Determining if ductwork will handle a new system?

[steve f]

I have 1200 s.f. serviced by seven 4"x10" supply registers with two 10" return ducts. I've posted before about my concern about a Trane XV95 60K furnace that I'm supposed to have installed in relation to return airflow, but I'm really concerned about supply also. From what I've been told, high stage airflow on the XV95 starts at around 1100 cfm. Is there some rule of thumb about how many supply registers I'd need for 1100 cfm?

I've had nine salesmen out, all from large dealers, most of the companies advertising NATE certified technicians. None have done any type of load calc, none have even mentioned doing one. Only one has checked the number of vents, he thought I was ok, and another one thought I might need another return.

The system is in a basement and serves the floor above. I have those removable celotex ceiling tile in the basement, so there's fairly easy access to the ductwork. The Trane salesman from the company I've decided to go with is advising that I get the system installed and that the technician will be able to adjust the airflow without any problem. When I told him another salesman mentioned I might need another return, he said if it came to that, since the guys would already be out here doing the install it would 'only' be $ to add a return.

I'm thinking of putting the whole process on hold until I understand what I might need as far as ductwork. I'm not comfortable with getting the system installed first and then worrying about airflow capacity later.

[HVAC9900]

Wish I had my ductulator handy,to be precise.
Need more specific info.

You mention a 4X10,do you mean the register is that size, or is it the actual duct for the register?
Also, the 10" ducts,are those round ? If not will need to know the other side measurement.

Is this all metal duct,is it insulated(and is that measurement you are giving the duct or the duct plus the insulation)?
Or is it flex or fiberglass duct?

Assuming my ductulator memory is functioning well ,and the 4X10 is the supply duct size, and you have 7,and you want to run 1100cfm, I'd say you are going to need more duct and supplies,or larger ducts(probably including the main trunk as well),unless you like a noisy air supply and drafts.

Does this have a main supply duct,or is it a big spider with the supplies coming off the top of the furnace(or AC coil)?

[beenthere]

Good chance your short on supply and return duct work.

Ask them to check the CFM and static pressure of your current system.

From that, they can determine if either or both your supply and return need remediation.

PS: No prices in post.
I edit it out already.

[BaldLoonie]

I have a similar furnace in a similiar house. No overheating issues at 1100 CFM on high fire but boy is it noisy! I have a switch on the furnace to keep high off except in the rare events I need a fast warmup in bitter weather. Really need a smaller furnace but hard to come by in high end.

[motoguy128]

Is 1100 CFM nessesary? On high stage my 60k BTU 80 furnace is only 900CFM. @ 950CFM you'd have a 56F temp rise on a 95% 60k furnace. The "low" fan setting for that furnace might be around 950 CFM.

I wonder if there is a compromise that wouldn't risk the HE... rather than making major ductwork modifications.

Still might be better to go ahead and add another return and supply if possible. Two 10" returns and 7 4x10" supplies are still a little small for even 950 CFM. The 4x10" registers should probably stay under 100 CFM each to keep noise down.

[vstech]

a rule of thumb for # of registers? well, sorta. it depends on length of the runs, and plenum distribution, but at .2" static a 6" round run handles up to 150cfm.
the more runs you have, the lower the static pressure, and fpm will be.
for your example, assuming you have 6" round supplies, 7 runs are capable of 1050cfm.
if 7" runs are used, it will handle the load. if 8" are used, even better.
the two 10" runs are capable of 1200 cfm at .1 which is where I figure maximum return pressures...

[SmackDabAOK]

[quote=HVAC9900;2920382]

I'd say you are going to need more duct and supplies,or larger ducts........unless you like a noisy air supply and drafts.quote]


Does the noise & drafts factor, hold much consideration when sales persons do workups on high end furnaces?
I reallize that high stage doesn't run often.

But the problem for the homeowner though, is they won't know 'til after installation if this would be a problem.

[steve f]

The install went well, for me anyway. The guys doing the install were at the house for about 7 hours and worked hard. The noise level on the AC & furnace are fine, about the same for the AC vs the old system (and never a problem on the old system), a little less noise on the new furnace. Both move more air compared to the old system. The air from the furnace is hotter than the 12 year old system. The new AC is putting out air with a 22 degree temp difference vs outside, so that's good from what I understand.

Since the furnace is in the basement and the system services the first floor, noise from the furnace itself was never a big issue anyway, I was concerned about noise from the vents and returns, but as I said, not an issue. The wife likes to feel the warmth asap, so 60K btu @ 96.7% efficiency vs a 12 year old 80% 60K btu furnace means more heat.

I would still have liked to have tried an XL15i heat pump with the XV80 furnace, but heatpump = unhappy wife, so that was out.

[motoguy128]

The new AC is putting out air with a 22 degree temp difference vs outside, so that's good from what I understand.

Maybe, maybe not. It depends on the humidity indoors. A 22F temp drop might be bad in high humidity because it means you latent capacity (energy used to remove moisture) could be low and more energy is going into reducing the air temperature. This time of year, it's likely fairly dry so it's probably doign a good job. I know I was amazed at how cold my air temps were on my new system compared to my old unit.