tmpitures

[mira]

dry bulb verus wet bulb.Why use wet bulb for inside readings
how much diference between the two

[BaldLoonie]

Wet bulb tells latent load too. When checking charge using superheat charge, really need to know that or you aren't charging accurately. 72 db with 60 wb has low load but 72 db with 70 wb is a very high load and will have high superheat on a piston coil.

Difference, 100% humidity they will be the same! Otherwise, difference will be based on how humid it is.

[mira]

I have a Goodman unit tha uses dry bulb on its chart
i know you know your stuff as i have followed your replys
thanks for your imput

[BaldLoonie]

The most accurate charts ask you to take wetbulb reading. That will be much better at setting superheat than looking only at dry bulb. If Goodman doesn't care, you won't be as accurate and risk overcharing or undercharging if humidity is particularly high or low.

[pulaskihvacr]

Originally posted by BaldLoonie
The most accurate charts ask you to take wetbulb reading. That will be much better at setting superheat than looking only at dry bulb. If Goodman doesn't care, you won't be as accurate and risk overcharing or undercharging if humidity is particularly high or low.

I agree. Most "dry bulb only" charts assume a 50% rh. If the wet bulb differs from that, the system will not be accurately charged.

[Carnak]

Wet bulb indicates the total heat

[ljbrandt]

I agree. Most "dry bulb only" charts assume a 50% rh. If the wet bulb differs from that, the system will not be accurately charged.

So, what do you do if the manufactuer has a "dry bulb only" superheat table?
If it's safe to say that most "dry bulb only" charts are assuming 50% rh, would it be acceptable to convert the dry bulb figures into a wet bulb using the 50%rh?

[JJDH]

I have not seen one goodman with a dry bulb only chart, then again I keep the slide charts in my bag and use em each time.

[chuckcrj]

My Goodman chart uses wet bulb.
If you have TXV you set the charge by SC so you don't need to know wet bulb to set/verify the charge.

[ljbrandt]

Just for reference, here is the service reference guide that came with my unit. As you can see, it has a dry bulb only superheat table.

[hvacrmedic]

Just for reference, here is the service reference guide that came with my unit. As you can see, it has a dry bulb only superheat table.

That chart appears to assume a constant dewpoint rather than a constant RH. That basis will actually make it more accurate on average than a chart based upon constant RH. FWIW, you'd still be better off using a generic slide chart based upon return air wet bulb temp than the one in that manual because exceptions to its assumed conditions will most definitely be encountered in the field.

[ljbrandt]

It appears that that chart assumes a constant dewpoint rather than constant RH. That will actually make it a bit more accurate on average than a chart based upon constant RH. You be better off still, to use a generic slide chart based upon return air wet bulb temp.

Thanks for the advice. I agree...doing some looking on the net, Fieldpiece uses the California title 24 target superheat tables as default and I think I'll do the same.

EDIT - I wonder if it would be easier (and more accurate) to just recover all the R-22, vacuum the system and weight in a proper charge based on manufacturer specs and line length rather than deal with all the variables using superheat to validate a proper charge. What do you guys think?

[Gunslinger]

EDIT - I wonder if it would be easier (and more accurate) to just recover all the R-22, vacuum the system and weight in a proper charge based on manufacturer specs and line length rather than deal with all the variables using superheat to validate a proper charge. What do you guys think?

I wouldn't do it that way. Airflow can make a difference. Take real time measurements and use them for each system.