Replaced my unit about 2 years ago. The first thing that went wrong was the TXV. "They just don't make them like they use to" comes to mind and this would be correct thinking.

Laymen terms... Think of a TXV like the gas peddle on your car. You push it down and the throttle body opens to let more gas in. As your speed equalized you back off the peddle, when you go up hill you push harder.

TXV has a sensor that mounts on the suction line coming out of the evap coil. That sensor is like your gas peddle. When your unit is warm the gas peddle is pushed and your throttle body is opened, when you equalize or cool off it starts to throttle back, when there is a load it opens up the throttle again to get more gas.

This opening and closing effect will affect your superheat. If the TXV is going bad or restricted then yes your superheat can be 0.

Is it what's wrong with your unit... don't know but there are the laymen terms.

 

Rheem put out some coils where the TXV manufacturer got some wax in the valves which held them open flooding the coil and causing very low superheat. Usually didn't show up on install. Most showed themselves during our last super heat wave when people couldn't cool the homes. Fix was to blow the wax out by boosting the refrigerant pressure. That worked every time and all systems put out good, cold air. I wouldn't suggest that here but odds are good that a new, working TXV will solve it.

 

Sounds like a reasonable diagnosis to me. Refrigerant does the most cooling when it changes from liquid to gas and 0 superheat means that the change isnt happening. As long as your contractor is honoring his warranty just let them work on it. They want it fixed as bad as you do believe me. Nobody likes working for free.

 

We've seen bad txv with zero degrees superheat and sub cooling. Lower than normal high side pressure and higher than normal low side pressure. Luckily for the customer it's been under warranty. Not good for profitability if your the tech doing the warranty repairs. But as long as the customer gets taken care of that's what really matters. Where have all the orifices gone? Lol.

 

i hate that i have to come to this board.....but i like that it exists!

thanks for all the feedback

 

Residential TXV superheat

One thing to remember about residential TXVs; they do not control superheat with the precision of a commercial TXV. Refrigerant charge has an big effect on their performance; overcharge and you will see flood back (low superheat) These valves have a wide range (from 3 to 20 degrees superheat) depending on what mode the system is running in- low, high, dehum. The superheat will also be affected by: what stage the unit is in (high, or low) and if its running in dehumidification mode ( a very low airflow, around 275 cfm per ton)
Also, the Infinity system has various system airflow settings- efficiency, high, max, that can affect superheat.
That being said, The charge needs to be correct before worrying about valve performance. about 11 degrees subcooling is typical of a properly charged system. Run the system in high stage, non dehum mode, dial in the subcooling to the nameplate value, then check the superheat.:.02:

 

just like zen said u need to make sure ur sub cooling is on with what the condensing unit calls for. a fellow tech found one the other day 7 yo and had perfect sc but sh was way high, bad txv. changed it today and all is golden, let the tech do his job if its under warranty ur in the clear atleast they found it now instead of after warranty expired. u did buy the cadillac and high performance systems need more acurate pressures to deliver what they are meant to do, when it is dialed in u will love these systems, guaranteed .

 

....Service guy comes today. Here is some verbatim as its all greek to me. "I wasn't getting pressures i liked so i let some refrigerant out. Something is wrong with the TXV and i'm getting zero degrees superheat. I'm going to order a new TXV and have the guys install next week." I prompted him to say how confident he was that if the TXV was replaced that we would start seeing cooling to thermo temp. He hemmed a little bit and was like "one step at a time, but yes i think it might fix it." That is a good responsible answer.

Guy was perfectly nice, but i want some feedback on...
a) does this sound reasonable? That a dysfunctional valve might cause this and that replacing the valve will be the silver bullet? Yes and yes.
b) i read somewhere that zero degree superheat could damage a compressor. Yes. Am I at risk of more damage or is waiting until next week to get this TXV replaced ok? You probably have a scroll compressor that tolerates liquid refrigerant flooding better than the older piston compressors. Replace it ASAP is the best plan.

also, if anyone wants to give me the one sentence layman version of whats going on, thats nice, too. What i got from the web is that a TXV is a valve that controls the amount of refrigerant flow into the evaporator and that super heating has something to do with the refrigerant changing states and therefore translates into cooling ability. Or something.

thanks for your time!

Boiling refrigerant absorb heat just like boiling water on a stove absorbs heat from the burner. Your air conditiong system keeps the refrigerant contained and recycles it. Heat is transferred into the system to the cooling coil and rejected outside in the other coil during opperation.

The cooling in your system occurs when the liquid refrigerant boils inside of the cooling coil and turns into a vapor. After the liquid is all boiled away in the cooling coil, the vapor is heated above the boiling point. This additional heating of the vapor is superheat. The expansion valve controls the flow of refrigerant liquid into the cooling coil so that all of the liquid boils away and the slightly superheated vapor is safe to return to the compressor. Compressors are designed to compress vapor.

Compressors cannot compress a liquid. Zero superheat means that liquid refrigerant is returning to the compressor instead of boiling away in the cooling coil and absorbing heat. (That is why your system isn't cooling as it should.)

 

From the information provided, there's no guarantee that the TXV is at fault. The system could be grossly overcharged which can lead one to believe the TXV isn't doing its job when in fact, the evaporator is flooded and the TXV is hunting in an attempt to control a gas temperature when in fact it's actually trying to control a liquid temperature. More info would be needed for a definitive diagnosis.

