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Central Air question and maintenance
Old 07-01-2008, 05:56 AM   #1
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Central Air question and maintenance

We have a central air unit that seems to be working but the house isn't getting cool enough. I know very little about this kind of stuff but that doesn't stop me from trying to diagnose the problem.

The fan and compressor are running on the outside unit and the big insulated pipe is cool to the touch but not ice cold. The little pipe is about ambient temperature. Does this mean that there's a leak with the refrigerant?

The last time I called a tech was a few years back and the problem them was that there were a bunch of dead ants near the compressor (?). This leads me to my next question:

I obviously need to be more diligent about maintenance. What can I do to keep the A/C running smoothly? As far as the outdoor unit is concerned, can I just remove the top of the unit and clean up the leaves and other dirt next to the compressor? The evaporator unit inside seems to be sealed with a silver tape. How does one go about cleaning this?

The outdoor unit is a Trane XE900 that was manufactured almost 20 years ago. The indoor unit is a Carrier, don't know how old it is.

Thanks!
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Old 07-01-2008, 06:23 AM   #2
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Between 1971 and 1973 I did heating and A/C work. There is some maintenance you can do for your A/C. Keep the outside unit (the condenser) clean of debris, as that will interfere with air flow and will have the same effect as dust bunnies on your refrigerator's condenser.

The skinny pipe is high-pressure (about 220-250 psi if I remember right) refrigerant that has exited the compressor as a high-pressure, high-temperature gas and been condensed into a high-pressure, low-temperature liquid. This happens in the outside unit.

The refrigerant then goes into the unit in the house where it's flow is restricted and it goes into the evaporator as a low-pressure, low-temperature liquid and literally boils ("evaporates") absorbing heat in the process. The pressure at this point will be about 80 psi.

It then goes back outside via the fat pipe as a low-temperature, low-pressure gas and is compressed, goes to the condenser and it starts over.

What you can do is make sure your air filter inside is clean (I saw some that hadn't been changed since Eisenhower).

If the blower motor inside or the fan outside has oil ports put ONE drop of oil in there every six months. If the blower motor has a belt (few do anymore) check it for cracks, shredding, etc. One going bad is pretty obvious.

Other than that there isn't much the homeowner can do without investing in about $3k - $4k in specialized tools to get inside the sealed system. It could be the compressor just got tired or there's a leak in the refrigeration line. At 20 years you definitely got your nickel's worth and the only way to tell is put a set of gauges on it to measure the pressures.
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Old 07-01-2008, 07:03 AM   #3
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Check freon level - of course if it is very low that suggests a leak. Maybe it just need to be "topped off" with a pound or so of freon. At 20 years you could be "beating a dead horse" to be putting more money into it. The new ones are much more efficient and, while expensive, are a good investment if you plan to stay in the home and a good selling point if you plan to leave soon.

As far as maintenance goes you can vacuum out the coils outside CAREFULLY so as not to bend the fins. You can also flush out the outside coils by spraying in the opposite direction of the air flow. The inside A unit, if you can get to it by removing the tape or through a access hole (you can purchase replacement tape at HD or Lowes) and using a shop vacuum clean the coils (again reverse of the air flow). In the base of the A unit clean it out of anything that is impeding the flow of the condensation water that collects there and be sure the pipe that takes the condensation water to the drain is clean and allow for the flow of the water. Beyond that, as already been mentioned, that is about all you can do. Additionally, you may want to be sure your duct work through out the home is not clogged or otherwise restricting the air flow. If you have metal duct work look for any small levers that are visible as they are used to "balance" your air flow. If some of them got turned the wrong way they could restrict air flow to an entire duct circuit.
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Old 07-01-2008, 07:31 AM   #4
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Quite often when the freon is low the inside coils will freeze up with condensation. A sign of this will be the circulating fan will be running but no air will be coming out of the registers throuhout the house.

A lot of the service places don't like working on this age stuff with the old freon. Most around here will only charge it up once.
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Old 07-01-2008, 07:54 AM   #5
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Many of the older systems used precharged refrigerant lines that had seals and threaded connections, over time the seals tend to leak.

If you have good air flow through the outside unit and good air flow through your ducts inside, I'd bet on low freon.

My 1980 vintage system (Trane) had leaky seals and I replaced the whole thing in 02. New system has all silver brazed connections.
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Old 07-01-2008, 05:16 PM   #6
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Thanks for all the replies!

As far as cleaning the outside unit, do I just remove all the screws on top on the unit? Or do people usually just blow compressed air in through the sides.

So the standard way to clean the evaporator coils to peel back the tape and then just vacuum the coils and tape it back up with new tape?

The air flow through the registers seems pretty good, so I think this points towards low freon as some of you have suggested.

My air filter is below the furnace unit and it's a metal air filter that I clean off with water. Is there another filter that I need to clean/change? The way my system is setup, we have a electric air cleaner (which supposedly "zaps" dust particles) and then the metal air filter element and then the furnace and the evaporator is on top. Come to think of it, I'll check to see if the electric air cleaner is functioning, maybe that's clogged up.

