New 7” Higher Air Handler: Give Up Electrostatic Filter Or Reduce Return Plenum?

[jjnet]

Because of a coil capacity mismatch between our Trane XL19i two compressor heat pump and the supposedly matched, but actually too small, variable speed air handler which we were sold by our original contractor, we’re having a new contractor replace our TWE037 with a TWE040. The new one is 2 ½” wider (26” instead of 23 ½”) and 7” higher (52” rather than 45”). The supply plenum is already the full 26” width and can be easily modified to fit the new air handler by removing a 2 ½” by 21” cover plate that previously overhung the top of the air handler.

But the supply plenum is literally bumping into some beams and cannot be moved any higher. We have a Trane electrostatic air filter below the air handler which coincidentally takes up exactly 7 vertical inches. We can fit the new air handler vertically either of two ways: (1) ditch the 7” high filter and switch to a high-quality 1” high filter, or (2) reduce the return plenum height between floor and filter from 14” to 7”.

The air handler is installed vertically with bottom input and top output. The return plenum comes down on the left side of the air handler and is 16” wide by 21” deep where it makes the 180 degree turn to come up under the filter frame. Trane’s install manual for these air handlers recommends 14” of vertical plenum space below the air handler. We are trying to determine whether it is reasonable to modify the bottom of the return plenum so that it is only 7” high where it briefly turns horizontal before expanding upward again to attach to the filter. If the filter’s height can be considered part of the 14” clearance, then it might still meet Trane’s spec.

Regardless of whether it’s considered within Trane’s spec, this means that the return plenum would have a very brief constriction (as it makes its 180 degree turn) from 16” down to 7” and then back up to 26” at the filter (all by 21” deep). Manual D’s equivalent length tables suggest that a brief constriction adds only a small amount of equivalent length. But since this is a constriction combined with the 180 degree turn, the combination might be more costly in terms of airflow reduction. When you add in the static pressure drop across the electrostatic filter, the effect of the constriction could be even more severe.

We are trying to find someone at Trane who can provide us with the pros and cons of the two choices, but so far we have not succeeded. Everyone we’ve spoken to so far has admitted that they just don’t know. In our area, virtually no one has a lot of direct experience with heat pump’s in general and Trane’s XL19i in particular. Ditto with their variable speed air handlers, and zoning setup. Any input or suggestions on who to contact at Trane would be very much appreciated.

[dash]

"The return plenum is 16"X21" and comes down the left side of the air handler.."


Can you install the air cleaner in the return plenum,with a little sheet metal work??


I wouldn't lower it to the 7" you mentioned,too restrictive and air won't flow evenly thru the filter after that turn.

[trane]

I would recommend the filter be moved to the return duct to the side of the air handler if room permits. If there is not room to do this my choice would be the 1 inch filter and have the ESP checked before using the pleated filters.

[trane]

Sorry Dash, you type faster.

[dash]

Originally posted by trane
Sorry Dash, you type faster.

That can't be ,I just started typing sooner.

[kim]

How far is the return from the side of the furnace?
If you have enough room, you could put the filter on that side.

[jjnet]

Moving the filter to the vertical part of the return plenum will work. I just measured the space and we have plenty of room to add transitions between the filter and the existing plenum shaft. We will have to incorporate a stand or support of some sort to take the weight of the filter off the sheet metal of the plenum. The plenum faces have the normal diagonal creases for some rigidity under differing pressures, but I don’t think that’s near enough to support the heavy frame and cells of the electrostatic filter.

A bonus is that, with the filter up a bit higher off the floor, it will be easier to access its components – no more dropping to the knees to remove and insert filter components. Also, we might be able to use our current electrostatic filter rather than replace it with a larger one to match the larger base of the new air handler.

Does anyone have an opinion on whether, to maximize airflow through the filter, we should replace it with the larger one anyway? We have space for a 26” wide version of the filter, so we can obtain a larger one if it looks like we need to. Also, Trane also has a new Clean Effects filter which supposedly makes our electrostatic filter obsolete. “New technology” eliminates the zapping and air flow sensor issues with the older design, plus does a better job of filtering as well. Trane’s web site compares a “clean air delivery rate” of 1200 for Clean Effects vs. 660 for the electrostatic filters. If this reflects a significant static pressure improvement, it may make sense to get the newer filter anyway.

Thanks much to Trane and Dash for this solution. I know that, for experienced installers, this kind of thing is very simple, but no one I’ve spoken to at Trane so far had suggested this approach.

[jjnet]

Thanks also to Kim, who snuck in there while I was posting my reply.

[dash]

Spunds like you have an Electronic rather then Electrostic,which is a good thng.just FYI before someone starts on all the problems asociated with Electrostatic.

A larger version of your filter will reduce the ESP(External Static Pressure),which is a good thing.

Per our Trane Rep the Clean Effects,is not a low pressure drop ,at least not in 4 or 5 tons.


They should test the ESP as it is now, and decide on the best filter, considering the presssure drop.

If the ESP is high they can install "turning vanes" where the return drop(plenum) makes the 90° under the air handler and where it makes another 90° back up.This will reduce the Equivalent feet of duct by 140' ,may be enough to allow Clean Effects .They may have to look at other options as well,this was just an easy to "see" suggestion.

[jjnet]

Dash, we do have the electrostatic filter with high static pressure drop. We also have turning vanes in the return plenum. Our original contractor bid us a price on the plenums according to our plans which included the turning vanes. Then he made them up without the vanes and refused to add them. I put them in myself, because the Trane installer’s manual for the filter indicated it was important to spread the airflow around the whole filter, and also because of the dramatic reduction in equivalent length for the 180 turn.

If there's no significant pressure drop advantage of the Clean Effects, we may just keep the one we've got. I have to verify the exact size of the next larger filter frame from Trane. I've been assuming it's around 26", but the one we have is 2" smaller than our current air handler, so the next one up may be only 24". If that's available, we may decide to get it in Clean Effects version.

[trane]

I would not buy the clean effects filter yet. They are new and there are likely changes that will be made as problems present themselves.

[docholiday]

can we get a picture?

[jjnet]

Photo of utility room. Sorry for the imperfect merge borders and angled perspective. The room is only 4 1/2 feet wide, so I couldn't get a straight on shot that would show much of the equipment. The image is small and low-res for instant viewing. (Lots of us are still on low-speed connections.)

The return plenum is covered in Reflectix insulation held one inch off the metal surface by spacers all around. Below the air handler you can see the electrostatic filter. Above the air handler you see some of the takeoffs on the front of the supply plenum. All supply surfaces (except the air handler itself) are covered with 2" of foil-backed fiberglass insulation.