Central AC issues and questions

[Qs Laptop]

Decided to try it again this morning and all worked fine...... Go Figure.

I am still getting the Tech over to check it out as diagnostic visits are free under my contract.

No idea why it failed. I will find out hopefully later today.

Nice job of diagnosis. Shorting out huge capacitors to make sure all the voltage is dissipated is not one of my favorite things to do.

Could the problem be your thermostat?

I sure hope you get an "all clear" from the service tech. Still, there will be that niggling doubt in the back of your mind. Here's hoping it keeps working. Mine's been working great since I posted this:
https://www.early-retirement.org/forums/f27/need-your-air-conditioner-serviced-good-luck-114364.html

 

[ShokWaveRider]

Could the problem be your thermostat?

The Thermostat seemed happy, operated the fan OK, I only turned the condenser off overnight, not the air handler that powers the thermostat, so I have to assume it is good. Still waiting for the tech to call, but it is still working after 4 hours of use.

 

[NW-Bound]

Yes, but OP's system is R22 that operates lower pressures than R410a.

Yes. I know that you know.

Just wanted to point out that new ACs use much higher pressures, and are more susceptible to leaks.

It's a similar situation for car ACs between the old R12 and the current R134a.

 

[ncbill]

Actually the P-T (Pressure-Temperature) curve for propane (R-290) is fairly close to R-22. It is also compatible with the mineral oil in R-22 systems. It can work as alternative for R-22. However, and this is a big however, propane is flammable and requires a leak-free system, and is illegal for home HVAC systems due to the flammability issue. Since R-22 is no longer manufactured, there are blended refrigerants that can mimic the R-22 P-T curve and are considered non-flammable.
^^^^^^Above provided for discussion purposes, not saying to use propane in place of R-22 in your home system.

From the link the reason R-44b is the best "drop-in" replacement for R-22 is that unlike previous proposed replacements R-44b is compatible with the non-synthetic (e.g. mineral oil) lubricants used with R-22.

So, at around only $30/lb. R-44b works just fine as replacement refrigerant once the leak is fixed and the system has been evacuated.

And if "fixing the leak" means replacing the coil, TXV valve, dryer, etc. that's still cheaper than installing a new system with a refrigerant (R-410A) that is only in use for just a few more months.

 

[travelover]

.........
2) Checked the 220v going into the Contactor - Good
3) Checked the output of the Contactor - (Not Good, No 220v to the compressor)
.........

Sounds like the relay (contactor) has failed. Mine failed twice. Once it had dead ants in it, and the second time the points were burned. I filed them until I could receive a new contactor. You can buy a new contactor off Amazon or at Graingers, though the SOBs at the HVAC parts store often won't sell to you

 

[NW-Bound]

Sounds like the relay (contactor) has failed. Mine failed twice. Once it had dead ants in it, and the second time the points were burned. I filed them until I could receive a new contactor. You can buy a new contactor off Amazon or at Graingers, though the SOBs at the HVAC parts store often won't sell to you

So, 220V got to the contactor, but no 220V getting out.

OK. I would check to see if the coil of the contactor get energized. Usually, they use 24VAC to control the relay or contactor. Or if you have someone inside the home turning the thermostat on/off, you may be able to hear if it clicks on/off or not. It would tell you if the contact was burned and you might be able to get access to it to burnish it for a makeshift repair.

If there's no 24VAC to the coil, then it takes some tracing of the circuit to see if an interlock sensor has cut out the power, such as the low refrigerant switch.

 

[Rianne]

We have a 2016 high-efficiency Trane. When the temp outside is close to 75, and we know it's going to get hot that day we turn it on at 77 degrees. It starts to cycle. On 2 minutes, off 2 minutes for a period of time. Our HVAC guy came over 3 times. One of the times he sat outside and listened to the cycling. Could not figure it out. Everything seems to work. New filter, electrical is good. Every morning it cycles before the temp outside gets hot. Once it's hot above our thermostat temp, it runs normally and has kept us cool during these 90-degree days.

Will the cycling shorten the life of the A/C? It has a 10-year warranty on certain parts. Google says the cycling is bad but do high-efficiency A/C units do this?

 

[ShokWaveRider]

Sounds like the relay (contactor) has failed. Mine failed twice. Once it had dead ants in it, and the second time the points were burned. I filed them until I could receive a new contactor. You can buy a new contactor off Amazon or at Graingers, though the SOBs at the HVAC parts store often won't sell to you

It has been working fine for the last 2 days, go Figure. It is a XP21, 21 Seer condenser,

I ordered a new 40a 24v contactor from Amazon, just in case and .... wait for it.... a mere $9.95 including shipping. I know the AC Company charges $190 to replace a contactor, even with our special service contract rates. It would take me a whole 20 minutes to replace. LOL.

