New Luxaire furnace w/old Rheem a/c

I Replaced a 1989 Rheem furnace 120k btu input with a Luxaire LP9c100c16mp11. I kept the Rheem racc042jas condensing unit. The new furnace has selectable settings for the blower fan to run. 1st choice is 1655 cfm but when the fan runs at this speed it sounds much louder than the original furnace did. The next setting would be 1345 cfm but that may be too slow for a 3.5 ton a/c. Not sure what I should run the blower at? Also anyway to fiqure out what that old Rheem furnace would of been running at? Of course I don't have the model # but it was a plain Jane 120k btu unit.

The installer of the furnace should have set up the air program for what is best for your situation. It is not as easy as just saying this way or that way is best without having the benefit of knowing all parameters of your system.

Did you install the unit Joe, or did an HVAC contactor do the work?

I did the install myself.

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Originally Posted by Handyjoe

I did the install myself.

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Then, whoever you bought the furnace from should be responsible for showing you the best application for your specific needs. That is not a plug and play furnace you have there. It needs to be set up for your specific needs.

Okay, but the bottom line is it is installed and I have the issue of should i run it at 1655 cfm for an 23 year old a/c or at 1345 cfm. i know to much cfm and the refrigerant may get too warm but too little cfm and it may ice up. i quess I will just put my gauges on it and decide for myself. Thanks for the open discussion.

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Originally Posted by Handyjoe

Okay, but the bottom line is it is installed and I have the issue of should i run it at 1655 cfm for an 23 year old a/c or at 1345 cfm. i know to much cfm and the refrigerant may get too warm but too little cfm and it may ice up. i quess I will just put my gauges on it and decide for myself. Thanks for the open discussion.

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None of what you think you know applies to that furnace or it's capabilities. If that furnace were in my house, in the area where I live, I would be setting the air closer to 1200 cfm.

Once again, not knowing anything about your circumstances, we cannot advise you.

There is nothing you will be able to ascertain with gauges for what you are asking about. Just what good is it going to do you to know the system pressures?

Why can't you get help from where you bought the furnace?

By using the quages you can check the pressure/temperature of the refrigerent going back to the compressor. So if you are running to high of cfm on the blower this could increase the pressure/temp thus the temp would go up and if it is too much it could prevent the compressor from getting the proper cooling it needs. Bottom line is just looking for some ideas and opinions and some good discussion.

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Originally Posted by Handyjoe

By using the quages you can check the pressure/temperature of the refrigerent going back to the compressor. So if you are running to high of cfm on the blower this could increase the pressure/temp thus the temp would go up and if it is too much it could prevent the compressor from getting the proper cooling it needs. Bottom line is just looking for some ideas and opinions and some good discussion.

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Joe, I'm sure you are a handy person, but you really do not understand the refrigeration system or how it relates to air flow. You also do not understand the variables that your new furnace can provide as far as comfort and efficiency. If you have the serial number of the furnace, I can check to see exactly what components are in your specific furnace.

We should limit putting gauges on a system to an absolute minimum. Anything that pressures alone can tell us, temperatures alone can tell us without taking the chance of introducing contaminants into the system or changing the charge.

Also Joe, what part of the world do you live in? I'm in Eastern PA where humidity is a real issue because we have high humidity but not a constant load. That is why I mentioned that I would get the air volume down to about 1200 cfm. You can do a lot more with the air on that furnace then you can with other furnaces, including reducing the air volume quite a bit without having a freezing issue. It depends on what exact features your furnace has in it and what your specific needs are according to the climate where you live and your specific living requirements.