Do Line Restrictions Cause High Head?

In this video, we address the common myth that liquid line restrictions cause high head. We bust this myth with a few simple explanations.
Common liquid line restrictions include clogged filter-driers or metering device restrictions. Many techs think that refrigerant will exit the compressor and back up against the source of the restriction, which would raise the head pressure. However, that's not true.
Our diagnostic sheet shows that liquid line restrictions cause low suction pressure and normal to low head pressure, not high head pressure. That's because the compressor requires refrigerant of a specific density to pump refrigerant into the high side of the system; when the suction pressure is low, the compressor can't move as much pressurized refrigerant to the high side, so a liquid line restriction doesn't have a huge impact on the head pressure when you have reduced amounts of high-pressure refrigerant in the liquid line in the first place.
Some thought experiments that can help you grapple with that idea include thinking about refrigerant in a tank and the pump down cycle (used to prevent refrigerant migration). Note: don't pump down a microchannel condenser!
However, when you have a restriction, its location has a significant effect on the head pressure. The amount of charge also matters, as overcharged systems are more likely to have a significant impact on the head pressure (raising it and causing hydrostatic pressure buildup within the system).
The most common liquid line restrictions are clogged filter-driers, metering device inlet screens, a kinked liquid line, or a TXV with an empty bulb. These very rarely cause high head for more than a short period of time.
The most common causes of high head pressure include high load, overcharge, and poor heat rejection (such as from dirty or obstructed condensers).
Read all the tech tips, take the quizzes, and find our handy calculators at www.hvacrschool.com/

[@RangerRick97]

Thank you for this. I am starting out in this industry and was wondering if I could find that diagnostic chart that you made anywhere online. It is very informative.

[@VinnyTerra1]

Ranger Rick I also would like a copy of your troubleshooting guide. Anyway you can email or provide yours so I may also get a copy?

[@RangerRick97]

As soon as I figure out how to get one I will provide it to you no problem

[@RangerRick97]

Diagnosis Cheat Sheet. Can be found on the hvacrschool.com resources section. Scroll down and you will see the cheat sheet. Thank you Brian once again for all this great information. hvacrschool.com/wp-content/uploads/2017/12/Diagnosis_Chart.pdf

[@obiemains1367]

I've never heard this explained in such a simple way. Kudos and thank you.

[@sknight0391]

I'm always gone by the general rule of thumb that if the discharge line temperature is "below" outdoor ambient temperature you have a restriction.

[@LightSound369]

Your approach is correct and your delivery well communicated. My preference is "heat exchangers".

[@lilb34n_]

The exact explanation I was looking for. Thank you!!!

[@andrewhicks6494]

Thank you for the video, I am going to make everyone at work watch this you don’t know how many times I’ve heard it’s a bad txv and it’s slammed shut because it’s hot high head and low suction...... It’s irritating to try to beat this idea into the guys but maybe your analogies will hopefully sink in better than mine

[@williamwood176]

Liquid line head pressure will be high at a constant increase depending on restriction severity ( closing king valve on condencer during pump down will show high side deminishing pressure as pump down acures because king valve port is open to side going towards metering device and not reading inside condenser as compressor pumps down whats in line set) During a restriction your connected at all times to what is happening in the system thus able to see when a full or partial restriction is accuring lower than normal suction high superheat for low side and high head or higher than normal pressure and high subcooling for high side.

[@theackid7887]

Shut up William

[@michaelcostello6991]

Very well explained what is going on here. Often on courses the explanation for whats really going on is not discussed.

[@MeltonMonster]

Great informative video!

[@zackdowns9854]

Amazing video… very helpful, thank you!

[@marvinostman522]

I like your explanation or discription of the components in the system. One thing that can mess people up is thinking pumps create pressure. They don't. Pumps create flow, resistance to flow creates pressure. Your metering device be it a TXV, or an orifice is a restriction. The TXV because it opens and closes changes the restriction to flow either more or less which will change the pressure. Thanks

[@spacemanwithraygun3933]

I did a job as a FNG long ago and brazed in a 7/8ths street el that had a locking cap jammed into it. We did not leave work at 3:30 as planned...

