Low Evap Coil Saturation Temp on system

gene2

We recently had to replace a leaking evap coil on system we installed 4 years ago - 10 SEER with all temps & pressures good. The system was empty when we arrived, the original coil used an orifice & the replacement uses TXV. Straight AC on Gas Furnace. No problems reported untill this event.

With the new coil, the low side sat temp is running 25-30 deg. causing freeze up. Replaced TXV, same problem. Check airflow with velometer come up with 1000 cfm - need 1300 cfm minimum. Pull new coil, put bubble gum on old just to see if problem with new. Same condition. Look through furnace for restriction, straight unobstructed duct runs with no dampers or point of restriction. Replaced blower motor showing excess current draw. Charge has been weighed in, adjusted up & down just hunting for a clue. Supply & Return plenums have been opened to free flow with no change.

Of course, the job is 1 1/2 hr away so it kills a day to do anything. Next step is to measure blower wheel & compare to specs but everything appears good to go. Why a problem now after 4 years.

Any suggestions will be appreciated. Not blowing my own horn, but not used to this as I'm usually the go to guy in this area, even by the factory.

Yuri

"Looked thru furnace for obstruction, excess motor draw". Is it a high efficiency unit with a condensing coil above the blower which may be dirty? Have to pull the fan to find out. Had a furnace where the insulation and tin foil was pulling off the walls inside it and severely blocking air flow. Sounds like the problem is internal to the furnace.

Good Luck

Idaho Joe

I have found items installed behind filters (in ceilings and walls) as well as in the return duct just out of site that someone was/is trying to hide (x-mas presents, guns, ect.), that are easily accessable and they would restrict the amount of air returning to the unit.

Just a thought.

__________________
Joe

Learn what you can, when you can and share the knowledge.

mo-flo

got to have all the numbers such as sh,sc, etc.,...i will say this,stick with it and you'll probably find it...just when you think you've seen it all in this trade....a guy see's something he hasn't seen before,....such has been my experience.

gene2

SH, SC etc.numbers have been pushed all around by adding-reducing charge, secondary heat exchanger clear, all supplies open proven by velometer, returns untouched evidenced by original wall paint still intact. Supply & Return plenums have been opened up, blower motor replaced due to original drawing excess amps. Problem appears to be in furnace but short of complete disassembly ( have visually observed clear path through while blower & coil out ) no obvious problem. Have run into loose furnace insulation before causing similiar problems on another brand. This brand tech support insists no similiar problem exists. We will stay on it but for the interim will install a freezestat due to other pressing service needs.

Extreme need to make hay while the sunshines due to the decline in service & installtion in our area. Economy & long cool wet spring has decimated our financial reserves. Winter is coming & not enough time to build reserves.

Fortunately the client is understanding & appreciative of our efforts. We don't charge for unproductive time & parts, or lay off key employees so these past months have really hurt our financial reserves. The freezestat is just to buy time untill our slower period. Usually in August when it's hot. Our service is slower during the weather extremes due to our emphasis on maintenace. We get pressed at the beginning of season when equipment is needed by those who wait untill it breaks for service.

Many thanks for the ideas and will post the brand & problem if ever found. The factory is stumped too. Our problems usually stump the factory so we're used to this.

Yuri

Does the return duct attach to the back of the furnace? Lennox will only allow side cutouts now as back cutouts reduce airflow (blower blocks part of the opening). Blower wheel not spinning on the hub? Loose fins on blower wheel? Some problems I have run into. Freeze stats are a pain/unreliable and can short cycle compressors in my experience.

Good Luck

gene2

Nope, nope, nope & nope. Thanks though.

beenthere

Instead of pushing the SH and SC around by adding and subtracting charge.

Recover charge. Put factory charge plus line set length adjustment back in.
Then read Sh and SC. And use them to diagnose the problem.

EG: With proper charge, is it running low or high SH, and or low or high SC.

Use a manometer and check to see what your static pressure is on both the return and supply side of the furnace.
If its a problem in the furnace. It that will tell you.

__________________
How many times must one fix something before it is fixed?

nicktech

if yer looking for 1300cfm, and ya only got 1000, thats close to a ton's worth of air flow. use a magnehelic guage to determine you static pressure to see if there is a duct issue.
dont rule out however an inefficient compressor. after running low to empty for a while your valves on the 10 seer could be damaged and not moving the right amount of refrigerant mass into the evap. calculate a volumetric efficency test on the hi and low side of the system. it's the ratio between hi side absolute(hi psi + 15) and the low side absolute (low psi + 15) your looking for a 3:1 ratio. anything more tells you the comp. is not doing all it can.
if this is the case an EPR could help back refrig. up in the evap and bring the boiling point up some away from freezing. hope this helps!