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aalendi 06-04-2012 06:58 PM

AC problem. Not enough supply/return?
I need a little expert advice with an AC problem I'm having. My house is a 1995 build and was not originally configured for AC. It has a 135k 5 ton Carrier furnace, an adequate number of registers (every window), two large floor returns on the main floor and one small high wall return upstairs. The high wall return is reduced to about 2" x 5" as it passes the staircase in my wall cavity. The house is 1770 sqft above grade and 1050 of that is on the main floor. I had my ducts and returns professionally cleaned in the fall.

Just last summer when I moved in, I installed a Rheem 2.5 ton R410A unit and A coil. The house takes forever to cool down if it cools down at all. An example would be today:

-I installed a brand new filter before conducting this study.
-I live in Denver and it was a 90F day today with lots of sun.
-By 4pm my house was 80F inside.
-I turned on the AC and measured my Delta T at the furnace. My return was 65F and the output was 42F giving a 23 degree yield. Not bad I thought.
-At 545pm (After 1hr 45m) the temperature at the main floor thermostat was 79.5F. The return still measured 65F, however the output had surprisingly dropped to 40F.
-Both times I measured, my mainfloor registers showed ~45F while my upstairs registers showed ~50F.

I have trouble understanding why my house isn't cooling down when its apparent that the AC unit itself is performing great and actually improving as it runs. One observation I've made (and I do not own an airflow meter) is that the upstairs vents have very poor supply pressure in comparison to the downstairs vents. All vents in the house are in-floor. This still doesn't explain to me how the main floor temperature would remain so high.

My initial thought was that the massive floor returns were simply recirculating the same cold air over and over again, but that was dis-proven by my return measurements being 65F both times. My original plan was to pull more high return air from the second floor.

Before I go running new ductwork and tearing my house apart, I thought I'd see if anyone has any input to offer. Thanks in advance!

Doc Holliday 06-04-2012 07:50 PM

5 ton blower (furnace) on a 2.5 ton condenser/coil. That mix match in size should be your first and most prevelent concern.

aalendi 06-04-2012 08:19 PM

I tried a tissue at each return air vent and all three seemed to have the same amount of pull. This seems to indicate that all of them have the same negative air flow, but I'm still not certain that the high wall is carrying the volume it should.

aalendi 06-04-2012 08:25 PM

@Doc.. From what I understand (and I'm no professional) is that its a maximum of 5 ton on the blower. I can't say I've seen a matched tonnage on any setup before and it always appears that the blower is higher than the compressor/coil. Unless I've got something wrong, if the blower was too much for the coil, I'd see a problem with my Delta T. Is that not correct? The 23-25 degree difference I'm seeing seems pretty good considering most people on here are reporting 16-20 on a properly operating system. If you feel I'm wrong, please let me know. Like I said, I don't claim to be an expert, just a fast learner. Thanks.

aalendi 06-04-2012 08:27 PM

@Poparoach. The motor doesn't appear to have additional taps and is a single speed. I'd believe that it could handle a 5 ton compressor, but that would seem overkill for the size of my house (at least to me). Thanks!

REP 06-04-2012 08:42 PM

The air in your duct system is probably moving too slow because the system is undersized.This could account for it giving up so much heat to get a 23degree delta T.
Its not your fault but the original installer didn't use proper design and inatallation methods.
It is not rocket science to figure out what each room needs in order to supply comfort.It is however simple math in the form of chiefly two engineering calculations.These are called a Manual "J" and a Manual "D"
A proper easy breathing system will supply enough conditioned cfm s and AND enough return air going out in each room to not only provide the most comfort but also that the whole system runs as ecconomically as possible.
Your return system is a joke in that reguard as the return probably isn't even enough to allow the furnace to run properly to say nothing about comfort.
It dfoesn't appear that anybody has done anything for your benifit in reguards to your HVAC system .
You may be able to make it work but without redesigning it you will never have what you should have.
As far as the equipment sizing,I could be wrong but both the furnaces appears to be the wrong size,but thats what the manuals are for before anything is installed.

Doc Holliday 06-04-2012 08:42 PM

What size condenser and a/c coil did you replace?

aalendi 06-04-2012 08:46 PM

There are two vents in the basement and I keep their dampers closed. The basement, even on a hot day is very cool. The house does have a lot of windows and presently takes a lot of sun as my trees are small. If I used the 1000sqft/ton rule, I'd be well over what's required.

I did just get done checking and found that my blower is in fact a multitap (not sure how I missed that). It was set to High for cooling and Med-Low for Heating. I jumpered the Med-Low to cooling and Med-High to heating. I just fired the AC back up after turning it off an hour ago. I immediately upon starting it got a 25 degree td. I'm curious to see if the lower airflow makes my difference. What is a standard td for you?

aalendi 06-04-2012 08:48 PM

@Doc, there had never been a condensor/coil prior to my installing one. I used a calculator in which I plugged in my square footage, area of sides exposed to sun, window dimensions, etc to return a suggested condensor size for my climate. The resulting size was something like 2.3 ton and I picked a 2.5 as it was closest.

aalendi 06-04-2012 08:54 PM

@REP.. I agree on the return air. My upstairs return looks (by the grille) much bigger than it is (the obstructed tiny cavity in the wall). I am planning to run another return to the upstairs and add another supply line to split the load to the upstairs rooms. There are three bedrooms and two bathrooms on the upper floor and from the basement I can't seem to find more than one 6" line running from the main trunk upstairs. That would explain my almost non-existent airflow up there. I just need to make sure I do my calculations right so that I don't put too much return air into the system. Any suggested books/sites for proper formulas?

REP 06-04-2012 09:03 PM

I don't think I have ever seen a house with too much return air.Anything is possible but it would be noticibly gross.
Also I hope we are talking metal duct,otherwise you will need to change that too.
There should be balancing dampers on each run .so that when everything is in place you can adjust them to put more pressure to the second floor.
Google Manuaql "J" HVAC and manual "D" hvac and read that.the only other things I've seen online are Put a wet fingure up in the air rule of thumbs used when equipment had pilot lights LOL.

aalendi 06-04-2012 09:04 PM

Poparoach's idea to explore the blower taps may have been a good start for my system. My main floor registers are now coming in at 40F and my upstairs at 45F. My td is still ~25F, but the 5 degree difference at the vents is encouraging. I know I still need to upgrade and reconfigure my returns/supplies, but for now we'll see if this makes my home even a little more bearable. Thanks Popa!

aalendi 06-04-2012 09:19 PM

1 Attachment(s)
@REP. I made (and attached) a quick and crude sketch of possible ideas for volume dampers in my branch ducts. Would this be how you'd recommend installing them? My thought is that reducing the air flow on either side of the upstairs duct would increase the airflow to the second floor where it is needed in the summer. Please let me know if you'd suggest a better means of doing so. There are already louvers with dampers on each register. Thank you!

REP 06-05-2012 12:22 AM

Balancing dampers are usually on each round takeoff.They are usually right at the round pipe take offs.
The kind in your picture are raely used as you get a much finer adjustment by working each run rather than a whole section of duct.

beenthere 06-05-2012 04:52 AM

Since you were already getting a 20 to 25 degree delta. it indicates you have low air flow. Probably need more supply to the upstairs, and more return also.

Dampers like you drew, would probably reduce air flow to low and freeze your coil.

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