Differences between fixed orifice and TXV

stevescivicJuly 16, 2007

what are the advantages and disadvantages of having a a/c evap coil equipped with a fixed orifice or txv valve?

I know the charging methods for them is different but some contractors I'm talking to say TXV is always the best way to install a/c systems.

Is this true and why?


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Capilary tubes (fixed orific) are an inexpensive device for control of refrigerant and is often used in small equipment. It does not have a valve and does not stop the liquid from moving to the low side of the system during the off cycle so the pressures will equalize during the off cycle.

As it has no moving parts it will not wear out, but since the bore is fixed, it may become clogged up with particles or oil logged.

TXV (thermostatic expansion valve) meters the refrigerant to the evaporator using a thermal sensing element to moniter the superheat. This valve opens & closes in response to a thermal element. The txv can be adjusted to maintain a low superheat to ensure that the majority of the evaporator surface is being used. This will give the refrigeration system a higher net refrigerant effect and higher capacities & efficiencies.

Hope this helps!

I vote for the txv.

    Bookmark   July 16, 2007 at 8:08PM
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how can I tell if I have a fixed orifice or txv in my system? When I look at the copper lines going to my evap coil all I see is copper lines that are brazed on. No screw fittings or anything.

I have a 3 ton trane condensor and a 3 ton "cub coil". The manual book I found said fixed orifice but instructions may only be used as a guideline and my contractor may have used txv.


    Bookmark   July 16, 2007 at 8:50PM
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A thermostatic expansion valve works efficiently across the wide range of load that an air conditioner may have to operate through and constantly adjusts, or meters, the correct amount of refrigerant to the evap coil as the load changes. A fixed oriface, either cap tube or piston meters the same amount constantly. Its efficiency can never really be determined because the load is never constant. One drawback with TXV's sometime is as energy rater la mentioned is at shutdown they stop the flow of refrigerant to the low side. On hot days when the system is running a fairly high head pressure this can make it difficult for the compressor to start again as it is now trying to start against a much higher resistance within the cylinders. Thats why you are seeing higher starting torque compressors in newer systems. When the pressure can equalize like in a fixed oriface system its much easier for that compressor motor to get cranking. But you can never have a fixed oriface system and expect a good SEER rating.

    Bookmark   July 16, 2007 at 8:55PM
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Unless you have access to the evaporator coil from a panel in the plenum you really cannot tell.

If you do have access, look for a bulb and capillary tube on the suction (large line) that feeds back to a valve in the liquid (small) line.

TXVs greatly improve system performance.

    Bookmark   July 17, 2007 at 9:10AM
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Everything posted coincides with the very little that I know about TXV. My contractor installed a fixed-orfice coil and told me I was getting the efficiency to be expected from a TXV. He was not trying to deceive, but was just ignorant. The factory rep set him straight and my (new) system was upgraded. One of my units sounds like a gentle rain at just one location near the unit. This is as soon as it starts, before condensation dripping. Two technicians have told me this is normal with TXV.

    Bookmark   July 17, 2007 at 4:52PM
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On some systems the sticker or plate that has the model number and serial number will have a place for txv.
Some say installed or not installed.
Some list location of txv.
It varies from brand to brand & some of these stickers
make no mention of txv, although it does seem that Trane
lists the location of txv.

    Bookmark   July 17, 2007 at 10:08PM
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CUB is a cap tube (fixed meter) coil.

The idea of a TXV is to maintain a fixed superheat (how much heat the cool gas contains) over all conditions. In HVAC, they usually shoot for about 15°. That means if the refrigerant enters the coil at 40° it leaves around 55°. On higher SEER equipment, the entering temp is probably more like 45° to 50°. By sensing the heat picked up and adjusting the refrigerant flow accordingly, the coil stays colder than with a fixed meter.

On a fixed meter, refrigerant flow is a function of high side pressure which is really based on outside temp. Hot out, cool in, there's plenty of juice flowing and the indoor coil stays very cold. The problem comes with mild temps outside and a load (warm) inside. The refrigerant could boil away fairly quickly and part or much of the coil becomes warm and doesn't remove much heat and does no dehumidifying. This is worst in mild humid weather. Refrigerant flow is low since it isn't that warm out but with humidity inside, there's a good load. A TXV would allow more refrigerant to flow, a fixed orifice can't.

    Bookmark   July 18, 2007 at 5:41PM
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My new Goodman 14 SEER heat pump has the TXV field installed on the outside of the air handler. It is wrapped with a black foam blanket that looks like a beer can insulator. The TXV (made by Parker) was an additional $55 from Goodman - money well spent in my opinion. The 14 SEER / 9.0 HSPF rating on Goodman's product spec sheet is specifically for TXV equipped units. I also used a hard-start capacitor to help with the starting issues mentioned in previous threads. The installation manual for the heat pump calls for 7 to 9 degrees of superheat.

