Bathroom Heat - a more direct approach!

[Snackchaser]

Like others, we’ve been disappointed with the lack heating in the bathroom.  In our Oliver, even the kitchen had minimal air flow and hot air was blasting under the bed.  When I took a closer look at the duct routing from the furnace, the problems became obvious.  

There were two 4” ducts coming out of the furnace, one from the front outlet facing towards the registers, and the other from the rear outlet facing away from the registers.  The front outlet was connected to the bedroom register with one 90⁰ bend.  It had the shortest run with the least turns, and twice as much air flow as the other two combined.  The rear furnace outlet was connected to the kitchen register after making a 180⁰ u-turn.  From there it was Tee’d and reduced to 3” to feed the bathroom.  The 3” duct then takes a long and torturous path to reach the bathroom.  The resistance from over 280⁰ of turns, and the longer run, resulted in much lower air flow to both the kitchen and the bathroom.  I also found a hole from where the duct had been rubbing on the hot water valve, and the kitchen had a restrictively tight bend at the register.

Fortunately this was a relatively easy fix, and the results were significant.  Our 2022 Elite II has the Dometic furnace, but I suspect that other furnace brands could have similar ducting issues and it might be worth taking a look.

The sketch shows how the duct routing was modified for better balance to all three registers.  The bedroom and kitchen registers now feed from the high resistance u-turn at the rear furnace outlet.  The bathroom has its own feed from the front outlet giving it the minimal number of turns possible.

This modification involved moving the Tee which is under the kitchen cabinet.  There is an access port under the forward kitchen drawers to reach the Tee and kitchen register, and it’s best to pull them right out of the access port to work on them.  The 3” bathroom reducer stays in place and it gets a new duct directly to the front furnace outlet.  The bedroom/kitchen gets connected to the rear furnace outlet.  Rather than using flexible duct to make the 180⁰ u-turn, I used two 90⁰ rigid fittings for less flow resistance.

All of the parts needed for this modification are readily available at hardware stores.  They usually sell duct by the foot, I bought 5’.  Use sheet metal screws and foil duct tape for the connections to make sure they don't come apart.  The kitchen register can be screwed together from the inside of the register rather than trying to do it from under the cabinet.

I used an anemometer before and after the modification to measure flow, and a on-line calculator was used to get Cubic Feet per Minute (CFM):

                               Before   After

Bedroom - 4”       100.7      53.2

Kitchen - 4"          39.3         65

Bathroom - 3"     11.8         30.2

Total                     151.8       148.5

 

Cheers, Geoff

[mossemi]

Very nice mod with a simple approach and excellent results!  Thanks for sharing.

Mossey

[aaronorange]

Thanks, I have a new project. Betty was excited to hear your modification & solution 🙂

[Rivernerd]

On 2/3/2024 at 12:02 AM, Snackchaser said:

Our 2022 Elite II has the Dometic furnace, but I suspect that other furnace brands could have similar ducting issues and it might be worth taking a look.

Our 2022 Elite II has the Truma Varioheat furnace.  The Truma ducting is smaller, more substantial and, from what I can see in the photos, better installed.  

In addition to the mods detailed above, I recommend you consider adding a return air vent from the bath to the underside of the front dinette seat.   Our Truma Varioheat system came with that additional return air vent.  It really helps the flow of heated air.

[jd1923]

@Snackchaser were you able to get to or see the "reducer" on the way to the bathroom duct? I guess it just goes to smaller diameter ducting, so nothing you can do about it anyway.

[Mainiac]

It is "standard" operating procedure, in hot air ducting, to decrease duct size the further it gets from the heat source. The same amount of air in the 4" duct gets put into the 3" duct. This increases the velocity and helps move the heated air along. I just hope the is a direct path out of the bathroom and back to the return duct. Also hope that air is returned through the cabin and not through the basement. The return air is going to suck the air with the least resistance. Hopefully it is not the air from the drain 'scuppers'. That air might be 40°, or more,  cooler than cabin air, and the basement air might be 20° or more. The warmer the return air the more efficient the furnace is at heating. 

[Snackchaser]

5 hours ago, jd1923 said:

@Snackchaser were you able to get to or see the "reducer" on the way to the bathroom duct? I guess it just goes to smaller diameter ducting, so nothing you can do about it anyway.

Yes, it's directly attached to the Tee for the kitchen register.   The 3" duct goes on to the bathroom through some sort of firewall and it's not easily reached.

There was enough slack in the 3" duct to pull the Tee (with attached reducer) out through the access port that's under the forward kitchen drawers.  The space is tight, and you will need to detach the 4" side from the furnace to get enough slack on that side.  You can make most of the new connections while it's out.

You'll need to disconnect the kitchen register first.  The screw holding the duct to the register can be reached through the access port, but it's easier to replace it from the inside of the register, rather than trying to do it blindly from inside the access port.

Cheers, Geoff

 Here's a photo that I forgot to add in the original post: