Performance opinions- diagnosis

[mpatrick]

Expert opinions please:

I have low performance and high electric bills. Moved in to the house 4 months ago.

3.5 Ton system 11 SEER Rheem (7 years old)
Horizontal A coil
Outdoor Ambient 78 DB 66WB
CFM 1585
RAT DB 71
RAT WB 65
SAT DB 60
SAT WB 59
Evap TD 11


Condensing Unit
Suction P 59
Suction T 45
Liquid P 199
Evap temp 33
Superheat 12
Subcooling 22
FLA 19.9 - Unit running at 18

Evap coil is fixed orfice.
Evap coil has been hacked into with what could only be described at the jaws of life. Opened panel observed clean coil, no air bypassing coil (no missing baffle or anything unusual other than the crappy patch job on the coil casing). It's like the previous owner had a problem but either did not have it fixed or the tech didn't know what to do.

Would appreciate any help, it's been about 15 years since I was in the mechanical side of the business (been in the controls business since then).

[beenthere]

How did you measure your CFM?

Slow it down.
Pump the unit down, and check for the right piston.
Check for a restriction in the liquid line.

Post the model and serial #s of your units.
That may help.
Is this a heat pump.

[appltech1]

Originally posted by mpatrick Expert opinions please: I have low performance and high electric bills.

I aint fallin for it. She said it wasn't that good and ever since you've had to keep the lights on.

[Shophound]

Without knowing more about your system, such as coil size and whether it's matched correctly with rest of system (since it was "hacked in"...who knows what was done?), sounds to me like you have excessive liquid stacking up in condenser (high subcooling) but at the same time a low superheat. Higher SEER rating could account for this partly, but you're reporting subpar performance and high utility bills.

How clean are all the coils and blower wheel? And did I read your outdoor ambient temp right as being 78 degrees dry bulb? TO be pulling near FLA at that mild of an ambient...something's up.

Post more info such as model nos. of coil, air handler, and condensing unit. And condition of all coils, dirty or clean. Just casually you look overcharged but not enough info to say with more confidence.

[cooltec]

Just so you know im still a new tec

check your filters, and make sure your condenser is clean.

is the blower cleaned and door closed

last is ther any frost on the line

[Mr Bill]

Originally posted by mpatrick
FLA 19.9 - Unit running at 18

Is that a FLA on the unit or RLA? Ruud, Rheem same difference usually measures theres in Rated Load Amps so 18 would be ok.

[mpatrick]

Conventional split system (not a heatpump)
Rheem Model RALB042JAZ
Serial 535fo7960712
Blower filter and coil clean, blower door closed.
Cleaned condensor, not perfect but reasonably clean.

Estimated CFM from fan curve on inside of gas furnace door using .5" static.

No frost on line.

System appears to be original as everything matches the other system (2 ton).

The evap coil is missing the model tag due to the hacking, but appears to be the same brand/model as the 2 ton only a little bigger. The 2 ton coil is a Benchmark B34d24-h.

[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.

[me75006]

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.

Check out electric rates by other providers. Here in Dallas area, I use Reliant, (used to use TXU) monthly rate is currently around 8.0 to 8.4 cents per kwh. Thats a big percentage difference from your 11.1/kwh.

[jdenyer]

Even if the A/C unit was malfuntioning this wouldn't cause you to use 145 kwh a day! At 18 amps at 220 volts this would be 4kw per hour, if unit were to run 10 hours a day thats 40 kwh per day. Figure in another 20 to 30 kwh/day to run the rest of your house and you would only be at 70kwh/day. Have you had your electricity supplier check this out for you? Maybe you have a bad meter, or a problem thats allowing current to flow to ground. Have power company check stuff out, then get a good electrician. There is something really wrong here and I don't think its your A/C. Try turning off all appliances and unplug everything in your house and see if that disc inside your electric meter stops moving, it should. Make sure you unplug everything as small plug in transformers will use some juice while plugged in. If meter disc keeps moving after everything is unplugged and turned off you have problem for the power company or an electrician to figure out. Hope this helps.

[Mr Bill]

That seems to be a lot of home for a 3.5 ton to try and cool
especially if your in Houston, I am paying 10 cents pkw in Houston right now unless they have gone up since last bill with reliant. You said 71 RAT you keep it pretty cool in there, I find with the majority of my customers that 78 is an average setting and it will make a difference on the light bill every degree you go down, how much? to many varibles but Reliant has some sort of average they say For every one degree you set the thermostat warmer in the summer, you can save 2 to 3 percent off your monthly cooling bill.I don't know pressures look ok but I just think you keep it to cold and personally 3.5 tons in Houston for that size home I just don't think will work unless you have no heat load.

[mpatrick]

I have managed to lower the KWH usage to 80/day, by doing the following:
1- Run pool pump early in the morning so the pool is clean when the wife and kids get in. I found that they were manually turning it on when they got up because I had it running in the evening.

