[siltout]

OK guys, this situation doesn't seem right to me. I've been out of the residential side for over 5 years now, so maybe I'm missing something. My wife and I just bought a renovated house, that was built in 1900. My wife liked the house and I liked the garage. I knew there were some issues with the HVAC, but I knew I could fix any of that (no, I'm a DIYer, I'm a licensed technician).

I knew the idiots who installed it did not put in enough RA, but I didn't realize the duct system was this bad. To give you a quick summary, after reviewing the airflow design specifications of the gas furnace, AC, and calculating the airflow of the duct system, this is what I found:

Gas Furnace CFM (Cooling) - 2010
Gas Furnace CFM (Heating) - 1505
AC Coil Nominal CFM - 1000
Duct Supply branch calculated CFM - 640
Duct Supply take off CFM - 1020
Duct Return Air CFM - 360

Can anyone tell me what the hell these people where thinking?!

So now, do I re-size all the duct and upgrade the AC? Re-size some of the duct and down size the gas furnace? Start from scratch and do a heat load/cooling load calculation and re-do the whole damn thing?

 

[dandyme]

how did you come up with those cfm #'s

 

[siltout]

The furnace and A coil came from manufactures literature. The duct cfms came from measuring them and using my old fashioned dial ductolator.

 

[siltout]

I know the duct numbers seem absurd. That's why I'm frustrated.

 

[bruced]

Static pressure

Check your static pressures along w/ cfms, you will be able to determine where your lack of airflow is. Your airflow blower charts should show cfm at certain certain static pressures, a furnace usually has .05 as normal, air handler .08, biggest problem is usually in return. Coil clean? Get a duct calculate and see if you can determine how much duct you need, how many leads, figure rule of thumb 100cfm per register and see what you get. No real way to determine w/o static pressure tests. Send a picture or two. Too small of duct will kill you. Tests show most 90+ furnaces are operating in the 70 and 80%'s because of in proper duct systems. That is not serving your customer well. Also how much return is being drawn from unconditioned or out of the envelope areas? How much supply duct leakage to unconditioned areas? If not right, you must make right, DO NOT try to change furnace operation till this is done, your furnace is designed to operate under certain conditions to get efficiency. It may work but at what price. Pay me now,pay me later. AIR FLOW IS KING!

 

[dlove]

http://hvac-talk.com/vbb/showthread.php?t=1033971


check out the post in commercial. good work sheets to follow

 

[71CHOPS]

I would start from the beginning. Do your load calc. and see what size system is required.

 

[BaldLoonie]

What size A/C?

 

[siltout]

The AC is a 2 1/2 ton system

 

[craig1]

The AC is a 2 1/2 ton system

so you need approximately 1000 CFM in cooling mode. It looks like your furnace is capable of 2000 (5 tons) so you'll want to use one of the lower speeds for cooling. Of course if the ducts are severely undersized, you might need to leave it on high to jam as much air through as possible until you fix the ducts.

 

[siltout]

Because of the inconsistencies with the initial installation, it makes me wonder if the furnace and/or AC sizing were done correctly to begin with. I think the furnace might actually be bigger then what the house needs, and I suspect the AC might be smaller then what the house needs. I'm thinking I just might do the following:
1.) check the static pressure of the system
2.) perform a heat/cooling load calculation to check for proper equipment sizing.
3.) determine if I'm going to replace either/or or both of the systems
4.) replace the duct as determined by the system.

I just hate having to completely replace the systems, because of the stupidity of others.

 

[rdholder]

Systems with greater than .08” pressure lost per 100’ of equivalent feet of supply ducts and .06” per 100’ of equivalent feet of return air ducts will have a lower capacity and efficiency. Velocity in feet per minutes of a main duct is 1000 FPM; return air 800 FPM and branch duct 600FPM. The manufacture may be speaking in foot square, 1000 CFM would be 2 feet square for 2000CFM

 

[JPJ13]

I believe in starting at the beginning by doing the proper heat load calculations on the home, then comparing what you currently have to that. I'm willing to bet that you are going to find the equipment and the ducting is incorrect.