Thanks!
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Old 07-01-2008, 06:03 PM   #7
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Well, I'm no expert, but I've had a bit of AC work done (& done some of it myself).

Your cold pipe on the outside should be pretty cold to the touch. If not, you may need freon. If it it loses freon in a short period of time after topping it off, you'll need to have it leak tested. Not likely if none of the pipes/fittings are in locations where they've been subject to damage over the years.

Clean your inside coils yourself if you can access them - they likely have never been cleaned. (or have them cleaned) Can be expensive to have them cleaned, esp if hard to access. I've met AC repair techs who really didn't want to talk about my inside coils because they knew they were a real PITB to get to on my downstairs unit. If your inside coils are pretty clogged up it can reduce your cooling capacity significantly & make your system very inefficient.

Be very, very careful not to damage the fins when cleaning inside & outside coils. Compressed air at too high of a pressure can bend the fins. There is a special fin cleaning foam you can spray on & rinse off - they should have it at Lowes, Home Depot, etc.
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Old 07-01-2008, 07:57 PM   #8
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Another quick check if you have an infared thermometer is to measure the temperature of the suction(large pipe) it should be about 48 50 degrees where it comes out of the indoor unit. Also measure the temperatures of the air coming out of the vent closest to the indoor unit and the temperature of the air in the house. If there is a 20 degree difference the a/c is working at capacity. If it is more than 20 degrees a possible shortage of refrigerant.
A couple of other things to check/do. Turn on the a/c early, turning it on when you get home is too late. Set the t-stat at 72-74 in cool and go to work in the morning. An a/c is not designed to "catch up" with a hot house. Also when you get home at 5pm check an outdoor thermometer and then check the indoor house temp if the temp inside is 20 degrees or more cooler than the outside the a/c is working as well as it should.

I made a good living as a HVAC mechanic for 6 years before going to work for the state.
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Old 07-02-2008, 09:57 AM   #9
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What you can do is make sure your air filter inside is clean (I saw some that hadn't been changed since Eisenhower).
Best story I ever heard from an HVAC guy was the customer who was having an air flow problem. Guy asked him if he ever replaced the filter.

"I put one in whenever it needs one".

Eventually they opened up one of the big sheet metal junction boxes in the crawl space and found about a dozen of the cheap fiberglass filters in it, crumpled up and jammed with dust.

The guy was putting in a filter, it'd fill up completely with dirt and eventually the system would crush the cardboard rim and suck the filter in and down the intake into the box. The guy would see that there was no filter in there and stick another one in.

Now, I know that not everyone is Albert Einstein, but wouldnt it ever occur any reasonably functioning person to wonder what happened to the old one? Dissolved?

Wanderalot...the normal effective service life of a well maintained HVAC system is generally 15-17 years. Other than replacing air filters often, lubricating any non-sealed motors (mostly older ones) and spraying the outdoor unit clean with a hose, there isnt much maintenance that can be done by the homeowner.

A trained pro can clean your coils, check your refrigerant pressure and add some, and check other aspects of the system such as the gas valves and igniter systems to make sure they're operating properly.

While many systems are known for operating past 30 years, they're well made older systems that have a very low efficiency rating.

At 20 years, almost anything new you might put in would be so much more efficient that you'd end up getting a payback in lowered utility bills in around 7-10 years. Thats providing you have a significant heating or cooling season.

If you live in a very moderate climate, get someone to make a service call and clean/recharge the system and forget about it for a few more years.

Otherwise plan on a 4-7k expense to replace your system with a new one.

Only advice I can offer on that is to pick a factory authorized, NATE certified installer, that most of the equipment is pretty much the same as far as capabilities and reliability, and to get a 10 year factory parts and labor warranty.

That way if theres anything wrong with the install, its on the manufacturer to make it right. Your installer is authorized and you have a full warranty.
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Old 07-02-2008, 10:27 AM   #10
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The guy was putting in a filter, it'd fill up completely with dirt and eventually the system would crush the cardboard rim and suck the filter in and down the intake into the box. The guy would see that there was no filter in there and stick another one in.
That's priceless. I'll be chuckling about that one all day.
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Old 07-02-2008, 07:24 PM   #11
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Now, I know that not everyone is Albert Einstein, but wouldnt it ever occur any reasonably functioning person to wonder what happened to the old one? Dissolved?
These are the same people who shouldn't be allowed to use septic tanks or under-sink disposals.

And I'm not so sure about caring for pets & children, either...
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Old 07-03-2008, 10:54 AM   #12
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Oh definitely. My old neighborhood was all septic and nobody knew you're supposed to pump the things. You can get away with not pumping if you have a huge downslope lot and 3-500' of leech line. These were .12-.2 acre lots and a lot of people had poured big concrete patios over the tank and leech field, oblivious to their existence.

