I have two small adjoining rooms in my house that are furthest from the HVAC air handler. They are constantly running about 2-3 degrees warmer than the rest of the house. There is the same amount of blown-in insulation in the attic over this area as the rest of the house. A 9" duct splits and supplies a ~50sqft laundry room and a ~180sqft study that share a wall.

I installed this duct fan into the main 9" duct supplying the rooms:

Amazon.com: AC Infinity RAXIAL S8, Inline Booster Duct Fan 8" with Speed Controller - Low Noise Inline HVAC Blower Can Fan for Basements, Bathrooms, Kitchens, Workshops : Tools & Home Improvement

Running it on high, I do notice a small amount of supply airflow increase coming through the vents. However, there is little to no change in the average room temperature.

I really don't know what to do next. Maybe install a 12"x12" return in the larger room? Run separate 9" supply ducts to each room register?

Any suggestions would be greatly appreciated.

Do you have jumper ducts between the room and hallway/common area? They're short ductwork in your ceiling that have a vent in the room and a vent just on the other side of the doorway. If you don't have these (or a return air in the room) when you shut the door that room, its effectively closing off the duct because air can't escape back to the return, it just hits the wall. At best its severely restricted as air flow can only get out from under the door. So that room has a higher pressure compared to the rest of the ducts and cold air gets rerouted to other ducts, because flow is always a difference in pressure between two locations.

https://basc.pnnl.gov/resource-guides/jump-ducts

Have you considered a mini split heat pump unit for the larger room and removing the larger room duct so that the laundry room gets more air flow?

All, thanks for the suggestions. MEEN - there are actually no doors to close off, so it really should not be restricting airflow back to the main return in the hallway.

I have uploaded an image so folks can see what I am dealing with:

https://imgur.com/a/P01dB53

I like both suggestions thus far
The cheapo way would be

• LG window AC 110v whose white noise makes work in Study less distracting, drowns out neighbors; oversize 125% if room is facng West or has lots of glass ... which would also beg the question of exterior shading via evergreens/other
• If needed, then 110v Study quartz radiant heater w/auto shut-off
• block off vent to Study (more air/cooling for Laundry, is hoped)

Past extra supply ducts (you mentioned) does work when the air is cold enough but am not reading this is the case. Further, even if air cold enough to add a twin, this will be defeated on the West side or by many windows (R-2 not much insulation)

Bonfire,

Looking at your drawing, there is a couple things that could be causing this issue. In general you have un airconditioned space in your carport up against those rooms, that can be a source of uneven cooling/heating as outdoor weather warms your garage which then warms your rooms. Also, laundry rooms just are warmer because of the heating element. Usually they dont have jumpers and people keep those doors closed. You're also double outdoor walled in that corner. So lots of sources of excess heat compared to the rest of the building. There is usually a damper within the AC duct in the kitchen, try closing that a bit more to see if you can push more into that far room to cool it off.

If you want to go full mechanical engineer (and you never go full mechanical engineer according to tropic thunder) go rent a FLIR to see the uneven heating in your house as well as anemeter to measure airflow out of the ducts.


Your AC's cooling capacity is limited by the evaporator coil/compressor size. Adding another duct run won't mean much because you're still limited in how much cool air you can produce and push around your house. I would try closing the kitchen duct more and see how it pushes more air into that room. Also, look into air sealing. There might be some big gaps around any roof/drywall penetrations that were covered up with ducts/lighting that is letting in warm attic air.

I have one of these that we use in the summertime to help cool our 80 year old house. Super quiet, variable speed (automatically slows/speeds to maintain temp).

Midea U-Shaped Window Unit

Thanks for all the suggestions. I told my wife about putting a small window unit in (back facing window) and just removing it if someone comes over. This idea was also suggested by someone at work. She said I can't do that. LOL

Our last house had a very hot Living room so hubby added another duct from the air handler. With twice the air is was perfect afterward. He also added another return as the door in that room was frequently closed.

had a similar situation, where we had an 8" supply duct hit a cheese box that went two directions, and THEN another cheese box for each, 1 reaching the bathroom, 1 to the bathroom closet (8" and 6" I think) and then the other was two 8" to spit to MBR. So:



We added an air return and then replaced the one 8" supply duct with two 10" ducts. Each 10" then only splits once - on split to cover the bathroom and closet and the other to the two supply vents for the MBR. Solved the hot room problem.

Please see me for more professional schematics.

My last 2 places have benefitted from me adding a 2nd return air vent.

Both systems were in the corner of the garage and matched up with a corner in the den. That made it easy to add a second one "around the corner".

My HVAC guy suggested it on the first house and basically said "you can never have too many return air vents.

I've noticed a big difference when I added the one at my current house. Even more so when I had the ducts replaced but that was another problem.

Quote:

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My HVAC guy suggested it on the first house and basically said "you can never have too many return air vents.

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same for my scenario. Chances are there *might* have been enough air getting pumped into the room originally, but nothing getting pulled out and it just gets stale. but while there, lets just add the duct and rework it with larger ones and make sure.

Thanks for the additional responses. I think the real problem is due to what MEEN mentioned. There is too much heat load over there due to the 3 exterior walls. It probably doesn't help that this part of the house was an enclosure of a portion of the original garage (I doubt the walls were insulated correctly).

Next, I think I am going to add a 12"x12" return in the "study" room. If that does not work, I will probably run another dedicated 9" duct to the laundry room and re-duct the existing 9" main duct to the study (leaving the duct fan in since it did seem to slightly increase flow). I would have to think all of this would provide some improvement.

A 9 inch duct is good for 200 CFM. A 10 inch is good for 265 CFM. "Generally speaking" you need 1 CFM per foot so without doing a normal heat load if you targeted 230 CFM total for both rooms you would be ok.

I would suggest a 10 inch duct from the supply plenum and then wye off to a 9 inch duct for the study and a 6 for the laundry room. Make sure you install a 9 inch and a 6 inch damper on the respective branches so you can properly balance out the airflow

Thanks 88. That's great info.

88, do you have a suggestion for a register box that accepts a 9" duct? I have had issues finding anything besides a 14"x8". Also, do they sell wye's to do the 10"x9"x6" transition or would I have to get some collars and build a "cheese box"? I am just wondering if it would be easier to run another 8" duct over there and put in another 12"x6" register a few feet from the other one. I am worried it might look odd, though.

Thanks in advance.

needs a return in the room

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