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Mis-matching HVAC system (replacing outside unit without touching inside)

Old 01-12-2008, 05:09 AM
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Default Mis-matching HVAC system (replacing outside unit without touching inside)

Guys -

I have a basic HVAC question. I've "got a feeling" that my AC compressor is nearing the end of its life. Have had some odd issues lately, the thing is getting noisy as heck, etc. System is a 3 (?) ton 12 SEER Rheem, 11 years old. Electric heat. Accelerating its demise is the crappy job a local company did when relocating it - but the good news is that there is a nearly new lineset, electric hookup, etc, so it will be a "plug and play" swap.

As far as I've been able to find, the scroll compressor itself to repair this one when it goes will run me $900.

Given that, I often see ads, or heck even ebay listings, for new condensing units for $1000-1500, give or take.

Is there any disadvantage to replacing my whole condensing unit but leaving the air handler alone? Aside from the ton capacity is there anything else that needs to "match" to make the two halves work properly together?

Thanks. There is actually a huge HVAC forum out there - but they hate homeowners trying to get DIY info. So last time when I needed to move my unit I took their advice and hired a pro - and got completely bent over.
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Old 01-12-2008, 05:58 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

Replace the outside unit and ensure the tech leaves the vacuum on the line for some time to ensure no leaks.

Air handlers are often only replaced when they either go out, get too old and are very inefficient (restricted coils, poor transfers) and/or the heating and cooling bills are creeping up which relates the former.

Here in Texas where AC is king, just like Florida, outside units go out all the time. The good thing is this, your new unit will likely be a 13 SEER as I believe there were laws put into effect 1 Jan 08 stating all new units will have at least a 13 SEER rating.

You will get bent over too, especially when you call for them to finish a job like you're talking.. Techs make their money not from the equipment (they barely break even there), but from the service provided. I personally would replace it myself, wire everything and solder all the pipes, then call a tech for an annual check up. Once he shows up, drop him a little extra cash so he doesn't tell you to grab your ankles!
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Old 01-12-2008, 06:29 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

Most of the newer units I've seen have expansion valves on the air handlers. Don't rule out replacing the air handler..the condensate pans and insulation deteriorate, and the fan motors will eventually fail. Plus, the air handlers get REALLY nasty with mold and gunk over the years. 11 years..I'd do the whole thing if you can afford it.
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Old 01-12-2008, 06:50 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

Replace the whole system if you can afford it. Systems are ARI rated as a system not as individual components. If you mismatch there is no way of actually knowing the systems ratings. And manufacturers are starting to offer zero warranty on their equipment unless it is purchased as a matched set.
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Old 01-12-2008, 06:57 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

I had a similar problem on a 10 year old Trane unit - the compressor seized up from what I believe was a lightening strike. I had the original installer and one other tech come out to diagnose the unit and provide an estimate. They both initially recommended replacing both the inside the house blower unit and the entire outside unit, not because there was anything wrong with the inside unit, but because they said it would be impossible to match up the ratings of a new outside unit with the older inside unit.

While I am sure there is some truth to that, I got the sense I was getting a snow job. I did a little research, and found out that the tolerances were not so precise that a "closest match" would cause risk of premature failure or any material inefficiency. Certainly not to the extent where I would be willing to put in a whole new system. Watch out for that scam. This is the equivalent to the brakes guys selling you new calipers with a basic pad replacement. I am sure we will hear from the HVAC guys who will come on and defend their turf.

I ended up calling a third tech, and told him to replace the compressor, which he did. Part - $600, labor/refrigerant - $400, and it has been running fine for the last 7 years.

The blower motors have a long life. If the coils and piping are still in reasonably good shape, I would just replace the compressor. They are the weak link in the system, so you could easily be in the situation again in 5 years.

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Old 01-12-2008, 07:24 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

I would replace the compressor and air handler try and get a 14 seer and a variable speed air handler that is the most efficient you can get
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Old 01-12-2008, 07:37 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

Hmm.. thanks for the comments guys. Turns out problem #1 was a failing contactor, I just finished replacing that and will see if that helps things. Of course the contactor going bad surely accelerated the demise of the compressor. I'm trying to be proactive about it, since I'd rather shop around and get a deal on a condensing unit during feb-mar then have it go out in the middle of August and pay $3k as an emergency replacement.

Trying to avoid replacing the whole thing - not necessarily staying in the house that long and for the size of the house (tiny) and the simplicity of the system I don't think I'm going to see any substantial benefit from investing a lot in a higher end A/C unit.

