Air conditioner & refrigeration equipment capillary tubes questions & answers.

This article series explains the function and installation of capillary tubes or "cap tubes" used to control or meter the release of refrigerant into the evaporator coil of an air conditioning or heat pump system, home refrigerator, freezer, dehumidifier, or other refrigeration equipment of up to 6 tons in cooling capacity. We define and explain the function, installation, servicing, and replacement procedures for capillary tubes.

Air Conditioner, Heat Pump, & Refrigeration System Capillary Tube Q&A

Our page top sketch of common visibly detectable capillary tube defects is provided courtesy of Carson Dunlop Associates as is the cap tube explanation at left.

[Click to enlarge any image]

On 2019-06-13 14:20:05.777130 by (mod) -

Check for abnormal voltage or high amps draw

On 2019-06-13 12:58:45.740565 by Chetanvvnagar

I don't have any inside in system thermostatic expansion valve but i do have refrigerant metering devices with 6 capilari distributor.
Sometimes run at around 70 psig and that time run stays longer and no freezing compressor but most of time running at 55 psig that time freezing compressor in 15 min.
Thanks

On 2019-06-13 11:18:34.037029 by (mod) -

That sounds like the thermostatic expansion valve is not adjusted properly and is passing too much refrigerant.

Watch out: that can damage or destroy the compressor motor.

On 2019-06-12 13:19:50.072785 by chetanvvnagar

Hi, installed new compressor on my 4 ton air conditioner with 407c weighted charge system.
Low side and high side matches pressure on my gauge. System has refrigerant metering devices with 6 distributors capillary tube.
My problem is my compressor start freezing up as soonest start air conditioner and within 10 or 15 min compressor turn off but evoporater fan still on inside.
Need solutions asap

On 2019-05-23 14:41:53.842746 by (mod) -

J

I'm unsure exactly which refrigerant metering device you're using. It's possible that the piston is jamming or icing. Are you seeing abnormal icing anywhere in the system?

Below is an example of a more-sophisticated capillary tube refrigerant control: York Part# S1-022-06844-051 as an example but we need more specifics.

On 2019-05-21 21:30:45.903487 by J

I have a piston metering device on an R-22 system. the high side pressure is good with very little change. The low side pressure hunts between 70 psig and 77 psig.

On 2018-05-12 03:14:06.212829 by (mod) -

Leo

I don't think so. If the cap tube isn't working the thermostatic expansion valve is not going to fix the problem. It's an either-or type device.

On 2018-05-11 10:31:53.615272 by Leo

Hi, can I add a txv before the capilry tube? 1999 5 ton Payne split Recidential thank you.

On 2016-07-11 19:57:52.066207 by (mod) -

Sid

Unless you're a trained HVAC repair person and have proper equipment, I doubt that you can successfully replace the capillary tube yourself. Doing so requires knowing how to remove refrigerant from the entire system without releasing any to the environment, cutting out the old cap tube, brazing in a new cap tube without blocking the tiny tube with excessive brazing compound or solder, installing a filter or two on the refrigerant line to capture any debris introduced into the system, pulling a vacuum on the system, checking for leaks and fixing any that are present, the installing a proper refrigerant charge and testing the system for proper operation.

So while you could match the diameter and length of an existing capillary tube by simply taking the old one to your HVAC supplier, that's not going to cut it.

Sorry.

On 2016-07-11 18:25:57.026508 by siddiq

hi sir, i have 3ton central split unit, and i want fix new capillary tube, please tell me which size and length i can use?

On 2016-07-03 21:23:49.901126 by (mod) -

Hi Bob. I don't understand from your question just what is wrong with your system. I'm not worried about a warm cap tube: the device is receiving liquid refrigerant from the compressor/condenser side of the system. The coolness should happen starting downstream from the cap tube, at or in the cooling coil in the air handler.

If the system is not cooling and the refrigerant charge is correct, then the cap tube could be plugged.

On 2016-07-03 04:29:45.563519 by BIG Bob Hansen

Hello, when I start my A/C unit, the pipe after the capillary tube gets a little cool, then the 'coolness' quickly goes away. There is no frosting on the line going into the evaporator, or in the evaporator at all. The tech guy was puzzled, because there were no leaks, and he said they put the right amount of [R410a] into the system. What could be happening?

On 2016-07-02 23:44:31.432074 by (mod) -

Jeff: there may be a method but I don't know of one. You're trying to solder a very fine, small-diameter tube; the chances of doing that without solder-blocking it seem miniscule to me, and adding a coupling or changing the length and volume would change the cap tubes performance anyway.

On 2016-07-02 22:36:58.027336 by Jeff Micono

can a capillary tube that is separated be soldered back together using a coupling?

