HVAC duct issues

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RustyShackleford

Minister of Fire
Hearth Supporter
Jan 6, 2009
1,364
NC
I've long known that the ductwork system of my HVAC is un-balanced. In summer, the MBR gets 5-10 degrees cooler than the rest of the house; that's fine, and the reverse in the winter is fine too, since I heat almost exclusively with wood - and the MBR is cooler, not warmer. (In other words, the system directs way too much air to the MBR, in both winter and summer. It the summer, that's, good, because we like sleeping in a chilly room. It'd be a bad thing in the winter, because the bedroom would be too hot. Except we don't run the HVAC in the winter, because we heat with wood. And since the woodstove is in the living area, the bedroom is still cooler than the living area, which is what we prefer). So it's not a problem now, but still it needs to be addressed.

Examining the ducts, I can see what appears to be a simple way of at least partially remediating the issue. Here are a couple of photos ...

The lefthand photo shows the main trunk, where the furnace and indoor heat-pump coil feed into it. It's probably 8x24" in size. All the ducts for the MBR attach to it (to the right of the photo frame). Then, in the righthand photo, you can see where the trunk doglegs to the side to clear a foundation pier, and gets much smaller. Note where a couple of runs of 6" flexduct takeoff right after it narrows. Seems to me it'd be simple to move those takeoffs to the larger part of the trunk. One of them would even become a much shorter run (it's probably close to 20ft long now).

So this might not solve the problem, but it sure seems like it would help. But should I not do it, is it possible it might make things worse ? I believe I understand the "two foot rule", that takeoffs should be separated by at least 2ft (unless they're on opposite sides of the trunk ?) and separated by 2ft from where the supply attaches to the trunk.

Another factor, I bought a cheap digital manometer, and measured about 0.25 inH2O between the furnace output and the evaporator coil, and aboiut 0.57 where the duct from the filter grill attaches to the return side of the furnace. That seems kinda backwards. I have one of those washable filters (20x25" for a 2-1/2 ton system).

I suppose I should get some HVAC pros in, but I know some of you here know a lot about HVAC, and the engineer in me can't resist trying to fix it myself.

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I'm still trying to puzzle out the imbalance. The system cools too much in the summer but is inadequate in the winter, correct? If so, that doesn't sound like imbalance. Sealing and insulating the supply and return plenums should make a notable difference. Heat pump ducting is much lower temp than oil or gas so duct losses need to be minimized. Our heat pump hvac also runs through an insulated crawlspace and everything is insulated.
 
I'm still trying to puzzle out the imbalance. The system cools too much in the summer but is inadequate in the winter, correct?
Sorry, I was un-clear. The system directs way too much air to the MBR, in both winter and summer. It the summer, that's, good, because we like sleeping in a chilly room. It'd be a bad thing in the winter, because the bedroom would be too hot. Except we don't run the HVAC in the winter, because we heat with wood. And since the woodstove is in the living area, the bedroom is still cooler than the living area, which is what we prefer.
 
The easiest thing might be to put a draft damper after the takeoff collar to the MBR duct and throttle it a bit, maybe 50%. The system will perform better with the trunks sealed and insulated.
 
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The easiest thing might be to put a draft damper after the takeoff collar to the MBR duct and throttle it a bit, maybe 50%.
There's several ducts to the MBR (it's a big suite), but yeah, I thought of doing that. I guess it'd increase the static pressure, but since it's only 0.27" now, I guess that'd be ok. The 0.57" I'm measuring on the return side seems too high, but I guess that's a separate issue.
 
Can you slow the blower at the same time as closing dampers?

The clown that did the HVAC in my house had the furnace blower set to max for heat, so not only was the air barely warm coming out of the furnace it was mostly going to the main floor due to it being the path of least resistance and the upstairs got some barely warm air.

I slowed the blower (still within safe temperature output range and flow rates per the manual) and balanced the registers. Now I get hot air to every duct, and the upstairs gets enough air to keep an even temperature with the rest of the house. The AC contractor came in after this and setup the AC the same way, with the blower on max, I fixed that a couple weeks after install too, then the AC the cooled the house evenly and dropped the air temperature enough across the coil to lower the humidity.
 
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How many runs/vents does the MBR have?
 
Other thoughts total static pressure of .84 iwc should be addressed IMO. How many ton system and how much return area do you have? What CFM is the blower set to?

My thoughts are add balance damper(s) to MBR to restrict flow and add another run or three to the under ventilated room(s). Find somewhere to add a return. In-line booster fans can help but are really a bandaid.

