HVAC Problems

[srfdude]

Got a 2008 Rheem AC unit outside, and it will no longer keep the house, in 100+ degree weather at a reasonable temp. If it runs all day it will maintain 81 deg. at best, when it gets over 105 not so much.
So a quick investigation shows that the 1" copper line from the unit to the house, about 18' long, has insulation that needs repair/replacement. I took an IR thermometer and the temp as the line leaves the unit is around 75° rising to around 85-88° as it enters the house. What should the temp be as it leaves the AC unit?

 

[BMWTurboDzl]

Got a 2008 Rheem AC unit outside, and it will no longer keep the house, in 100+ degree weather at a reasonable temp. If it runs all day it will maintain 81 deg. at best, when it gets over 105 not so much.
So a quick investigation shows that the 1" copper line from the unit to the house, about 18' long, has insulation that needs repair/replacement. I took an IR thermometer and the temp as the line leaves the unit is around 75° rising to around 85-88° as it enters the house. What should the temp be as it leaves the AC unit?

There are two lines leading to the outdoor condenser. The suction line (leading to your compressor) is the larger of the two and is typically insulated. It should be cold. You can have someone check the refrigerant level and you can insulate the pipe yourself.

 

[bullwinkle]

The liquid line normally isn't insulated, but the return/suction/vapor line should be insulated, and sweating from the cool return refrigerant returning to the condensing unit. If the return line is above 70F you have a major refrigerant shortage/ leak, or the expansion valve/device is plugged.

 

[atikovi]

Got a 2008 Rheem AC unit outside, and it will no longer keep the house, in 100+ degree weather at a reasonable temp. If it runs all day it will maintain 81 deg. at best, when it gets over 105 not so much.

When I asked a similar question recently, someone said the difference between the outside temperature and the temp of the air coming out of vents should about 20°. So if it's 101 outside and 81 inside, that's normal. You really need to connect a set of gauges to read system pressures to see if there is an actual problem. It could just be low on freon. And be forwarned that if you call an a/c service, they will tell you your 16 year old system is old and outdated and needs to be replaced. Or they will charge you $300 for the few pounds of freon to top it up, when you can buy a 30 pound drum for less than that.

 

[Nukeman7]

When I asked a similar question recently, someone said the difference between the outside temperature and the temp of the air coming out of vents should about 20°. So if it's 101 outside and 81 inside, that's normal. You really need to connect a set of gauges to read system pressures to see if there is an actual problem. It could just be low on freon. And be forwarned that if you call an a/c service, they will tell you your 16 year old system is old and outdated and needs to be replaced.

This is not correct. On a home HVAC system, the temperature differential between the cold air exiting the air handler assembly should be at least 20 degrees F cooler than the ambient air temperature at the return air duct inlet. Therefore, if the air inside the home entering the return duct is 72 degrees F, the chilled air coming out of the air handler just downstream of the evaporator coil should be 52 degrees F (or cooler) if the system is operating properly. However, the most accurate way to determine proper operation is by performing Superheat & Subcooling calculations as shown in this video.

Or they will charge you $300 for the few pounds of freon to top it up, when you can buy a 30 pound drum for less than that.

A 2008 system likely has R-22 since R410a was mandated on January 1, 2010. You are not legally buying R22 or R410a without a refrigeration license and it is likely going to be ~$700 for a 30 lb. tank.

 

[zzyzzx]

the difference between the outside temperature and the temp of the air coming out of vents should about 20°.

I thought that the 20 degree rule was the temperature difference between the input air to the air handler and output air of the air handler.

 

[atikovi]

Ok that maybe how it's calculated, I'll have search for the post.

 

[atikovi]

You are not legally buying R22 or R410a without a refrigeration license and it is likely going to be ~$700 for a 30 lb. tank.

Yea but I've seen them on Facebook for as low as $250 a sealed tank in winter.

 

[mk378]

I took an IR thermometer and the temp as the line leaves the unit is around 75° rising to around 85-88° as it enters the house. What should the temp be as it leaves the AC unit?

The "unit" being the condenser unit outside?
It doesn't work the way you think it does. The cold comes from inside. The large line is used to return "spent" low pressure gas refrigerant to the compressor for another round. The flow through this line is from inside to outside. The line may warm up slightly as it goes outside. Insulation is mostly to prevent sweating and it also improves the efficiency of the system slightly.

The most likely explanation is that you have high superheat due to a low charge. There isn't enough refrigerant in the system to keep the indoor coil properly full of boiling liquid refrigerant. If that is the case, some effort should be made to find the leak.