    Bookmark   July 20, 2007 at 12:11PM
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Gary, how did the initial subcooling and superheat shake out on your installation? Did your TXV require much adjustment?

    Bookmark   July 20, 2007 at 4:56PM
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I calculated 6 degrees of superheat and 6 degrees of subcooling:

80 psi suction pressure = 48 degrees saturated suction temp
210 psi liq pressure = 105 degrees sat liq temp.
Measured 54 degrees suction line temp (54 - 48 = 6 degrees superheat).
Measured 99 degress liq line temp (105 - 99 = 6 degrees subcooling).

I've made no adjustment (yet) to the TXV even though as you know the installation manual calls for 7 to 9 degrees superheat and 5 to 9 degrees subcooling. I used 25 feet of line but added no refrigerant to make up for the extra 10 feet.

I'll probably take more readings and re-check the figures before I decide to tweak. I realize that getting accurate liq and suction line temps are critical and I want to make sure that they're corrrect.

Take care.

    Bookmark   July 20, 2007 at 9:27PM
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Did you take the superheat pressure and temp at the evaporator output?
Measuring subcooling is usually easy at the condenser, but if you are measuring superheat at the condenser you have some line pressure loss to account for (and also temp rise)

I do not recall using both to set equipment without an adjustable valve.

    Bookmark   July 20, 2007 at 9:53PM
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Thanks Gary. I've installed a very similar system to yours, and from the same supplier, but it's the 13 SEER heat pump with variable speed air handler and the TX3N2.

It's been an adventure. The condenser came grossly overcharged for 15' of line. We had to remove 1½ lbs of the factory charge right off the bat. Since then my readings have been all over the map. We've had really crazy weather. Hot one day and cold rain the next. On install day it was cold and rainy. The tech initially got 17 for superheat which went down to 11 before we called it a day.

The first week, the system thought my house was a meat locker and was operating on some ridiculously short run times. Like running 20 minutes an hour when it was close to 100.

This week the weather is back to consistent blazing hot and humid but the compressor has settled down to more reasonable cycles. Today, at 95 ambient and sweat in the pool humidity, it was running 8 minutes on and 11 minutes off to maintain 76 in the house. Subcooling was 9 and superheat 4. My TXV needs a tweak but, like you, I'm waiting a while.

I'll shoot you an email through the Gardenweb network and give you a good email address if you want to compare notes and swap stories.

    Bookmark   July 21, 2007 at 12:19AM
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Both measurements were made at the service valves on the outside unit. When I make my next set of measurements, I'll check superheat at the evap coil - thanx for that info - much appreciated.


I also have a TX3N2 and I'm interested to hear your adventures. I decided on the multispeed fan and have it set to low which is the factory setting and it works well in my 1800 sq-ft split foyer w/metal ductwork. I have noticed that the new 3-ton unit (same size as the old 21 year old Trane) really has a lot of a$$ and has moved the indoor temp from 83 to 75 in 2.5 hours constant run time on a 90 degree day when I was away. Typically, the unit will run 5 hours/day total for indoor temp at 74 and outside at 90 with Baltimore humidity. I don't pay attention much to compressor run time as I keep the indoor fan running most of the time when I'm home.

I am also pondering the accuracy of the temp measurements. Even with a Fluke multimeter w/t-couple, a degree or two off on the measurement makes a big difference in the calcs.

    Bookmark   July 21, 2007 at 9:46PM
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Garyg, you can check the accuracy of your temperature analyzer by sticking it in a glass of ice cubes that has been filled with water. Stir it up for several minutes then stick the probe in. The iced water is always 32 degrees. BTW use the same thermometer for all your measurements and make sure you insulate the line where you are taking them.

    Bookmark   July 22, 2007 at 12:17AM
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Thanks for that reminder mr havac. I've been neglecting to account for the half degree error in my thermometer. Duh!

    Bookmark   July 22, 2007 at 12:17PM
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Thanx from me too, Havac Man. I used a co-workers Fluke meter and he told me that he checked it in ice water as well as boiling water.

I need to check the suction temp at the evap coil. I am assuming, based on Brickeye's post, that this is the typical location to check it (although the pressure readings were made at the condensor as this where the ports are located).

    Bookmark   July 23, 2007 at 2:43PM
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"although the pressure readings were made at the condensor as this where the ports are located)."

Now you know the problem with superheat measurements.
There is at least a few PSI between the evap output and the condenser input depending on line length and turns.
I put a port at evap outputs just out of habit.

    Bookmark   July 23, 2007 at 7:09PM
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Goodman specifies that superheat and subcooling be taken at the condenser with temps read near the service valves. Seems like a superheat taken at the evap may be different but that's not what they're looking for.

    Bookmark   July 23, 2007 at 7:48PM
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