I have Have you been tracking dew point and duct temperatures while AC is on thinking outside temps are 85 and below. Chart says at 71 degrees humidity at 51% dew point is 51F. Depending where you have your thermostat set a delta T of 21 degrees is possible. Just things to keep in mind.
 
There are 4 ducts in the MBR and only 5 in the living/kitchen/dining area, which is 50% more floor area, plus cathedral ceiling sloping up to 16ft high with a loft. Crappily designed system, I guess. I suppose I could simply delete one of the MBR ducts. It still seems like I ought to move at least one of those takeoffs I showed in the OP photos, certainly the one where the flexduct is 15-20ft long; do you see any reason NOT to do that ? But yeah, probably need to do more than that.

I am bewildered why I'm seeing 0.57 iwc on the return. It's a 20x25" filter grill, 2.5-ton system, and a very short piece of really large diameter flexduct between the filter grill and the furnace. I guess I could see what it is with the filter removed, and also try a disposable filter (right now it's a washable one). The furnace spec says 16x25" should be enough - even for a washable filter IF it's a "high velocity" one, which I have no idea if mine is.

I'm not quite sure about the CFM setting, But it'll be somewhere between 1000-1250 cfm, from the manual. I'll check how it's set now.

Haven't really been tracking dewpoints and temperatures. but I can say for sure that the dewpoint hovers very close to 70 degrees all summer here (as you know, I'm sure). Oh, you mean the indoor dewpoint ?
 
I slowed the blower (still within safe temperature output range and flow rates per the manual) and balanced the registers. Now I get hot air to every duct ...
So slowing down the blower made the airflow more even ?
 
There are 4 ducts in the MBR and only 5 in the living/kitchen/dining area, which is 50% more floor area, plus cathedral ceiling sloping up to 16ft high with a loft. Crappily designed system, I guess. I suppose I could simply delete one of the MBR ducts. It still seems like I ought to move at least one of those takeoffs I showed in the OP photos, certainly the one where the flexduct is 15-20ft long; do you see any reason NOT to do that ? But yeah, probably need to do more than that.

I am bewildered why I'm seeing 0.57 iwc on the return. It's a 20x25" filter grill, 2.5-ton system, and a very short piece of really large diameter flexduct between the filter grill and the furnace. I guess I could see what it is with the filter removed, and also try a disposable filter (right now it's a washable one). The

furnace spec says 16x25" should be enough - even for a washable filter IF it's a "high velocity" one, which I have no idea if mine is.

I'm not quite sure about the CFM setting, But it'll be somewhere between 1000-1250 cfm, from the manual. I'll check how it's set now.

Haven't really been tracking dewpoints and temperatures. but I can say for sure that the dewpoint hovers very close to 70 degrees all summer here (as you know, I'm sure). Oh, you mean the indoor dewpoint ?
Yes crawl space dew point.

My new 4 ton ductwork has almost 1200 sq in of filter. 290 is in per ton was the exact amount. I have restrictive filters. Merv 11 behind a fiberglass filter for dog hair.
 
So slowing down the blower made the airflow more even ?

In combination with adjusting the ducts, yes. I guess I should say heat distribution more than airflow.

My ducts to the upstairs are undersized, and can only flow so much air, it better to have that air warmer. Slowing the blower and partially shutting all the main floor registers evened out the heat distribution
 
Who ever did the install should come back and fix it.. each one of those runs should have a damper on them so you can balance and control the flow between rooms. Also every single seam on your main lines should be sealed also those patches should be sealed as well.. I always like to use the tinfoil tape to seal all my joints. You have a lot of air loss with those not being sealed.
 
Who ever did the install should come back and fix it.. each one of those runs should have a damper on them so you can balance and control the flow between rooms. Also every single seam on your main lines should be sealed also those patches should be sealed as well.. I always like to use the tinfoil tape to seal all my joints. You have a lot of air loss with those not being sealed.
It was installed 35 years ago when the house was built. Not even sure what company it was.

Crawlspace is encapsulated, so leaks aren't as harmful, but should still be fixed.
 
It was installed 35 years ago when the house was built. Not even sure what company it was.

Crawlspace is encapsulated, so leaks aren't as harmful, but should still be fixed.
Remember those leaks mean your living space is at a lower pressure than the outside. Ie you’re sucking more air from the return than you are blowing back in. That means you are pulling unnecessary outside air in. Could be as much as 25+% of 1200 cfm.
 
