# Pre-charge on goodman straight cool



## hvaclover (Oct 2, 2008)

You will have to measure the superheat if the coil is a fixed orifice to determine the refrig volume.

If the coil comes with a TXV you must use the sub-cooling chart to determine refrig volume.

This reading will only be valid with with the correct cfm airflow.


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## protechplumbing (Sep 22, 2008)

So I can't just run 15', vac out moisture and open the charge in the condenser? The condenser says it's precharged with r-22 and oil for 15 of line set. It's an orifice type. No TXV.


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## SD515 (Aug 17, 2008)

Most pre-charges are set for the length of the line-set (15 ft in this case) and the coil of matching tonnage, but superheat/cool test should be done after installation anyways.


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## protechplumbing (Sep 22, 2008)

Let me clarify. I'm doing a "cut in". There wasn't any central air before. I'm cutting in registers, Running flex w/ duct board plenums, Air handler is going in the attic thru a gable cut. Condenser is going on the ground below the gable cut. I understand the basics of the vapor compression refrigerant cycle but I don't have a recovery machine. I do have gauges and a vac pump. silfos and oxy/acetylene. I have to go pick up a nitro bottle and reg. (I need one anyway for leak detection in plumbing)


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## protechplumbing (Sep 22, 2008)

Cool. Will do. Many thanks.



SD515 said:


> Most pre-charges are set for the length of the line-set (15 ft in this case) and the coil of matching tonnage, but superheat/cool test should be done after installation anyways.


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## hvaclover (Oct 2, 2008)

Ok I see where you are coming from now.


As an HVAC techs we did not "assume" the right amount of refrig is going to be in the system. (especially from Goodman) from the factory.

We confirm the volume of refrig by performing the superheat or sub-cooling test.

Make sure the AHU is not blowing more CFM than the nominal tonnage of the AC. That will give you false reading.


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## protechplumbing (Sep 22, 2008)

I ended up using close to 20 feet of line set. Did super heat test. Its 7 deg above goodman's number. About how much r-22 would be needed?


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## hvaclover (Oct 2, 2008)

Add small amounts and wait half an hour before before rechecking SH.


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## beenthere (Oct 11, 2008)

protechplumbing said:


> I ended up using close to 20 feet of line set. Did super heat test. Its 7 deg above goodman's number. About how much r-22 would be needed?


There is no one set amount that will lower the Superheat by X degreees.

The install directions however. Should have contained a section that told you how much charge to add per additional foot of line set and for the coil match you used. If it was an approved coil match.


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## protechplumbing (Sep 22, 2008)

I got the charge right. Now for the ducts. I'm planning on running quiet flex. see pics. If you think this design will fail catastrophically let me know as I'm making this up as I go.


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## hvaclover (Oct 2, 2008)

Bwahhahahahahahahahahahahahahahah!

Think of that as a plumbing lay out and then ask that question.:no:


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## protechplumbing (Sep 22, 2008)

I was hoping for help rather than laughs.

My line of thinking was a "flow balanced manifold"


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## hvaclover (Oct 2, 2008)

protechplumbing said:


> I was hoping for help rather than laughs.
> 
> My line of thinking was a "flow balanced manifold"



Just ribbing 'ya. 

To me it does not look exactly right. We got some guys, one named Beenthere, who can guide you thru this. Flex duct is unforgiving and I have to confess I am more a sheet metal guy myself. I am just worried that with all the takeoffs from diff trunks may not give you good airflow.


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## beenthere (Oct 11, 2008)

First.
How can you have the charge right without the duct work hooked?
Second.
16" round sheet metal is too small for 4 tons, let alone 16" flex duct.

Do you plan on using tee's, wye's, or what method for tapping in your supply branches to your trunk line.
Same question on return.


PS:
Also, you only show 10-6" supplies. Thats a lot of air to try and move through a 6' round duct.
Your going to get air noise.


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## hvaclover (Oct 2, 2008)

beenthere said:


> First.
> H_*ow can you have the charge right without the duct work hooked?
> Second.
> 16" round sheet metal is too small for 4 tons, let alone 16" flex duct.
> ...


:yes::yes::yes:

You charged that ac with no duct work hooked up? Either I missed that or you never mentioned that when i told you to make sure air flow was adequate. 

Your probably over charged like hell with that fan running full bore.


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## hvaclover (Oct 2, 2008)

Did you add those numbers after your original post? I sure don't remeber seeing them 'til now.

You need seven inch flex to make up for friction loss. Six won't do it.

Google some flex sizing charts for your system.


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## protechplumbing (Sep 22, 2008)

If the fan puts out 2000cfm and I have ten registers and assuming I can get the flow balanced, that would be 200cfm or less per 6" duct. 200cfm doesn't seem like much for 6" duct. I did look up the "ductulator" charts from quietflex. 200cfm works out to be 902 fpm for 6". Is that too fast? It just gives ranges. I'm confused about the friction. The fan gives output based friction. Friction is based on speed. Speed is based on cfm put out by fan. WTF!?!? It's a circle?


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## beenthere (Oct 11, 2008)

This just an example of a duct lay out. That is a bit better at giving you air flow to your furtherest supplies.

