RMD Exhaust Flue

[tractors1]

Originally Posted by skiman

If I was pulling the Frig, I would do this. The only thing I would turn the T a bit sideways to get it away from the plywood and trailer wall. I think this would induce a nice draft with the boiler heat introduced above the coils.

I plan on getting rid of the "T" when I pull the fridge. I think Dometic sticks it on all their smaller fridges as the "caravans" in Europe can also install their "through-the-side wall" vent kit - they don't have roof vents. When I checked into my two local big RV/Trailers service shops, I was told none of the larger Dometic fridges have that "T" on top.

I'm a retired mechanical engineer with HVAC training, and that "T" impedes the flow compared to no "T". When I put my hand over either end of the "T" running on both electric and gas, hot air flowed outward from both ends.

[Klem]

Originally Posted by tractors1

I plan on getting rid of the "T" when I pull the fridge. I think Dometic sticks it on all their smaller fridges as the "caravans" in Europe can also install their "through-the-side wall" vent kit - they don't have roof vents. When I checked into my two local big RV/Trailers service shops, I was told none of the larger Dometic fridges have that "T" on top.

I'm a retired mechanical engineer with HVAC training, and that "T" impedes the flow compared to no "T". When I put my hand over either end of the "T" running on both electric and gas, hot air flowed outward from both ends.

The way I understand it is that the T and the baffle in the tube is there to slow the rising air and transfer more heat to the gases in the system. Like tuned exhaust exhaust the purpose is to raise efficiency.

Didn't seem that important to either but a frig tech told me not to change the flue. That's why I used the dryer vent to build a chimney around the T and up.

Food for thought, if there are installations without the T then it's a moot point.

[Parker]

Interesting about exhaust coming out of both ends of the tee. They only connect the upper opening to the wall vents in the Euro models. I was guessing the tee acted to bring in air from the low side for dilution. That leaves the lower end continuing to dump exhaust right behind the fridge.

[tractors1]

Originally Posted by Parker

Interesting about exhaust coming out of both ends of the tee. They only connect the upper opening to the wall vents in the Euro models. I was guessing the tee acted to bring in air from the low side for dilution. That leaves the lower end continuing to dump exhaust right behind the fridge.

Actually, I think with the added length of pipe on the upper side of the "T" there probably would be an induced draft after the piping warmed up. Like starting a fire in a fireplace, once the flames get going there's a lot of draft up the chimney.

With just a short "T" both openings are about the same distance from the center pipe. Not much to set a direction for the flow of hot gasses.

[Parker]

Could be. It would be interesting to test that idea. Too bad one of us doesn't have an extra RMD fridge sitting around to play with.

[tractors1]

Before I added the flue factor (and doing every other thing folks recommended) with this RMD I had to run it on either the 4 or 5 setting to keep the fridge cool (40F or lower) in usually shaded and temperate (60-75F) ambient conditions.

Did all my testing with an empty fridge on max cool (5) and concluded the flue was good for around 10 degrees reduction in both freezer and fridge. So that's the stress end of the testing. No overheating or warping of the Dometic top cover.

Now I'm focused on settings to have the freezer freeze and the fridge not freeze (it was as low as 24F.) Today at 104F ambient, with full sun on the side behind the fridge running electric, I tried setting at 4 and got freezer at 5F and fridge at 33F. I dropped it down to setting at 3 and had freezer at 7F and fridge at 39F. I'm going to leave it there overnight and see how low the fridge goes when ambient drops to 70s with the goal of not freezing lettuce.

I've never had this RMD run well at a 3 setting.

Also noticed the Texas fan switch light is on sometimes regardless of which way I throw the switch. Is there a thermal switch in there somewhere? Since I don't know where it would be located I can only surmise that perhaps my duct pipe is mushed up against it causing it to activate......

[Parker]

Originally Posted by tractors1

Also noticed the Texas fan switch light is on sometimes regardless of which way I throw the switch. Is there a thermal switch in there somewhere? Since I don't know where it would be located I can only surmise that perhaps my duct pipe is mushed up against it causing it to activate......

Oh no! A confounded experiment. Now you have to start all over.

[tractors1]

I didn't try to remove the "T" from the boiler exhaust as I assumed it was either soldered or held in place with screws. Just got an email from Gene, the webmaster at casitaforum, who added a hose to his fridge. Looks like I was wrong - if you can jerk that thing off installing a hose will be a WHOLE lot easier.

He added an 8 inch hard pipe extension to get the hot gasses above the condenser fins.

Done with my tests. If I run on "4" during the day and drop to "3" at night, the freezer stays froze and the fridge doesn't freeze. These tests were run with an empty fridge so I could see changes quicker. With stuff in the fridge the swings won't be so rapid. Hoping to run on one setting while camping next week.

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Charlie, It just pulls up and out no screw. If you have the vent on the roof you don't need this mod as the hot air will go up to that vent unlike our Casitas.
Gene

[Parker]

That's good to know. I really thought there's a screw into the side of it. I wonder if there are any issues with changing the draft with these tubes, or if that would even matter? You've got me thinking I need to try this mod before our next trip, but I'm a little bit wary of committing to it just yet.

