Anyone familiar with HVAC troubleshooting?

[Player2]

My central AC unit is acting up again, seems to crap out every year when it starts to really heat up. I'm sick of calling the repairman every year and pouring more money down the drain every time. The darn thing is 5 years old and has had issues the past three years! I'm hoping someone more knowledgeable and experienced than I may have some pointers.

The thermostat seems fully functional, as far as setting the temp, displaying ambient temp, switching between cooling/heating/off. The outside unit also seems fine, except it only comes on for about a minute most of the time, sometimes it will stay on for a few minutes. I think the issue lies with the air handler inside. When the outside unit comes on, I can hear a hissing/pressure sound from the inside unit, but then the fan never kicks in and there is no air coming out of the vents. From some cursory reading online, I think I am going to check out the fan relay, but if the fan or relay is the problem would that cause the outside unit to shut off prematurely?

Thanks in advance for any suggestions/advice!

[severach]

Re: Anyone familiar with HVAC troubleshooting?

Any history of what the service man has been fixin all these years? AC that breaks every year is usually due to a refrigerant leak slow enough to last all summer and your repairman is doing the easy fix to keep the bill low. There's usually a low pressure switch that turns the system off if there's not enough refrigerant.

Your first step is to find the wiring diagram. It might be pasted inside a panel or found in your owners manual.

[kc8adu]

Re: Anyone familiar with HVAC troubleshooting?

try setting the stat to constant fan.
does it run then?
in the few minutes it does run does the bigger of the 2 lines at the compressor frost up?
no air over evaporator=freeze up.
your unit may have a sensor on the evap to tell the board to shut down if the evap drops below 32f.this will happen fast if the fan is not running!

[paul_h]

Re: Anyone familiar with HVAC troubleshooting?

Any history of what the service man has been fixin all these years? AC that breaks every year is usually due to a refrigerant leak slow enough to last all summer and your repairman is doing the easy fix to keep the bill low. There's usually a low pressure switch that turns the system off if there's not enough refrigerant.

Your first step is to find the wiring diagram. It might be pasted inside a panel or found in your owners manual.

agree

try setting the stat to constant fan.
does it run then?
in the few minutes it does run does the bigger of the 2 lines at the compressor frost up?
no air over evaporator=freeze up.
your unit may have a sensor on the evap to tell the board to shut down if the evap drops below 32f.this will happen fast if the fan is not running!

agree

I'm a fridgie, but things work differently in the USA. So whether the fan is on auto or not is important, and any lines icing up.

If this unit has always broken down every year, and it's been fixed each year pretty easily, that looks like a refrigerant leak issue to me.

But forums and tech info works differently as well, www.HVAC-talk.com won't talk to me, and it's almost impossible for me to get info for your crazy USA style units sometimes.

[Player2]

Re: Anyone familiar with HVAC troubleshooting?

Thanks everyone for your responses thus far.

Last summer when it stopped working, the repair man did recharge the refrigerant and advised me that there could be a pinhole leak somewhere. He said that if the coils (not sure if that's the correct term) started icing up again it was probably leaking. At that time, the fan would still run, and if I flipped the breaker and let it thaw it would cool for a while, until it iced up again.

Since I made my original post, I checked it again and it is icing up a little bit, but not like last time when the coils(?) were completely encased in a block of ice. Also, it took it several days before ice formed this time, last time it would happen within a day. I tried shutting it off again to let it thaw, and when I turned it back on the fan still did not run.

I have tried switching the fan from 'Auto' to 'On' at the thermostat, but it still doesn't come on. It sounds like it is very possible that it is kicking the compressor off because the temp is dropping too low since the fan isn't running. I'm going to try to dig into it tonight, hopefully I can find the wiring diagram, but if not it shouldn't be too hard to identify the wires going to the fan and at least see if it's getting voltage.

Thanks everyone! I'll post an update after I check it out.

[smason]

Re: Anyone familiar with HVAC troubleshooting?

A/C problems can be a pain for sure. However I really can't sympathise much, as I look outside and see and inch of *&^%ing SNOW!!!! GRRR..

Sorry, like a bad cap, I had to vent.

