Brand new central AC freezing, canNOT find cause
Hi all. Thanks in advance for any advice you can offer, and apologies for the length. This is seemingly an extremely unique, complex, and frustrating problem. I'll do my best to describe as briefly as possible -- please feel free to ask any clarifying questions you find necessary.
My wife and I live in a typical 1400 sq ft suburban house in Atlanta, with a 2.5-ton AC unit. House is 13 years old, and until a few weeks ago, all HVAC was original equipment. Two storeys, with an open living room to second floor. A/C system is NOT zoned. Plus our master bedroom is over the garage, and faces south. So our upstairs (especially master) tends to be 5-10 degrees warmer than down.
Thermostat is downstairs, and we like to sleep cold, so it's usually set to 65 or 66 down at night, so we can hit 68-ish upstairs (ideal sleep). Then when we wake up, we set it to 70 or 71 (wife works at home and needs it comfy in upstairs office), and it usually doesn't run again until midday. This has more or less worked fine until the last 8-9 months...
The old system had been running more or less OK until August 2011, when we discovered it had a freon leak. Had it topped off at that time -- but just a month or so back (only 7 months since top-off), it was already down 3 pounds, and freezing up again. At that point, we decided to replace the coil and outdoor unit with a new R410 system, rather than trying to repair an old R22. New equipment is Ruud, still 2.5 ton. We assumed this would solve our freeze-up issues, and independent professional opinions agreed with that.
Unfortunately, the regular freezing-up has continued on the new system. Our contractor has been GREAT -- he's been back out several times at no charge to try to diagnose this, but with no success. He calls this a "one-in-200" type problem. He's basically tested everything he knows could cause freezing, and nothing is wrong. Literally everything he's checked is working like clockwork.
Here is what has already been ruled out, both by me as well as my contractor:
* Refrigerant leak. My guys have vacuum tested the whole system -- coil, outdoor unit, line set -- at least twice. Waited for over an hour to see if the needle moved, it didn't. Checked the charge again after over a week, no change. No leaks.
* Insufficient or collapsed ductwork. They checked all of it. Nothing is collapsed or pinched, and the ductwork system is sized 10-20% over what is needed for a 2.5-ton system.
* Upstairs dampers closing off in error. We have a very primitive thermostat-based damper system for upstairs vents, and we thought perhaps it was getting stuck shut. Verified that it is open when it's supposed to be, it's responding to our settings correctly -- and in any event the damper will default to open if there were a problem.
* Bad blower motor. The vents still put out plenty of air. You can feel the force 6-12 inches away from the vent. Additionally, Chris asked me to touch the motor once when the system was frozen, to see if it was hot (an indication it's going bad). It was comfortable to the touch.
* Dead transformers, capacitors, etc. I don't even know all of these, but I know my guy has replaced several of these parts just trying to rule stuff out.
* Dirty coil. It's brand new and squeaky clean.
* Dirty filters. They're brand new and clean.
* I might even be leaving something out... honestly, they've checked so much stuff that I can't even keep track of or understand all of it.
**** Aside from the freezing problems, the past 2-3 days I've also had an issue with the outside fan not coming on when cooling is called for. I can hear the outside compressor kick on, and see the fan run for a few seconds or minutes, but then shut off. But the compressor is still running out there. If I let it rest for a while, sometimes it will resume working correctly...?? I do have a low-ambient controller on this new unit, which I understand could shut the fan off when it's cold out -- but it's not cold out. This just happened tonight, and it's only in the upper 60s. Yesterday it happened in the middle of the afternoon, probably 80 out. So, honestly not sure if this is related to the freezing issues, or just coincidental weirdness.
I completely trust Chris, my install contractor. He's been in business for 25 years, solely from recommendations, no advertising. He came personally recommended from several independent sources. But I think he is getting to the point where he's running out of things to check from his experience.
So I just want to know if anyone has any suggestions that might help him know where to look next. ANY ideas would be HUGELY appreciated. I feel so frustrated that we just spent money on a (presumably) good new system, and we're still freezing the unit and struggling to cool the house each day -- when it's barely 75-80 degrees outside so far. Dreading what the summer will bring if we can't figure this out.
