination heat/cool thermostat usually has a fan switch with two settings "auto" and "on". In the auto position the fan motor will run at the slower "heat" speed when the bonnet (where the heat exchangers are) temp reaches a fairly high temp (like 120-150F) for heat delivery and it will also run at the higher "cool" speed whenever the AC is on. The "on" setting should cause the blower to run at the fast "cool" speed continuously as long as the thermostat mode isn't "off". The fast speed is always commanded by the thermostat itself, that is there is no other connection between the furnace blower control and the AC compressor/condensor outside other than the thermostat itself. As a result the blower will run if you select "cool" and the thermostat temp is above the target cooling temp or if you put the fan switch to "on" whether or not the outdoor AC is even powered let alone working.

Assuming your AC compressor isn't running when you have the problem the potential causes for that are:

1> No AC to the rooftop unit.
2> AC compressor motor is overtemp.
3> The low pressure side of the freon circuit is too low due to a leak.
4> The high pressure side is too high. Usually due to the evaporator coil (the thing that cools the air) icing up. More often than not this happens when the airflow is insufficient or the expansion valve is acting up.
5> Either the high or low pressure limit switchs are malfunctioning (pretty rare)
6> The contactor for the compressor is defective.

IIRC on most of the residential units the same contactor controls the compressor and the condensor fan so if the fan (in the rooftop unit) is turning you can probably assume the contactor is OK. Some high pressure limits have a manual reset and if yours does (usually a red button on the exterior of the rooftop unit) your problem wouldn't correct itself after a while.

 

[mikea]

Mike, something you ought to know about this. Almost any residential AC compressor will fail to start if it has recently run and shut down. It can take several minutes for the high pressure to bleed off and until it does, the compressor motor will remain stalled when you turn it on. The usual result of that is for a the motor to overheat until the thermal overload protection kicks in removing power from the motor. Recovery from this can take a long time (5-15 minutes) especially if the compressor sits where it's really hot to start with (like on a roof next to a bunch of other units).

...And FWIW the typical residential combination furnace AC blower has two operational speeds, one for heat and one for AC.
Assuming your AC compressor isn't running when you have the problem the potential causes for that are:

1> No AC to the rooftop unit.
2> AC compressor motor is overtemp.
3> The low pressure side of the freon circuit is too low due to a leak.
4> The high pressure side is too high. Usually due to the evaporator coil (the thing that cools the air) icing up. More often than not this happens when the airflow is insufficient or the expansion valve is acting up.
5> Either the high or low pressure limit switchs are malfunctioning (pretty rare)
6> The contactor for the compressor is defective.

IIRC on most of the residential units the same contactor controls the compressor and the condensor fan so if the fan (in the rooftop unit) is turning you can probably assume the contactor is OK. Some high pressure limits have a manual reset and if yours does (usually a red button on the exterior of the rooftop unit) your problem wouldn't correct itself after a while.

Thanks, Lance, I think you nailed it. It makes sense since I remember as I shorted the cooling lead I heard a relay close but I didn't hear a (compressor) motor run... or is the compresser be on the roof with the evaporator?

Anyway I think it makes sense that the compressor does overpressure and overheat, where a thermo protector shuts it off and it has to have no demand an hour of two to cool down. It must be on this one that if you keep up the demand the compressor stays hot.

I guess we have nothing to fix until we can't get the thing to reset after powerdown if needed. As I recall we have yet to have it fail in normal use. That faliure mode is even less likely with this stat having the delay feature.

 

[Skip Miller]

Thanks, Lance, I think you nailed it.

Agree, lots of good info in Lance's post.

It makes sense since I remember as I shorted the cooling lead I heard a relay close but I didn't hear a (compressor) motor run... or is the compresser be on the roof with the evaporator?

The condenser is on the roof, the evaporator is in your unit.

They want the compressor remote-mounted to cut down in-unit noise. So it, too, is on the roof (in your installation).

Anyway I think it makes sense that the compressor does overpressure and overheat, where a thermo protector shuts it off and it has to have no demand an hour of two to cool down. It must be on this one that if you keep up the demand the compressor stays hot.

