Incorrect size Ducting, help


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Old 04-01-23, 04:54 PM
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Incorrect size Ducting, help

Hello, Hope someone can help here. We built a 1200sqft ADU 6 years ago and it has a furnace in it with no a/c. The heater has always cycled on and off and I thought that was normal behavior. If its 65'F, and we set it to 72'F, it would get to around 67, cycle off, then cycle on in a few minutes and eventually it would get to 72.

When we tried to sell the house, the inspector said that was incorrect and it was malfunctioning. It turns out there is an airflow issue. We had an HVAC company come out to look at they said return duct is only 8" and needs to be at least 12". There are 3 supply vents which appear to be 6". Of the 3 vents, 2 are up stairs in the ceilings which is inaccessible and one is downstairs and easy to access. We would use the hvac company but they are charging many thousands.

So I had a contractor friend replace the return duct from 8" to 12". He also replaced the supply duct to be 12". The supply vent in the downstairs is now 12" duct as well, but the two upstairs supply vents are only 6" ducts because they are not accessible. The supply duct is 12" but it was split to 6" duct going upstairs. , so the downstairs duct is 12" but the upstairs ducts are 6" if that makes sense.

After we did these duct changes, it seem to be better but still the furnace cycles on and off after 5-10 minutes.

Do we need to upsize the supply ducts going to the upstairs bedrooms too? I'm not sure how to do that because there is no access to the upstairs ducts other then this starting point.

Also, another thing I noticed is the downstairs vent blasts out tons of heat while the upstairs vents barely push out any heat. It was this way before we made changes to the ducts, and still this way after the changes.

 
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Old 04-01-23, 05:13 PM
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I'm not a HVAC expert but want to make a few comments. First, while it's desireable, I don't believe it's necessary for supply and return ducts to be sized equally in each location. Second, the photo shows ducts bending at the hangers. This could be a source of airfkow issues. See if you could sraighten them out.
 
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Old 04-01-23, 05:49 PM
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Thanks Tony! I did just check the bending on the hangers and made some adjustments, but still not much air going into the upstairs' rooms, and furnace still cycles on and off.

 
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Old 04-02-23, 06:31 PM
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This is getting frustrating. We cut open the ducts going upstairs so that it just just blasts out the heat just to test if the furnace still cycles off, and it does!!

Question right now is, if I run one single 12" supply duct to one vent, and no other vents exist, there is no reason to have an airflow issue or there still can be?
 
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Old 04-02-23, 06:57 PM
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so why is the furnace cycling off exactly I would not think it would really be a duct issue is it throwing a code is the thermostat shutting it off? you may need to get more information on your furnace brand and model and figure out why.
 
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Old 04-02-23, 08:21 PM
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The code said it was "airflow" when the HVAC guy came to look at it.

I dont know how to look at the codes. Do I open up the panel and look at the codes?
 
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Old 04-03-23, 10:52 AM
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We had a friend that has an HVAC contractor come out to tell us how to fix it..

So he said the furnace is too big and we need to change the 12" return to 16", and to add two more 10" ducts to the supply and make some vents for them..

Guess we have some work.
 
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Old 05-16-23, 07:57 PM
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1200 sq should have been a 50,000 btu ( smallest unit) with todays construction methods.
Any future hvac dealing you must have Manual J load calc.
 
 

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