On 2016-06-23 14:47:01.301449 by Anonymous

Return section not cooling

On 2016-06-23 14:44:25.061848 by Anonymous

Sir 5ton york a.c. compressor trip after 15minted

On 2015-07-09 21:32:55.577380 by (mod) -

Seems possible: if the cap tube is partly blocked the amount or rate of refrigerant release into the coil might simulate "low refrigerant level" (such as when there's a leak) and that condition can cause coil icing.

On 2015-07-09 18:35:36.998330 by Rudy

will a defected cap tube cause the evap coil to freeze?

Question: Coil has cap tube metering

(May 8, 2012) Wayne said:
I just replaced a compressor (R22 wet) and condenser unit on an old system. The coil has what I believe is cap tube metering. The liquid line (3/8") enters a manifold of sorts which splits into four small line imediatly before the coil. The manifold as I call it can be dis-assembled (no I did not take it apart) it look like a couple of plumbing fitting. The HVAC vendor I bought the condenser unit from said to release all the freon into the system. I think it has to be charged just right to get the high and low pressure correct. The house is out in west texas so its difficult to get a tech (nearest) to drive 100 miles. Sound like I'm going to have to beg one to make the trip unless someone can help on line. Please advise and thank you.

Reply:

Wayne I'm not certain I have a clear understanding - perhaps you could send us some sharp photos of the cap tube system you describe? Use the CONTACT US link found at page top or bottom of any of our web pages.

Often a cap tube is branched into several smaller distribution lines that meter refrigerant into different sections of a cooling coil - it's simply a means of efficient distribution to improve coil efficiency, and shouldn't affect the charge quantity.

Question: piston kit with new AC condenser size 065

(July 27, 2012) Anonymous said:
received a piston kit with the new ac condenser size 065 my a coil from another manufacture look to have a number 69 on it . Could this cause a problem with pressure or frost and are pistons interchangeable with different equipment makers

Reply:

Anon I may be missing something in your question or we are confused about why you are working with a piston kit.

An air conditioning system "piston kit" in my lingo refers to a piston and probably rings intended for use in rebuilding a compressor motor - not something we'd see on a residential cooling system (where techs replace the entire hermetically sealed compressor).

On some air conditioner compressor motors failures of the piston ring can lead to a compressor failure and damage to the piston cylinder wall. Rebuild kits can be similar to automobile engine rebuild kits - the cylinder is bored, smoothed and an oversized piston and ring kit are installed. Without a piston repair a commercial refrigeration compressor motor may be unable to develop full head pressure.

About interchangability, the answer may seem a bit obvious, but basically if the piston shape and measurements match the compressor motor design specs it's "interchangable". If not, not.

Question:

Oct 4, 2012) griggs said:
Motor home metering device---no pressure on low side

(Dec 28, 2012) john dillenger said:
why does my accumulator sweat during heat pump mode?

(Feb 13, 2013) Chuku Sunny said:
what is the possible effect of variation of diameter a particular flow line of an air conditioner system

(Feb 16, 2014) Anonymous said:
Sir! I've got frozen capillary tubes just beneath piston metering device. Can the orifice be plugged. The evaporator coil is warm. John

Reply:

If the cap tube is ice covered I'd start by checking the refrigerant charge level.

Question: Refrigerator cap tube problems vs other trouble spots

Mar 31, 2014) doug said:
My fridge just isn't cooling great - okay, but not great. I pulled the plastic tubing off of my refrigerator capillary and there was a rough feel to the metal and what I think was a white substance on the tube, maybe some moisture. I'm not sure because I wasn't paying too close attention because I had not found this excellent article by then. Would those things I mentioned possible pertain to a leak in the capillary?
thanks.

Reply:

Doug,

Usually a leak in refrigerant piping will produce an oily substance (lubricant in the refrigerant oil), and further, if you had a leak in the piping most likely you'd lose all refrigerant and get no cooling at all.

Is it possible that the cap tube was intended to be in good thermal contact with another refrigerant tubing section and that the white substance you found was a thermal grease?

Last (for now) check the obvious things first: blocked air flow, wrong control settings, imablanced setting of controls for a freezer compartment vs. refrigeration compartment; or a stuck defrost timer;

Question: damaged or crimped cap tube may cause compressor over-pressure and shut down

(June 1, 2014) Anonymous said:
if capilary tube is choked ,then frost will form on sution pipe & then compresor will trip

Reply:

Thanks for the comment, anon, but I'm not quite sure I follow the reasoning for all cases. I think you mean that refrigerant flow is reduced but not stopped entirely. That condition can also occur with TEVs and indeed can look like low refrigerant - frosting at the cooling coil

Question:

(June 8, 2014) allen said:
whats the distance between tne tube and evaperatercoil?