Another possibility: check the evaporator to see if it is iced up. We had a contactor stick closed and the compressor ran continuously, although the blower shut off as intended. The evap coils iced up totally and when the blower turned back on, we weren't getting any air flow through the ducts! We had to secure the compressor at the main breaker and let it thaw out for 4-6 hours, IIRC. Your unit could also ice up if it has a depleted R-?? charge. Checking the evap when the problem exists is relatively easy.

-Skip

 

[SCCutler]

Jumping in late here, but I am seconding Skip's recommendation, big-time. Your initial issues sounded most like an iced-up evaporator.

How long has it been since her evap coils were cleaned? Lotsa dust on 'em, and they get very wet with condensate, then freeze.

If she has not had a general service done, now would be a good time.

 

[mikea]

You know how you put off going to the doc until you decide you should and then you end up sitting in front of the doc explaining that the problem is gone now?

Jann's A/C went into dumb mode again in normal use earlier this week, the last time simply because I took the thermostat out of auto mode to set it to fixed 73 degrees on a 75 degree day. Once again when it blew warm air for a few hours I turned it off and opened up the windows to cool the place off. After a few hours of rest it was fine. This happened on two consecutive days.

Jann called in a pro who is due here shortly. It's a sunny day and 90 degrees outside.

I tried to make the A/C fail again. It got so cold I had to set the temperature higher. That's never happend before.

I'm going with your theory that it ices up on cooler days but as I recall we've had it fail on 80+ degree days, too.



I hope we can make it fail for the repair guy. It's possessed.

 

[CJones]

When I worked as an on-call maint. person for a large apartment company in Ames, we had several A/C units freeze over at the beginning of the year. Students weren't used to the heat yet, so they would walk home from campus and crank the A/C wide open. About an hour later they would call saying that the fan was blowing, but no cold air. 99% of the time it was b/c the compressor had frosted over. Let it rest for a bit and it would work fine. If I ever had it happen more than once at one apartment, I would ask how hard they were running it. Most of the time they were trying to cool a 3rd story 1100 sq. ft apartment from 85 to 65 degrees in about 1 hr which caused the unit to frost over. I would tell them not to set it so friggin low and let it work itself down slowly. Then I would go outside and make sure nothing was blocking the ventilation around the compressor. If that didn't fix it, I would call the A/C contractor and let him deal with it.

Interested to find what the 'real' problem is.

-Chris

 

[mikea]

Interested to find what the 'real' problem is.

It was down a pound of freon. When I said "That means it has a leak." He said that chasing down a leak is 'nother whole, expensive project.

He was seeing temps of 74 degrees until he added the freon and was getting 52 degrees.

He also told me if it freezes up - and it could have due to the low levels - I'll see ice at the furnace. I think we had freeze ups. He said the ice blocks the flow so the air coming out would be weak and that's what I was seeing.

I guess being that as far as we know this is the first recharge in 20 years, we'll forego believing it has a bad leak.

It was only $102. Not bad.

I'll send Jann my bill.

BTW, once I told him had replaced it, he checked my wiring at the thermostat. I disappointed him. The only thing he changed was what side of the R-Rc jumper the wire to the furnace went. A whole .0000001 ohm difference. That's the way the pros wire it.

Thanks all for the edumacation.

 

[gismo]

It was down a pound of freon. When I said "That means it has a leak." He said that chasing down a leak is 'nother whole, expensive project.

He was seeing temps of 74 degrees until he added the freon and was getting 52 degrees.

He also told me if it freezes up - and it could have due to the low levels - I'll see ice at the furnace. I think we had freeze ups. He said the ice blocks the flow so the air coming out would be weak and that's what I was seeing.

I guess being that as far as we know this is the first recharge in 20 years, we'll forego believing it has a bad leak.

It was only $102. Not bad.

I'll send Jann my bill.

BTW, once I told him had replaced it, he checked my wiring at the thermostat. I disappointed him. The only thing he changed was what side of the R-Rc jumper the wire to the furnace went. A whole .0000001 ohm difference. That's the way the pros wire it.

Thanks all for the edumacation.

I think you got off pretty cheaply. The A/C on my car quit two weeks ago and after spending something like $1300 it's still not fixed. According to the shop (who's been really good to me over several years) two high pressure hoses were leaking but when they replaced them this week, they found another leak in one of the pressure switches. A ($180) replacement switch is coming monday.