Air handler and ductwork is in reasonably good shape and clean. 11 Years sounds like a long time but my neighbor just replaced his system - which was installed in the early 70s!
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Old 01-12-2008, 08:04 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

Sounds like you just need to replace the compressor. Just how did the contractor make your compressor go bad? I'm curious to hear that. The 13 minimum rating was in effect in 07. Manufacturers could not build anything smaller than 13 after that point. If you could find the equipment, you could install lower than that though. With an older coil and new unit, things CAN NOT be warranteed. Mixing the two is not a scam. If it were, I'd tell you. It doesn't affect my business any which way. Fact is things are made differently today as opposed to ten, twenty years ago. As far as installing yourself, and paying cash........ I'll give you a guess what happens if something goes wrong. No one will work on it under warrantee. Everything is contingent on install. Keeping the pump on it for HOURS is kind of useless also. Once the vacuum pulls down 29.5 lbs, guess what, there's no leaks. A little longer is'nt bad because it cleans the lines out. But as long as it's on for at least an hour your fine. 11 years isn't bad at all. You still have many years to go on that system. Just replace the compressor and you'll be fine. Just get a good company in there to do it. Call many companies and ask questions to find the right guys that's all.

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Old 01-12-2008, 10:08 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

tinmarine - 1/12/2008 11:04 AM

Sounds like you just need to replace the compressor. Just how did the contractor make your compressor go bad? I'm curious to hear that. The 13 minimum rating was in effect in 07. Manufacturers could not build anything smaller than 13 after that point. If you could find the equipment, you could install lower than that though. With an older coil and new unit, things CAN NOT be warranteed. Mixing the two is not a scam. If it were, I'd tell you. It doesn't affect my business any which way. Fact is things are made differently today as opposed to ten, twenty years ago. As far as installing yourself, and paying cash........ I'll give you a guess what happens if something goes wrong. No one will work on it under warrantee. Everything is contingent on install. Keeping the pump on it for HOURS is kind of useless also. Once the vacuum pulls down 29.5 lbs, guess what, there's no leaks. A little longer is'nt bad because it cleans the lines out. But as long as it's on for at least an hour your fine. 11 years isn't bad at all. You still have many years to go on that system. Just replace the compressor and you'll be fine. Just get a good company in there to do it. Call many companies and ask questions to find the right guys that's all.
Tin - I came home after a week on vacation and found the fuse to the condensor blown. In retrospect I'm assuming the contactor failed closed and kept it running until it heated up enough to blow the fuse. The other night, woke up to hear the compressor running and running and running and the fan inside turned off. Took me a while to put 2+2 together to realize the contactor was sticking - my AC is on 360 days a year on an electronic thermostat so there's no telling how many times it may have stuck one way or the other. Also from the looks of the contacts I'm not confident it was passing the voltage it should have been, which from my understanding could also damage the compressor.

It's a different world down here in South Florida... I'm not sure I'd even count on getting a viable warranty with a brand new system. The "pros" I paid $1k to move my unit 50 feet to the side of my house sent a couple of $15/hr jackasses out here whose idea of freon recovery was to vent it through a coke bottle full of water (no lie) and then rather than vacuuming out the system, simply blew freon through it and then closed the valves. They "advised me" that my compressor was "probably almost shot" since it was not pumping down as fast as they thought it should, and did their best to sell me a new system since mine was "on its last legs." (that was a year ago)

In retrospect my understanding is that when opening and moving the system they surely should have kept a vaccum on the line for a while - and probably also replaced the dryer (?) as part of opening it up. But what do I expect for $1000 cash? (again - this was the co that did the ORIGINAL install, pretty big outfit)
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Old 01-12-2008, 01:07 PM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

Flot - 1/12/2008 1:08 PM

vent it through a coke bottle full of water (no lie) and then rather than vacuuming out the system, simply blew freon through it and then closed the valves.
That's funny right there.


They "advised me" that my compressor was "probably almost shot" since it was not pumping down as fast as they thought it should, and did their best to sell me a new system since mine was "on its last legs." (that was a year ago)

In retrospect my understanding is that when opening and moving the system they surely should have kept a vaccum on the line for a while - and probably also replaced the dryer (?) as part of opening it up. But what do I expect for $1000 cash? (again - this was the co that did the ORIGINAL install, pretty big outfit)
Pumping down the system is the process of vacumming the lines, NOTHING to do with the compressor.
The dryer they should have replaced, but you don't have to KEEP a vacuum on the system while moving it, just close the valves, particularly that short a distance.
For $1000 you should have recieved EXCELLENT service. Figuring they only used about 20 bucks worth of freon.
Shop around for a GOOD company. They are out there.

