HVAC airflow issue

1713 Views 16 Replies 6 Participants Last post by beenthere, Feb 10, 2016

ruhappy69

Discussion Starter · Feb 7, 2016

So I have a Goodman unit that was installed 2 years ago. Its 90K BTU @ 80% efficiency. I had the guy run 2 additional vents in the basement where the unit is located (now a finished basement), yet these 2 vents put out almost zero airflow. The takeoffs are very close to the Plenum and I'm told that is the issue (the air isn't given a chance to run far enough). Is it safe to assume that if I remove these takeoffs and reseal them then cut in these 2 takeoffs further down the trunk that it will start pumping real air flow?

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yuri

· #2 · Feb 7, 2016

No. The plenum is under high pressure from the fan so any takeoffs must get some airflow. However air flows out the point of least resistance. You may have to get dampers put in the other pipes or if you have them then they may need to be partly closed to restrict the flow thru them and build up more pressure in the plenum which will force it thru the new pipes.

throrope

· #3 · Feb 7, 2016

Well, I've seen otherwise with respect to yuri's post.

We had a connection too close to the inside of an elbow. No air came from that outlet. Often air does what it wants no matter what we need. The inside of elbows due to the momentum of air often have a negative pressure that can pull air into the duct. Something like a siphon.

With respect to your basement comfort, I doubt you will get satisfaction no mater how many outlets you create. Basements have completely different heating and cooling needs than the rest of the house and the thermostat upstairs is unlikely to respond.

I suspect the complaint is heating, not cooling. I suggest installing a completely separate heating source. Others have used a direct vented gas fireplace. I ran a loop of fin-tube from our hot water heater. The heat needed is not much, but without it, you'll be cold.

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yuri

· #4 · Feb 7, 2016

It is possible as horizontal units like that are rare where I am. There are takeoffs that have a lip inside to scoop air into them. Where I am we always have at least 3 feet of plenum before we start any takeoffs and then it goes horizontal into ductwork.

ruhappy69

Discussion Starter · #5 · Feb 9, 2016

So Yuri

You stated: 'However air flows out the point of least resistance.'. If that is the case then the take off closest to the plenum (which is a simple straight 10ft run - that doesn't have any airflow) would have the most airflow coming out. The one that is like 3rd in the line that goes to the new bathroom pumps out very good CFM but the other 2 before pump nothing..

yuri

· #6 · Feb 9, 2016

It depends. If you have a 3 foot high vertical plenum with takeoffs at the top and then horizontal ductwork with takeoffs and rooms straight above the furnace they get the most air and the further away rooms less. Your problem is you need turning vanes or takeoffs with scoops to direct the air towards them. You are getting a swirling venturi effect I suspect and they are in the eddy current.

ruhappy69

Discussion Starter · #7 · Feb 9, 2016

What if

I spider 2 of the ducts into the one duct farthest from the plenum and seal off the 2 other holes. Would that be a solution?

ruhappy69

Discussion Starter · #8 · Feb 9, 2016

here is an updated pic

here is an updated pic of the setup. The one coming out of the top is the short 10 ft run into the basement ceiling that provides near zero air flow.

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beenthere

· #9 · Feb 10, 2016

First, make sure it isn't kinked where it goes into the ceiling.
Second, that is about the worst place to have tapped it into. Moving it down the trunk line should help.

Can't tell by the pic. Is the trunk line wider then the transition from case coil to trunk line? If not, not a good thing either.

ruhappy69

Discussion Starter · #10 · Feb 10, 2016

6"takeoffs are on first two feet of duct work. Two takeoffs on last five feet. One plenum 17x20. One five foot by 14x12. Two pieces.

Another quick question... If I move that further down how do I cover the existing hole? Just the tape?

Technow

· #11 · Feb 10, 2016

I would be more worried about that vent pipe.....

Freakin death trap.

ruhappy69

Discussion Starter · #12 · Feb 10, 2016

Why? Please explain?

yuri

· #13 · Feb 10, 2016

Looks OK other than if you stand up and walk into it you can break your neck or get whiplash. In a crawlspace sometimes you got no choice but to run it that way. Won't affect the furnace.

Technow

· #14 · Feb 10, 2016

yuri said:
Looks OK other than if you stand up and walk into it you can break your neck or get whiplash. In a crawlspace sometimes you got no choice but to run it that way. Won't affect the furnace.

You guys in canada are supposed to have the strict building code.......:vs_worry:

4" goes to what looks like 6" single wall....all taped together.... If you say its alright...it must be.

user_12345a

· #15 · Feb 10, 2016

Furnace venting looks like it's horizontal, maybe its just the pic, but isn't it supposed to slope up?

yuri

· #16 · Feb 10, 2016

We used to do it that way and it meets the minimum code and is GrandFathered in.

However in a new install they would have to use B vent and downsize it to meet the vent size for the furnace and 1" bigger depending on the length of run.

However it is not a death trap. How is that?

The furnace will still run and maybe a bit cooler exhaust and in a cold basement you may end up with condensation in the ventor / premature corrosion. If too big it can have pressure switch tripping issues but those are rare. Still not deadly.

I would use aluminum instead of galv. but it meets the code. In the US the codes all different than in Canada and I see all kinds of different setups. We used to tape the joints but it is not code.

Unless it was not inspected it meets the code in Chicago.

Its all history now as we don't have any new mid efficiency furnace installs as we are all 92%+.

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beenthere

· #17 · Feb 10, 2016

ruhappy69 said:
Why? Please explain?

Looks like single wall pipe in an unconditioned space. It should be B vents.

That transition piece isn't really a plenum. But some contractors will call it that.

Use Sheet metal to patch those holes. And then seal and insulate the patches.

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