Just installed Goodman 2.5 ton condensing unit with 4/5 ton evaporator coil (AHRI #200814817). Added 8 ounces 410a to make up for additional line set but sub cool is only four degrees. One question...does the oversize evaporator coil require additional refrigerant to properly set up system? I do not want to overcharge if it can be avoided (no reclaimer).
Subcooling Woes
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I believe it would, more tubing in coil = more refrigerant.
I think as long as you charge to the manufacturer recommended subcooling value, no danger of over-charging, as long as the txv is working properly and not overfeeding (superheat would tell you that) and the indoor airflow is set correctly.
I think as long as you charge to the manufacturer recommended subcooling value, no danger of over-charging, as long as the txv is working properly and not overfeeding (superheat would tell you that) and the indoor airflow is set correctly.
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Why would you install a 4 to 5 ton coil with a 2.5 ton condenser?
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^Nominal coil tonnages are for 13 SEER units.
To hit 15-16, a much larger coil is required. Heck, even for a 14 the coil is normally 0.5 to 1 ton nominal larger than the condenser.
To hit 15-16, a much larger coil is required. Heck, even for a 14 the coil is normally 0.5 to 1 ton nominal larger than the condenser.
Thanks very much for the responses. I really appreciate having someone to "talk" to. One subscriber asked why such a large coil. I admit I hesitated on that but that is what the seller kept pushing...and it had an AHRI number so i finally went with it. My concern was its ability to dehumidify and will eventually get to that. Anyway, here are yesterdays readings after 45 minutes runtime:
Outdoor Air Temperature 80 degrees
Indoor Air Temperature 72 degrees
Liquid Line 282 PSI
Suction Line 135 PSI
Liquid Line 91 Degrees
Suction Line 62 Degrees
Sub Cooling 4 Degrees
Super Heat 14 Degrees
Delta T 15 Degrees
I do not have a pitot tube (about the only tool I lack!) so not sure of CFM. Furnace is on lowest of the four speed selections right now. I am 608 Universal Card certified BTW...just not an HVAC specialist (master electrician retired).
Any further comments greatly appreciated.
Outdoor Air Temperature 80 degrees
Indoor Air Temperature 72 degrees
Liquid Line 282 PSI
Suction Line 135 PSI
Liquid Line 91 Degrees
Suction Line 62 Degrees
Sub Cooling 4 Degrees
Super Heat 14 Degrees
Delta T 15 Degrees
I do not have a pitot tube (about the only tool I lack!) so not sure of CFM. Furnace is on lowest of the four speed selections right now. I am 608 Universal Card certified BTW...just not an HVAC specialist (master electrician retired).
Any further comments greatly appreciated.
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25,111 Posts
The AHRI number is just a model # with some technical specs attached to it. If Goodman recommends/approves you use it with a 2.5 ton unit AND you get a specific higher SEER then it may be OK as it won't void any warranty from messing with oil return etc.
Subcooling just means that there is some liquid refrigerant in the bottom 2-3 rows of tubing in the condensor. As long as you have 1 deg of subcooling you won't flash vapor into the liquid line which is a big problem.
Ideally you want 2 tubes full but in your case probably only 1 is full but still safe. More full tubes = more area of liquid exposed to the cooling air from the fan and more subcooling. You don't want 3-4 tubes full of liquid or else the discharge pressure rises due to the reduced area to cool the discharge gas.
Point is you want some subcooling but not too much and the ideal amount is determined in a lab by Goodman to get the best SEER.
What your SEER is with 4 deg I don't know and whether it is messed up/lower I don't know.
To get rebates for higher SEER packages our power company had matched sets of larger coils and condensors according to AHRI but I usually see them being with .5 larger not 1 ton.
Subcooling just means that there is some liquid refrigerant in the bottom 2-3 rows of tubing in the condensor. As long as you have 1 deg of subcooling you won't flash vapor into the liquid line which is a big problem.
Ideally you want 2 tubes full but in your case probably only 1 is full but still safe. More full tubes = more area of liquid exposed to the cooling air from the fan and more subcooling. You don't want 3-4 tubes full of liquid or else the discharge pressure rises due to the reduced area to cool the discharge gas.
Point is you want some subcooling but not too much and the ideal amount is determined in a lab by Goodman to get the best SEER.
What your SEER is with 4 deg I don't know and whether it is messed up/lower I don't know.
To get rebates for higher SEER packages our power company had matched sets of larger coils and condensors according to AHRI but I usually see them being with .5 larger not 1 ton.
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17,805 Posts
I looked up the reference number and it's valid, 15-16 seer machine (ecm needed to hit 16) needs large coil.
I assume you added the txv required if it didn't come with the coil?
