couldn't get the suction pressure up

**badboyheel** · 06-12-2008, 09:39 PM

fellas did an install today on a 5 ton lennox ac13 with an old lennox oil furnace (don't have model number). dropped charge after pulling vac. started unit up and it was running a 64 and a 200 ambient temp 85. went in side checked air filter and it was clean, it was an older merv. 16. i don't have the tools to check cfm. lineset was about 25 ft. long 3/8 by 1 1/8 i added 1lb. suction did not rise past 64 head rose to 225. by the way it was a txv system. what could be the prob? added another 12 oz and nothing.

and yes just the ac was replaced not my decision, the HO did not want to pay the xtra dough. i informed him about possible effects of this but he didn't want to hear it.

and yes i should apply for pro membership, just waiting to get a new copy of epa card ( lost the orig.)

thank in advance guys

**beenthere** · 06-12-2008, 09:55 PM

Well, when you added that pound, you probably over charged it.
You got low air flow most likely.

You should have checked the firing rate of the furnace. Then fired it up and check temp rise and figured out your CFM from that. You might only be moving 325 CFM per ton.

**green jumper** · 06-12-2008, 09:56 PM

Add to that you should check the charge, you didnt check the charge. You just saw what the preasures were.

**ar_hvac_man** · 06-12-2008, 09:59 PM

Not enough info.

**Jay 41** · 06-12-2008, 10:03 PM

Originally Posted by ar_hvac_man

Not enough info.

We're getting to that.

, But you're right could be undersized ducts.

**toy501** · 06-12-2008, 10:09 PM

mismatched system gives no guarantee of proper system operation. check esp, i bet it's way too high. probablly never get the refrigerant side balanced out correctly.

**badboyheel** · 06-12-2008, 10:12 PM

well i checked my approach temp and it was right on. if remember correctly my superheat was on the high side 25. beenthere your right i could've checked cfm using temp rise, damn. still learning and that leaked out of my brain. jay it wasn't a belt drive. i thought it was lack of air flow, and say that was the case, whats the next step if furnace blower does not produce enough airflow.

**beenthere** · 06-13-2008, 12:31 AM

High SH isn't from low air flow. Low air flow gives a low SH.
Do a temp rise. Find out if you are low on air flow.
Then you have to determine why. Most 5 ton coils are very restrictive. It may be the coil causing most of the restriction. A static pressure test will tell you if its teh coil, return, or supply duct.
You may be able to set the blower to a higher speed.
After you have your air flow to 1750 or more. Yopu can take your enterID WD, and Db temps, and start the A/C again. Recheck your SC/approach and see what your SH and coil delta are, and check your discharge air temp.
If its still not inline, check the TXVs powerhead.

Is the TXV adjustable.