Performance opinions- diagnosis

[espock]

Originally posted by mpatrick
Just to calibrate what I mean by high electric bills.

$543 last month for a 2700 sq ft house in Houston. Rate is 11.1/kwh, usage is average of 145kwh per day. Other large loads have like the pool pump etc have been investigated and every thing points to this unit as the energy hog.

Whuh ! I live in Houston and my last summer bill was ~$200/month and my wife complains it's high (2300 ft2 1-story 5-ton AC, trying to keep @ 78F , <50%RH). I feel your pain.

On the elect. rate, on the Reliant bill, ~7c/KWh, but watch out the "energy cost" line, additional ~5c/KWh and this would change as fuel cost goes up.

[midhvac]

It does sound possible that the unit's a little overcharged.

It also sounds like your thermostat habits could be part of the problem, depending on the number of degrees of setback you're trying to use and the number of changes per day, and the setpoint of each thermostat in relation to the setpoint of the other thermostat. You could be creating higher latent loads on the system.

[karsthuntr]

Rheem requires a txv on an 11seer a/c unit. With a RCBA 4878 coil it only gives you 10.5 seer. Your superheat is way to low it should be a 54 deg SLT at parameters given.

[Edited by karsthuntr on 07-01-2004 at 08:23 PM]

[HVAC Pro]

I matched a couple of Benchmark coils with Rheem heat pumps and had low suction and low delta temps. on both. Nothing I could do would correct the poor performance. Now I will only match a TXV Rheem coil against their heat pumps or condensers, if a Rheem OEM piston coil is not available.

[mpatrick]

I have decided to replace the evaporator coil. I will be installing a new coil with a TXV this weekend.

Before anyone asks:
1. I have CFC universal certification (got it years ago)
2. I will recover refrigerant (borrowed recovery unit from work)


Thanks for all of your help and opinions.

I will post the results and performance when I return from vacation in 2 weeks.

[HVAC Pro]

I bet you will be amazed at the difference!

[smokin68]

I may be off-base here, but with a "hacked in coil" you might actually have a 6 seer instead of a 11. Depending on age/condition of condenser, either replace with a real high effeciency unit or just a match A/H for that condenser. If you think all signs point to this a/c, and it's costing you $500+/month,I wouldn't hesitate to replace it. Make sure you have duct system evaluated. You're right about 11 DT, that's not good.good luck.

[JohnB]

Originally posted by lmtd
First guess, and it is a PUNT, overcharge.

Need more data.

Air entering condenser
Air leaving condenser
Saturated condensing temperature (psig is fine if you do not have a saturation chart)
Liquid line leaving condenser
Liquid line at metering device (about 6" in front of it but after all accessories, right before it enters the air handler)
Air entering the evap
Air leaving evap
Suction saturation temp (psig is fine if you do not have a saturation chart)
Suction line temp at the evap (leaving air handler)
Suction line at the compressor (condenser service valve works too)
Compressor Amperage actual reading
Compressor RLA or FLA from the modle plate.

Get it all at the same time and you will get what you seek.

This will only take you 1/2 an hour, it would be nice to see the numbers before you change the coil.

[Mr Bill]

Originally posted by mpatrick
I have decided to replace the evaporator coil. I will be installing a new coil with a TXV this weekend.

Don't forget that hard start for the condenser with that txv.If that's a recip. it will bust a gut trying to crank-up.

[midhvac]

Hackety hackety hack
a chill ran up my back

[sadlier]

Originally posted by mpatrick
Expert opinions please:

Outdoor Ambient 78 DB 66WB
CFM 1585
RAT WB 65
Evap TD 11

Condensing Unit
Suction P 59
Liquid P 199
Superheat 12
Subcooling 22
FLA 19.9 - Unit running at 18

Evap coil is fixed orfice.

With the numbers provided the unit is overcharged according to the superheat (should be about 18°), overcharged according to the subcooling, but the 59# suction pressure and 11° evap TD indicates that there is something more than just an overcharge. I would vote looking for a restriction such as a kink in a line or a plugged refrigerant filter/drier.

[mech890]

I had a goodman unit do something similar to me. Superheat was crazy low, pressures low, subcooling high, only a 10* drop across the coil. Tracked it down to the distributor tubes to the left side of the coil were totally clogged. Was only feeding 1/2 of the evap. Been in there 2 years, last guy just charged the hell out of it to get the pressure up.

[mpatrick]

Changed the coil today.

16 degrees across coil
11 degrees superheat (of course TXV)
12 degrees subcooling

Evap pressure 72
High side 225

I will post complete results next week when I return from vacation and can hook up the service tool I borrowed from work to take the original readings. I want to be able to chart the data against the data I took before the change.


I did notice that the coil I removed had a leak repair done/attempted at the u-tube, I suspect they plugged it full of solder. I was going to cut into it to verify, but my wife insisted that it go to the curb since today was trash day.