Questions about return

I have a 3.5 ton unit in a 1500 sq ft under air home. Insulation is not the best (R13 ish with almost 20% of the home being under a 'flat' roof), and it's located in S. Florida. It's a bit oversized still I believe.

What appears to have happened was this unit was upgraded with a new addition to the home (275 sq ft room with the flat roof), from a 2.5 ton prior.

It's a split system with the air handler right on top of the old return.

The return takes a 18.5 x 18.5" filter and has a 14" duct going about 24 inches to the air handler. The air handler sits on one part of the duct knocking is down to about a 12" opening. I can't raise the unit higher than it is.

Cooling is good and the return is not too loud. My a/c runs 7-9 hours a day at 6am-8am 78, ->4pm 85, -> 12am 78, ->6am at 82 degrees.

Still I am thinking if I went to a 18-20" duct and expanded the return size to 18.5 x 24 or so it would improve cooling a bit. By expanding the return/air box I think I could get the duct from being right under the unit. At the same time it will fit between the existing beams and should be a pretty transparent install into my ceiling.

My second repair on this will be to put a second return in the office adjacent to the living room with it's own filter.

Is there any good how-to's on building a return box/plenium?

I am pretty handy and this all came up when an A/C guy I hired to fix a dripping pan brought up everything as a problem except fixing it. I was able to go in the attic, pull the side panel off the unit and see a coil to drain pan hose visibly with a clump of algae in the bottom of it. I popped it off and ton of water came out. The pan was nice and clean.

Thanks
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Welcome Alkemyst:
No, the unit is not oversized. Considering your insulation you would do a slow roast with any less.
Any sheet metal shop will make you a return air box.
Your A/C man sounds like one of those who says "it just needed a little gas".
Apperantly you were watching him though and he couldn't use that little white lie. Did he charge you for the visit?
Glenn

Actually when I called they told me for their $79 'tune up' they'd clear the line. I had said I didn't think it was the line as I had flushed it / vacuumed it pretty well.

Guy gets there revacuums it, tells me even though I have a good angle to the drain line that it was too long. I had extended it about 18' to get it off the concrete pad...it drains very well though, I was watching it once I went up in the attic the next day. I poured some water down it with a little bleach in it.

He then checks the freon outside with a guage that was not reading right. I didn't get a good feeling on this, esp when he told me it hasn't been reading right for weeks. He ended up taking the cover off the gauge to get a 'better reading'.

He said I was overcharged and that's why it was dripping. He removed freon. Later he took another reading and it was too low so he added what he took out back. He claims it was still off but he overcompensated.

He then told me he couldn't do the $79 tune up since the unit was faulty and he'd have to charge $90/hr for the time to repair / troubleshoot.

He went inside and I had a good temp. differential...then he identified the duct/return as too small and told me the static pressure was holding the water in the unit. He identified the handler was on the duct further causing a restriction (he estimated I only had a good 12" of duct opening with the restriction). He also told me my house was 75 degrees when the thermostat was reading 79. That made sense as we keep it at 78 and it felt a lot colder.

During this I stated I hadn't had any problems in the 4 months I have been here and now that its cooled down a bit the unit has been running less. He said it was building up until now.

He went in the attic said he drained the pan...problem is that pan was not really a pan but just the support for the handler to the rafters. He also adjusted the pitch.

Told me there was nothing he could do and the unit needed replacement ($2600 roughly), a new return (about $300) and to figure about $3600 with taxes. I wouldn't qualify for a rebate since the outside unit would not be replaced.

It was still dripping when he left.

I got up in the morning and figured it out...he didn't even open the unit.

They charged me $150...I have left 3 messages since and got one follow up call from a secretary that said a manager would call me back by yesterday.

The current return box is made of fiberglass duct material in the ceiling, along with the plenium to the handler.

Temps have been really good though since I changed the thermostat and I have averaged about 5 hours of cooling a day with temps in the mid to low 80's.

I didn't think of a sheetmetal shop...I know of a couple downtown. I will call and get an estimate...do I have them put the 'hole' in it for the duct, or do I cut it to fit? I am debating whether to do a 20" duct making it a little oblong to fit the 19" opening or just a 18" and keep it round. It's a short run basically a 180 degree bend, maybe 5' of ducting to keep it nice and smooth.

Would the plenium also be sheet metal (it's also currently fiberglass duct board too). It's only taped and not mastic'd either...I have read you should use mastic also.

Hello Alkemyst:
I would want to get rid of the fiberglass ductboard and any flex duct. The fiberglass ductboard flakes off inside the system and blows out to go into your pulmonary system; it causes lung cancer just as sure as asbestos does. The flex duct has wire in it to keep it open. The air has to hop over each winding of wire and wears itself out after about 3'.
I had a 35' run of flex duct once that would'nt move the boss' cigar smoke at the wall register; changed it to galvanized pipe with insulation sleeves on it and it blew out a candle 3' from the wall register.
The 18" round should be supplemented with another return duct which most sites need from another room.
Glenn