[@craigmarshall40]

Great video thank you for all you do 🙂

[@mwdub02]

It really depends, I’ve seen restrictions that can trip the internal safety relief valve on a scroll compressor.

[@ianmarthinus]

Great info to know! However in the field I always see high discharge pressures when it comes to restrictions in the liquid line. Would love to see you do a demo on your training equipment

[@josemartinez-jt6tw]

How?

[@kenthomas4668]

because you are not the first person to work on it, they added freon.

[@kennybakerhvacr5751]

​@@kenthomas4668this IS the answer to that

[@bormorzx]

Great education video. I’m checking the diagnostic quick sheet and I do have questions! How the suction pressure or head pressure line go down or up and the superheat or subcool “normal”?? In my thoughts if the pressure change the subcool or superheat also will change!! Or not?? Thanks

[@Bluezhealer]

Awesome 👏 @ 2:00 I was like yup yup;thanks for sharing this!!

[@jeanabrea5054]

hey bryan i very much appreciate your video. can you make another video on freeze point and freeze protection and what causes of it.

[@donnierobertson3088]

Nice job and video like always

[@philoso377]

Hope this comment doesn’t taken negatively. The word “reject” can be counter intuitive in HVAC operations. Example - reject heat. Reject often implies - oppose, against, repel and reflect something from outside that is not already in the system. In HVAC a more appropriate word may be - discharge, release, dump, unload etc. This could open up our student’s mind, a bit.

[@phillipjoy8191]

Great videos like always.😜

[@mackenziegray2090]

I came across a weird one on a carrier heat pump, while running in heat. My suction was 90psi and discharge was 250psi. Acted like a low charge but when adding refrigerant my suction psi would go up but my discharge wouldn't move.

[@mrcrux213]

You're correct mr. Compressor will pump what it gets.

[@kamozoren]

In my experience, only systems with sufficient liquid capacity(big condensers/receivers), will not raise head pressure during a significant liquid line restriction. A system with a capillary tube, (small freezer/refrigerator), will experience a very high head pressure. And that's what everyone else I've heard is experiencing(including Ty Branaman in his video about restriction). With AC units, I cannot personally say, since when shutting the liquid service valve(during pump-down), the gauge is connected the outer part of the tube, not the inside. So of course the reading is going down. Maybe the type of compressor(scroll, piston) is also affecting the result. Any way, for a piston(reciprocal) compressor with a capillary tube blockage, the head pressure will go sky rocketing.

[@District0fMisery]

Great video 🤘

[@donaldmccuddy3611]

Thanks

[@troymason4799]

My question is, how come when its 65-80 degrees outside i ALWAYS get a very low subcool and about 15-20 superheat (residential units). And the saturation temperature is always about 5-10 degrees above ambient. Is this normal?

[@caru3257]

Grayfurnaceman had one on this. Great video. I also watch under cu27.

[@michaelcostello6991]

post link

[@yujen725]

Love your videos, watched almost all of them and learned a lot!! Just a quick question, the graph that you made "HVAC AIR/REFRIGERANT Diagonistic Quick Sheet", when you use arrows to portray high and low, what is that high and low with respect to? What is the definition of High or low? are all these values fixed by the manufacturer? what if the manufacturer doesnt provide these pressure numbers?

[@allach1772]

Oh boy you got a lot to learn.

[@yongweiwang2386]

So what will be likely to happen if there’s restrictions in discharge line? Superheat.subcooling and comp amps?

[@carlwheeler3403]

Great video, what would cause a higher than normal LLT, 4 ton with a TXV. The subcooling stays at around 5 degrees regardless of charge, the Superheat does vary with charge. Thanks in advance.

