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Old 07-31-2010, 09:19 PM   #1
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Duct length - is there a "limit"?


Here is the scenario - Mater Bedroom cannot be kept cool. I have a two ton comfortmaker (2007) unit feeding MBR, master bath, regual bath, and one other bedroom - total ~750sq ft. Air handler is FSU2X2400A, 13 SEER. I upgraded the fan to 1/2 HP. Now, I already know duct is a problem. For supply, they ran 50' 12" to a distribution box. It has feeds to second bedroom and 2nd bathe which are adequate. However, for MBR, they tap off this with 40' 10" flex, including a 90 degree turn to another distribution box, with 25' 6" flex into MBR, and 35' 7" flex to MBR. Return is 50' 14" flex. Now, it is obvious the problem, to start, 12" flex is not enough, let alone all the 10" and total lenght to MBR. Now, I have had many opinions, but it boils to two: 1st says no matter if you increase duct, still too far, move unit, got a fair price (at least I thought) of $2100 to relocate and tie in new duct, put unit in attic above MBR, 2nd opinion says they have seen even longer lenths of duct, I just need to cap off the 10" to the MBR and run another 12" strait to the distribution box in MBR. Note I do live in Florida, but just had attic faom sealed, so I don't have hardly any heat loss from flex. (although being from up north, I believe the inventer of flex should be shot!)

So, I know the unit isn't even running at full capacity due to limitations of duct. Am I better off relocating the unit for $2100, or will spening about $400 in adding a 2nd 12" work almost as well?

I would not hesitate, but I have to replace my 1991 Trane unit, which experts on this board helped me get going in another post. I am going witha 4 ton 2 stage to replace the old 3 ton trane (I have a spralling 3000' ranch), just had foam done, pockets are getting empty fast!

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Old 08-01-2010, 06:32 AM   #2
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Duct length - is there a "limit"?


Your duct system is undersized. Thats all.

Your 14" flex return is too small. I don't care what someone tells you a duculator says. Its too small.
So as you said. The 12" supply flex from the unit is too small then also.

While increasing the 10" to a 12" will help. It probably won't help much.

How big is your master bedroom?

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Old 08-01-2010, 06:37 AM   #3
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Duct length - is there a "limit"?


Also. On your other unit. Going from a 3 ton to a 4 ton. Requires a lot of duct moderation. Your current duct system won't be large enough a 4 ton unit. Are they adding supplies and returns when they install it?

Did the 3 ton have a lot of trouble maintaining set temp, that your increasing size by 33%? In hot humid Florida. Oversizing can make you very unhappy with the high humidity it can and usually does cause.

I hope the 2 stage Trane your getting is the 20i, and not the 16i. The 16i is not known as a good unit to keep humidity low. It rather poor at it. And when oversized. It can't be made to work any better.
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Old 08-01-2010, 10:18 AM   #4
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Duct length - is there a "limit"?


I may have put too much info - so, as I thought, it is also starved on the return. I thought so, when I pull the cover, I can feel the resistance from the suction. MBR is 13 X 22. Because they tapped the master bath right off first junction, it is fine, as well as other other bath and bedroom. It's also the largest room with most sun exposure.

What I may not have made clear, I was going to run a new 12" directly from the unit to the supply in the MBR. That would then mean I would have two 12" supplies from the unit. Does't help the 14" 50' return issue.

I am not going with the trane, heard about the isues, have to check with contractor what MFR for 4 ton he is installing, but I believe tempstar?

Professional energy rater, with real heat load calc's, (I havelots of glass) said that with the foam, I could get close to a total of just over 5 ton for entire house. Now, foam was not installed 100% correct, major issues with that contractor, but 95% what it should be (after his 3rd time out braking wires and ducts I won't lethim in the house, we negotiated a significatnly lower cost due to issues - long story there, but I would advise ANYONE in the general orlando area to stay away from foam, the jokers I had out (about 10 different bids) had no clue -spray and pray)

Back to the issue: I went with the 4 ton as the 3 ton does not keep up. It coud be definately be age, it's in bad shape. So, I am covering my bets by getting a 4 ton dual stage (2 ton on low, 4 ton on high) That way, if I really need it, I do have 4 ton, but if not, the foam working as it should, I only run on 2 ton. Yes, I have found a real contractor, and he is upgrading the ducts to handle the 4 ton.

So, I had 5 ton originally not properly installed, g0ing to 4/6 ton properly installed.

On the duct side, based on what you said, if I had one 12" directly to MBR distribution box, second 12" to cover other rooms (already installed), would that be enough supply? Then I would have to upgrade return to 16".

I may not have fully described return, but I went with most limiting element. The return to the unit is 14" to MBR, then has a tap of 12" to 2nd bedroom. But still, 50' 14" back to unit is limiting element, as everything flows back thru the 14", no matter how many taps.

I am staring to believe moving the unit would fix all the issues. Unit would be tied in right above MBR with 12" supply to MBR, and the return would then have a 14" return from MBR, 12" return from 2nd bedroom, both tied into unit in "parallel" rather than in series as I have it now.

Let me know your thoughts, they have been right on so far. Also, for what it's worth, we like a cool house (73).

