Checking return air system with ESP?

[Bill NC]

I just had a new Trane heat pump system installed in my house.

AHU: 4TEE3F40B1000AA
Cond: 4TWX5030A1000AA

The return air system for my old unit may have been undersized, however it seemed to work fine for 17 years.

The estimator told me that I would need 2 - 14x20 floor grates for returns (filter at the AHU). This seemed excessive to me, so I tried reading up on return air systems online (I'm a general contractor).

The new system is a 2.5 ton system (AHU is 3.5 ton to bump up the efficiency). Installed in a non-vented crawlspace.

From what I picked up online, using rules of thumb, I came up with the following:

We need approx. 200 sq in of filter rack area per ton.

2.5 tons x 200 = 500 sq inches of filter (so probably a 20 x 25 filter).

The free area of a filter is about 50% of the actual size, so I need about 250 sq inches of return duct and 250 sq inches of net return grill area.

Two sources of return air (1) oak return air grills at the risers of the stairs and (2) a panned section of floor joists with an existing 14 x 14 grill (no filter) at one end.

(1) The wood grills at the stair risers have a total free area of 40 sq inches.

(2) The panned joist plenum will have a grill at either end, and a drop to the return air box at the AHU almost directly below an existing 14 x 14 grill witl 156 sq in oc free area per the grill manufacturers website.

Their installer installed about a 3-4' section of 14" flex almost directly under the existing 14 x 14 return air grill. The cross sectional area of a 14" circle is 154 sq in. And the flex has a very sharp bend in it that significantly reduces it's area.

So, it would seem logical that right now the 14" flex is the bottleneck, and I may or may not need to add a second grill at the other end of the panned joists.

I've asked the installer to bring a manometer on Monday to help figure out what we need to do.

The tag on the AHU reads: "Tested at 0.5: W C external static pressure".

When he gets here, what would you suggest that we do?

Exactly where would we test for the pressure in the duct system?

What results would indicate that we are getting enough return air flow?

Should we upsize the piece of flex as a first step, retest, then add another floor grill only if we still are not getting enough return air to the AHU?

Thanks in advance for your help with this.

Bill

[sktn77a]

Watch this:

http://www.bacharach-training.com/ESP100.wmv

Keith

[Bill NC]

Keith,

I'm downloading it now. (I'm on dial-up, so it will take a little while.)

Thanks,

Bill

[dash]

What you came up with for free area of grills is fine.

Duct size from the equipment to the return grille is totally different.

It is calculated using Manual D, from www.acca.org

Size is dependent on the turns, fitting,type grilles,elbows,etc.,etc.,plus the cfms of air flow required.No rules of thumb.

[udarrell]

I wanted to see their presentation , however, I'm on slow dial-up & it would have taken 5 hrs to download the video.

Maybe Bill's dial-up is faster than mine.

If not, here is a link to one of my pages:
http://www.udarrell.com/external_static_pressure_readings.html

There is a link to duct sizing & a little of what Dash mentioned.
------------
Solving for Available Static Pressure (ASP) - When Designing or Redesigning Duct Systems, Finding TEL, Fiction Rate:
Total Equivalent Length (TEL) Find the Total Return length, then find the longest Supply Equivalent Length (EL) by finding the longest measuring duct length, number of EL in the turning elbows, trunk take-offs, boots, etc.

Once you have all the correct Device Pressure Losses (DPLs) on the longest Supply Air run, evaporator coil, diffuser, etc.

Use the manufacturer's nameplate pressure (IWC) or .5" ESP and subtract all airstream device pressure losses in the longest TEL duct run (supply diffuser, damper, wet coil, etc. all available in manual D) from that given value. That will leave the "Available Static Pressure" - ASP for duct & blower design purposes.

You can figure the Total Equivalent Length (TEL) by using the Manual D length additives for the various fittings, then use your Duct Designer ductulator to properly size the duct system to meet the required CFM, velocities, FR as required in respect to the blower's Nameplate ESP, normally, 0.5" and its performance graph in relationship the remaining (ASP) Available Static Pressure.

