bgavin Posted June 30, 2021 Share Posted June 30, 2021 Q: Why does the fan in the condensing unit run in Exhaust mode? In Exhaust mode, the fan draws external air in through the heat exchange cooling fins. This warms the air noticeably, and also exposes the compressor motor to this hotter air. If the fan ran in the opposite direction, i.e. Intake mode, it would flood the interior of the unit with cooler external air. This would expose the compressor to the cooler air, and (maybe) help it run a wee bit less hot. The airflow through the heat exchanger fins would be the same. If I take an IR temp measurement of my compressor... it is REAL hot. Thoughts? Link to comment Share on other sites More sharing options...
No Show Joe Posted June 30, 2021 Share Posted June 30, 2021 That doesn’t sound right, Post the wiring diagram or a model number of the unit. Link to comment Share on other sites More sharing options...
Sedalia Dave Posted June 30, 2021 Share Posted June 30, 2021 By pulling air through the coils any dirt or debris collects on the outside of the unit. More likely to get noticed and easier to wash off. Also a lot less likely to have the airflow completely blocked. By pushing air into the unit a large bag could significantly restrict the airflow. Link to comment Share on other sites More sharing options...
bgavin Posted June 30, 2021 Author Share Posted June 30, 2021 https://www.bryant.com/en/us/products/air-conditioners/116b/index.html Bryant 116BNA042A = 16 SEER, Puron, 4.2 Ton My question is focused on the air flow direction affecting the surface temperature of the compressor. Yes, the fan could be blocked. Yes, the coils could be externally blocked. This is my 2nd unit, and it is 1 month and 1 day away from failing just outside the 10 year warranty, same as the first one did. As with all things Mechanical or Electrical... heat is the enemy. I was tinkering with the idea of fabricating a copper heat sink girdle ( or ?? ) to wick away heat from the compressor, and thus increase its life. This is one of those academic questions like Alpo is famous for raising. Link to comment Share on other sites More sharing options...
Cholla Posted July 1, 2021 Share Posted July 1, 2021 Most coils are draw through because the air pattern is more even, as opposed to push through in which the air flow pattern is strongest in front of the fan. And in most cases it is following the natural tendency for hot air to rise. Your compressor is already internally cooled by the returning (suction) gas refrigerant from your evaporator coil, which should be in the 50s-60s in temperature. The top of the shell (on a scroll) is the discharge gas, which is going to be hot. The discharge gas has to be hot in order for it to be able to reject the heat into the surrounding air, even if it is 110 degrees out. A general rule of thumb is the discharge temperature is 30 degrees above the outside ambient temperature. In the old days, the shell on recip compressors was the suction gas, so it felt cool. Scroll compressors are not built that way. I would not mess with your air flow or design. If they thought they could reverse the air flow and make it more efficient, they would have so they could boost the SEER rating. Trust the engineers that designed the unit. Link to comment Share on other sites More sharing options...
No Show Joe Posted July 1, 2021 Share Posted July 1, 2021 I looked at the wiring diagram, the fan should only blow One way , drawing air though the coils and out the top. Link to comment Share on other sites More sharing options...
bgavin Posted July 1, 2021 Author Share Posted July 1, 2021 This is just an academic tire kicking question. Cholla, thanks for the lucid reasons for running in Exhaust mode. Trust the engineers... I remember the Challenger and the Tacoma Narrows bridge engineering. I also remember a lifetime of engineering screwups over my career in the computer business. These resulted from either engineering incompetence or bean counter overrides. I figure my 2nd Bryant unit will die another $2300 death, just after the 10 year warranty expires. So, I will look at this is an A/C rental.. where I pay $230 annually to rent the condenser unit and replace it on a 10 year cycle. I've not dissected a scroll compressor, so I have no understanding of how/why they fail. Link to comment Share on other sites More sharing options...
Cypress Sun Posted July 1, 2021 Share Posted July 1, 2021 If you got 10 years out of an condenser, you should be somewhat happy. If you can find a new one and replace it for $2,300, you should buy a lotto ticket. Link to comment Share on other sites More sharing options...
Cholla Posted July 1, 2021 Share Posted July 1, 2021 1 hour ago, bgavin said: This is just an academic tire kicking question. Cholla, thanks for the lucid reasons for running in Exhaust mode. Trust the engineers... I remember the Challenger and the Tacoma Narrows bridge engineering. I also remember a lifetime of engineering screwups over my career in the computer business. These resulted from either engineering incompetence or bean counter overrides. I figure my 2nd Bryant unit will die another $2300 death, just after the 10 year warranty expires. So, I will look at this is an A/C rental.. where I pay $230 annually to rent the condenser unit and replace it on a 10 year cycle. I've not dissected a scroll compressor, so I have no understanding of how/why they fail. The most likely reason a compressor fails is not the engineering but the person installing it. There are specific standards that need to be taken to keep the refrigerant system clean during installation. (nitrogen purge, evacuation, line sizing, line cleanliness, etc.) If these steps are not taken, moisture and contaminates will slowly create acid and sludge. If properly installed, a system should last decades. I have seen units rusting away on the outside but the compressors still chugging away inside. In theory, if industry standards are followed, nothing should EVER cause the oil to breakdown and cause a failure because it is hermetically sealed. It's when the filters are allowed to get dirty which allows liquid refrigerant to get back to the compressor and wash the bearings (liquid refrigerant acts like solvent), the condenser coils get clogged with cottonwood seeds or grass clipping that causes the unit to over heat, contactors to get worn and cause excessive amperage, and dozens of other maintenance items that cause a unit to fail. I work on units daily that are 20-30 years old and working as designed. I have worked on compressors built in the 1960s that as of two years ago were still going as strong as the day they were built. The only reason some of these units get changed is because newer units are more efficient/cheaper to run or the refrigerant is getting harder and expensive to buy. The vast majority of failures falls squarely on the shoulders of the person that first installed it. Next is the person that maintains it. The industry average for a unit is 17 years. Provided you old system wasn't a burn out and they used the same copper lines over again, there is no reason to expect it to fail now. In all seriousness, a good tech should be able to use a megger to check the compressor motor insulation, do an acid test, check the sub-cooling, the superheat, and check the amp draws and give you a good idea if there is a developing problem. Scroll compressors are very reliable, 100-year old technology that modern materials finally made possible in the 1980s. The weak link in any hermetic compressor is the motor bearings and windings. Link to comment Share on other sites More sharing options...
bgavin Posted July 1, 2021 Author Share Posted July 1, 2021 I knew I asked the question in the right place... many thanks. Link to comment Share on other sites More sharing options...
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