[@HVACS]

I have noted your question and will answer it in Q/A video

[@cliffordpace616]

That is a fantastic troubleshooting chart, I would like one as well. I do believe I have a different issue. Outdoor ambient temperature is 75 degrees. This is a Trane 7.5 ton built in 2002 R22 heat pump system. There are times when the system will run for 10 to 15 minutes with no issues whatsoever. Then after this period of time the system will do the following. The head pressure will modulate. Meaning it will go from 250 psi to let say 325 psi. This varies greatly up to 375 then the high pressure switch opens and with in seconds the cycle repeats over and over and sometimes it will operate correctly for a brief period of time. All the while the suction pressure remains the same 75 psi. Any help on this would be greatly appreciated.

[@awesomeaustin2348]

These units have split coils, I would pull the fan housing off and split the coils, check and clean, this is the most common problem with these units. If this doesn't fix the issue, then I would recover the refrigerant, replace both filter/driers and check valves that are on the floor of unit, some would go ahead and replace TXV while refrigerant is out of system, then I would after a proper vacuum, recharge unit with new refrigerant using the subcool & superheat method. Thats my 2 cents anyway.

[@toddpollock73]

Would you have a high sub cool with a liquid line restriction?

[@armandolopez6365]

Can someone explain to me the meaning R/A temp on this chart sorry I’m new to this ?

[@DeAnda92]

Since we are talking about restriction. I would like to add a question to a person of your knowledge and expiriance that can help me asnswer it. How can i prove that a the coil condensor is restricting refrigerent and causing an issue or change from original standing point? IF lets say hail hit the condensor coils and dents the refrigerent pipes inside. I can comb the coil but that only hides the restricitions dents inside and/or potential micro leak. So would i check these with a refrigerant leak detector tool and gauges?? What would i be looking for?? Thank you!

[@rickl4214]

What if you have high suction pressure and high temp?

[@mohamadsaidi7252]

how can I find the difference between line restriction and low charge in the mini split that there is not any service valve to measure head pressure???

[@tyebundlez8262]

Would love to take screenshot of that chart do you mind posting that chart without your visual blocking the other half??

[@JayArenstein]

hvacrschool.com/wp-content/uploads/2017/12/Diagnosis_Chart.pdf

[@jcaniban6243]

Hello Sir Bryan Im always watching your videos about HVAC and also a subscriber. Sir may I request for the diagnostic sheet that you made as a technician in reefer container it will help me more to diagnos the system . Thanks sir Bry

[@alvoelkle]

Of course a inop condenser fan, or fan that is beginning to fail and possibly running slow cause Temps to rise in the condenser, which raises pressure. Right?

[@tootfish1]

What would indicate my head pressure if is at 450 and low side at 120? Could it be a dirty co denser coul or a bad txv

[@tjjns5]

Bad Txv. but make sure you have blower working running are across the coil , and make sure there is nothing stopping or slowing down the air flow

[@joffrecueva5662]

I hope so 😁

[@mtbbiker6401]

My R410A 5 ton, 14 SEER unit was running 510 PSIG (136 degree sat temp) when it was 112 degrees outside. According to diagnostic chart CTOA that would actually be about right. 20-25 degrees over ambient temperature. Is that correct? I was worried the head pressure was way too high and I'm overcharged. compressor amp draw was 23A. Plate lists 26 RLA.

[@seek3n]

Dirty condenser? Incondensibles?

[@mtbbiker6401]

@@seek3n I am worried about incondensables but when I measured again when it was cooler outside (90 degrees) H 380 psig (113 sat temp), 102 liquid line, 11 subcooling (calls for 10), L 143 psig (50 sat temp), 62 vapor line, 12 superheat. 18A draw. Those numbers are within proper range. In this case COTA is 23 degrees which seems about right for a 14 SEER heat pump. As long as my condenser sat temp is about 20-25 degrees above ambient temp and I have 10 degrees subcooling, the system seems to be running at proper temps. I may be a little over charged. I'm going to check my subcooling again at multiple ambient temps. I cleaned condenser coil a couple of times so I think it's okay.