There are charts available to determine what the total pressure drop will be then when you figure the "Total Equivalent Length" run of the longest Supply duct runs; ALL lengths of duct and ALL fittings trunk duct take-offs, etc., to get the Effective Length (EL) additive to that duct run for the 'Total Effective or (TEL) Total Equivalent Length.'

Subtract the total Device Pressure Losses (DPL) from the AH equipment's "Available Static Pressure," usually .5” ESP because most furnaces are designed for .5" ESP to achieve the desired high speed CFM, if needed, you can always use a lower blower speed.

Then get a total equivalent length of your ductwork most ductulators have this on the back of them. Friction Rate = Available Static Pressure times X’s 100; divided by the TEL, that is the friction rate per 100 ft of SA &/or RA ducting. There is an example linked on rhe page. - Darrell

[Bill NC]

The download of the video does look like it is going to take at least 5 hours.

Udarrell, good site.

The system is a split system heat pump using the factory air filter slot in the AHU. Since the filter is integral to the AHU, do I test just upstream from the AHU and also just downstream from the AHU?

If, for example I get - 0.2" on the return side and + 0.3" on the supply side for a total of 0.5" I'm good, right?

And if I get - 0.4" and + 0.3" for a 0.7" total then either my return duct system and/or my supply duct system is too restrictive, right?

Thanks for the help guys.

Bill

[udarrell]

The download of the video does look like it is going to take at least 5 hours.

Udarrell, good site.


The system is a split system heat pump using the factory air filter slot in the AHU. Since the filter is integral to the AHU, do I test just upstream from the AHU and also just downstream from the AHU?

If possible probe test Return between the filter & the blower; if not you could add the increased negative of the clean type filter U are using. Some rate them at a specified FPM velocity.


If, for example I get - 0.2" on the return side and + 0.3" on the supply side for a total of 0.5" I'm good, right?
(Right.)


And if I get - 0.4" and + 0.3" for a 0.7" total then either my return duct system and/or my supply duct system is too restrictive, right?
The Return would be too restrictive.

Thanks for the help guys.

Bill
We had some lightening here so I shut my PC down; just came back. - Darrell

[beenthere]

The download of the video does look like it is going to take at least 5 hours.

Udarrell, good site.

The system is a split system heat pump using the factory air filter slot in the AHU. Since the filter is integral to the AHU, do I test just upstream from the AHU and also just downstream from the AHU?



Bill

Downstream of air filter.

Its included in the fan chart.

[dash]

Downstream of air filter.

Its included in the fan chart.

True for many, but you have to read the fine print and footnotes of the brands fan data to know for sure.Then there's the adjustment for a wet coil too.

[beachtech]

4TEE3F40 is a V/S blower :) there is some wiggle room there :)

[beenthere]

True for many, but you have to read the fine print and footnotes of the brands fan data to know for sure.Then there's the adjustment for a wet coil too.
The sheet I have for the Trane, says includes air filter and wet coil, no heater.

[Bill NC]

Dash, BeachTech, Beenthere,

Many thanks!

I'll print this thread out for my installer.

Bill

[dash]

The sheet I have for the Trane, says includes air filter and wet coil, no heater.

No doubt,just pointing out all brands are not the same,when it comes to their fan data.

So with this Trane,you need to adjust for the heater used,and if the filters are elsewhere,another adjustment to your tested ESP.

[Bill NC]

If the return air system is changed to increase air flow to the unit, will the charge on the unit have to be checked?

I seem to remember reading someplace that if the static pressure is not within spec that the charge is hard to get right.

Thanks again,

Bill

[beenthere]

That would depend on how low the air flow was.

[beachtech]

The sheet I have for the Trane, says includes air filter and wet coil, no heater.

yup, thats standard for all resi trane sys

[beachtech]

Dash, BeachTech, Beenthere,

Many thanks!

I'll print this thread out for my installer.

Bill

:eek: i bet he will be happy about that :o