 

[JoeWras]

We have a 2016 high-efficiency Trane. When the temp outside is close to 75, and we know it's going to get hot that day we turn it on at 77 degrees. It starts to cycle. On 2 minutes, off 2 minutes for a period of time. Our HVAC guy came over 3 times. One of the times he sat outside and listened to the cycling. Could not figure it out. Everything seems to work. New filter, electrical is good. Every morning it cycles before the temp outside gets hot. Once it's hot above our thermostat temp, it runs normally and has kept us cool during these 90-degree days.

Will the cycling shorten the life of the A/C? It has a 10-year warranty on certain parts. Google says the cycling is bad but do high-efficiency A/C units do this?

Do you have humidity control? Some systems will attempt to bring down humidity even if the temperature is in proper range. When they are in this mode, they do weird things.

BTW: some people have humidity control and don't even know it because it isn't the default display on the thermostat.

 

[Winemaker]

If you have a 2 story house, another trick is to keep the upstairs doors closed. If you think of your AC cooled air as water, you want to "fill" the upstairs rooms with the cool air, if the doors are open, the cool air sinks and flows downstairs. Cool air sinks, hot air rises.

We also have an upper and lower cold air return in each room. During the cooling season, we close the lower return, and the warmer air is pulled in the upper return.

 

[ERD50]

We have a 2016 high-efficiency Trane. When the temp outside is close to 75, and we know it's going to get hot that day we turn it on at 77 degrees. It starts to cycle. On 2 minutes, off 2 minutes for a period of time. Our HVAC guy came over 3 times. One of the times he sat outside and listened to the cycling. Could not figure it out. Everything seems to work. New filter, electrical is good. Every morning it cycles before the temp outside gets hot. Once it's hot above our thermostat temp, it runs normally and has kept us cool during these 90-degree days.

Will the cycling shorten the life of the A/C? It has a 10-year warranty on certain parts. Google says the cycling is bad but do high-efficiency A/C units do this?

Do you have humidity control? Some systems will attempt to bring down humidity even if the temperature is in proper range. When they are in this mode, they do weird things.

BTW: some people have humidity control and don't even know it because it isn't the default display on the thermostat.

Yes, but even with humidity control, a 2 minute cycle does not sound right, way too short, IMO.

The thermostat for my high efficiency unit (also in 2016) had a zillion settings to optimize it for the particular installation. I'd bet the installer did not get this configured correctly - I'd push them on it, 2 minute cycles don't sound right.

Try searching on "short cycling" or "normal ac cycle time" and/or maybe call Trane to get an opinion. Your installer might need a push to correct this.

The refs I found considered anything less than a 10 minute ON time to be short cycling. A contributing factor would be an AC unit that is too large for the home. A smaller one runs longer cycles, and does a better job of taking humidity out.

Is you AC a multi-stage, or variable stage (some high efficiency units are)? If so, it should be running at a lower level to keep a longer cycle under moderate temperatures. Again, a wrong configuration in the thermostat can cause this to go out of whack.


-ERD50

 

[Rianne]

Yes, but even with humidity control, a 2 minute cycle does not sound right, way too short, IMO.

The thermostat for my high efficiency unit (also in 2016) had a zillion settings to optimize it for the particular installation. I'd bet the installer did not get this configured correctly - I'd push them on it, 2 minute cycles don't sound right.

Try searching on "short cycling" or "normal ac cycle time" and/or maybe call Trane to get an opinion. Your installer might need a push to correct this.

The refs I found considered anything less than a 10 minute ON time to be short cycling. A contributing factor would be an AC unit that is too large for the home. A smaller one runs longer cycles, and does a better job of taking humidity out.

Is you AC a multi-stage, or variable stage (some high efficiency units are)? If so, it should be running at a lower level to keep a longer cycle under moderate temperatures. Again, a wrong configuration in the thermostat can cause this to go out of whack.


-ERD50

Thanks, we have a ranch (2500 sq ft), crawl space. All the venting, ducts, etc cleaned. Sealant and insulation installed. Humidity control. The HVAC came and installed an app on both of our phones so we could alert the office when it started doing this. Somehow, they can read the thermostat and see what's going on. They got our wifi password to install it. The unit cost $9500 but we got energy rebates that brought it down to 1/2 the cost.

We're keeping it on one temp because we don't want to confuse it.

 

[JoeWras]

It's a good thing we didn't have A/C when I was a kid. I loved to spin the thermostat and watch the eerie green sparks through the mercury contacts. That was just with the furnace, it could take the abuse. If we had A/C, I'd be burning it out in no time.

