Consider installing an automatic register booster fan. Do pick the right register size!

https://www.amazon.com/AC-Infinity-Register-Thermostat-Control/dp/B0792QR5YT/ref=sr_1_2?crid=3CN8S931VXK8Y&keywords=air+vent+blower&qid=1685973552&refinements=p_89%3AAC+Infinity&rnid=2528832011&s=hi&sprefix=air+vent+blower%2Caps%2C109&sr=1-2

https://www.amazon.com/AC-Infinity-Register-Thermostat-Control/dp/B07H8R5G1W/ref=sr_1_3?crid=3CN8S931VXK8Y&keywords=air+vent+blower&qid=1685973640&refinements=p_89%3AAC+Infinity&rnid=2528832011&s=hi&sprefix=air+vent+blower%2Caps%2C109&sr=1-3

I am so glad you came here before wasting money hiring pros to remedy this. The wall between garage and br #4 is missing insulation and there is air infiltration from laundry to br#4. Other than that and poor roof flashing the home appears well built.

@klem,

I realize your post is intended to mock others attempting to help the OP, but you need to at least get the physics right. The air infiltration issue is concerned with warm, moist outdoor air entering the home via windows, doors, and penetrations in exterior walls--not room-to-room air exchange within the home. The comment about the roof flashing was a nice touch.

It was home built by another builder who claimed that HVAC wasn't rocket science.

Attempting to help the OP?

Helping would be going to the house and fixing it, not arguing over what you think is wrong from a forum board.

There are many other possibilities for this kind of problem. HVAC is a job done in person not a job performed in a forum board by those who don't even do the job themselves. Our job is to guess and appear like we are helping.

Let's guess!

The OP asked for help on an internet forum. They appear to be intelligent enough to understand the limitations of working remotely and with limited information. Houzzers who want to help (the vast majority in my opinion) will provide ideas of what might be wrong and where to look to investigate and what options there are for solutions. If you're unwilling or unable to contribute anything in the "helpful" category, you might need to explore other forums than houzz.com to spread your "cheer."

I'm just merely telling the OP that guesses are a dime a dozen.

How is informing the OP what a guess is worth, not helping?

false hope? We guessers and enthusiasts are here for clicks, we don't care about false hope only the impression of appearing like we are the only ones offering help.

It's helping because we say it is.

There was a gas leak and it was pitch black, we showed up to "help" find the leak. We handed the guy a bic lighter so he could see. That helped him because we never heard back from him.

Carry on, caped crusader. For truth! Justice! and the Raymerican way!

Put a cheap, single blowup bed in bedroom #4. When Billy, Suzie, or Grandpa don't behave, make him sleep there.

Just an average Joe here, but agree with Charles Ross Homes in that the difference I see between the rooms is the shared wall with the garage. Do you know if there's insulation in that wall? Are the exterior garage walls insulated?

The room is close to the furnace/blower, but do you know the ducting is run? Is bed4 the last register on a run off the blower? First? The answers to these questions, plus the airflow, settings of balancing dampers, etc, are all potential factors.

Ignore the bickering above. It's an ongoing issue between these two.

I came here to post the exact same question. My problem bedroom (son's room) is also the closest to the HVAC and shares a wall with the garage. My bedroom at the other end of the house has two exterior walls and it doesn't get hot like my son's bedroom. I've had a new unit and ducts installed and that didn't help. I've asked two different technicians about this and neither was sure what would cause the problem. They both suggested adding a return to his room just to see if it would help. They told me they didn't have everything they needed, so would return at a later date. Neither came back. Does your problem room have a return?

There should be a HVAC return in every bedroom in a home for comfort.

My house is small (1,400 sf), so I only have two returns. My mother's house is 1,700 sq and she also has two returns, but doesn't have any "hot" rooms.

Noel - Does your "hot" room by any chance have a return right out side of it? Mine does and someone mentioned they may be causing the problem, althought I'm not exactly sure why.

Msrose I wager "someone" is kin to a couple other blowhards named I Heard and They Said. Them two come around here frequently passing off lies and hearsay as fact. You can rest easy the return in question isn't causing nearby room to not cool.

klem1 - I should have clarified the "someone" in reference was one of the A/C technicians that I paid for their professional opinion. I understand this may not be the best place to ask for help, but when two A/C technicians don't have a clue what to do, it made me wonder if anyone else had ever dealt with this and possibly had a suggestion or a guess that I could investigate.