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  1. #1
    [See UPDATE on Manual J results, page 2 of thread.]

    I am in a 2000 sq. foot, three story townhouse in Boston with a 19-year old Lennox hot air furnace, with 137,000 BTUs. The a/c is 5 ton. Thinking it is near the end of its life, I have started shopping for replacements. From reading this board, I am leaning toward a Rheem modulating furnace, but would consider a Trane or American Standard. Being a cheapstake, I only heat the place to 60 degrees day and night in the winter, and only use the a/c five days a year. (I use window a/c's instead a lot.)

    The best I can tell, the sheet metal ductwork was improperly routed and sized when this place was built, and realistically is NOT changeable because it is all contained behind walls and ceilings.

    From the top of the furnace, the two primary metal ducts going out are 18x12, and 18x9, with one flexible soft round duct, maybe 8". A few outlying registers give little or no output. The two main trunks are zoned ass-backwards with two dampers: front half of the house and back half, rather than by floor, so I basically keep them open wide.

    The air returns, from what I can tell are grossly undersized. As it enters the furnace, the sheetmetal return duct is 16"x18". Working backwards, that leads up the three stories, with a few bends. With internal insulation, the actual internal dimension 12" x 17.5".

    Given these issues, I have a few questions:

    (1) Is the Rheem modulating furnace (RGFD) a good or bad idea? Those slower fan speeds may suck in too little air, no? Or, is it because it can self-adjust, it will be smart enough to run higher fan speed to suck in more air?

    (2) Given my ducting problems, realistically NOT changeable without tearing up the walls of the whole house, do you have any other general recommendations?

    (3) One installer said I should stick to the 5 ton a/c (which I think more right-sized should be 3.5 to 4 tons), because a smaller unit won't have the power for those large outgoing ducts. That doesn't make sense to me. Does it to you?

    (4) From a few phone calls, and the one installer who visited so far, I get the sense that NO ONE will do a real heat load/loss calculation. They see what the ducting is, see how large the old unit is, speak to the distributor, and come up with a size. What do I realistically do, and how should my duct problems be factored into figuring out what the right size units are?

    Thanks in advance.

    Edgar

    [Edited by mrconsumer on 08-25-2006 at 07:22 AM]

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,158
    Somebody better do it right. 5 tons for a 2000 sq ft townhouse sounds about twice what a place in Boston would need!!!!!

    Variable speed blowers don't like to be choked for air. The Mod will flash an error code if it tries to move more air than it can. You can cause motor or control damage too. Roughly calculating, your return is good for about 3 tons of air.

    We don't have a contractor locator on here so I'll risk wrath and refer you to http://www.heatinghelp.com where they do. Those guys are boiler experts from the northeast but I bet you'll find someone good to handle your forced air situation.

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    mrconsumer,

    Have a Pro ,that can do the Manual J,S,and D calculations (hard to find ,keep calling till you do),run the calculations to size it correctly.


    You may be able to add insulation,to reduce the size of the system that will be needed.Improvements to windows,doors etc ., can reduce the load as well,since your duct system is undersized,this would be the best way to go.

  4. #4
    >>You may be able to add insulation,to reduce the size of the system <

    Dash,

    The construction of the townhouse is with 2x4's in the outer walls, so they are not even 6" thick. There is insulation in the walls, and none can be added.

    The windows are thermopane, and it is not realistic to say I am going to change windows.

    Given the conditions I have, I really need to know the general type of furnace that will work best given an undersized air in-take.

    I am sure some are better or worse than others. (With my current Lennox, I have had to keep the front metal door of the furnace open slightly to let in extra air, to keep the limit switch from tripping every 10 minutes.

    Edgar

  5. #5
    Join Date
    Aug 2002
    Location
    Southold.calm
    Posts
    5,685
    You need to size the equipment with Manual J
    Select the equipment with Manual S
    And size the ducts for the above with Manual D

    If you want to size your equipment to the duct size and thats not correct for the building, then what do you give up? Your comfort. And thats what your paying for isnt it?

  6. #6
    >>And size the ducts for the above with Manual D
    >>

    Pegmsg...

    I can't say it any more clearly: my ducts are buried behind walls and in between floors and ceilings. I cannot realistically rip out all the walls of my condo to change the ducting.

    I have to live with this ducting, so the question is what to replace my current equipment with given the constraint of the existing ducting.

    Edgar

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    First,have the load calculation done to see what size furnace and A/C you Really need ,yours sonuds oversized.A smaller system will work better on yur ducts,if it can do the job.