[Klem]

I also added this little fan to move freezer air into the frig when things go south ... (we go south). Notice holes are on either side of the center channel, you see looking up in the frig, so no wires were damaged.

I added a resistor to the fan so it just slowly circulates the air. Works great when we taking a ferry and you must turn the propane off!

I have a little plastic box that sets over the fan and hole when traveling.

Back to back conduit parts.

[Parker]

Originally Posted by tractors1

I didn't try to remove the "T" from the boiler exhaust as I assumed it was either soldered or held in place with screws. Just got an email from Gene, the webmaster at casitaforum, who added a hose to his fridge. Looks like I was wrong - if you can jerk that thing off installing a hose will be a WHOLE lot easier.

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Charlie, It just pulls up and out no screw. If you have the vent on the roof you don't need this mod as the hot air will go up to that vent unlike our Casitas.
Gene

Look at Myron's post #104 here about "Cooler Question." Sure looks like a screw in the side of the tee.

[tractors1]

Originally Posted by Parker

That's good to know. I really thought there's a screw into the side of it. I wonder if there are any issues with changing the draft with these tubes, or if that would even matter? You've got me thinking I need to try this mod before our next trip, but I'm a little bit wary of committing to it just yet.

Now I don't know if there is a screw or not - in this recent post (#104) if you blow up the pic of the rear of the fridge I think there is a screw head holding the T on top. Looks like the T has a vertical slot and may not be fully seated to the boiler tube.

http://www.escapeforum.org/forums/f3...tml#post210634

[Parker]

Originally Posted by Klem

I also added this little fan to move freezer air into the frig when things go south ... (we go south). Notice holes are on either side of the center channel, you see looking up in the frig, so no wires were damaged.

Ever try it without the fans to see if gravity results in enough circulation? Looks like a useful mod.

[Ron in BC]

Originally Posted by Parker

Look at Myron's post #104 here about "Cooler Question." Sure looks like a screw in the side of the tee.

My recollection, such as it is, is that there is a screw holding it on.

That mod, adding a short length of copper tube, was done by someone on the forum a couple of years ago. I think that its a good mod to do, getting the hot flue gases at least clear of the top of the fridge. I'd still replace the Tee on top of it.

Ron

[Klem]

Originally Posted by Parker

Ever try it without the fans to see if gravity results in enough circulation? Looks like a useful mod.

Thought about it but 'no'. I wanted 'control' of the flow to test different velocities by changing the resistor. My intention was IF I needed to have the freezer just be a source of cool air that would be fine.

So my focus has been keeping the frig cool and all the food (and Irma) happy. We toss an ice cube tray, frozen fruit, etc. in the freezer but IF it thaws I really don't care. I put the ice cube tray in a plastic tray so if it melts the water doesn't go wild.

Sad to lower the bar to I just want a reliable frig, got tired of putting ice cubes in the frig to save food ...

Seeing so many frig fans that just move the air around in one cavity or the other I decided to scavenge the freezer.

[Kountrykamper]

Originally Posted by Parker

That's good to know. I really thought there's a screw into the side of it. I wonder if there are any issues with changing the draft with these tubes, or if that would even matter? You've got me thinking I need to try this mod before our next trip, but I'm a little bit wary of committing to it just yet.

There was a screw in the 8551.

[Parker]

Ah, there it is. I'm wondering about the trade-off between putting a larger duct around the tee to carry the heat to the top vs making up a new tee like you did. I'm a little bit uneasy about putting the aluminum vent line in there, but then I haven't tried it to really know. Also, I wonder if I can reach that screw from the top if I pull the whole vent assembly or if it means pulling the fridge out. That's where the 3" vent line has the advantage.

[elongest]

The plumbing on my 8555 is different from the 8551 pictured. The line out of the boiler comes up and bends slightly over the T exhaust. It then turns to the front of the trailer and runs by then enters the condenser from the front of the trailer. With the T installed there is not a straight shot up to the roof vent with a dryer hose. That is why the flexible tube is needed with the T installed. With my arm and hand I can barely touch the T much less remove a screw while reaching through the roof vent. Like Charlie I tried to pre-form the hose and positioned the hose over the T with a yardstick.
Eddie

[Parker]

Oh...that's too bad. Sitting here in front of the computer, it's easy to imagine just reaching in there to remove the screw and heading off to a welding shop to make a taller tee or an extension. That's how I get myself in trouble sometimes. Things are pointing me in the direction of leaving it as it is until after our fall travels and then maybe pulling the fridge, adding insulation, and doing a tee extension at that time. I should have learned that lesson with the Casita. I opened up the stack wrapping once to install a new heating element, thinking I could do it all while still installed. There was just no way to work in that space and I ended up with the fridge on the table top to get it back together. Thankfully, it was much smaller.

[tractors1]

Well, one could always drill a hole in the side for access with a long screwdriver, then plug it when done. Think I prefer to pull the fridge.