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Old 11-29-2008, 11:46 PM   #1
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Can I get more heat?


My existing oil furnace return plenum is smaller than the supply plenum. Rooms furthest away from the furnace are cold most of the time. If I make the return at least the same size or slightly larger, would I have better heat distribution? Would do you think?

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Old 11-30-2008, 06:15 AM   #2
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Can I get more heat?


May or may not help.
Some times just increasing return size, only makes it worse.
You may need both your return and supply ducts altered.

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Old 11-30-2008, 07:18 AM   #3
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your return should be the size of the filter section off the furnace right up into the space...hallway or ceiling grill.might consider catching the rooms that are the coldest with a return duct from each into the nearest return point.Supply temp....see how the temp is from the nearest discharge grill compared to the furthest one away.
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Old 11-30-2008, 11:28 AM   #4
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Our house was built in 1990 and is logs on the 1st floor and a combination of trusses and stick-built for the 2nd floor.
Our cold air returns are in the center of the house, mounted in hallway walls.
The hot air ducts come off (2) main arteries and enter the rooms at the outside walls, mostly in front of windows.
I measured (in degrees fahrenheit) closest (104) to the furnace and furthest away (97) on 1st floor. On the 2nd floor all of the registers are about the same distance, but travel through a un-heated air-space (trusses) and range from warmest (88) to coldest at (75).
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Old 11-30-2008, 11:33 AM   #5
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insulate those pipes running through the cold space.venting should never be on the other side of the barrier!
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Old 11-30-2008, 11:38 AM   #6
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Sounds like poor insulation value on the duct work in the trusses.

If you can run your furnace with the blower door off, and remeasure those air temps(first and second floor), and see if the second floor regs start to even out with closer to the first floor.
If so, you need more return.

If not, you need more then just a bigger supply plenum.
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Old 11-30-2008, 02:29 PM   #7
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beenthere,
I knew that there were problems with the ducts in the un-heated airspace between the 1st & 2nd floors. A number of years ago during a time when I was installing proper-vent in between the trusses on the underside of the roof, while the eaves were open, I notice the ducts were not insulated and basically open to the outside of the house. While everything was opened up, I added 4-feet of insulation into all of the 2nd floor truss ends on both sides of the house. Unfortunately, we ran out of money and I was not able to do the gable ends at that time. Next spring/summer prior to shingling the gable ends of the house I plan to remove the exterior siding, covering over the trusses above the logs and install insulation across them as well; hopefully blocking off the un-heated airspace. The only alternative looks too costly; in pulling down the sheetrock ceilings in all of the rooms and add insulation directly to the ductwork.
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Old 11-30-2008, 02:33 PM   #8
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They may be lined.

If they wouldn't have any insulation, they would sweat when you run the A/C, and destroy your ceilings.
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Old 11-30-2008, 03:44 PM   #9
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beenthere,
I could see a condensation problem, but we don't have A/C. And the reason I was concerned about the return plenum possibly being too small; was that a sheetmetal shop said that I needed to get a larger volume of air back into the furnace. One of their mechanics said if you have 1000 cfm going out you need 1000 cfm coming in. I wasn't sure, so I posted here for some advice.
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Old 11-30-2008, 07:33 PM   #10
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Quote:
Originally Posted by 1parkpointer View Post
beenthere,
I could see a condensation problem, but we don't have A/C. And the reason I was concerned about the return plenum possibly being too small; was that a sheetmetal shop said that I needed to get a larger volume of air back into the furnace. One of their mechanics said if you have 1000 cfm going out you need 1000 cfm coming in. I wasn't sure, so I posted here for some advice.
the flow must be matched to the flow of the furnace .a little back preasure isn't bad but there's a fine line.You don't want too much back preasure it's hard on the fan and the heat exchanger in the furnace!but you need to move heat ducting to the heated side of the vapour barrier aswell!it is hard to heat rooms when the first 30 seconds of your heat cycle blows in the air that has been cooled sitting in pipes in the attic!!
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Old 11-30-2008, 09:11 PM   #11
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This weekend, I looked at a few hot air furnaces. Most of the supply plenum openings were of the 20" x 22" variety. Yet their corresponding returns measured in at 16" x 20". In looking about, I'm learning that the supply plenum size in area is usually made larger than the return air plenum, basically to allow output routes to the many rooms that the hot air system ultimately supplies heated air to.
Regarding the flow, is there a way to calculate the flow in cfm that is created by the circulator blower which sits in or very close to the return plenum, verses the flow which is pushed out or eminates from the supply plenum by the very same blower? Obviously, the output velocity is less than the input of cold air returning if the openings are different. I still question the size necessary for proper air flow.
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Old 11-30-2008, 09:34 PM   #12
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Can I get more heat?


Quote:
Originally Posted by 1parkpointer View Post
This weekend, I looked at a few hot air furnaces. Most of the supply plenum openings were of the 20" x 22" variety. Yet their corresponding returns measured in at 16" x 20". In looking about, I'm learning that the supply plenum size in area is usually made larger than the return air plenum, basically to allow output routes to the many rooms that the hot air system ultimately supplies heated air to.
Regarding the flow, is there a way to calculate the flow in cfm that is created by the circulator blower which sits in or very close to the return plenum, verses the flow which is pushed out or eminates from the supply plenum by the very same blower? Obviously, the output velocity is less than the input of cold air returning if the openings are different. I still question the size necessary for proper air flow.
you should have a tech come and do a test with a gauge they will make one hole in the return and another in the preasure side of the furnace that will tell you once and for all if the plenum is too small or return too small!
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Old 11-30-2008, 10:01 PM   #13
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orangeguy,
Sounds good! I'll get one to check them out. Thanks!
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Old 11-30-2008, 10:15 PM   #14
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You would need a flow hood, or manometer to determine CFM.
A measure of the static pressure of both ducts, tells you which one or if both are undersized.

Its possible to make your own inclined manometer. But, its hard to make it as easy to read as it should be.

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