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Discussion Starter #1
Anyone know what the cause might be?
House has central air and heat. One story house with Unit in attic. Heating using natural gas. There is only one intake, and it’s in the hallway, 20”x30”.
In the winter, I often get cold air coming out of multiple registers (maybe all of them) when hvac is not running. It’s not a little, you can actually really feel it even with 10 foot ceiling. There does not seem to be a leak in the ductwork.
The only thing I can think of is that somehow the air is actually circulating from intake, cooling off in planum or ducts and coming down through registers, but why would it be doing that in this house and not others, I’m not sure.
Anyone has this problem? Thanks
 

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You are correct in your assumption (convection) and my guess would be all of the ducts and plenum need to be air sealed and SUPER insulated.
I said air sealing because unless it has been specifically sealed it is always leaking. Sealing air ducts takes a special effort and few contractors will spend the time. They just cover everything with foil faced bubble wrap. Had one hvac guy insisting that bubble wrap was r-30, r-3 would be closer. but he was right in a sense, it needs r-30.

The truth is, the mechanical equipment doesn't belong in the attic.
Bud
 
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Hot air rises and cold air settles.

Try block the return air grille with some insulated wood or material. That way you will not have airlfow thru it from the warm air rising thru the supply vents or vice versa.

Your problem happens where I am so you are not alone.
 

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I don't think he can block it as he has heat through the same system.

Bud
 
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Discussion Starter #7
Thanks all. Yeah you are right that, I can’t block it as it’s used for heat.
I thought about blocking the intake w foil just to confirm the theory, as I’ve already looked for leaks - sounds like you all are thinking same/similar, so I’ll try that - the problem is that I usually feel it when I’m either already in bed, or late night done for the day and on the couch :smile:
The intake duct actually curves high up in attic, and it’s definitely above hvac, then comes down into hvac intake. By high up, I think maybe 1-2 ft above hvac At its highest.
I’m wondering if that’s causing excessive circulation?
If I built an intake planum box, I could keep it level, and also provide returns from bedrooms, so maybe that would help overall.

On a different topic, if fan provides 1600 com max, how should ducts be designed? To be close to supporting 1600 or some percantege less, or use some other rule?
Thank you all.
 

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With the thermostat in the off position, as a test, consider covering the return grill to determine if the air flow ceases.
Allow me to run this by 1 more time worded a little different.

We're doing a test with some plastic sheet and tape to determine if there is convection at the return when the unit is off. This is a temporary less than 5 minute test to determine if sealing the return off stops the cold air from falling from the supply registers. No, we're not going to run the unit with the return sealed off.:wink2:
 

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You may have a negative pressure in your house as most houses do.

Meaning you are sucking air in and it comes in the easiest place which may be your ductwork if it is leaking.

If your doors and windows and weatherstripping are more airtight ( which saves NRG $$ also ) then you may have less air movement and less drafts and maybe less of a negative pressure.

Negative pressure is caused by open chimneys and exhaust fan openings etc etc and is natural.
 

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Discussion Starter #10
Thanks all.
I’m gonna try to test it by blocking intake.
Another thought, this effect positively correlates with outside temp, earning, the colder it is outside, the more cold air seems to be going through room vents

Any thoughts on if putting intake planum and returns from each bedroom would help this?
Thanks
 

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idea: when you feel the air blowing from the duct work, is the fan blowing in the unit? if it is also used as the heating, blocking the intake could cause the furnace to overheat.
when you feel this air moving, I assume everything is off. but What was off? was it cooling or heating when it shut down and the air continues to move? that would indicate where to start looking. if the furnace was running, then shuts off, but the air / blower continues to move you might take a look at the sequential switch, to see if the snap disc has failed. if it was cooling, then shuts down, but the air keeps blowing, you might want to check your thermostat setting or wiring.
I would be very cautioned about blocking an intake. not that you couldn't do it, but if the circumstance was right, there could be some really bad consequences . use your best judgment.
 

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Discussion Starter #12 (Edited)
Recently I realized we are getting attic air, and fire smoke in the house, so it was very clear that hvac system has leaks. I found 3 areas where system was leaking big time (intake to blower and between heater and air handler) and wow..all had to have always been there due to how the system was put together. Just unbelievable workmanship, or lack of rather. Not to mention lack of inspection.
I don’t know if it’s causing cold draft from registers in winter, but will find out soon enough, however, after I fixed them all properly, I noticed that we still get attack air through our vents into the house for the first minute or so. So went up again, and found two minor leaks, and noticed that when system starts, the air clearly gets pulled into it for first 10seconds or so through those “holes”. So makes sense that we get attic air coming from vents initially only and then it’s clean air, bc eventually as the airflow through intake ramps up, the air flows out of those holes and not in. However, those leaks were so small, and air was pulled in for maybe 5-10s that it makes no sense it would affect air coming out of all vents and for a full minute.
After patching those, we still get a little bit of questionable air initially but now only 10s or so. So seems that was the cause, at least partially.
This is real onion pealing. Any thoughts on what to do next?
I’m thinking to spray some air freshener around intake first and run it. Then later spray around air handler and see which one produces more smell in the house or not.
 

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The initial air temperature coming from the registers in winter will be whatever the attic temperature and attic duct work is at that time the blower starts. If the attic is 44°F that's what you'll get until room temperature and or the heater does some warming.
 
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