Thread: Best Advice EVER- Replace 31 year old condenser with new 13 SEER unit.

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Jun 2009
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Best Advice EVER- Replace 31 year old condenser with new 13 SEER unit.

Thanks you so much for the advice to replace my old 31 year old Carreir unit with a new unit- rather than try a temporary repair!

Summary: New unit uses about 50% of the kWH of the old unit.

I hope making measurements and doing math does not fall into the DIY category?

Here's measured data from the two day old Nordyne 5 Ton 13 SEER (if matched with new coil).

System Performance... Check my Math: (I convert to SI units. I hope that doesn't seem odd.)

Outside air temp 97.5F
Air leaving condenser 114F
Delta T = 16.5F
For my 5 Ton (Nordyne JS5BD060KA) factory spec list 4000CFM for condenser fan. How accurate is that 4000 CFM number??

Measured total watts when compressor, condenser fan and evaporator fan are running: 4950 Watt.

Measured Watts with just interior air handler (evaporator) fan 550 Watts.

So to measure the total BTU output into the atmosphere by the condenser unit:

Convert to SI units.

4000CFM = 113.2 m3/min
air at around 104F has a specific density of 1.127 kg/m3
so 113.2 m3/min * 1.127 kg/m3 = 127.6kg/min
Air entering unit at 97.5F and exits at 114F = delta 16.5F
16.5F = 9.17C
Specific Heat capacity of air 1.005 kJ/kg C
127.6 kg/min * 1.005 kJ/kg C * 9.17 C = 1176 KJ/min

Convert to per second gives kJ/Sec which is Watts:

1176 kJ/min * 1min/60 seconds = 19.6Kj/sec or 19.6KWatts of energy TOTAL pumped out of the condenser.

Now compressor and condenser fan contribute 4.400kWatts

So 19.6kW - 4.40kW = 15.2kW pumped out of the house. Convert to BTU/hour for fun:
15.2kW /0.293 = 51,900 BTU (Sounds about right for a 5 Ton unit).

And SEER is supposed to be BTU/WATTS so 51,900BTU/4400 watt = 11.8 SEER (Should be EER- See post below) (Worse if I include the watts of the interior air handler fan)
51,900BTU/4950 = 10.5 SEER (oooops! Should be EER!!- See post below.)

Now the effective cooling in the house:
Return air temp 79
Vent temperature 59

Delta 20F = 11.1 C

Now here's where it gets screwy. I have to GUESS as to CFM of the air handler...
I'm going to GUESS 2000CFM, and then figure out how many BTU are being wasted due to air leaks and conduction into the attic.

The interior air handler fan is 550 Watts so 1877 BTU goes to the fan.
2000 CFM = 56.5 m3/min
air at around 70F has a specific density of 1.205 kg/m3
so 56.6 m3/min * 1.205 kg/m3 = 68.2 kg/min
specific heat capacity of air 1.005 kJ/kg C
Air enters at 79F exits at 59F= Delta 20F = 11.1C
so 68.2 kg/min * 1.005 kJ/kg C * 11.1 C = 760kJ/min

Convert to per scond to give kJ/sec which is Watts:
760kJ/min * 1min/60 seconds = 12.68 kJ/sec or Watts.
Convert to BTU:
12.68 kJ/sec / 0.293 = 43,300BTU/hour

Which means that IF I have an air flow of 2000CFM, 43,300 BTU is coming out of the house interior, and
the remainder (51,900 - 1,877 - 43,300) = 6723 BTU is wasted, cooling air in/from the attic.

After two days of operating the new 5 Ton Nordyne system that replaced the 31 year old Carrier system, it seems to be running about 1/2 as often, and consumes 5000 watts instead of 6000 watts.
Thank you so much for advising me to REPLACE the old Carrier unit, rather then trying to (mickymouse) repair the old aluminum coil. You guys give the best advice!
Last edited by blank_disk; 08-17-2009 at 06:52 PM. Reason: Should be EER not SEER. From beenthere

2. EER would be BTU/watt.

SEER is a little different.

Also, you have latent heat being removed. Which you can'tmeasue by drybulb readings. So your indoor isn't as bad as your calc makes it to be.

3. Wow, that's a lot of number crunching. I thought I got into HVAC formulas, but you rock!

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