(June 16, 2014) Patti Greenwood said:
As soon as our system kicks on, the capillary tubes frost over. Earlier in the day, the larger tubes were frozen over. The fins are near spotless clean as we had them cleaned about six months ago and we replace the filter monthly. What could be the problem?

Reply:

Patti

Some frost formation is normal; if however you are describing a change in how the system operates I suspect a refrigerant leak.

Question: how condensation of the heated freon gas takes place

(july 14, 2014) anonymous said:
Sir. Could you please explain how condensation of the now heated freon gas takes place

Reply:

Anon,

At the outdoor compressor-condenser unit the returning freon is compressed to a high temperature high pressure gas. That gas is circulated through the outdoor condensing coil. Key is that the gas is at high temperature AND high pressure. Blowing outdoor air over the outdoor condensing coil transfers heat from the gas to outdoor air, cooling the gas back to a liquid.

Question: use a compressor burn-out filter

(July 26, 2014) Bill said:
Oustside compressor-condensor connected to the indoor gas furnace. It appears the de compressor had a burn-out. It was replaced with a new one. I had the system (circuit)flushed with nitrogen, vacuumed pumped and re-filled with coolant. All pressures seem "OK" Systems runs fine but no refrigeration is present. The air coming out of the ducts at ambiant air temperature. What is the possible cause of this?

Reply:

Bill,

I'm not sure what you mean by "no refigeration is present" - you say the system runs fine. I think from the rest of your note you mean the system is not cooling.

If the system was properly charged with refrigerant and the compressor is indeed working and developinng proper pressures (which I doubt) then perhaps debris from the burned out compressor has clogged the refrigerant metering device.

A burnout filter should have been installed on both suction and supply sides. The system may need further cleaning and a TEV may need replacement.

Aug 2, 2014) Bill said:
Thanks for responding to my query. He is some additional information. Only a expantion device was present at the plenum coil. An TEV was installed. The circuit was put under vacuum ( down to 940 on the vacuum scale) and refilled with coolant. System was turned and running. The high pressure guage that was connected at the oustide unit was not showing more than a value of 50 when the technician was expexting about 100 ( not certain if this mean is in PSI) No refrigeration was present inside de plenum the air emperature being at ambiant (around 71f) Looks like something is "blocked".. Do you have an opinion?

Question:

(sept 23, 2014) mahmud molla said:
How use 500l glass top chest freezer capillary size

(Sept 23, 2014) Anonymous said:
hello sir
have a good day sir how to check 18000+24000 BTU WINDOW A/C CAPILLARY ?

Comment on refrigerants vs "Freon":

(Sept 23, 2014) Daniel said:

I think it is important to note that we are talking about refrigerants here, not coolants, and not Freons.

Coolants don't change phase to do their job of transferring heat, the liquid coolant in your car's cooling system is a liquid and is supposed to stay a liquid.

Refrigerants do change phase, usually from a liquid to a gas and back again, but even the ice in a drink can be seen as a refrigerant since it is changing phases, from solid to liquid, to keep your drink cool.

Freons are certain refrigerants, usually CFCs (like R11, R12) and HCFCs (Like R22) made by DuPont Chemicals.

It is somewhat OK to call say R22 from any manufacturer Freon, it's the same as calling any adjustable locking pliers Vise-Grip whether or not they were made by Irwin.

What is not OK is to call refrigerants that DuPont has never called Freon by that name, for instance calling R134a a Freon is wrong, because DuPont calls it Suva.

Reply:

Excellently put, Daniel. I agree completely. However because many readers are not familiar with proper refrigerant terminology we find that people looking for information about refrigerants search on all sorts of words that we might agree are not quite correct. "Freon" has become about as generic and mis-used as "kleenex" for "tissues". Your note is helpful to readers who want to speak accurately, while keeping the word "freon" around helps people find it. I'll copy your comments into our text on refrigerants

at REFRIGERANTS & PIPING

Question: frequent choking at the time deepfrezer gas charging of R-134 A

(Nov 15, 2014) Anonymous said:
iam facenig frequent choking at the time deepfrezer gas charging of R-134 A What precautions i have to take

Reply:

Try charging from the low side using gas only.

...

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Recommended Articles

• EXPANSION VALVES, REFRIGERANT
• CONTROLS & SWITCHES, A/C - HEAT PUMP
• LOST COOLING CAPACITY the air conditioning system seems to be "running" but not enough cool air, or no cool air at all is being delivered to the occupied space.

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CAPILLARY TUBE FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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