2. Raised programmable stats to 76 during the day from 73.

3. Hot tub has an electric heater(50 amp/230v)- when the power flashes (happens often) it defaults to 100 degrees and can not be reprogrammed. I designed/installed a lockout circuit that shuts the controls down on a power failure. Requires manual restart.

4.Replaced 80% of incandecent bulbs with compact flourescent.

5. During the investigation of all of this, I discovered that this particular AC only had a 10 degree Delta T and warranted more investigation.

6. I am in one of the few regulated sections of Texas and cannot change power companies.

7. Electrical system/meter is in good working order. Verified with digital KWH meter I have access to.

8. This is one of two AC units. I have a 2 ton upstairs.

9. I know 71 is cold for the return air, I had it turned down low while I was taking measurements. It has been cool and rainy for 3 weeks here.

[Edited by mpatrick on 07-01-2004 at 09:42 AM]

[Mr Bill]

Sorry I did not catch the other unit on my first cup of coffee, of course TEV would be better I am surprised that rheem could get 11 seer without one but maybe so. I tell you have a lot of power consuming stuff going on, my home is about 200 sq.ft. smaller than yours single level and other than a/c the only other power consumers i use is my MIG Welder sometimes, and my air compressor and a fridge in the garage and i run a blower out of a furnace in my garage 110 volt i think it runs around 6.8 amps probably run all this an average of 8 times a month for a short period and my bill last month was 245.00 and i felt pretty blessed. I know I have a lot of customers that have those 500.00 light bills but they have bigger homes and keep it a lot cooler than i do but none of us in my home really like it real cold 78 during the day is fine and we turn it to 75 at night.

[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.

[rosspg]

I have the exact same unit as MPatrick (which is how I found this thread)...hoping someone can help me out even though this thread is a year old now.

We live in NW Florida about a mile from the gulf. The internal square footage is considered to be around 2150 (+- 50 s.f.) but our ceilings range from 9 to 12 feet high (probably averages about 10.25'). It is a single story home and while we had a very reputable builder, at the end they do an energy efficiency rating on the home...out of 0 to 100, it was 96 or so, with 100 being worst.

We got the same unit as MPatrick as we wanted to cool the garage for additional usable space, though we took no other measures to make that space energy efficient. One drawback, though, is that we usually do keep the garage door closed (leading to the house) and when doing so, the return air draw is enough to pull one door in line-of-sight closed on occasion.

We have very low energy costs per kWh, which is part of the reason I believe most builders here are so poor on energy efficiency. Our cost is about 7.6 cents per kWh.

We are also VERY liberal with our house cooling and with our pool heating (both due to health necessity). The house temp is typically 71 - 73...occasionally, 74 or 75. The pool is usually kept around 93 degrees. The pool is covered with a scroll compressor heat pump of high efficiency (sorry, but I do not have the rating handy), so while it is still somewhat pricey to heat, I estimate it contributes an average of $110/mo during the 7.5 months/year we operate it.

For one year, we were in a program where we could buy our energy at different prices (based on consumption and time of day)...however, it required having a land line....so when the phone line went, so did that program.

We have gas and electric (the gas is also of poor efficiency....80.5), but the electric accounts for roughly $3000 annually. However, it seems to be worsening year-over-year. We just had a record $410 electric bill at the rate I described (7.6 cents/kWh) in addition to a $45 gas bill. We have had our Rheem (11 seer) since building the house in 1999 and I have never been impressed with it other than its ability to cool the entire house even when temps are in the upper-90s day after day. That is where our temps peak out...we do not break 100.

I am strongly considering any and all measures to increase the efficiency of our house/appliances. The washer, pool scroll compressor heat pump, and refrigerator are the only high efficiency devices in the house. From what I can tell, the A/C and possibly the dryer (though that is occasional in usage) appear to be the largest consumers in studying the meter. While just about everything was running last night, we consumed an entire kWh in 3 minutes 6 seconds! We do have a number of computers which are always on (not the monitors, though), but during periods where the energy guzzlers are not running, the wheel will make a 1/12 kWh rotation in about 13 seconds. That seems to be our low rate that I have witnessed. Still, that would come out to far more than the 140 - 160 kWh/day we are averaging...I'm not sure what changes when we go to bed to slow the rate down farther.

Unfortunately, you can see I know next to nothing in this area save trial and error...I know nothing of the internals other than that the same model Rheem indicates it also has a scroll compressor. That's basically where my [lack of] knowledge ends.

Please advise.
Thanks,
Ross

[beenthere]

Stop cooling the garage.
You make it sound like it has no return in it, so that means the system is putting your house in to a negitive pressure and pulling in outside air, when you have the door closed to it.

Pools around here aren't heated above 87.

Add insulation to your attic, see about upgrading your windows and door, weather stripping.