Check your wetbulb return air temperature - can use it to determine target supply temperature.
15F drop may be okay if it's relatively humid or the blower speed is set high.
What's the full model number of the furnace? (to check blower size)
On a 3 ton drive furnace the fan should be on high or medium high for 2.5 tons, depends on your air ducts. On a 4 ton drive - lower.
I assume you added the txv required if it didn't come with the coil?
Check your wetbulb return air temperature - can use it to determine target supply temperature.
15F drop may be okay if it's relatively humid or the blower speed is set high.
What's the full model number of the furnace? (to check blower size)
On a 3 ton drive furnace the fan should be on high or medium high for 2.5 tons, depends on your air ducts. On a 4 ton drive - lower.
I did install the TXV; sized for the 30,000 condensing unit (along with the proper orifice). Just started the unit up and wet bulb (next to RA register) is 65 degrees; dry bulb 74 degrees. The Goodman furnace is a GMSS92080, condensing unit GSX160301, and coil CAPF4961C6. I placed unit on "LO" last fall attempting to slow the CFM down a a bit. The total static on low right now is around 0.8" water column. Advice please.
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"(along with the proper orifice)"
Either you use the orifice or the txv, never both.
The full furnace model is required to know blower capacity, there are usually two versions of the 80k.
You can determine proper cfm based on static pressure.
It's best to measure wetbulb and return air temp in the return itself as it can differ. (there's leakage, etc)
Either you use the orifice or the txv, never both.
The full furnace model is required to know blower capacity, there are usually two versions of the 80k.
You can determine proper cfm based on static pressure.
It's best to measure wetbulb and return air temp in the return itself as it can differ. (there's leakage, etc)
Yikes...a thousand apologies! My heart skipped several beats as I ran to the basement to check my parts bin. Thankfully both orifices were there. I only installed the TXV. The furnace is a Goodman GMSS920804CN. Total static is around .8 right now. Thanks for your support.
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So that's a 4-5 ton drive and at 0.8", the table shows 1000 cfm for low so you have it set correctly.
If the humidity is normal the temperature drop is slightly low assuming your static reading is accurate.
If the humidity is normal the temperature drop is slightly low assuming your static reading is accurate.
Thanks again...the static reading number is pretty much correct. I have a Dwyer Mark II stationary manometer attached and am using Dwyer static pressure tips. I also verified with digital instrument. Gonna keep plugging away. Doesn't high superheat along with low subcooling usually indicate low charge? The suction service valve has never had condensation on it also.
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I don't think 14 degrees superheat is that high, coil sat temp is a little high (47-48F?) potentially due to a high load - ie high humidity. (hence needing to check wetbulb, not just drybulb return air temps)
Also, having a large coil to hit seer rating also raises the coil temp.
Less efficient units run lower suction temperature/saturation.
It makes sense to charge it to the correct subcooling required by goodman, the txv should automatically maintain proper sh. It may or may not be adjustable.
Also, having a large coil to hit seer rating also raises the coil temp.
Less efficient units run lower suction temperature/saturation.
It makes sense to charge it to the correct subcooling required by goodman, the txv should automatically maintain proper sh. It may or may not be adjustable.
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That furnace is not suppose to be paired with a 2.5 ton A/C.
Your SC, should probably be 7°, so you might be a few ounces low on charge. The install manual should have the amount of additional charge the coil needs listed in it.
Your duct work is grossly under sized if you have a TESP of .8" on low speed.
Your SC, should probably be 7°, so you might be a few ounces low on charge. The install manual should have the amount of additional charge the coil needs listed in it.
Your duct work is grossly under sized if you have a TESP of .8" on low speed.
I agree 100% on all three of your points! Thanks much for the input. Seems to be working fairly well right now but definitely needs a bit more attention. My HVAC tech friend conveyed to my last evening that the unit is very much undercharged so I will be adding some real soon. By the way, I'm looking at Goodman Expanded Performance Data for the GSX160301 condensing unit...can anyone tell me what the abbreviation S/T means? The WWW just does not recognize it as an HVAC term.
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Ducts may be undersized but as long as it gets proper airflow in heating mode, should be okay as long as you can tolerate the extra noise in heating mode.
The 5 ton drive (maxes at 1900 cfm at 0.5") should be able to move enough air for heat on medium low or medium, but static will be through the roof.
Minimum heating cfm is 1050 for 65F temp rise but really it should be 1200+ to not stress the heat exchanger.
The 5 ton drive (maxes at 1900 cfm at 0.5") should be able to move enough air for heat on medium low or medium, but static will be through the roof.
Minimum heating cfm is 1050 for 65F temp rise but really it should be 1200+ to not stress the heat exchanger.
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