HVAC Heat Pump

[Paulbstockton6@hotmail.co]

Well, almost all new manufacturer's air handlers have ECM blowers, so many, it's likely mandated. They sound great, especially in setups that allow for long duty cycles. Some people have allergies and leave the blower fan on 99% of the time and thats where the ECM blowers shine with their 30% better efficiency. Where they don't shine is in systems with high static pressure, or lousy ducts, as then they will only supply as much air as its pressure sensors allow.
Alas, it's possible a HVAC contractor will assess your actual needs, possibly suggest a 60-80k BTU high efficiency heat pump unit that will work in a 3.5 ton unit...which means...a 3.5k blower would be about 1400 cfm....possibly at 1400 cfm, your ducts would be sufficient.
And, there is still time to do return and supply Temps for the furnace to calculate temperature rise. Should be in the range of 40°-70°. Say, if the rise was close to 40°, you could lower the fan speed which could possibly circulate air better...being that the blower is operating longer.

[redpoint5]

Originally Posted by Paulbstockton6@hotmail.co

17x20 is good for about 1600cfm and 12" is good for about 500cfm..... that checks out

Grille size 17x20 = 340sqin x 2 = 680cfm... problem
So the grille for the 12" round duct (capacitu = 500cfm) vs grille (680cfm) good.
Grille size for the 17x20 square duct (capacity = 1600cfm) vs grille capacity (680 cfm)..... not so good and this return is noises than the other.one.
So you have. .500+680cfm= 1180cfm. (Choked) for a 5 ton system which likely set to 1500 to 1700cfm....
What just happened...so now you will have excessive pressure (big deal, I'm in the same boat) This will make the blower motor work hard...drawing more amps.
I predict that if you tested static pressure you will be exceeding the .50wc spec...

So, since we are deficient in return air, look at the return plenum, or that 17x20 duct....can you tap into that and run an additional return somewhere where it might be benificial....?

I took a closer look, and the return is even worse than calculated, and it is way more noisy as you predicted.



The knuckleheads have the plenum placed well below the grill, and didn't seal anything. The foam is my attempt to seal it up. So the excess capacity of the return duct is made even worse by the grill being misaligned with it. Maybe I can bend the plenum up to the grill? Don't know how these are normally joined/sealed to it.

Could I simply add a 2nd 17x20 grill next to the existing one and feed it into the return?

Here's the other side of the wall, showing the return plenum sitting on a 2x4. Don't know why the forum put the image upside down.


My furnace control board schematic if that's useful at all. I can't find specs on the motor anywhere. Strangely, the more restriction there is, the less power it draws. I turned the fan only on, and it draws 750 watts. When I remove the filter and open the grill, it draws 800 watts.

[Paulbstockton6@hotmail.co]

To answer your earlier question about how much air flow is needed for a 125,000btu furnace... 1600cfm. And the airflow required for a 5 ton a/c is 2000cfm.... (and..17x20" duct is about 1600cfm).
So yes, your biggest gain with your current equipment would be to fill that 17x20" with as much air as possible. I think it would be very easy to open up the return grille to be large enough...(even going a bit bigger, say 2 20x30" return grilles.)
It also looks like you could tap the right side of the return just behind the grille to penatrate the bedroom wall, adding a return to each bedroom is recommended, It looks like you have high ceilings (with fresh paint ).ducting some return air from up high would be good if you didn't mind tearing out drywall....a standard 2x4 wall cavity lined with a 3.5x14" duct is only good for 100cfm.
Is it also possible to run a other 12" round return into the other floor somewhere?
Now I'm curious about the number of and sizes of your supply vent registers....