HVAC Vent Re-direct

So the master stateroom is over ventilated (too cool) whilst the salon directly overhead, with an undersized vent, is not cool enough. When the new A/C unit was installed a few years back, the PO was told its not a capacity issue but a vent issue which could be resolved albeit with $$. He did not go for it, as his health was failing and the boat was soon to be on the market.

In lieu of adding another A/C unit and its attendant installation challenges and costs, I thought I could re-direct some of the air flow from the master to the salon (directly overhead) to a to-be-installed floor register proximate to the outer-wall. The master duct and the new ducting are thankfully in the end of a large hanging locker and out of the way both functionally and visually.

The floor register in the salon may be fan powered to pull the air away from the master air flow or failing that, install a new master vent that can be closed or partially open to re-direct the air to the salon

Other than cutting through the salon floor/master ceiling being a mystery, am I missing anything? I don't see plumbing or wiring being an issue. And I don' think cutting a 10" x 2" cut into the floor structure would be structurally compromising.

Thanks in advance.

Before going to the trouble, remove the grill in the area under air conditioned. Air conditioning multi compartments with one unit is all about air flow. The grill generates significant flow resistance (reduced CFM of cooling air).

If you find that greatly improves the situation, consider changing the duct size (diameter) by an inch or two. Increasing the diameter reduces frictional line loss which increases air flow to that room. Generally, blocking air flow to the over cooled room will accomplish the same thing, but at the risk of creating other problems, most commonly, the evaporator freezing up.

Ted

Not sure how your two areas are attached to the AC unit, or even what kind of AC you have. If it's a single drop-in unit that's pretty common on boats under 50-feet or so, there's probably insulated flex ducting. If there's a T or Y that directs flow, there's typically a diverter flap that can adjust flow from one side to the other. See attached.

Some vents close more fully than others to allow air to be directed between rooms. Here's a 4-inch round one that will shut-off almost completely

https://www.suremarineservice.com/Cool/Round-Plastic-Grilles/06-2023-12.html

Peter

Agree with the above. Most cooling issues I see are not insufficient unit capacity but rather restricted airflow both supply and return. it's hard to get sufficient grille space and move the proper volume of air. Too often the supply duct is undersized and squeezed in with return air ignored or left to "find it's way back."

Also the molded marine duct fittings are important to maintain air velocity, don't rust and can be cleaned periodically to prevent mold.

O C Diver said:

Before going to the trouble, remove the grill in the area under air conditioned. Air conditioning multi compartments with one unit is all about air flow. The grill generates significant flow resistance (reduced CFM of cooling air).

If you find that greatly improves the situation, consider changing the duct size (diameter) by an inch or two. Increasing the diameter reduces frictional line loss which increases air flow to that room. Generally, blocking air flow to the over cooled room will accomplish the same thing, but at the risk of creating other problems, most commonly, the evaporator freezing up.

Ted

Click to expand...

Thanks

It is one unit with multiple compartments. Not a A/C capacity issue in the salon, but a vent size issue with no room/access for increasing size.

Removed the grill and it might have made a marginal increase as the grill woodwork reduced flow by maybe 10%.

And I still need to put a grill on the hole which by definition reduces flow by some %. So not a long term solution.

Did try the restrict the flow to other colder compartments, and it did indeed shut down due to freezing.

mvweebles said:

Not sure how your two areas are attached to the AC unit, or even what kind of AC you have. If it's a single drop-in unit that's pretty common on boats under 50-feet or so, there's probably insulated flex ducting. If there's a T or Y that directs flow, there's typically a diverter flap that can adjust flow from one side to the other. See attached.

Some vents close more fully than others to allow air to be directed between rooms. Here's a 4-inch round one that will shut-off almost completely

https://www.suremarineservice.com/Cool/Round-Plastic-Grilles/06-2023-12.html

Peter View attachment 128872View attachment 128873

Click to expand...

Thank Peter...

I cannot get into the ducting to check for Ts or Ys and in my very amateurish HVAC opinion, a single 4" x 12" vent cannot handle the volume of air to adequately cool a fully windowed 15' x 13' salon. My only (practical) solution seems to adding the additional vent fed by the over cooled master.

And I fear for pushing to much air into that restricted vent space causing a freeze shut-down (back pressure I presume).

Wish me luck...I am going in

Osprey69 said:

Thanks

It is one unit with multiple compartments. Not a A/C capacity issue in the salon, but a vent size issue with no room/access for increasing size.

Removed the grill and it might have made a marginal increase as the grill woodwork reduced flow by maybe 10%.

And I still need to put a grill on the hole which by definition reduces flow by some %. So not a long term solution.

Did try the restrict the flow to other colder compartments, and it did indeed shut down due to freezing.

Click to expand...

Have you considered a duct fan to increase the flow to the under cooled area? Might be worth a test to put a fan in front of the vent to simulate a duct fan.

Ted

Air is flowing fine out of the original vent. Just one vent and its too small for the space.

Went ahead and cut into the floor. 1 1/4" of sandwich construction. Installed master stateroom to overhead salon duct bypass. Replaced master stateroom vent with a closable vent. With stateroom 90% closed A/C is now flowing freely into salon passively and the temp is steadily dropping.

I ordered an active fan register and will install tomorrow to pull more air up into the salon. Should be a better result.

Thanks to all for feedback.

Keep us posted. Would like to hear a follow up after you’ve used it a while.

John

So the powered register is in. It is 90 degrees outside and 75 degrees in the salon. The Admiral is pleased as is the first mate.

The flow seemed fine without the powered register, and I do not have CFM #s to compare, but I feel better knowing we are pulling the air from below vs. pushing it up. Not scientific, but for short money worth the add. Also makes sure it gets the air up and around the end table nearby.

Now on to removing the non-functioning Icerette...