[cadiman]

Re: Anyone familiar with HVAC troubleshooting?

your low on freon (r22)or R134a) Your " A " coil in the house is iced up It only holds a few pounds of freon. This coil has to run above 32 degrees but a good indicator is the two lines at the A coil are the same temp when you have the proper amount of freon. above 32 and below 40 degrees. The leak is almost always outside in the "B" coil or lines to the b coil or where the copper lines are soldered. The first place to look is at the cap on the shraider valve ( looks like a car tire vavle stem) on the B coil where the lines enter. the cap could be loose and the valve could be leaking You need to find out what is in your system and buy a can or a 30 pounder and a set of guages.
You may need a license to buy it
I do automotive repair so I have all the stuff already If you have a buddy thats a mechanic he can help you ( we like to work for cash )

a good temp gun is what I use for this around 80 bucks
Good luck!
Cadiman (usa)

[Krankshaft]

Re: Anyone familiar with HVAC troubleshooting?

A very good tip when you have low refrigerant was mentioned above if the evap intake pipe (known as the liquid line) is as cold as the evap output (low pressure vapor line). Then the refrigerant charge is fine if the evap input is colder and the output is warmer you have a low charge.

Just make sure when checking the liquid line you feel after the expansion valve. Before the EV it's a high pressure liquid from the condenser and is hot.

If the AC guy said there was a leak he should have put UV dye in there. IMO it's the easiest way to find AC leaks I have had mixed results with electronic leak detectors.

The hissing could be the expansion valve they tend to hiss loudly when the charge is low.

Coil freezing is either the result of improper airflow over the evap or a low charge.

As for the fan I don't know how your system is wired does the thermostat trigger the fan or does the air handler?

[larrymoencurly]

Re: Anyone familiar with HVAC troubleshooting?

Your indoor coil has lots of pinhole leaks if you see black streaks running down in it, and I read they're usually caused by not pumping down the system enough before charging it, but in older units, improper cleaning of the coils at the factory leaves acid residue.

[Player2]

Re: Anyone familiar with HVAC troubleshooting?

your low on freon (r22)or R134a) Your " A " coil in the house is iced up It only holds a few pounds of freon. This coil has to run above 32 degrees but a good indicator is the two lines at the A coil are the same temp when you have the proper amount of freon. above 32 and below 40 degrees. The leak is almost always outside in the "B" coil or lines to the b coil or where the copper lines are soldered. The first place to look is at the cap on the shraider valve ( looks like a car tire vavle stem) on the B coil where the lines enter. the cap could be loose and the valve could be leaking You need to find out what is in your system and buy a can or a 30 pounder and a set of guages.
You may need a license to buy it
I do automotive repair so I have all the stuff already If you have a buddy thats a mechanic he can help you ( we like to work for cash )

a good temp gun is what I use for this around 80 bucks
Good luck!
Cadiman (usa)

The two lines at the coil are not the same temp, the larger line is cold and the small line is approx. room temp. If recharging the freon for a home unit is similar to a car it should be no problem. I'll try to find out what type of freon it uses, but I'm guessing it is R134a since the unit was installed in 2005.

Would low freon cause the blower (what I've been referring to as the 'fan') at the inside unit not to run? It seems odd that the blower is getting voltage but not running. There is not any kind of 'trigger' wire or anything that seems to be shutting it off. The wire that runs to the delayed relay has voltage.

Thanks!

[Player2]

Re: Anyone familiar with HVAC troubleshooting?

A very good tip when you have low refrigerant was mentioned above if the evap intake pipe (known as the liquid line) is as cold as the evap output (low pressure vapor line). Then the refrigerant charge is fine if the evap input is colder and the output is warmer you have a low charge.

Just make sure when checking the liquid line you feel after the expansion valve. Before the EV it's a high pressure liquid from the condenser and is hot.

If the AC guy said there was a leak he should have put UV dye in there. IMO it's the easiest way to find AC leaks I have had mixed results with electronic leak detectors.

The hissing could be the expansion valve they tend to hiss loudly when the charge is low.

Coil freezing is either the result of improper airflow over the evap or a low charge.

As for the fan I don't know how your system is wired does the thermostat trigger the fan or does the air handler?

Whereabouts would the expansion valve be located? The documentation I have on this unit is very limited (basically a wiring diagram and guidelines for mounting). The sound I described as hissing isn't abnormal, it's the sound I normally hear when the AC is working properly (minus the noise of the blower). Hissing probably was not the right word, it's not like pressurized air escaping from a tire for example.

AFAIK, the repair guy did not add dye to the system when he recharged it. He just said if it happened again there was a leak.