Dad: "I hear the air conditioner going on and off, have you noticed, honey?"

Mom: "It's Joey again, he's messing with the thermostat."

 

[ShokWaveRider]

Here is an AC Tip. If you are not sure of the Health of Your start or Run Capacitors and you do not have a Meter with a Capacitance setting. Most meters, especially the cheap clamp on type have Current and Voltage.

Measure the Current (in Amps) going through the Capacitor to be tested, divide it by the voltage across the Capacitor and multiply it by the constant 2652. The result will be in Microfarads.

In my case, the capacitor should be 30 Microfarads.

I/V x 2652 = 4.65/402 = .01157 x 2652 = 30.67 Microfarads. This is close enough as most caps are +/- 5% - 10% So my Cap is good. Results may vary slightly, but the Time constant of 2652 worked good for my 30 Microfarad Capacitor. In reality constants will vary between capacitors.

Or, You simply measure the current (amps) of the motor start winding coming off of the capacitor and multiply it times 2652 (on 60 Hz power; 3183 on 50 Hz power). Then, divide that number by the voltage you measure across the capacitor

This saves putting an Ammeter in Series with the Capacitor supply feed.

Here is a sample clamp on Meter from Harbor Freight.
https://www.harborfreight.com/digital-clamp-meter-96308.html

 

[travelover]

.........
Here is a sample clamp on Meter from Harbor Freight.
https://www.harborfreight.com/digital-clamp-meter-96308.html

I have this linked $17 multimeter and it measures microfarads directly.

LINK

 

[ERD50]

Here is an AC Tip. If you are not sure of the Health of Your start or Run Capacitors and you do not have a Meter with a Capacitance setting. Most meters, especially the cheap clamp on type have Current and Voltage.

Measure the Current (in Amps) going through the Capacitor to be tested, divide it by the voltage across the Capacitor and multiply it by the constant 2652. The result will be in Microfarads.

In my case, the capacitor should be 30 Microfarads.

I/V x 2652 = 4.65/402 = .01157 x 2652 = 30.67 Microfarads. This is close enough as most caps are +/- 5% - 10% So my Cap is good. Results may vary slightly, but the Time constant of 2652 worked good for my 30 Microfarad Capacitor. In reality time constants will vary between capacitors.

Or, You simply measure the current (amps) of the motor start winding coming off of the capacitor and multiply it times 2652 (on 60 Hz power; 3183 on 50 Hz power). Then, divide that number by the voltage you measure across the capacitor

This saves putting an Ammeter in Series with the Capacitor supply feed.

Here is a sample clamp on Meter from Harbor Freight.
https://www.harborfreight.com/digital-clamp-meter-96308.html

Good info. Just FYI, the "2652" is not a time constant that will vary with the capacitor, it is just a simplification of the formula for working backwards from Reactance (obtained with the Voltage and Current readings) to Capacitance. In the process, you come up with 2652.58 for 60 Hz, from 2(π)f, inverse, times 1 million to get microfarads. No part of the capacitance (real or imaginary) comes into play in that part of the equation, it's just straight algebraic manipulation of the constants.

1e6×1/(π×2×60) = 2652.58

I have this linked $17 multimeter and it measure microfarads directly.

LINK

Yes, a good first check, but I like the current, voltage check as it is done under full voltage and current conditions. Just in case something is breaking down at those higher voltages (though that would probably blow the cap anyway?).

-ERD50

 

[iloveyoga]

Meta-observation: people are really passionate about HVAC issues on this forum and bogleheads.

OK, back to our regularly scheduled thread...

Agree!

 

[NW-Bound]

Here is a sample clamp on Meter from Harbor Freight.
https://www.harborfreight.com/digital-clamp-meter-96308.html

That's a good price!

Better yet, spend a bit more to get a clamp meter that can also measure DC currents. Indispensable for working with DC wiring circuits, such as on RVs and PV solar systems.

I have this linked $17 multimeter and it measure microfarads directly.

LINK

That too.

Get everything.

 

[NW-Bound]

We have a 2016 high-efficiency Trane. When the temp outside is close to 75, and we know it's going to get hot that day we turn it on at 77 degrees. It starts to cycle. On 2 minutes, off 2 minutes for a period of time. Our HVAC guy came over 3 times. One of the times he sat outside and listened to the cycling. Could not figure it out. Everything seems to work. New filter, electrical is good. Every morning it cycles before the temp outside gets hot. Once it's hot above our thermostat temp, it runs normally and has kept us cool during these 90-degree days.

Will the cycling shorten the life of the A/C? It has a 10-year warranty on certain parts. Google says the cycling is bad but do high-efficiency A/C units do this?

