Under Cabinet Register Weak Blow


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Old 12-26-16, 06:58 AM
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Under Cabinet Register Weak Blow

Long story short, while remodeling my kitchen my contractor ran a heat run from the basement to under a base cabinet with a register on the front of the cabinet. You can barely feel the register expelling warm air. I have a brand new furnace and great pressure from every other register in the house. But instead of having a warm kitchen, I essentially have a heated cabinet.

I suspect the problem may be due to the ducting from the basement emptying into the dead space under the cabinet and not being directly attached to the register. The cabinet is large and can hold a lot of warm air before it is forced out the register. Can under-the-cabinet flexible ducting help?

Also, the contractor used 6inch pipe from an existing duct branch in the basement. Shouldn't this pipe be smaller? If I change it to 3inch pipe, will that increase the force the air is pushed out?

Thanks in advance. I'm somewhat handy but never dealt with HVAC before so I can use all the advice I can get.
 
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Old 12-26-16, 07:55 AM
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You have the diagnosis correct, the air flow is dumping behind the toe kick.

However, the amount of air flowing in equals the amount flowing out but as noted your cabinets are gaining the majority of the heat gain.

If this is at the end of the run you may just not have enough air flow available. If somewhere in the middle a smaller duct will just deliver less air flow.

That application is not unusual but I have installed a cabinet and did install a duct from the floor to the toe kick so that's an option, but you will need to remove the cabinet which may not be an option!
 
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Old 12-26-16, 08:00 AM
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What i did in my kitchen was to add a 1x under the cabinet that both blocked off the back portion and since it was installed at an angle it directs the air to the register. Mine works great this way. As mentioned above you'd need to remove the cabinet to do that. Not sure if it would be possible to remove the grille and insert something to direct the air flow.
 
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Old 12-26-16, 08:02 AM
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It's always a good idea to make a ductwork box to transition to a 2x12 toekick register so that airflow is not lost. As mentioned, this has to be done before the cabinet is set.
 
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Old 12-26-16, 09:26 AM
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The cabinet is installed, but it is not permanent yet. The counter top has not been installed yet and the cabinet stands alone all by itself, so it is not too much of a pain to remove the cabinet.

Should I look at reducing the size of the 6inch duct pipe also, or will the duckwork box be enough to improve the force?
 
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Old 12-26-16, 09:36 AM
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Does a smaller diameter pipe blow harder or is it the opposite?
 
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Old 12-26-16, 11:08 AM
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Also, the contractor used 6inch pipe from an existing duct branch in the basement.
Unless the existing branch duct was significantly oversized AND the new duct was connect before the balancing damper, assuming there even IS a balancing damper, this was absolutely wrong.

The "pressure" you feel at the register is a function of airflow. Reducing the size of the duct will reduce the airflow and give you even less 'pressure" at the register. There are very few instances where a branch duct smaller than six inch diameter is preferred.
 
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Old 12-26-16, 11:10 AM
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I don't know about duct sizes but when I remodeled our kitchen I moved the duct so it would fall under the cabinet. All I did was move the duct [didn't change it's size] My wife was skeptical and didn't think it would work but she really enjoys the heat coming out on her feet in the winter time.

I'm not positive but I think all the flex duct coming off of the trunk on my system is 6"
 
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Old 12-26-16, 11:22 AM
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Furd, the existing duct branch was 3.25 by 12 rectangular and capped off right after the new 6inch pipe. So it only feeds this new register in the kitchen. It is the first branch off the trunk. There is no damper. Is that proper?
 
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Old 12-26-16, 11:29 AM
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The rectangular duct has a cross sectional area of about 39 square inches and the 6 inch round duct is about 28 square inches. Since the rectangular duct serves nothing but the round duct and toe-space register that is acceptable. Truth is, you may want to increase the size of the round duct if you are not getting enough airflow through it.

Properly, All branch ducts should have balancing dampers installed near to where they branch off of the main trunk or plenum. These allow adjustment of airflow through the various branches to keep temperatures more consistent throughout the house. Unfortunately, as a cost saving measure balancing dampers are all too often not installed.
 
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Old 12-30-16, 06:55 AM
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You might have already solved this problem but I would also go back to where duct comes off the trunk and make sure that it has taped seams, runs relatively straight and is otherwise unobstructed and tight. You could be losing warm air into the basement.
 
 

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