Rumor has it the city was going to come around in a couple of years and charge everyone 12k to hook them all up to city sewer. Once the tanks overflowed and started backing up into the drains and creating their own surface leech areas...
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Old 07-03-2008, 01:23 PM   #13
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With our old system, every time our house couldn't get cold enough it was because we had to add some freon (the service people could never find the leak). The symptoms were sometimes different though.

Once, the house would not get cold at all and the flow of air coming from the registers was reduced to almost nothing. It was because the exchanger was completely frozen solid and therefore the air couldn't go through it. We also had ice chunks forming around the pipes traveling between the exchanger and the compressor. Once we topped off the freon, everything came back to normal.

Another time, the flow of air was fine but there was no cooling at all. No other visible symptoms. That time there was almost no freon left in the system.

To clean the exchanger use extreme gingerness because it is so easy to poke a hole in it. If you do, it's pretty much over. I used a soft bristle brush to clean between the fins and I also use a vacuum cleaner (again use the soft brush attachment, not a hard plastic one). While you're in there clean up the condensation pan and make sure it is not clogged. Finally, clean up the p-trap on the drain line.

To clean up the compressor, I never go inside the enclosure, I just make sure that there are no leaves or debris blocking the air intake slits on the side and that nothing is blocking the cooling fan at the top.
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Old 07-03-2008, 01:29 PM   #14
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We were living with a friend about 8 years ago... his house would not get cool at all in the summer. He was complaining since the house, at the time, was only 7 years old. I asked him when the last time was that he had replaced the filter. He said "what filter?". The only reason it was still blowing some air is because the dirt had put so much weight on the filter that it tore a little opening along part of the top.

It got much better after that.
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Old 07-03-2008, 01:54 PM   #15
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We are currently visiting Son in Northern Virgina and it just so happened he had his HVAC (Elect AC - Gas Heat) serviced today for $89.95. The fellow that did it looked in his little book and proceeded to do almost exactly what I mentioned above (message 3). He did pull the Squirrel Cage fan and motor out of the inside unit - something I would not do since I can get to mine without doing that - but it added about 30 minutes to something that could be done in 5. Seemed to me to be almost exactly what I do every year to mine. Helps to cover the AC (exterior unit) with a small tarp to prevent leaves and those pesky pine needles from getting in over the winter.
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Old 07-03-2008, 03:24 PM   #16
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Here's another fun tip. Turn off the breaker to your air conditioning unit in the winter. Some of the units have a little heater in them to keep the refrigerant at the proper operating temperature and they all have some sort of control board in them thats sitting there powered on at the ready to do absolutely nothing for 3-6 months.

Might save you a few bucks in electricity over the winter.
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Old 07-03-2008, 03:33 PM   #17
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Here's another fun tip. Turn off the breaker to your air conditioning unit in the winter.
If it's a heat pump this will really save you some bucks!
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Old 07-03-2008, 03:33 PM   #18
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If it's a heat pump this will really save you some bucks!
Yeah, I was going to say that. We have a heat pump, so we'd be spending some of our savings on winter coats and thick blankets.
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Old 07-03-2008, 04:00 PM   #19
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Had the same problem three years in a row until I found a competent HVAC guy who actually found the leak and fixed it rather than just recharging...
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Old 07-04-2008, 12:41 AM   #20
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CFB mentioned the heater and winter current draw. Few people know about this.

Reciprocating compressors often had a crankcase heater that was wired directly across the 240 volt line in the outdoor unit. As such, it was powered whenever the AC breaker and the outdoor disconnect switch were both ON. It doesn't matter if the AC is actually running, the heater is on anyway. The heater was usually a resistance wire in a metal band that was strapped externally around the compressor's case.

In colder climates, the usual instructions were to turn on the AC breaker in the Springtime (and thereby turning on the crankcase heater) at least 24 hours before using the system. But most people didn't know about it, and usually left the AC breaker ON year-round. So it was drawing current year-round. I forgot to turn them off sometimes after cooling season was over, too.

Scroll compressors which are in just about every residential compressor unit made today have variable tolerances and are more forgiving. I haven't seen a heater on a scroll compressor.

Around here, we get cottonwood cotton and dust (dirt) from the air on the condensor fins. The only way to do a good job and restore efficient condensing action is to hose it off from inside out. This backflushes all the stuff out of the fins.
Turn off the outside disconnect, take the top off, tilting and holding it up if the fan motor is attached. Use a hose nozzle that will give a tighter spray pattern with pressure, and spray from inside-out holding the nozzle perpendicular to the condensing coil face. Move slowly along with the nozzle. Most condensing coils are two-row, so you need a tight water pattern to flush through both coil layers. Needless to say, avoid spraying the electrical control box if it gets exposed when you remove the top. If you do get water in with the contactor and start/run/fan capacitors, let it dry out completely before turning the disconnect back on.

And when lawn mowing, try not to nail the unit with grass clippings.
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