[@seek3n]

@mtbbiker6401 what's your indoor return temp? And is it a txv? The design for cooling is vapor Sat at 30-35 below return temp.

[@mtbbiker6401]

@@seek3n Thanks. It's a TXV heat pump. I've been playing around with the placement and positioning of the indoor bulb. It was mounted inside the handler but I moved it to the outside. I didn't know about the 30-35 below return temp. I will check it again tomorrow but I think that was about right.

[@mikecummings2188]

Aft of the restriction will be a refrigeration effect is moisture on the tube

[@danielzanetti9977]

where can I get that diagnostic quick sheet???

[@brugotti]

hvacrschool.com/wp-content/uploads/2017/12/Diagnosis_Chart.pdf

[@tjjns5]

look up model number and look for manual

[@cristobalpaz6687]

I have a question what about if the low side is normal air flow is good but the high side is high what is going on there

[@x-man4702]

Will this explanation apply to a Heat Pump system?

[@renecuevas4128]

Yes

[@tjjns5]

@@renecuevas4128 yes

[@jericosha2842]

Why is the superheat so high though? if it's not absorbing much heat, then you would assume superheat would be low? I understand everything else, but I can't figure that out. Maybe it's too late at night.

[@coreyrodgerson4853]

Superheat is a measure of how much heat the gas has absorbed above it's saturation temp/change of state. It is easier to add heat to a vapor than it is to add heat to change the state from liquid to gas. If you've got lower pressure of refrigerant in the evaporator, the liquid is more prone to absorb heat. It changes to a gas quickly and spends more time in the coil as a gas. That enables it to absorb a lower quantity of heat, but it moves the sensible temperature up. You end up with a lower amount of BTUs absorbed by the refrigerant, but more of it is sensible heat. It shows up as higher superheat.

[@Oscarbm1234]

This is that one comment you made on that TXV podcast that mind fucked me. Been trying to make sense of it.

[@boongoon92]

Can't see the stat sheet for my current problem lol high head normal suction pressure

[@toli0968]

410 system. low suction 98 psi, very high superheat .... head pressure 410 psi with 0 degree subcool????? no receiver clean condenser??? restricted txv with low charge??

[@kevinsklow3473]

Are microchannel condensers clearly marked as such, or is there a way to easily tell? Still learning here.

[@seek3n]

I have high suction low liquid. Subcool at 10. 20 degree temp split. 90% of carrier i install

[@justdissin7340]

What can cause high side to be at 110 psi,low side is 65 psi,system is r22

[@jesseluna4731]

Need a bit more information than that but just a guess.......possible compressor blow by(compressor valves) or maby a stuck reversing valve.

[@justdissin7340]

@@jesseluna4731 ..i replaced compressor & did the same thing,i took out original dryer filter from inside condenser & it had alot of debris,i end up replacing the condenser & now is working, im just curious what caused that issues in 1st place,maybe to much oil in condenser?

[@DW-vl2wi]

@@justdissin7340 Would need to know that ID temperature and tonnage of the system. If you indoor temperature was low and you were working a unit under 3 tons possibly nothing was wrong with the charge.

[@troymason4799]

The key here is subcool. The more severe the restriction the higher the subcool

[@averagebloke4474]

Coils that need cleaning cause high head

[@bigkidband5731]

I’m trying to determine whether or not my compressor is going bad. The unit will run fine when outdoor temperatures are below 80 degrees, but outdoor temperatures above 80 and above cause the compressor to shut down. The air handler is still blowing fine. I have to hit the red reset button on the compressor In order to get it to restart. Compressor fins are clean. Any help would be appreciated.

[@jsosa2727]

No they would cause a pump down.

[@kosworkshop]

Love the video, but it would help if you slow down a bit. It was like trying to listen to an auctioneer explain the troubleshoot. Had to watch it several times to take it all in. I do understand that for seasoned vets it may have been just fine. However, a lot of people seek education through videos.

[@Whatda25]

Your video would have been fantastic if your face was not superimposed on the chart.