You see the CFM's don't add up to the 1600 that, that 4 ton wants. But then again, there are no room sizes.
And you never said you did a load calc.


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## beenthere (Oct 11, 2008)

You don't want to hear the noise that a 6" will have at 200 CFM.

Plus, you don't want a 4 ton system moving 500 CFM per ton.
Unless you live in a desert area, that you need to add humidity when you run the A/C.

How did you determine what size system you need?


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## hvaclover (Oct 2, 2008)

Plus you are assuming that the air will be evenly distributed to each register.

That ain't gonna happen. Some rooms may end up requiring more air than you thought and that will mean closing dampers to other rooms ect.

Or you just won't get the airflow strength you were hoping for after it passes thru all the bends and turns the fiter and the ac coil.


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## hvaclover (Oct 2, 2008)

protechplumbing said:


> If the fan puts out 2000cfm and I have ten registers and assuming I can get the flow balanced, that would be 200cfm or less per 6" duct. 200cfm doesn't seem like much for 6" duct. I did look up the "ductulator" charts from quietflex. 200cfm works out to be 902 fpm for 6". Is that too fast? It just gives ranges. _*I'm confused about the friction. The fan gives output based friction. Friction is based on speed. Speed is based on cfm put out by fan. WTF!?!? It's a circle?*_


_*
*_ 

It's as much as art as it is a science. That's why there are guys who make a living doing only flex and sheet metal duct systems.


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## protechplumbing (Sep 22, 2008)

The houses to the left and to the right have the same square footage and very similar construction. They both have 4 tons and seem to work fine. Though both systems have undersized returns and bad temperature distribution they do keep the houses cool in the middle of summer.



beenthere said:


> You don't want to hear the noise that a 6" will have at 200 CFM.
> 
> Plus, you don't want a 4 ton system moving 500 CFM per ton.
> Unless you live in a desert area, that you need to add humidity when you run the A/C.
> ...


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## hvaclover (Oct 2, 2008)

protechplumbing said:


> The houses to the left and to the right have the same square footage and very similar construction. They both have 4 tons and seem to work fine. Though both systems have undersized returns and bad temperature distribution they do keep the houses cool in the middle of summer.


Don't count on what other homes are like. Too many variables. Sun side exposure, color of roof shingles, ect. Since your stuck with the four ton might as well get the duct sized right.


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## protechplumbing (Sep 22, 2008)

Ok, let’s talk about the opposite extreme with the ducts then. What's the down side in over sizing other than cost? More heat gains in the ducts?


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## beenthere (Oct 11, 2008)

Those other houses may be cooling.
But, are those systems dehumidifying.
Or, do those people have to set their thermostat set point lower, to keep the humidify in check?

An over sized duct system, in an unconditioned space, will have higher heat gain/loss.

But, even with an oversized trunk, the branches can be designed to have static regain, to get the velocity up for register throw.

A common misconception, is that a large duct won't throw the air as far as a smaller duct with higher pressure.

A standard PSC blower, can't deliver rated air flow, on undersized duct work.

With your design. You will have high static pressure. Loud registers. And still not get the proper air flow.


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## beenthere (Oct 11, 2008)

A 6" duct moving 200 CFM, would have a Friction Rate of .32"(thats not static pressure, static will be much much higher), at a velocity 1050 FPM.

Add all the pressure drops of all the devices, and and the length of the run. You'll be over 1" of static. 
No way that blower can move 1600 CFM against that static pressure.


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## hvaclover (Oct 2, 2008)

I remember the old 1600 sq ft homes from the early to late fifties.

Low boy 125kbtu furnace 1/5 or 1/4 belt drive motor 800 rpm, blower wheel 20x12.

The plenum was 20x18 and the trunk 28x8.

Moved a WHOLE bunch of air but so slow you never noticed it. The static was so low by today's standard it was laughable.

Now everything is high velocity thru ducts barely the right size or the static is so high the ducts scream.:furious:


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## beenthere (Oct 11, 2008)

Those old units were set up for about 1000 to 1250 CFM, at 75 to 90° temp rise.

Had to use a 1/3 to 1/2 HP motor if you put more then a 3 ton A/C on it.


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## hvaclover (Oct 2, 2008)

max temp rise was 100*f on the ones I worked on. to get three ton 1/3 hp was all it needed. 

Tried a half horse one time. Went back on a noise problem. Opened the the filter section panel (can you imagine! The smoke pipe was welded to the heat exchanger and exited thru the cold air stream to connect to the chimney!).

Anyway, as i was saying I opened the blower section (was making hell of a racket.) and the blower shaft let go and shot the blower wheel out the cold air opening like a bullet and bounced off the far wall.

Just too much juice with a half horse. 1/3 was plenty and the air just poured out of those wall heats. Had to use deflectors to get the air above waist.

Had a great one about twenty seven years ago. Lady called complaining 
that her dog had not stopped shivering since we put the ac in.

went to check it out. Stupid helper didn't change the pulley and didn't add deflectors. All the air was pooling at floor level and much colder cause the 
the blower was not moving fast enough.

AAhhh...the good old days


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## beenthere (Oct 11, 2008)

LOL...

Sidewall diffusers, they can make life interesting for cooling.


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## hvaclover (Oct 2, 2008)

No wonder they went to floor registers


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