 

In the old days we could force the closing of a TXV by spraying the sensing bulb with liquid refriterant during system operation (R22 was 40°F Below Zero). If you carry a can of computer "compressed air" cleaner you can invert the can and spray R410A on the bulb legally. Read the label. Go figure.

 

so, they cant replace my TXV until monday. things have seemed better since the tech came out*, but the temps have been much lower outside.
Can someone tell me how i can check my superheating myself? just curious to check before and after the fix. Can this be checked via my thermo? when i hit the advanced buttton, i get a limited menu. If this can be checked by me, what should the number be? Is that number supposed to be constant or does it fluctuate with the outside temp?

*he said he released some refrigerant..."overcharged" means too much refrigerant in the lines/too high pressure, is that right?

thanks again

 

You can't check it yourself. You would need manifolds to do so.

He released some of the charge? Hmmm.

A little while back. I was out on a second opinion call. The TXV had been condemned by the other company. My readings were 4°F SC, 3°F SH, 362PSIG head, and 143 Vapor(R410A system) It wasn't the TXV that was the problem. Most of the guys on this board will know/or be able to guess what the problem was.

Ask the tech that comes out to do temp readings on the line set at both the in door and outdoor unit before changing the TXV.

 

I havent been to school on this since 1968 but 0 super heat to me means a fully flooded evapourator pulling max heat from system.Super heat is for compressor safety not efficiency. Have I missed something.

 

I think we're looking for saturation rather than flooding or starving the coil

 

most times i find a unit with a txv at 0 degrees of superheat i start looking at airflow issues first

 

so....here's a funny story. Wish i could say a funny ending to the story, but i suspect the saga will continue.

my attic is a nightmare to enter and work in so they send two guys, one to climb up there and the other to stay below and help send stuff up and down. 2 hours in they decide they need the second guy up there and have to cut the sides out of the return ducts to get him access. almost 4 hours in and they seal everything up and go outside to run their tests. He is still getting crap superheating readings so hes now saying maybe they have a 2nd bum TXV, but now they'll check airflow at all vents. 10 minutes later they inform me that in 3 of the 4 bedrooms, there is cardboard stuck up in the vents! :eek2: 2 of them are almost completely obstructed, the 3rd half so. Previous owners must have decided those rooms were too cold, or more likley they didnt use them - the old vents have no louvers or way to shut them to they opened them up and put in sheets or cardboard. They had since been painted over, so the way to open them wasnt even visible, you couldn't see the cardboard, and they had the vent pattern spray painted onto them from the renovation. So, now i know why my kids have been sweating their asses off.

Flow obviously dramatically improved and it was night and day in the kids rooms.

Unfortunately, the superheating was still around 2 degrees. He said that he was getting a 20 degree temp drop and everything else looked good, so he's going to see if it continues to cool well when it gets hotter out. He said he has seen some houses that never get a good superheating reading even if they cool well. He also said the rain could be preventing him from getting an accurate reading. I have no idea if the system can operate well at that reading.....we shall see.

 

I don't find it to funny of a story, tech condemns TXV before checking for proper airflow, still only 2* superheat? I think I would be doing some CFM calc's to make sure the proper amount of air is going through the coil.

 

You mentioned superheat after they removed the cardboard, but not the subcooling, which will tell the rest of the story.

 

Originally Posted by jbooker56
I havent been to school on this since 1968 but 0 super heat to me means a fully flooded evapourator pulling max heat from system.Super heat is for compressor safety not efficiency. Have I missed something.

Just processing outloud... 0 degrees superheat would indicate that the liquid refrigerant did not change states within the evaporator (although theoretically I suppose it could have), thus pulling the least amount of heat from the system. Super heat is a term used to describe a vapor heated above its saturation temperature, and because of that, the presence of superheat confirms that the refrigerant is indeed in its vapor state. Superheat only has to do with compressor protection in the sense that its presence likewise confirms that liquid refrigerant is not getting back to the inlet of the compressor.

 

There are many scenarios that illustrate the importance of clean blower wheel blades, a clean evaporator, while optimizing the airflow heatload through the heat absorbing evaporator coil into the liquid refrigerant to boil it off.

If those areas were properly checked on systems that are a number of years old, in a fairly high percent of them, you'd find way too low heat absorption by the coil to boil off the required amount of liquid refrigerant. The result; half to a ton lower delivered BTUH...

We should all strive to make the unit achieve its Nominal BTUH Rating in accordance with a proper heatload presented to & absorbed by the evaporator coil. Under many varied conditions you can't do that with a fixed orifice or cap-tube system...

The big advantage of a TXV is that it will try to adjust the rate of liquid flow to meet the rate of the liquid boil-off demand while achieving the Superheat & Subcooling targets of the mfg'ers Temp-Ratings.

Example: with a fixed orifice, it's a cool 65-F outdoors with a large indoor crowd & boiling sweetcorn generating a huge sensible & latent indoor heatload; the fixed orifice or cap-tube won't have enough head-pressure to flow liquid fast enough to keep-up with the boil-off & superheat will skyrocket with a lack of BTUH heat absorption.
The compressor might also eventually get too hot due lack of cold enough suction vapor.

Put a TXV on there, also with or without a suction-line accumulator, with a 8 to 12-F Superheat setting & the system will, of course, far exceed its ARI BTUH tonnage Rating!
More liquid will be boiled off & thus more heat absorbed & ejected by the condenser...

Those factors ought to be checked & properly adjusted on every service call; you'll improve performance & both you & your customers will be much better-off for it...