I thank you very much from a previous post, I did jumper out the RV, and I disconnected the heat strips just in case.
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Old 08-01-2010, 10:52 AM   #5
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Duct length - is there a "limit"?


How much air is being supplied to that second bedroom now? A 12" return to it can't bring back anymore then is being supplied to that room.

A separate 12" flex to feed your master bedroom will get you around 400 CFM. If you branch it off to atleast 4-7" or 3-8". Or add an 8" to your existing configuration.

A 4 ton 2 stage Tempstar is between 2.8 and 3.2 tons in first stage. Might not remove enough humidity.

What is your humidity with your current 3 ton?

Your current 3 ton may only need the coils cleaned. Or could be low on charge, and need the coils cleaned.

And running the returns back separately is a good idea.
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Old 08-01-2010, 12:47 PM   #6
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Duct length - is there a "limit"?


thanks for the info

Humidty is much better now that I have the foam, don't have anything to measure, but can tell the comfort level.

No, it does not maintain temp as it is.

The 1991 Trane is killing me in $$. I have a baseline in this heat of 150kW/day. When it was down for two days to figure out issue, my usage dropped to 80KW/day. The unit is shot, I cleaned coils a year or so ago, went to do it again, the "fins" in the air handler actually fall off when touched, bad, bad shape. Previous owners each smoked 3 packs a day for the ~15years in the house, nicotine is incredible. The air handler is covered, even coil cleaner doesnt help. As an example, I tried refinishing cabinets, paint stripper wont take it off. So, it is really hard to tell if I am getting the full 3 ton. I highly doubt it based on visual coil inspection. That means I am left with a true heat load calc that was done. With all my glass, and without foam, it showed I need 6.5 ton (I have one room for business with 3 servers running) With foam, heat load came out to a total of 5.15 Tons, again, this was a pro that captured everything, I have a 19 page report with each rooms heat load. Thus my paranoia on the 4 ton dual stage. I will double check, as you hit it correct, better to have unit on low stage to remove humidity rather than not running. I will definately follow up with contractor as I only want 2 ton in low stage.

I also just go home from ~10 busines trip, so I am back to monitoring my usage, but based on 18hrs of data, I think you got it on the trane, it was going into defrost with heat strips. If I could get away with not replacing it, believe me, I would. I may have to relocate in 3 years, so I am running payback numbers the best I can.

Based on what I know:

3 ton is trashed, most impact on electric bill

2 ton is probably only running at 1 ton due to duct

There are just so many "wrongs" in this system, very hard to determine what causes are. Had I known a month ago what I know now, I would not have done the foam.

Any inexpensive two stage systems you reccomend? Again, I am hoping to stay here, but have to plan on the 3 year timeframe.

Replacing the 3 ton is a "must" due to it's condition, but I do want no more than 2 ton on low stage as I am not sure what the foam impact will be on totals.

Moving the 2 ton is a comfort issue
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Old 08-01-2010, 01:16 PM   #7
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Duct length - is there a "limit"?


Foaming. Greatly reduces load.

If the attic is foamed that it is a conditioned space. You knocked 1/2 ton off from your previous load, minimum. Possibly more, because you also reduced your infiltration, which was a big part of your total heat gain load.

What infiltration did they use for your house without the foam. And what is it now with the foam?

What design temps did they use? Often, contractors will add a safety top their calc, specially if you /the home owner says the old X ton system doesn't cool well. They will tend to increased the outdoor design temp to make it come out with a higher load, so they can show you that a bigger unit is needed.

The only 2 stage that is 4 ton in second stage, and 2 ton in first stage, is the Tran 20i. And it is one of the most expensive units there is. However, with the tax credit, it takes some of the sting out. And if your utility company offers rebates for high efficiency, it can come down the price of a high end single stage unit.

With your evap coil as bad as you said it is. When it loses temp in the house. Is it losing temp by just 4 or 6, or is it losing temp by 10 or more?
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Old 08-01-2010, 03:07 PM   #8
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Duct length - is there a "limit"?


Originnaly air infiltration was 4000 cfm, with foam down to 2750. Foam not 100%, but I have single pane windows 12' X 12' single pane sliding glass doors, standard set sliding glass doors ect

It 96 degree outside right now, main area maintains 78 degrees, spare bedrooms about 80 degrees, so it loses about 4 degrees at peak.

They used 93 degrees as basis. They did calcs with and without foam, 6.5/5.15.

There such a thing as a 3 1/2 ton dual stage, i get where you are headed, If it is tolerable with the unit in the shape it is (bad) I may only be getting 2 ton or so out of it
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Old 08-01-2010, 04:10 PM   #9
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Duct length - is there a "limit"?


4 at peak, with fins missing from the evap coil. Sounds like 3 tons is pretty much sized right.

No 3.5 ton 2 stage. Trane has the 15i. Its a single stage. Matched with a VS blower indoor unit. It can reach 16 SEER and 13 EER, and qualify for the tax credit. It has better humidity removal ability then Tranes 16i 2 stage.

You might want to get a hydrotherm. And measure your indoor humidity.

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