 

[97prizm]

Got a 2008 Rheem AC unit outside, and it will no longer keep the house, in 100+ degree weather at a reasonable temp. If it runs all day it will maintain 81 deg. at best, when it gets over 105 not so much.
So a quick investigation shows that the 1" copper line from the unit to the house, about 18' long, has insulation that needs repair/replacement. I took an IR thermometer and the temp as the line leaves the unit is around 75° rising to around 85-88° as it enters the house. What should the temp be as it leaves the AC unit?

Did the A/C unit work fine last year? If so are the fins/ coils dirty? My friend does hvac install and recommends using low pressure water to flush from the inside out. It could be low on refrigerant as well.

 

[CDX825]

Sounds like its low on refrigerant but it could be other issues too.

I would suggest washing the coil on the outdoor unit because if its plugged with dirt you will lose capacity. Take your garden hose and wash it out.

 

[PandaBear]

Got a 2008 Rheem AC unit outside, and it will no longer keep the house, in 100+ degree weather at a reasonable temp. If it runs all day it will maintain 81 deg. at best, when it gets over 105 not so much.
So a quick investigation shows that the 1" copper line from the unit to the house, about 18' long, has insulation that needs repair/replacement. I took an IR thermometer and the temp as the line leaves the unit is around 75° rising to around 85-88° as it enters the house. What should the temp be as it leaves the AC unit?

I would imagine the temperature of the high pressure line coming out of the condenser (outdoor) unit should be a certain degree above ambient outdoor temperature. Afterall it is air cooled with outside air and you will never be equal to or below outside temperature.

If it is just insulation problem you can always buy some insulation and add back on.

 

[Fitter30]

Suction line should be cold. Post 6 The 20° difference is across the return air and supply 20" to 30" from evap coil. Ir thermometers are not the best probe type do a better job. Temperature of the liquid line 3/8" bare copper line and outdoor temp?

 

[John105]

Just oversimplifying the situation, I had found a 1999 unit dropped the ambient temp 18F. This got worse as it began to fail. By 14F I knew something was wrong.

On the new unit 2020, it drops the ambient temp 21F.

I had always heard a 20F drop is max, properly sized. People who live in climates where it’s routinely over 95F would know best. Maybe units are oversized. Much of the comfort is achieved in dropping the humidity as well.

What’s blowing out the vents? I would say 60F max. But ideally lower, just maybe not when it’s 100F outside. Long story short is in the northeast our systems are not designed around 100F…

Another unscientific comment I have heard is don’t set your thermostat lower than 20F vs outside.

 

[SC Maintenance]

You have to measure the temperature drop across the coils.

If your attic is 130F, the 52 degree air can be 62 by the time it exits an internal duct..

Also, how humid can make a big difference. If your taking 72F air that has 75% Humidity it can take a lot of energy to get it down 20 degrees.

 

[KJSmith]

Yea but I've seen them on Facebook for as low as $250 a sealed tank in winter.

Last 30lb drum of R22 I bought was $1200.00 wholesale.
That was almost 3 yrs ago.

There are other/cheaper choices.

 

[KJSmith]

Maybe units are oversized. Much of the comfort is achieved in dropping the humidity as well.

That is the reason you do not oversize a unit.
It won't run enough to remove humidity.
They will sometimes oversize the evaporator coil by a half ton to help remove humidity.

 

[PWMDMD]

I thought that the 20 degree rule was the temperature difference between the input air to the air handler and output air of the air handler.

This is correct but I have to admit it as 95F here last week and it was really nice that my new unit was able to maintain 69F inside when the thermostat was set to 68F. The outside inverter compressor is capable of adjusting between 25% to 130% in 1% increments and on high load days that 130% comes in handy and on 80F days it still allows for long run times to dehumidify and no short cycling.

 

[oilcandave]

I wouldn't go too far down past the outside dew point if yer vapor barrier is on the inside wall.

 

[John105]

That is the reason you do not oversize a unit.
It won't run enough to remove humidity.
They will sometimes oversize the evaporator coil by a half ton to help remove humidity.

Also I heard that typically if oversizing the duct work is likely inadequate and the unit could freeze up since there isn’t enough airflow.

But what I am envisioning is that the sizing algorithms are different for say Portland ME, Philadelphia, Phoenix, and Las Vegas.

Maybe a good question for the Op is if anything has changed. If so, what, and by how much.

Here’s what our runtime looks like. It was 98F on Saturday and the unit ran 10 hours. That’s extreme for us.