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Old 01-12-2008, 05:27 PM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

If its an air handler in a closet with a filter grill below it take the filter out, stick your digital camera in there and take a picture of the coil, should give you a good idea of its condition. Replacing the condensor without doing the air hander isn't a problem. It won't reach the rated efficiency of the unit, but it won't be a big deal...its will still be better than what you had before. If its a heat pump its a different story...electric only heat tho, not a problem. If you give me the model # off the tag on the condenser I can usually tell you the size.

Purchase a good quality condenser and if the air handler is in good shape you should be fine for now. If the coil looks like crap go ahead and do both or it will create headaches for you later on. While you have the filter out check behind it and make sure the return is sealed up well. Also on a 3 ton its common to find a 20x20 filter...if its not atleast 18x24 get a bigger filter grille. A good company will give you all of this information when they come out along with a proposal detailing exactly what units they would use and materials included.
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Old 01-13-2008, 05:31 AM
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Default RE: Mis-matching HVAC system (replacing outside unit without touching inside)

You can replace the outdoor unit without replacing the indoor, but as they said it is not recommended and
most companies will not do it. Here in Alabama you could not get it permitted or inspected. The problem is
not the heater but the cooling coil, but it is probably a single unit and not a separate heater and coil.
The new units have thermostatic expansion valves on the indoor coils as opposed to a fixed orifice on
the older units. This helps to obtain the efficiency for the unit and also keeps the correct superheat to
the compressor which lets it live a long time. One of the dangers of not replacing the coil is that the
superheat is not correct and refrigerant floods back to the compressor which can damage or destroy
it. Then you have the issue of R-22 being fazed out, and replaced with r-410A or Puron as carrier calls
it. If you replace the indoor equipment you may consider going with the new refrigerant. I would say
replace the compressor if the rest of the outdoor unit is in good shape, or go ahead and replace
everything with the new refrigerant. They will also have to replace the line set again if you go with
the new refrigerant. The oils are not compatible. But I have heard of companies doing it without
replacing the lines, just flushing them out. I hope this helps.
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Old 01-13-2008, 09:51 AM
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Default RE: Mis-matching HVAC system (replacing outside unit without touching inside)

N D H - 1/13/2008 8:31 AM

The new units have thermostatic expansion valves on the indoor coils as opposed to a fixed orifice on
the older units.
I'm willing to bet with it being a 12 SEER that it already has a TXV. Unit isn't that old. I am surprised it has a scroll compressor as opposed to a piston though.

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Old 01-13-2008, 06:21 PM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

My company is in Connecticut so maybe building officials are different in Fla., but there is no permitting issues with a replacement you are discribing.
A 11 year old 12 SEER unit is most probably a R22 system. You should be able to match a 13 SEER R22 system with no problems. Your SEER may be a bit lower but close. Depending on the indoor unit you more than likely can get a TXV conversion kit if it isn't already a TXV. A 12 SEER probably is a TXV. This is assuming it is electric heat, not a heat pump. Heat pump may be possible but a bit more difficult. Based on what your local contractor experience has been I wouldn't attempt it if a heat pump.
As far as warranty, the equipment manufacturers have never given me trouble about a condenser swap attached to an existing indoor unit. Sounds like HVAC is a bit different in Fla.
To me the bigger question is what are your plans with the house , what is the shape of the indoor unit and what is the state of the real estate market and will a new indoor unit add to the vaue of the house enough to warrant the initial cost outlay.
Mechanically there is no problem with installing a 13SEER R22 condenser.
BTW, the point of the vacum pump is to draw a deep enough vacum in the system to cause any moisture to boil off (see Boyle's Law)and be removed. If they change the condenser ask to see the new drier - it's a cheap $20 insurance.
As far as warrantys go, the parts warranty is usally with the equipment manufacturer. I have gotten compressors warrantied for equipment I didn't install or sell. Some equipment companies also offer a labor warranty (ex: Tempstar) The contractors around here offer a 1 year warranty where the contractor deals with everything for you.
If you are not going to change the indoor unit, I would get competitive prices on a 13 SEER 3 ton condensing unit replacement. Ask neighbors who they use for HVAC.
Whew!Good Luck.
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Old 01-14-2008, 05:24 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

You can replace the outside unit only. It will be mismatched and be less efficient, have no warranty, and you may not have fixed the cause. If you go this route, make sure the indoor (evaporator) coil is clean. 11 years, I would bet it's partially clogged, this will contribute to compressor failure. I didn't read all the responses but Tin is correct, a system being vacuumed will not pull down to certain levels if there is a leak. When I pull a vacuum I know if there is a leak by how quickly the system reacts. If you replace just the compressor, here are a few things to also check: indoor fan motor run capacitor (can cause motor to be erratic) - clean evap. coil - keep most supply vents in house open - keep a clean filter in the return - CLEAN THE OUTDOOR CONDENSER COIL - REPLACE THE DUAL RUN CAPACITOR (in the outdoor unit) - INSTALL A NEW FILTER DRYER - For airfare, limo service to your house, room and board for 1 nite, and a free fishing trip the next day, I'll come down and replace that compressor for ya.



