Replies

1. 
   goldhiller | Jan 31, 2005 06:48pm | #1

   Define "extended", if you would.

   How many more feet of run, what size, any elbows, etc.?

   Did you yet try rebalancing the system?

   Knowledge is power, but only if applied in a timely fashion.
1. 
   homebaseboston | Feb 01, 2005 02:25am | #2

   Hi Gold,  Yea we added about 20 feet of 8X10 (or 8X12) of rectangular duct with 4 branches running off the top into the joist bays. (a 90 off the original system, then a 4 foot run with another 90 to go the rest of the way.  Long house with a few jogs).  The branches are only about 4-6 feet.  We used flexi for the branches, which I may be regretting soon if CFM is the issue.

   I've tinkered a little with the dampers on the orignal lines, but not much, for rebalancing.  Don't want to rob Peter to pay Paul, ideally. 

   There is a 90 degree elbow, and I was disappointed that the installer didn't use a nice raduis 90 (sweep, maybe?), but an angled 90.  (IOW, it hits a 45 degree angle at the corner).  Seems to me that every little bit would help with smooth air flow.  In fact, he retrofitted an oversized 90 at that! 

   I thought I recalled the installer saying that 1500 CFM might be undersized for the square footage, but he's a young guy so I don't know how much he really knows, and I only vaguely recall the conversation.  I got the impression we may be stretching the system to the limits of the motor's capacity.

   Appreciate any thoughts.  It's really my own fault... took my eye off the ball on this one.

   Brian

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   Edited 1/31/2005 6:29 pm ET by homebaseboston

1. 
   goldhiller | Feb 01, 2005 05:36am | #5

   Since you'e on an oil furnace, there's a chance that you don't have the largest nozzle installed that the unit can handle. Only you or your furnace guy would know that. But if you can up the nozzle size and the fan speed as well, you can get some more heat out of the thing. (If you increase nozzle size, you'll also have to readjust combustion air and over-fire draft........and the anticipator setting onthe thermotat.) Bear in mind this can only work well/efficiently if your return ductwork is large enough to supply the request for more air from the increased blower speed. Ideally, you get to capture all that extra heat that you're entitled to. If not, what you don't grab heads up the flue. Then again, if your guy thinks the motor is hauling on it's knees now, increased blower speeds might be out of the question........and it might also mean that the largest nozzle it can handle is in there already. Or maybe the return ductwork is a bit undersized and this is restricting the blowers ability to move enough air. Impossible to say from here. As far as robbing Peter to pay Paul.........that's exactly what balancing a system is all about. But from the sounds of this new ductwork, you might have a hard time ever getting enough coin in that particular pocket...with the current new setup. You don't have anything to lose by trying, eh? Pay particular attention to how much air is delivered to the area/room where the thermostat resides or that'll get satisfied before the other rooms get their fair share.Knowledge is power, but only if applied in a timely fashion.

2. 
   TRice | Feb 01, 2005 02:35am | #3

   Typically air handling equipment, including furnace are rated with an air flow (CFM) and a static pressure (usually expressed in "inches of water column" or just in.)

   Furnaces and all other types of equipment in this realm are sized on capacity (in btu/h or mbh, which is 1000's btu/hr), not CFM. Your furnce should have a rating plate inside the cabinet that will describe the rated input in btu/hr and the rated output. It will also have some air flow/rpressure and electrical/motor data as well.

   Based on what you've posted here, your best bet is to ask the advice of a local professional heating and air conditioning contractor. Way to many unknowns to try to give any useful advice here.

3. 
   Piffin | Feb 01, 2005 02:49am | #4

   I'm not a furnace or heating guy, but I have picked up an ounce or two of info along the way.

   And i love working puzzles. It's almost as much fun as the algebra things on the SAT tests...

   first, you are not heating any square feet. You are heating cubic feet of space - or attempting to do so.

   If you have about 2400 sq ft, then I will assume that you have nine foot high cielings since you are in Beantown, giving you 21600 cu ft to heat.

   The CFM is not a measure of the heat your furnace can produce. That would be measured in BTUs ( not to be confused with Beantown TUrnsignals, delivered with the center digit of an upraised fist) the CFM is the number of cubic feet of air the fan can move in a minute, in a perfect scenario. Add restrictions, ducting, elbows, etc, and the actual delivered CFM at the outlets can be half the rated CFM in practical terms.

   So I take your 21600 cu ft of air and move it across the plenum at an effective rate of say 800CFM, and you see that you have spent 27 minutes to have theoretically circulated teh air in your home.

   Depending on how much heat you are losing at the house skins and how effective the exchange at the plenum is, I can imagine why you feel like you are not getting enough out of it.

   Temporarily doing things like switching the fan to run 24/7 instead of automatic, and removeing the inline filter restrictions might help a wee bit. Make sure the ducting is clean inside to eliminate that restriction, and check to see if there is any way you can eliminate elbows in the lines. Flow is reduced with distance and elbows.

    

    

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1. 
   homebaseboston | Feb 01, 2005 06:45am | #6

   Aw, Pif, you've got us all wrong down here.  We don't even use the *middle finger* signals any more!! :) 

   Contrary to popular opinion, the real problem with Boston drivers isn't actually the miserable human beings we grow around here.  It's kind of a "lord of the flies" thing.  I'm a transplant from small-town CT, actually, and the truth is Boston has no street signs, confusing narrow roads and very few road directions at intersections/lights etc that actually tell people wear the heck to line up.  It's amazing.  Let's face it... absent some kind of actual "instruction" as to what the "right thing to do is" in a situation, even the mildest of our locals down here revert to "what's right for me to do" and it becomes sort of a downward spiral.  Of course... our yankee temperments probably don't help matters much, either!

   Thanks for the info... that's what I was getting after.  I'm pretty sure the furnace is cranking enough raw heat, but based on even the theoretical #'s you mentioned, I suspect I might be pushing the motor too hard.  It is a really looooonnngg run.  The total run is probably 40 feet, give or take, from the Plenum if you include the 20 odd feet we added.  The short run at the other side of the house is getting plenty of air flow, so it looks like -- absent an overhaul -- we'll have to adjust the dampers the best we can for now.  Thanks for the tip on that Gold... I was thinking the same thing... maybe trick the furnace into staying on a tad longer.

   Hey... the good thing about being in Boston right now is the the BuildersTrade is coming to town.  Should be a fun show.

   Thanks guys.

   Brian

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   Edited 1/31/2005 10:46 pm ET by homebaseboston