    Or do the load calc yourself,on this site right here,

    http://hvaccomputer.com/talkref.asp

    for a small fee.

    You should have a pro test the return and supply static ,this can be used to determine what the static of a smaller system will be,and give insight as to what mosifications can be done and what they would do.

    There may be duct improvements that can be made,where the furnace connects to the supply and return duct,turning vanes would help a lot.Increasing return grille and filter size would also help.

    Posting photos of what is accessable would help us help you.




    Not a good idea,but if you want to replace it without looking into the improvements that you need ,stay away from VS indoor fans ,as the can self destruct on severely undersized ducts.

    Find a furnace ,thru a Pro with the hisghest ESP(static)at 5 tons,one brand may be a little better then another,so check around.

  8. #8
    Join Date
    Aug 2006
    Location
    High Point, NC
    Posts
    155
    Have someone size the system with manual J and find out what you need. have them evaluate the ductwork.With a manufal J calculation to determine what you need, you might need less. They can evaluate your ducting based on what you need. Maybe the ducting will be OK if you need less. All we can do is specualte on your duct size since we do not know the size of equipment that you actually need.

  9. #9
    Join Date
    Nov 2000
    Location
    Memphis TN USA
    Posts
    6,945
    You might want to consider 2 small systems.

    But get an idea of how much capacity you need before doing all the work.
    If the superheat ain't right it ain't charged right.

  10. #10
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,158
    If you cycled on limit before, you WILL have trouble with a new furnace unless properly sized and the ductwork can support it. The old relics could run much hotter so if the old one needed the door cracked (very dangerous BTW) to keep off limit, you are guaranteed to limit out on a new one of similar size. AND today's limits which are $3 snap-discs at the end of a couple of posts, will not tolerate opening & closing like the old Honeywell L4064 did. A few times open and she'll stick open leaving you without heat. And today's lightweight heat exchangers don't take kindly to running hot either.

    We're convinced, ducts aren't changing. So get your load done, compare that to duct capability. If you need more equipment than the ducts can handle. maybe 2 systems will be the right way. But I highly doubt you need anywhere near 5 tons of cooling. Not knowing all about the place, I'd guess that around here 2000 sq ft townhouse, especially with no outside exposure on left & right, would cool on 2.5 tons and heat with 75K 80% furnace at most. Many stand-alone homes that size get by with that equipement. Bigger is NOT better. We size to 95 summer and -10 winter.

  11. #11
    Thank you all for continuing comments and suggestions.

    I just had a company here and insisted they do a Manual J... and he did. (Will figure results at the office, he said.)

    Someone wanted pictures to help in making suggestions.

    Here they are:

    1. Furnace in closet in laundry room on lowest living level:

    PICTURE NOTE: forum software seems not to bring you to the actual picture, but rather to my homepage. Place cut and paste the URL to view pix.

    http://www.consumerworld.org/gifs/furnace1.jpg

    2. Furnace with hot water heater showing (note outgoing duct splits into two (9x18", 12x18" and unseen 8" flexible):
    http://www.consumerworld.org/gifs/furnace2.jpg

    3. Return showing on the right:
    http://www.consumerworld.org/gifs/furnace3.jpg

    4. Return as it goes up into ceiling:
    http://www.consumerworld.org/gifs/furnace4.jpg

    Edgar

    [Edited by mrconsumer on 08-24-2006 at 01:21 PM]

  12. #12
    Join Date
    Jul 2006
    Location
    Lancaster,Ohio
    Posts
    464
    [QUOTE]Originally posted by mrconsumer



    I can't say it any more clearly: my ducts are buried behind walls and in between floors and ceilings. I cannot realistically rip out all the walls of my condo to change the ducting.

    >>>>> If you had a broken water pipe in the wall would you live with that too? Chances are you would NOT have to "rip out ALL your walls or floors to fix the problem"!

    I have to live with this ducting, so the question is what to replace my current equipment with given the constraint of the existing ducting.

    >>>>>Do don't HAVE TO live with that ducting! I don't know of any equipment manufacturer that designs their equipment for resrticted air flow...but rather restricted air flow will void every warrenty I ever read!
    IcyFlame

  13. #13
    Join Date
    May 2006
    Location
    Philadelphia, PA
    Posts
    1,391
    Are you an end townhouse or middle?

    If you are in the middle I would think your ductwork may be closer to what you need than you think. The equipment could be oversized for your home and climate.

    If you really had to insist the Man J be done I will suggest you do it yourself. It is $50 well spent and not real difficult to do especially since you are in the house.

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