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Old 01-16-2008, 05:30 PM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

The system will not be "mismatched". All that will happen is the SEER will actually fall between 12 and 13. Not a big deal. The primary difference is the size of the condenser coil. A larger coil yields a higher SEER rating.
After the condensing unit is installed, with a drier and properly leak tested, evacuated and charged the entire system should be evaluated. At that time you can check temperture splits on the evaporator (18 - 22 degrees) to determine proper airflow ( eliminating dirty coils, poor duct design, closed diffuser grills, dirty air filters). Then check superheat at the outlet of the evaporator(12 degees) and subcooling on the liquid line (10 degrees). Finally check refigerant pressures and amperages. If all this is acceptable your system should run fine. If I read the OP first post correctly, the system is functioning now. I would do the evaluation now and eliminate any indoor related problems.
I have NEVER had a warranty issue with a manufacurer or wholesaler by replacing a condensing unit on an existing indoor unit. Hell, they just give you the replacement parts and ask for proof of purchase date. If there are any warranty issues it's with the contractor, not the equipment manufacturer. Really, this is a simple job.
BTW, we used to do hydroair installs all the time. This involves a First Company indoor unit with a built in hot water coil. They didn't make an outdoor unit so we matched it to Tempstar condensing unit. No problems.
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Old 01-16-2008, 07:35 PM
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jwb - 1/16/2008 8:30 PM

The system will not be "mismatched". All that will happen is the SEER will actually fall between 12 and 13. Not a big deal. The primary difference is the size of the condenser coil. A larger coil yields a higher SEER rating.
After the condensing unit is installed, with a drier and properly leak tested, evacuated and charged the entire system should be evaluated. At that time you can check temperture splits on the evaporator (18 - 22 degrees) to determine proper airflow ( eliminating dirty coils, poor duct design, closed diffuser grills, dirty air filters). Then check superheat at the outlet of the evaporator(12 degees) and subcooling on the liquid line (10 degrees). Finally check refigerant pressures and amperages. If all this is acceptable your system should run fine. If I read the OP first post correctly, the system is functioning now. I would do the evaluation now and eliminate any indoor related problems.
I have NEVER had a warranty issue with a manufacurer or wholesaler by replacing a condensing unit on an existing indoor unit. Hell, they just give you the replacement parts and ask for proof of purchase date. If there are any warranty issues it's with the contractor, not the equipment manufacturer. Really, this is a simple job.
BTW, we used to do hydroair installs all the time. This involves a First Company indoor unit with a built in hot water coil. They didn't make an outdoor unit so we matched it to Tempstar condensing unit. No problems.
I think JWB needs to take a refresher class in hvac.
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Old 01-18-2008, 10:52 PM
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Tinmarine needs to go to HVAC school period!
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Old 01-19-2008, 07:35 AM
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Freezerman1 - 1/19/2008 1:52 AM

Tinmarine needs to go to HVAC school period!


Ok, I'll bite. Why is that? Is it when I told him his unit is fine and just replcae the compressor? Or how about when I said he should great service for $1000. Or perhaps pumping down the system? Replace dryer? Dates on federal regs? The only other thing I would have added is the fact that the tech that came out could have SAVED all his freon by pumping back into his condensor, and not filtering it through water (that still cracks me up). From the sounds of it, things are done different when it comes to warranties across the country. Everything here IS contingent on install. If it fails due to install it doesn't get warranted. Never mind the fact that when the installing company comes out and replaces the part, they charge insane amounts of money. My cousin moved into a house and her compressor went out. Still under warantee she had it fixed. Over $600 bucks to replace. Throw in another 100 and I would have gave her a whole new unit.
So let's hear it sport. Back your claim, let's have fun.

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Old 01-19-2008, 09:06 AM
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Default Re: Mis-matching HVAC system (replacing outside unit without touching inside)

A little off topic, but it came up earlier. There is no way anyone is going to open my system with out doing a Nitrogen sweep and a vacuum for at least 24 hours, regardless of closing valves prior to opening up the system. 10 years working with a bunch of hacks and suddenly we are swapping out units for a industrial maufacturing plant! Go figure. Watch the mechanics also, I had one show up that I was loaning my tools to, and advising. The new rules play into the HVAC techs. Get to sell you a whole new system when the shelves are clear of the lesser effecient stuff.
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