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Cold high pressure line


rlskfoster
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On an inspection today I had a temp differential of 5 degrees. The unit was blowing 65 degree air and I had 70 degrees at the return. The high pressure line was cold, does this have any significance? I suggested they have the unit checked, but, I was curious about the cold high pressure line.

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I am just guessing that you are talking about air conditioning and concerned about improper temperature differentials, right?

When the outside temps are too cool, temperature differentials are almost meaningless. If it is below 60 degrees F don't turn it on, especially with a cold night preceeding. The cold liquid line is not unusual with high eff units on mild days. With low ambient temps, I would not call for service based on temp. differentials alone.

Jim

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I was always taught that 15 to 22 degrees was the range for temperature differentials. I do take them, but, I know some people don't think much of the practice. I have thermometer with a lase rpointer. Technically I beleieve you are supposed to measure at the plenum just passed the evaporator coil. Usually I find the closest register.

The outside temperature on this day was 75 degrees. I just found it odd the high pressure line was cold. Usually it is warm and if there is some trouble the line can be very hot, indicating low freon, clogged coil fins, etc.

So I was just wondering if anyone knew what a cold high pressure line could indicate.

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You can also check the liquid and suction line temperature--with a laser thermometer--at the condensing unit to get a truer read on functionality when exterior temps are moderate. I learned this from a master HVAC friend. Even a bum unit may be able to keep a house comfy when it's only seventy or seventy-five outside, and the differential may mistakenly trick you into thinking a unit is operating properly. The refrigerant lines, though, are capable of telling one the true story.

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I hope no one is using an infrared thermometer for measuring the temperature difference between return and supply air.

I use something like this:

http://mannix-inst.com/index.php?sectio ... tegory=all

You can also check the liquid and suction line temperature--with a laser thermometer--at the condensing unit to get a truer read on functionality when exterior temps are moderate. I learned this from a master HVAC friend....The refrigerant lines, though, are capable of telling one the true story.

So what are the acceptable temperatures with that method?

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Bill,

I'll explain about the suction line after work. It involves making certain the A/C system is properly sized, which goes beyond the scope, but that's what most of us are about anyway. The industry standard for air-flow cfm is 400 per ton, and if the unit is missized or overcharged, it can produce proper temp. differentials even though it isn't operating properly.

John

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I hope no one is using an infrared thermometer for measuring the temperature difference between return and supply air.

Not the air temp, the plenum temps. Granted, there is some buffering of the plenum temps from ambient air, but the difference in working systems is usually sufficient to further indicate health of the system (the house is cool, there are no abnormal sounds and no one has Legionaire's disease are other signs of a working system)

I also take splits by measuring register temperatures and have discovered what will likely be inadequate cooling capacity on second and third storeys. I've reported these findingsto find out later that the third floor was indeed inadequately cooled.

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I simply hold my hand in the air stream at a register as far from the unit as possible. If it is cool, the unit is cooling. I then check the lines at the condenser unit, I want one cold and one warm. I listen for strange sounds, and vibrations. Good so far, the unit is cooling. If I find the need, I can test "TD" with temp probes, but I seldom find the need or desire. I also use an IR temp gun to shoot all of the registers to make sure I have some airflow from them. This can also give you a ballpark(not exact as you are measuring the surface temp of the register or duct) temp reading for your own information.

Then in my report this is all I say. The HVAC system was cooling at the time of the inspection.

This has worked great for for me over the past decade, and this has been in the deep South with high humidity.

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Okay, so this is how it was explained to me by someone I trust. It goes beyond the scope and standards, but here it is.

First, an HVAC system requires a minimum of 400 CFMs of air flow per ton of cooling and a maximum of 600 CFMs per ton. Using 400 CFMs per ton is pretty much an industry standard. A six-inch round supply run is considered as providing 100 CFMs, a seven-inch provides 150 CFMs, and an eight-inch duct provides 200 CFMS. The math is pretty simple once you begin checking out the runs coming off of the trunk duct (or distribution box, as some techs call it.). If the system doesn’t have adequately sized ductwork, the temperature differential is meaningless. Likewise if the indoor or outdoor coils are dirty, the filter is dirty, or if supply and/or return registers are closed or blocked.

