Ductwork and airflow question.


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Old 12-21-13, 09:57 AM
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Ductwork and airflow question.

My furnace is short cycling and overheating because of the restricted airflow. I'm being told my supply ductwork needs to be replaced. It is a downflow gas furnace, with a 15x13 plenum. All of the ductwork is 4" hard pipe that spiders out from the plenum.

My HVAC guy want to replace the plenum, trunk it out, run 5" flex from the trunk, then at the end of each duct reduce the flex to fit the existing 4" register boxes. Does this make sense?

The existing register boxes are on the lower part of the wall, and apparently very hard to remove.

Will adding the trunk and 5" flex actually increase airflow if it all leads to the same 4" registers?

Thank you!
 
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Old 12-21-13, 10:44 AM
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It will help the air from deadheading right out the furnace. But with out a heat load done there is no way to tell if the 4in boots should be replaced. I'll add 4in is very small
 
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Old 12-21-13, 06:19 PM
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How many 4" pipes are there and what size is the furnace?

If you're going to go through the expense of replacing the ductwork, it needs to be designed properly to deliver the correct amount of heat to each room -> some rooms may need 4", others may need 6".

If your ducts are in a conditioned space, metal should be used since it has less resistance to airflow than flex.

You need a room by room load calculation. Good luck finding a contractor who will actually do the job right.

You may want to consider replacing the furnace at the same time, since it might be oversized and has been operating on a bad duct system. If it's been tripping the limit for years, the heat exchanger might be cracked.
 
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Old 12-22-13, 06:20 AM
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There are eight 4" pipes and the house is a rancher - 1100 sqft. The furnace was 80%, 75000BTUs and we did find a crack in the heat exchanger. We wound up replacing the furnace with a 80%, 50000BTUs.

The new furnace is working fine and has not been short cycling nor is the limit switch popping. He still wants to do the ductwork, but I'm having a hard time justifying the cost because: 1 - there is currently no problem as far as I can tell. 2 - I don't think adding bigger ducts that lead to the same small registers will increase airflow.

He gave me a good deal on the furnace in anticipation of doing the ductwork. So we may need to come to some agreement, but is there any reason to do this ductwork? He keeps insisting it will increase airflow and efficiency, but I don't get it.

No load calculations were done.
 
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Old 12-22-13, 01:22 PM
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Five inch flex & four inch pipe deliver virtually the same amount of air. As important, if not more so, is return duct sizing. As an absolute minimum, you need 20% more return than supply.

Every furnace has temperature rise specs (usually on the data plate on the furnace). After the furnace has run at least 5 minutes, 10 minutes is better, measure the temperatures in the supply & return near the furnace. Do not measure temps at registers.
 
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Old 12-22-13, 04:24 PM
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He gave me a good deal on the furnace in anticipation of doing the ductwork. So we may need to come to some agreement, but is there any reason to do this ductwork? He keeps insisting it will increase airflow and efficiency, but I don't get it.
Just because the furnace is heating and not tripping the limit doesn't mean it's working properly. It will only trip the limit if your ducts are grossly undersized. The installer probably had to increase the blower speed to stop it from riding the limit. (higher electricity use, excessive noise)

The supply air is supposed to be around 120F +/- 10, while some limits may only trip at 160F+. The safety cuttoff is there to prevent your house from burning down if the blower fails, not detect bad duct system.

8 x 4" isn't nearly enough for the smallest furnace you can buy, especially with everything branching off the plenum. 8 x 5" probably isn't enough either. Low airflow shortens lifespan and reduces efficiency. Get the ductwork fixed.
 
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Old 12-23-13, 08:27 AM
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Thanks for all the responses!

I understand that the duct system is still not perfect, but I'm trying to get a sense if I'm being told bad info or not. Does replacing the ducts but leaving the 4" register boxes make sense?

The analogy in my head is a garden hose. If i want more water flow and upgrade to a fire hose, but there's a garden hose nozzle attached to the very end of it, I'm still only getting a garden hose's worth of water flow. Is this thinking wrong?

The HVAC guy claims it is wrong, but not matter how many times I ask I can't get a good explanation as to why.
 
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Old 12-23-13, 08:51 AM
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Sac....no HVAC guy, but I tend to agree with you. Using the fire/garden hose thing you may not get any more flow, since the velocity of the water/air will increase, but the volume will stay the same.

Try filling a bucket with a garden hose...then put a sprayer on the end...by our thinking it should probably fill just as quickly, but the water will be spraying at a higher velocity.

I'm probably thinking wrong since air is compressible and water isn't. In a furnace it would still feel like a restriction. Just like blocking half of a duct. If you need a 5 or 6" duct at a certain CFM. putting a 4" on the end will reduce it...right?

My head hurts....let's wait for a Pro to explain.
 
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Old 12-23-13, 09:56 AM
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Sounds like it was originally a Coleman high velocity design. Don't even think they were true 4 inch. Not a surprise that the old heat exchanger died. We did a changeout and kept some of the original lines, but had to add some floor registers in some rooms, using a new plenum. You only have about half of the needed airflow, even for a 50k IMO. Especially if you have or ever want air conditioning.
 
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Old 12-23-13, 01:54 PM
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The register 'boxes' are 4 by what? 4x8, 4x10, 4x12?
 
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Old 12-23-13, 06:08 PM
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I can't visualize your setup.

Helpful info:

Where is the furnace located?
Where are the ducts located?
Where are the duct boots located?

------------
Changing the ducts but leaving the boots will probably improve airflow, but the boots should be really be changed.

The alternative is to leave the boots, connect 4" ducts and add new 5-6" ducts/boots where necessary -> as called for by a room by room load calculation.

But like I said before, it's not a "it's working or it's not"* type thing; many systems work, but very poorly. A poorly operating furnace will still heat your house.
 
 

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