New blower needed?

[deeraeya]

As part of the negotiations for purchasing our house an HVAC company replaced the AC coil above our furnace, balancing it with the outside unit, retarding coils etc. It all looks good etc but I'm concerned that the static pressure drop of the unit is too high for the blower in our furnace. My question is whether they should/could have known that and whether I have any reasonable recourse?

The ducts are a mess and I'm getting quotes to fix those. As part of that process we had a Manual J done (assuming some improved insulation) and apparently we need 1150 cfm. The manual for the furnace says that the blower can deliver 1150 cfm @ 0.545 IWC. It's unclear if that was tested with a filter or not (no footnotes). FWIW it's a RUUD Silhouette II Model: UGPH-05EAUER.

The pressure drop for the new coil Wet @ 1150 is 0.415 (Goodman Model#: CAPF3636A6DB). Accounting for grills etc takes another 0.09 IWC.

If the blower was tested with a filter in place (which filter?) then the Available Static Pressure for the ducting appears to be 0.04 (0.545 - 0.415 - 0.09 = 0.04)

If the blower wasn't tested with a filter it seems that a reasonable estimate for the "True Blue" filters we are using (24"x12") at end of return is something like 0.08 (based on this link http://www.texairfilters.com/news/testsonpressuredrop.htm adjusting for smaller size filter). That would imply that there is no available static pressure @ 1150 and best approach (short of replacing the furnace) is to replace the fan with a higher powered one.

Presumably at the moment we're getting much less cfm, which accords with cooling difficulties throughout the house, but particularly at the end of the longest runs.

So that's a long way of asking: should I be annoyed with the company that put this coil in the system? On one hand they might have been operating with some assumption of less cfm needed (at 900 cfms the coil only has as a drop of 0.28), on the other hand I think it was irresponsible to specify this without knowing the pressure requirements of the system. Or should I just ignore them, work with what we have and just get some quotes for a replacement blower?

Thanks,
James

[hcong]

I am guessing the seller was the one who hired the company to replace the coils? Since there was originally a coil there the ac guy probably just replaced it with a similar coil. The installer probably could have done more due diligence and compared the static pressure loss, but time is money and they probably didn't quote to do all that work over a coil replacement.

Your furnace is the smallest one of the line, it is rated at 1200 CFM but at 0.5 esp with a high speed fan. You might have a medium-low speed fan (factory standard) that gives you only 860 CFM @ 0.5 esp. If you are not getting enough airflow, there is a chance that the furnace was never designed for the 1150 CFM.

You could ask your real estate agent if the original owner also had airflow issues... You said your ducts are a mess, so that might be a bigger issue. Without knowing the original coil esp, it is hard to say if the coil replacement caused the problem.

[beenthere]

The CAPF3636 I have specs on, is only .23"wc PD wet, at 1200 CFM.

[hcong]

The CAPF3636 I have specs on, is only .23"wc PD wet, at 1200 CFM.

I got around the same as the owner, maybe this is an old table... see attached.

[beenthere]

LOl... ooops, I was looking at the D.

With the A. that means that the the ESP to use to find AP is .6" or .7", have to do that with 5 tons coils a lot.