Once you ascertain that the system has adequate air flow—of course you can’t always see the evaporator coil—you can check the suction line. On air conditioning mode, the suction temperature should never be below 50 degrees or above 65 degrees, assuming the exterior ambient temperature is above 70 degrees and that the interior ambient is in the comfort zone of 68 to 75 degrees. The suction line temperature normally will read between 50 and 56 degrees when the system has been operating regularly and the outdoor temperature is between 75 and 85 degrees.

When a heat pump is operating on heat mode, the suction temperature should be between 75 and 100 degrees above the exterior outdoor temperature.

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Thanks for the information. I have questions.

. . . Using 400 CFMs per ton is pretty much an industry standard. A six-inch round supply run is considered as providing 100 CFMs, a seven-inch provides 150 CFMs, and an eight-inch duct provides 200 CFMS. The math is pretty simple once you begin checking out the runs coming off of the trunk duct (or distribution box, as some techs call it.).

Is this rigid duct or flex duct?

How do I calculate the difference between the two?

Are CFMs calculated differently when there's a large trunk duct feeding the branch ducts as opposed to the branch ducts tapping directly into a central plenum?

In systems from the 1960s, I see long ducts running the length of the house that step down in size after directly feeding each register. Why did they do that then and why don't they do that now?

. . . When a heat pump is operating on heat mode, the suction temperature should be between 75 and 100 degrees above the exterior outdoor temperature.

How do you go about measuring the temperature of that line? Is sticking a probe under the insulation accurate enough? (I've occasionally seen systems (Lennox?) where there's a well in the suction line. Someone told me that you're supposed to fill the well with glycerine and stick the probe in there. But I hardly ever see that.)

- Jim Katen, Oregon

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Hi Jim,

The duct calculations are for round flex ducts, and yes, they are generalizations. Clearly if a duct is two miles long, its cfm capacity is gonna be decreased. I asked the same question vis a vis flex v. rigid and was told they're similar enough to calculate interchangeably.

I use a laser thermometer to check the temperatures of the refrigerant lines, and it seems to work well.

As for your other questions, I'll pose them to my friend and get back to you. My impetus for asking a master HVAC contractor about this stuff was to do a better job for my clients. Clearly, there are limits to what one can accomplish during the time we have in a house. I got burned a time or two 'cause people would charge their leaky systems before I arrived and the temperature differential would read just like it was supposed to. Now I check the interior and exterior fittings with a refrigerant leak detector. Foolproof? Of course not. But I've found problems that I previously wouldn't have.

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Originally posted by Bain

I asked the same question vis a vis flex v. rigid and was told they're similar enough to calculate interchangeably.

I'm all ears & willing to hear, but that particular comment flies in the face of everything I've experienced.

I'd love to talk to your HVAC guy; how could flex & rigid ducts have similar air flow rates?

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I have done a couple of them. I took a college class in mechanical engineer and one of the projects was to do a manual J on your own home.

Now to the best of my recollection there was no imput for duct types.

If, big if, flex ducts are installed properly there should be little difference between them and rigid duct if the R value is the same.

Captain

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Originally posted by rlskfoster

I was still wondering if anybody knew of any significance of the pressure side refrigerant line being cold versus warm or hot?

Jim Luttrall mentioned it early on on this thread. If it's cool or cold outside, the liquid line is often cool. I don't know why, but I've observed it as well.

- Jim Katen, Oregon

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Originally posted by kurt

Similar to the Manual J, has anyone ever found flex duct installed properly? I never have. Ever.

No, but they are getting better. Down here almost all new homes are flex.

Example they use to lay the flex on top of truss, then they hang it with sting, then string and insulation, then chair webbing, now cardboard and chair webbing.

That took ten years can't wait for the next ten.

Captain

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  • 1 year later...
Originally posted by rlskfoster

On an inspection today I had a temp differential of 5 degrees. The unit was blowing 65 degree air and I had 70 degrees at the return. The high pressure line was cold, does this have any significance? I suggested they have the unit checked, but, I was curious about the cold high pressure line.

Was this the larger or smaller line?

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