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Also every single seam on your main lines should be sealed also those patches should be sealed as well..
I guess you mean the seams in the big metal trunks (in the photos).

Patches ? I see one on the main trunk where the furnace attaches. Maybe the last guy moved a piece of flexduct that was attached there, and it's one of the ones that's too long and oughta be on the fat part of the trunk. (The previous - and original - system was a "gas pack" that sat outside the house at the end of the fat trunk, off to the right of the frame in the photos).

Still wondering what people think of moving one or both of those runs of flexduct to the fatter part of the trunk. Esp since it'll make one of them way shorter.
 
I guess you mean the seams in the big metal trunks (in the photos).

Patches ? I see one on the main trunk where the furnace attaches. Maybe the last guy moved a piece of flexduct that was attached there, and it's one of the ones that's too long and oughta be on the fat part of the trunk. (The previous - and original - system was a "gas pack" that sat outside the house at the end of the fat trunk, off to the right of the frame in the photos).

Still wondering what people think of moving one or both of those runs of flexduct to the fatter part of the trunk. Esp since it'll make one of them way shorter.
I don’t think it would change much. What would have more of a change is making sure all the flex duct runs were pulled tight. I forget the numbers but the sag has much larger effect on flow than one would think.
 
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I guess you mean the seams in the big metal trunks (in the photos).

Patches ? I see one on the main trunk where the furnace attaches. Maybe the last guy moved a piece of flexduct that was attached there, and it's one of the ones that's too long and oughta be on the fat part of the trunk. (The previous - and original - system was a "gas pack" that sat outside the house at the end of the fat trunk, off to the right of the frame in the photos).

Still wondering what people think of moving one or both of those runs of flexduct to the fatter part of the trunk. Esp since it'll make one of them way shorter.
you got it... i bet if you did a smoke test on all those seams and the patch you would be surprised on how much was leaking..
 
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you got it... i bet if you did a smoke test on all those seams and the patch you would be surprised on how much was leaking..
Better to use foil tape, or mastic goop ?
 
Better to use foil tape, or mastic goop ?
The pros used mastic on anything wrapped up in insulation or on flex connections. And used this on everything metal. (Then it got wrapped and insulated and mastic).

It’s a mastic tape.

What HD has
Your friend has shared a link to a Home Depot product they think you would be interested in seeing.




image.jpg
 
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The pros used mastic on anything wrapped up in insulation or on flex connections. And used this on everything metal. (Then it got wrapped and insulated and mastic).

It’s a mastic tape.

What HD has
Your friend has shared a link to a Home Depot product they think you would be interested in seeing.

Ordered a couple rolls - that's a great price too, thanks for the tip.

I also got some of these takeoffs, great price (and 24ga, instead of 32ga from HD), re-attach a couple of the outlets to the over-supplied MBR, do the other two if that works out: https://www.zoro.com/greenseam-coll...teel-24-ga-8-in-w-x-gr6pocr6ga24d/i/G3001372/

Got a roll of hanger strapping, fix the sags (as recommended above): https://www.homedepot.com/p/Master-...ec63eaef243857ed430880fedd3e4cfd1cdd494f17638

I think I'm gonna move one of those ducts (in OP photo) to the fat part of the trunk. It'll make the run way shorter - it's probably close to 20ft now, that can't be good.
 
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I also got some of these takeoffs, great price (and 24ga, instead of 32ga from HD), re-attach a couple of the outlets to the over-supplied MBR, do the other two if that works out: https://www.zoro.com/greenseam-coll...teel-24-ga-8-in-w-x-gr6pocr6ga24d/i/G3001372/
BTW, when I install these, I guess the thing kinda sticks in place, then add a few sheet-metal screws, and maybe some of that mastik tape depending on how tight it seems to seal against the trunk.

If cutting a new hole in the trunk, what do you do about the insulation, just trim it around the hole and leave it be, or do you need to tape the edge somehow so fiberglass fibers don't get into the airstream ?
 
Shortening a duct and moving it closer to the supply plenum will definitely increase the flow out of that duct. Also have you considered just eliminating one of the vents going to the MBR since you have so many?
 
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Also have you considered just eliminating one of the vents going to the MBR since you have so many?
Yeah, maybe even two of them, because there are actually five now ! One in the walk-in closet (wife insisted, it gets musty in there in summer), one in toilet stall (rest of bathroom is open to bedrooom-proper), one by sliding-glass door, one by a smaller window, and one by an interior wall. So maybe the toilet one and the interior wall one.
 
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