HELP! Thanks! :-)
a BETTER low ambient control will cycle the fan by approach method or liquid line temperature. if the liquid line temperature is to low the fan will shutoff to try and raise the pressure/temperature. I do not know which low ambient control you have though. if it is an intermittent freeze up i would look more towards the indoor expansion valve failing. rheem/ruud are famous for bad TXV(thermal expansion valves) have them verify that the TXV bulb is secure, in good contact, and insulated. also have them look at any service bulletins on diagnosing or field repairing the expansion valve. from what you are describing i would have my focus on the TXV. did they replace the refrigerant tubing with the new installation? if not you may still have a leak and they are trying to cover it up by adding refrigerant and not telling you. if the unit wasnt low on charge then i dont know why they are trying to find a leak under a vacuum. a proper leak check can be done with 50psi of nitrogen and bubbles. then increase pressure to 400psi and let hold for an hour. a good vacuum gauge(micron gauge) would also tell you if there are any leaks. I hope they are as honest as you claim and would lean more towards failing expansion valve
gravity, thanks for the reply! I also am not sure what type or brand of low ambient this is. I requested it on the new install, because we do often call for cooling, even when it's relatively cool out -- and that has given us (unrelated) freezing problems years ago. So the goal was just to help prevent freezing in colder weather. But onto the main problem.....
Thanks for the recommendation on the TXV, I will have them look into it as you mentioned. Hopefully that goes somewhere!
As for the refrigerant line tubing, I'm assuming you mean the connection going from the attic, down through the outside wall, and out to the backyard unit? They did NOT replace that -- but in the past two weeks since install, they have isolated that section of tubing and checked it specifically for leaks, separate from the coils etc. I want to say they did what you mentioned (nitrogen etc) but not sure. They actually tested that section twice to verify, with me standing there -- because, in his words, "I really don't want to run a new line set up the outside of your house." :-)
Additionally, Chris has added small amounts of refrigerant 3-4 times in his testing. In fact tonight, he went "off the chart" and added more than is specifically required, in hopes that it will help keep pressure up and prevent freezing. And he has not charged me for any of these additions. I don't think he'd be pumping free gas into a line he wasn't SURE was leak-free...?
So I think it's safe to say there are no leaks in the line. So for now, I'll focus on your point about the TXV, and see where that goes. Thanks for the advice.
Still welcoming any other posts from others as well!
Forgot to mention... in addition to doing the vacuum testing, they also veeery slooowly ran a freon detector along the entire length of the line (at least, as much as is accessible). Also checked the new equipment. Nary a peep....... And, additionally they checked to make sure there was no debris in the lines.
Just some additional things they tried...
a failing TXV can sometimes show as low on charge. If he is avoiding running new refrigerant tubing then that is just wrong. there is refrigerant tubing ducting that hides the tubing. some manufacturers are fortress and slim duct. if he has tested those line 2 times then he is doubting himself. if the lines are old and were not flushed properly then you probably have a gummed up TXV because of the mix of oils. maybe you can come to a agreement of splitting the costs of running new refrigerant tubing. I run new tubing with all of my jobs. it is rare that you cannot run new tubing. if for some reason a customer denies running new tubing i will walk from the job or strictly document and have them sign off if a problem occurs.
has airflow been verified by doing a simple static pressure check and verifying with the blower charts provided by the manufacturer? simple readings should be taken.
return drybulb and wetbulb temperature
supply drybulb and wetbulb temperature
static pressure after filter on return -"w.c.
static pressure after unit on supply +"w.c.
has he measured superheat and subcooling or just looking at pressure?
The old lines are original to the house, so 13 years old. Don't know if that's considered "old" or not. I'm not sure if he flushed them during installation or not, but I wouldn't be surprised -- he seems very attentive and proud of doing good work. He has taken care of several small things other folks have missed, without asking, free of charge. Like I said, FWIW, he's highly recommended to me. Anyway, I'm fairly sure he HAS flushed the line at least in the past two weeks, during his post visits.
Sorry for creating a slightly wrong impression. I don't think Chris is opposed to new tubing on principle -- I think he was just trying to double- and triple-verify the absence of leaks, before charging me a few hundred dollars for new copper. But I'll ask him about it again.
Also, from your description, it sounds like you can run new tubing inside existing walls, directly in place of the old tubing? Chris seems to think if they ran new stuff, they would need to go up the outside of the house and through the soffit. Am I / is he missing something?
Finally, everything you said starting with "has airflow been verified..." and on, went waaay over my head. :-) So I'm honestly not sure if he has run those tests. I do know he has a little thermometer probe that he lets sit in various supply and return vents for several minutes, before checking it. He's also done this in the attic, right at the supply/return on the furnace. Is that what you mean?
THANK YOU for your help!
Assuming low airflow, and faulty system components and refrigerant charge have been eliminated, crazy low thermostat setting + mild outdoor temperatures are most likely the problem.
Originally Posted by jaredhamilton
This is simplified a bit, bu with the normal 400cfm of airflow per ton of cooling, the indoor coil in a typical residential system will be operating around 30-35º below the return air temperature under "normal" conditons.
So with 70º return air, the coil would be operating at 35-40º under "ideal" airflow conditions and normal 80º+ outdoor temperatures.