After looking at the wire diagram, the blower is not triggered directly by the thermostat. It runs to a delayed relay, and that is triggered by the thermostat. The blower motor is getting proper voltage from the relay.

[Player2]

Re: Anyone familiar with HVAC troubleshooting?

Your indoor coil has lots of pinhole leaks if you see black streaks running down in it, and I read they're usually caused by not pumping down the system enough before charging it, but in older units, improper cleaning of the coils at the factory leaves acid residue.

Checked it out and thankfully it does not have black streaks. Thanks for the tip!

[ratdude747]

Re: Anyone familiar with HVAC troubleshooting?

you cannot change your own freon, as it is a CFC. you must have a licence to possess it in a re-fillable form. think older AC units on cars.

[Resqueline]

Re: Anyone familiar with HVAC troubleshooting?

Does the fan motor get warm? Does the fan turn easily by hand?

[delaware74b]

Re: Anyone familiar with HVAC troubleshooting?

Chances are the system is still R-22. You should still be able to see the label on the outdoor unit - that will tell you the type of freon is in the system. FYI, the EPA is in the process of banning manufacture and/or import of R-22. They're also looking into replacement for R134a in automotive systems.

[cadiman]

Re: Anyone familiar with HVAC troubleshooting?

I was not worried about the fan because you are low on freon I am guessing you are talking about the fan on the outside at the compressor? If your system is r22 or r134a get a 30 pounder if you can still buy it. If you are like me go and get your license fix your stuff and pick up some side work. You don`t need to take a 2 year program just a course on taking the test.
Where I live you can buy a 30 pounder of r134a without a license, but you will still need a set of guages and some training on safe hook up and a good book on a/c to get started. There are some doos and don`ts to a/c just like tvs. good luck

Cadiman

[Krankshaft]

Re: Anyone familiar with HVAC troubleshooting?

I got my license a few years back no real brain buster. Took 3 days read the large PDF passed the test got the license. I'd been doing auto A/C work prior so I knew the basics already.

I believe they are replacing R22 with units that can handle R410 in central A/C systems due to the ban on new manufacture of R22. Although recovered R22 supplies can be reused indefinitely. But we all know how that caused the supply of R12 to dwindle which caused the price of R12 to rocket up.

Check for 12V drive from the thermostat at the blower relay when the system is running if the 120V supply is fine.

Also is it a capacitor start motor or split phase motor? If you can't find a cap hooked up to the output wires of the motor it's a split phase.

A bad start cap could cause the motor just to hum and not start give the motor a spin while its humming and it will start to verify.

[Player2]

Re: Anyone familiar with HVAC troubleshooting?

Does the fan motor get warm? Does the fan turn easily by hand?

The fan does turn easily by hand, the motor does not get warm (at least not within about 10 minutes before I shut it off).

[Player2]

Re: Anyone familiar with HVAC troubleshooting?

I was not worried about the fan because you are low on freon I am guessing you are talking about the fan on the outside at the compressor?

No, it's the fan at the inside unit which draws air over the coils to cool the house that is not working. The outdoor unit comes on fine, it just doesn't stay on for long.

It is getting 240v at the motor. It has two 120v lines (240v total) and a ground. That is what leads me to believe the motor is bad, there is no other line hooked up that could be kicking it off. The motor also has a capacitor, but it is passing voltage fine.

Granted I am not an expert, but it seems possible to me that the reason the coil is icing up is because the fan is not coming on and there is no airflow over the coils. Does that make sense or am I missing something?

[Player2]

Re: Anyone familiar with HVAC troubleshooting?

Check for 12V drive from the thermostat at the blower relay when the system is running if the 120V supply is fine.

Also is it a capacitor start motor or split phase motor? If you can't find a cap hooked up to the output wires of the motor it's a split phase.

A bad start cap could cause the motor just to hum and not start give the motor a spin while its humming and it will start to verify.

I will check the voltage from the thermostat. I think it should be ok, because when I try to run the system the relay is outputting 120v. The motor has one 120v line coming straight from the breaker, and a second 120v from the relay (it's a 240v motor).

It does have a cap, 7.5 uF 370v. The motor does not hum, and if I spin the fan it doesn't start either. The fan does spin freely. Basically the motor shows no signs of attempting to start whatsoever.

I am thinking the motor needs to be replaced. It's getting proper voltage and there is no trigger wire or anything to interrupt it from starting. What are your thoughts?