Short cycling does wear things out faster, such as the relay or contactor we talked about earlier in the thread. In addition, it hurts the efficiency, as the compressor needs some time to pump up the freon to the right pressure for the cooling to start, then it is turned down again. No different than starting up your car to drive one block, then stop to shut the engine down for 2 minutes. Start up the car and drive for 2 minutes then stop. Rinse and repeat. Your MPG would be shot to h3ll.

All thermostats have a parameter called temperature control hysteresis. It is simple. A thermostat must have a dead band in its temperature control. To keep the temperature at 78F, it waits until the temperature rises up to 78.5F before turning the AC on. Then, it waits till the temperature drops down to 77.5F to turn it off. That's a hysteresis of 1F, and the average temperature is 78F.

I have seen a thermostat with a switch to select between 1F and 2F for hysteresis. Most don't.

With a higher hysteresis, the cycling time is obviously longer. But the drawback is that you suffer a wider temperature swing.

Variable-speed ACs do not have this problem. They slow down the compressor as well as the fan, in order to match the cooling power to the rate of heat gain. The power draw of the whole system is of course reduced at the lower speeds.

My ACs run 24/7. If it gets cool down below my setting of 78F, the compressor turns off, but the fan runs at a low speed to circulate the air. This stirring of the air keeps the temperature even inside the house. I love it.

PS. The power draw of the fan of the interior air handler of my mini-splits is less than 25-50 watts. It's the same as a ceiling fan.

 

[ShokWaveRider]

Get everything.

I kinda like this one:

https://www.ebay.com/itm/3735905643...Kv1JvHu+1iW/5Vtd+9z5EH4Xg=|tkp:BlBMUNCEvcrFYA


BUT this one seems like a killer deal, a throw away meter.

https://www.amazon.com/Display-Mult...6671&sprefix=clamp+meter+with+,aps,101&sr=8-2

 

[NW-Bound]

I kinda like this one:

https://www.ebay.com/itm/3735905643...Kv1JvHu+1iW/5Vtd+9z5EH4Xg=|tkp:BlBMUNCEvcrFYA


BUT this one seems like a killer deal, a throw away meter.

https://www.amazon.com/Display-Mult...6671&sprefix=clamp+meter+with+,aps,101&sr=8-2

Ah, but read the descriptions carefully.

The more expensive one measures both AC and DC currents with the clamp.

The cheap one does not. Only AC currents with the clamp. Still a heck of a good deal though.

 

[Time2]

Ah, but read the descriptions carefully.

The more expensive one measures both AC and DC currents with the clamp.

The cheap one does not. Only AC currents with the clamp. Still a heck of a good deal though.

Also it has 60A and 600amp ranges. I don't know how it auto ranges, but 6A range would be nice.

 

[ArmchairMillionaire]

In short, this 23 year old AC is long past its expiry date, and needs to be replaced.

Not necessarily... Our central A/C and furnace came with the house when we bought it in 2009. According to the serial numbers, they were both from 1992. We replaced the furnace in 2020 due to the heat exchanger being partially plugged as well as the induction motor housing being cracked and the circuit board being water damaged from condensate. All that could have been fixed, but it still would have been only an 80% efficient furnace so we decided to replace it with a smaller two-stage 96% efficient unit.

This summer our A/C stopped working on a 100º day. Since it was 30 years old, I thought about replacing it as well but after troubleshooting it (the compressor wouldn't come on and the fan would only spin if you gave it a "push-start" with a screwdriver) I determined that the capacitor had failed. $15 later I had the capacitor replaced and it's blowing cold air again.

Just because something's old doesn't mean it needs to be replaced. Sometimes there's truth to the old adage "They don't build 'em like they used to".

 

[NW-Bound]

Also it has 60A and 600amp ranges. I don't know how it auto ranges, but 6A range would be nice.

No, it does not autorange, and you have to manually select either 60A or 600A.

My 10-year old Craftsman AC/DC clamp meter does not have autorange for the current measurement, and offers a 40A / 400A range select.

Not doing precision work, I find the 40A range plenty good because it reads out to 0.01A resolution. That's 10mA resolution.

PS. When working with electronics, I use old DVMs and wire them in circuit for current measurements. And that's why I have several DVMs.

 

[NW-Bound]

Speaking of DVMs, I am long due for a 6000-count DVM. All mine are 4000-count.

What's the big deal?

When working with lithium ion cells, when their voltage gets close to the 4.2V full voltage, the 4000-count DVMs have to switch from the 4V range to the 40V range. In the higher range, the readout of the cell voltage has a 10mV resolution.

If you want to do precise cell voltage balancing, then you need a 1mV resolution which is provided by the 6000-count DVM, which can stay in the 6V range, and not switch to the 60V range.