If the airflow through the indoor unit is lower than it should be for the size system, and/or the outdoor temperature is very mild <70º, or low <60º, the temperature the indoor coil is operating at can go down significantly below freezing.
Your old system may have been "ok" operating in those conditions because it likely had fixed metering for the refrigerant flow to the indoor coil, so under low load conditions will tend to flood the indoor coil with refrigerant.
Not great for the health of the compressor, but tends to keep the coil from freezing up to quickly if the run times are short.
The new system may have an expansion valve controlling the refrigerant flow into the indoor coil, which will throttle the refrigerant flow into the evaporator coil under low load conditions.
This tends to drop the coil temperature further below freezing than would happen with fixed metering under low load/ambient conditions, so the coil can start freezing up almost immediately.
The new system probably also has a much larger condenser coil, which can cause the condensing temperature/pressure to nose dive in mild and low ambient temperature conditions more than the old system did.
Once the condensing pressure nose dives, there is less pressure to push refrigerant through the expansion valve into the indoor coil, so the temperature of the coil goes way down.
I'd look at getting your contractor to install a freeze protection thermostat on the indoor coil, and a low ambient head pressure control in the outdoor unit.
The freeze protection thermostat will turn the outdoor unit off for a few minutes if the indoor coil temperature gets to low.
The low ambient head pressure control will slow or cycle the outdoor fan to keep the head pressure up in the system.
Between the two controls, the indoor coil will be less likely to run significantly below freezing, and if/when it does go below freezing, the outdoor unit will turn off for a few minutes to let the coil warm back up a little.
I have to agree with Mark on this!!!!!
I seriously doubt a 13 year old builders model furnace in Atlanta will have a 90+ furnace.
Originally Posted by klyons20
Was nitrogen used for the leak checks or solely vacumm pump?
The OP has stated that a low ambient kit is already installed on the condenser.
Do you have anymore news on the system?
The refrigerant tubing can be ran on the outside of the home and put in a guttering type system. It can be called, "Line Hide", "Lineset Duct", "Slim Duct". Do a search on "Fortress Lineset duct" or "slim duct" and you can find some pictures of it.
@skibme: I've heard a ton of jargon the past two weeks that is all new to me, so I'm having difficulty remembering some things. I think Chris and his team mentioned possibly running a nitrogen test at some point, but I can't remember if they ended up doing it. I do know they ended up isolating and doing a vacuum test for sure.
@gravity: No more news yet. Chris came today and observed the "fan not spinning" issue, and concluded there must be a problem with the brand-new low ambient control that he'd installed. So he took that offline for now, and is bringing a new one on Thursday.
Chris came by again today, and said we have a 25-degree differential between supply and return, and that is too high. Apparently it's supposed to be around 20 or less. He thinks some additional airflow capacity will help this.
His next suggestion is to add some additional ductwork to the master bedroom (the hot room on front of the house over the garage). He said although our ductwork is properly sized as a system, it's not distributing cooling well for where it's actually needed. Since we often end up freezing out the rest of the house to get the master comfortable, I think he is hoping that additional CFS in the master will:
1) help that room stay closer in temp to other rooms, enabling the unit to not run as long (hopefully avoiding freezing as well); and
2) help increase overall airflow across the coil, which would also help avoid freezing.
That seems to make sense to me... how 'bout y'all? Aside from the recent problems I've had, I've ALWAYS had trouble cooling the master relative to rest of house, so hopefully this will help that in any case.
I also need to mention the freeze control thermostat for the coil before he returns... forgot to bring that up today.
So many variables, and so many problems! So ready for my air to "just work". Thank y'all so much for your advice, hopefully it will accelerate some solutions!
I think your problem here is that you need your duct system revamped, exactly how is the sizing is correct or 10%-15% oversized and you still need to set the thermostat at 65 degrees to make the second floor 68 degrees? Why would your system include zone dampers only for the second floor? I would think these would never close in the cooling season.
Do these dampers control the system or are they only controlling the dampers.
Your issue is low thermostat setting coupled with low outdoor ambient. The low thermostat setting could be increased if the second floor was receiving the proper air flow. You might consider the advice of a freeze-stat, a low ambient control, and some zone dampers in the first floor to direct some air upstairs, I would also recommend adding a few more supply ducts to the second floor.
Your contractor is probably a nice guy, but he should not have been stumped by this problem. He seems to be blindly attempting to fix the problem, why search for a leak if there is no loss of charge? A 13 year old line set is perfectly reusable if it has no soft solder joints, and was not a leak problem before. When my 27 year old cooling system finally bites the dust I will be reusing my line set as I have never added an ounce of refrigerant in 27 years.
Most likely with the issues with R-410a and leaking coils I will be contemplating using a different refrigerant altogether.
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