What is an acceptable differental tmp

[Robert J. OConnor]

Originally Posted by cgoyette

I take temp readings at the supply and the FIRST duct into the home after the air handler, and the most distant register.

I think he means the air coming into the unit, or the "return"

[David A. Andersen]

So do I.

[Robert J. OConnor]

Originally Posted by jfarsetta

... measurement of the temperature split between the high and low pressure refrigerant lines at the compressor, and looking for between 15 and 18 degrees ...

Do you mean the temperature rise between the air coming into the condenser (outdoor coil) and the air being discharged out of the condenser (outdoor coil) ... or perhaps you are talking about the temperature rise of the refrigerant, which most specialists will check, but that split/rise would be much higher.

As a rough guide many HVAC specialists will check that the larger suction/return line feels very cold (like a cold can from the fridge) and that the smaller liquid/supply line feels very warm. A considerably greater temperature rise than 15 to 20 degrees, which is the split commonly used to roughly check the temperature drop across the indoor coil (after it's been running for a while, and as long as it's not extremely humid or dry).

See the attached diagram from the old TOD for a pretty good schematic of an AC system, with some very rough system temperatures.

Any HVAC specialist worth his salt wouldn't even hesitate grabbing the gages and thermometer to check superheat/subcooling ... but that is well beyond a typical home inspection.

[cgoyette]

Originally Posted by roconnor

I think he means the air coming into the unit, or the "return"

yes you are correct.

[Charley L. Bottger]

See the attached diagram from the old TOD for a pretty good schematic of an AC system, with some very rough system temperatures.

Yes Robert very rough I don't agree with some of them at all.

I have to go get my HI CE tonight and I think this thread will grow by the next time I look. Good Luck

[mbonvissuto]

Call me when you guys are in town, Im buyin. I'll show you guys some differential temp down here in DECEMBER.

[Charley L. Bottger]

Originally Posted by mbonvissuto

Call me when you guys are in town, Im buyin. I'll show you guys some differential temp down here in DECEMBER.

Mark why would anyone in their right mind want to come to NY especially in December to many people.

[Barry Adair]

Originally Posted by cbottger

Mark why would anyone in their right mind want to come to NY especially in December to many people.

Your cohort in crime will be there!

http://www.grinchmusical.com/?gclid=...FQ5YgAodjwoM6A

[cgoyette]

Originally Posted by badair

Your cohort in crime will be there!

http://www.grinchmusical.com/?gclid=...FQ5YgAodjwoM6A

man that was hilarious...I clicked the link but looked away and then heard the song...still laughing!!~!

[Gary Reecher]

The link below will give you a chart to use. You'll need an electronic thermometer that can measure both dry and wet bulb temperatures in the return and a thermometer to measure the supply air temperature. If the return air dry bulb temp is 76 and the wet bulb temp is 76 then the temp drop across the evaporator will be 6.5 degrees. Some will gawk why so low of a temperature drop, but at those db and wb temps the water would be almost fog. But it is a good chart to use. Target Temperature Split for Airflow

[Doug Edwards, CMI]

When someone takes a temperature at the return air grill or at a supply air register using an IR thermometer such as a Raytek, they are NOT measuring the temperature of the "air" but rather the temperature of the metal grill/register. That seems to be the thing that keeps getting missed in this never-ending saga of temperature splits. The IR thermometer can not and will not measure the "latent" heat in the air. You need a sling or electronic psychrometer to get the Wet bulb temp. I think David said that earlier. The latent heat can add as much as 20% to the "sensible heat" and if it isn't detected can skew the hell out of this mystical 20 degree split everyone seems to be looking for. I do not remember who said it but trying to determine if the system is performing by using this method is like trying to determine the horsepower of your truck engine by measuring the temperature of the exhaust gases. Before anyone goes into a conniption fit over my use of the word "heat", everything in HVAC is "relative" and is about the transfer of HEAT from one place to another.

[Doug Edwards, CMI]

To illustrate I took some pics at the closest supply register and at the return air grill. Note the significant difference in the two temps and also note the humidity at the two. If you really want to get into the physics of it, note that the humidity is "higher" at the supply registers than at the return. I also went and took the readings at the furtherest register and they were pretty consistant to the closest.

[cgoyette]

Originally Posted by dedwards

To illustrate I took some pics at the closest supply register and at the return air grill. Note the significant difference in the two temps and also note the humidity at the two. If you really want to get into the physics of it, note that the humidity is "higher" at the supply registers than at the return. I also went and took the readings at the furtherest register and they were pretty consistant to the closest.

Very educational, thank you. Greenie


Curt

[Robert J. OConnor]

Originally Posted by Gary Reecher

If the return air dry bulb temp is 76 and the wet bulb temp is 76 then the temp drop across the evaporator will be 6.5 degrees.

That would be 100% Relative Humidity (RH) ... essentially almost raining in the house ... just not realistic.

Common AC design parameters are return air of 75°F @ 50% RH, which gives you 62°F WB, and a target TD of 19.3°F. Or 70°F @ 50% RH, which gives you 58°F WB, and a target TD of 18.7°F

Even for 75°F @ 70% RH, which gives you 68°F WB, it's a target TD of 14°F ... and those are not considered comfortable indoor conditions (but possible if just starting up the system on a muggy day).


P.S. Dave ... if you ran the examples with more realistic numbers (0% RH just isn't realistic) you won't see those kinds of differences. Although I understand the point you were trying to make, which is that humidity is a big factor. Just something to keep in mind if your indoor conditions are at the extremes of humidity levels (either very dry or very humid).

JMO

[David A. Andersen]

The link below will give you a chart to use.

Target Temperature Split for Airflow

Thank you Gary!
A very productive approach to this subject.
That is why this subject should be rehashed.
The only problem, a wet bulb temp is needed and there are Inspectors that will consider spending $10k on IR, and there are those that won't kick in for a CHEAP hygrometer for $35 (but still feel the need to "evaluate" HVAC performance).