Then you might be able to get a higher eff smaller a/c.

[rosspg]

For the garage, we do use that as additional living space (though as I mentioned, it has standard insulation or lack thereof). We have parked a car in there only once or twice in 6 years. However, due to homeowner association issues, we cannot convert it to a room per se...the attic above it is commonly shared with the rest of the attic above the house. We installed a fan after H. Ivan repairs to provide some exhaust.

The insulation is, I believe, R-38 (for the thick areas)...but it is the lousy(?) blown fiberglass. I can only speculate how much that costs us and would absolutely consider redoing it if you think it would make a significant difference. I think most other walls are something like R-13 or whatever Florida standards are.

You are correct on the lack of the return air...the AC was upsized to that Rheem with additional output as well as an increase in the SEER from 10 to 11 (which to me seems anemic given the 12 SEER we had when living in MD beforehand).

Can you explain more about the negative air? We typically keep the garage door closed despite the fact that the air flow was designed (capacity and draw) for it to be open due to pest wars in the garage (home sweet home to the roaches and the like).

I know we keep the pool super-heated, but that is because we typically swim at night when it is cooler. I do use a solar blanket/hand reel.

Part of the problem is that we do heat the pool to high temps and cool the house to low temps, but we do not wish to give that up. When evaluated, our A/C system was putting out around the 50 - 60 degree mark, so it is working like it should at least in regard to output. However, I don't know if its poor efficiency and/or the house's poor efficiency are what cause the bills to be excessive. The gas bill is never an issue...I think its all-time record is perhaps $110 - $120. The stove top only, the water heater, the house heat, and the grill are the only devices using gas. All others use electric (not heat pump save the pool).

Would replacing the blown fiberglass make a notable difference? Can it be roughly quantified? We do not have any weatherstripping in place. The windows are double-paned. I recall when the house was being built that there tended to be significant gaps around the framing and the sliding doors. We have quite a bit of glass in the house, though the majority faces (in order) South, then North, then West, then East.

Ross

[beenthere]

Few garage doors close with a tight seal, so your loosing conditioned air aroung the door(my guess).
When you loose air to the outside, it has to be made up some how, and thats by drawing it in any where else it can, such as gaps around windows, and other doors.

I don't know how you got around code, to have a supply in the garage.

South windows, do you have good heavy drapes, and close them when the suns shining through.

They may have fluffed the insulation when they blew it in, meaning you don't have r38.
You can lay insulation over it, or have more blown in.

Again, around here 87 for the pool, and we swim at night also, so it can't be any warmer up here in PA. That extra 6 degrees can be alot of money in electric.

Some pools up here are only heated to 83.

You may want to have a contractor check to see if its feasable to zone you system, so youor not cooling areas of the house your not using for 8 hours. This can also save alot.

[hugodrax]

how do you get your home rated for efficiency?
I would do the following
1. swap all your lighting for fluorescent bulbs
2. put that white insulation powder in
3. put those roof turbines (the passive ones)
4. check for duct leaks
5. put the weather stripping to seal air leaks on windows/doors
6. swap out the AC for one of those fancy Carrier infinity systems
7. run your pool pump 4-6 hours only.


My electric bill now so far is averaging 120-130 a month and I have a large saltwater tank with PC lighting/pool with large pump.

my bills used to average 345 bucks a month

in 10 years thats going to be 25,800 dollars.
not a bad investment. [I had #2 though previously I did 1,3,4,5,6,7]

[suemarkp]

One thing to think about -- is there some way to utilize the cold air being produced from the pool heat pump to cool the air conditioning condensers (stack them and use a huge diversion duct)? Ideally, a common heat exchanger system would be used, but this is complicated. Perhaps find a way to blow the pool HP cool air into the converted garage space.

Also, do you have sun shining on the pool HP and shade on the house AC's?

[docholiday]

Originally posted by mpatrick
Expert opinions please:

CFM 1585
RAT DB 71
RAT WB 65
SAT DB 60
SAT WB 59
Evap TD 11


Accoding to these numbers you have a total capacity of 35,947 btuhs. That model with a matched piston coil has a capcity rating of about 40k. You are lacking 4,000 somewhere.

Your sensible BTUS however is only showing a 18,829 meaning the latent load is very high (almost 50% of the total system btuhs. Look at your return numbers...
65WB and 71DB? thats 73.11% rh!!!!!

I would expect with high airflow and such high humidity to see the 11 degree drop. The fact you are using a mismatched coil may have something to do with the cpacity loss. Hopefully with the new coil, dropping the airflow to 1400 and finding out how a unit doing that much latent work can not be reducing that humidity.

:( I just realized you guesed at the CFM by looking at the blower door chart without actually reading the true static.
"Thanks Doc for looking that up anyway"
"NP dude, I wasnt doing anything anyway"

Your airflow may be higher or lower in which case all my response is bogus.