What can be done about uneven heating in fifth wheel?

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By Chris Dougherty
Chris Dougherty is a certified RV technician. Here is a letter he received from a reader while he was serving as RVtravel.com’s technical editor.

thermometer humorDear Chris,
We have the same problem with our Crossroads 34′ fifth wheel as we had with our Jayco 30′. The furnace (which is located near the steps going up to the front) keeps the bedroom and bathroom toasty, but the living area is still cold. Would covering certain vents to force the heat to the back cause an unsafe situation? If so, is there any way to get more heat to the rear of the unit? —Astrid


Dear Astrid,
This is a common issue with fifth wheels. The answer and the fix relate to the simple fact that heat rises. As the furnace dispenses hot air from the registers, the warm air rises to the highest point, and the cooler air falls to the floor. The problem is the floor of the upper bedroom and bathroom (or whatever room is in the higher level of the fifth wheel) is much higher than the downstairs floor. So, the warm air layers from the ceiling down.

What enhances this problem is that most of the ‘lower end’ fifth wheels have only a single zone for heat and air conditioning. The thermostat is usually located in a ‘central’ spot which will ‘average’ the temperature between upstairs and down. However, as I have found in my 5’er, and I assume you have as well, that average just doesn’t work when the temperatures outside start to get extreme. In this case, the upstairs gets downright warm, while the downstairs gets cool.

If it were a two-zone system, the fix would be easy: keep the door to the upstairs closed. The downstairs zone would stay at its set temperature as would the upstairs. But since it’s one zone, it requires some ingenuity to keep ahead.

Closing off registers in an RV is NEVER recommended. The reason is because the furnace is designed to work properly with a certain minimum amount of airflow to make it more efficient, and to keep it from overheating. Closing off registers reduces airflow and can lead to furnace overheating and cycling, as well as overheating of other components like duct work and other registers.

A small electric fan is one option to help circulate the warmer air from upstairs back downstairs. An RV ceiling fan would be another option.

##rvt758 #RVDT1218

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TravelingMan

OK…Here are a few suggestions I recently completed to correct our problems. You may have 1 or more of the same symptoms.

1) Start by checking the air flow leaving the unit first. Pull ducting from the back of the unit. If there is little or no flow, you may have a squirrel cage blower problem. There are typically two blowers. The lower squirrel cage blower is usually metal (due to higher temperatures of the intake/exhaust of the burner chamber). Then, there is the upper double cage which is made out of cheap plastic. This is the blower that provides air flow to the RV. Once the blades start to break off, air flow is reduced. Enough blades break off and there will be no flow. If this is your problem, you can purchase a metal squirrel cage to replace the plastic one, but I have not found one yet that has the same air flow rate. The metal cage works less efficient than the plastic cage. Catch twenty-two.

2) Next, check the configuration of the flex and floor ducting. Ours was so out of balance, the bedroom fried us out. All you need is one 2″ duct to the bedroom. NOT (2) 4″ ducts! A 2″ for the bathroom is also sufficient as well. Re-route all ducting to reduce air flow these locations. Be careful with 90 degree bends and tees as it can make a difference in how much air will flows thru a ducting due to restrictions. Leave the adjustable register so that you can throttle to your taste.

3) Then here is the biggy…The living room. BEFORE you can fix this problem, you’ll need to understand how the duct work is put together. Typically, they use either round flex duct (on smaller units) or rectangular ducting for in-floor larger units. Since we have the latter (a 42′ 5th wheel), I’ll discuss our issue and resolution. After replacing the failed plastic squirrel cage, and re-configuring the duct work, I had more than enough air flow to accommodate the living room. Even with just 2 floor registers.

More on floor ducting…The Floor Ducting is cheap!…It is fabricated from thin foam board. The manufacturers could remedy this problem by installing inter-locking metal ducting (DRV Mobile Suites is the only one I am aware of so far that has metal ducting for 5th wheels. Maybe a Luxe or some other custom brand as well). The foam board comes in large, pre-bent sheets. Once on site, the RV Manufacturer folds the ducting over and places a seam of cheap, non-sticking tape on the seam. I say non-sticking because that is where the failure comes from. It comes loose after about a year or two. Then, all of the air flows into the basement area and not into your cabin. In order to fix this issue, I had to remove the underbelly and slice open all of the hidden ducting in the insulation package. There, I found that about 18′ (yes feet) of ducting had come undone. So, although I had a blast furnace in the bedroom, there was no flow in the living room. We suffered thru this for two winters. It starts slowly and gets worse until there is no flow left. You waste a ton of propane as a result. The worst part to reach was the area above the black and water tanks. For most people, the only way to fix it there is to remove the tanks. And that is another problem. But I found a way to do it without having to remove the tanks. I don’t recommend it as its a fight. Taking the tanks down would certainly be the lesser of the two evils, so use that one. In addition, not only were the tape seams undone, but the foam ducting had pulled apart from each other. They come in what appears to be 4′ or 5′ sections. With foam board, you cannot lock them together like you can with metal ducting. They just add more tape. So, in all, it took about 5 days to complete the work and make it right (No thanks to the RV Manufacturer who doesn’t seem to know what they are doing)…I wonder how expensive this would have been if I had taken it to a dealer?

With that problem now fixed, now you can go back to the (2) flex ducts feeding your floor ducting. Since you have worked thru all of the other problems, now you can probably add one or two more flex ducts to the floor duct. I would recommend the installation of a curved plenum to reduce air flow restrictions for even a better increased flow rate. I now have a total of (4) 4″ flex ducts in the floor ducting (instead of 2).

And after going thru the math calculations on proper duct sizing, taking into consideration heat gain, insulation properties of the wall, floor, and ceiling, ceiling heights, window loss, duct resistance, etc., this solved our problems. If we ever purchase another RV, it will have metal ducting or we won’t buy.

Our RV is now balanced and about two weeks ago we experienced a 12 degree day in Indiana with no problems. We are now able to keep the front and back of the RV within 1 degree of each other at 80 degrees. We could have even set it higher with no issues if we wanted to.

For those having a problem balancing your unit, maybe these suggestions will help.

Peggy

We had this same problem. Our solution was to add a duct to the wall by the stairs. This duct was provided heat from a t connection at the bedroom duct. This reduced the amount of heat into the bedroom and increased the amount to the living area. Now there is not such a huge difference in temperature between the two spaces.

Abe Loughin

A possible work around, or hack as the kids say today, is to get a tension rod and floor length curtain and place it at the top of the steps. We prefer our bedroom cooler than the living space (downstairs). This solution works for us as the heat vents in the bedroom provide enough heat but the Lions share of the heat stays in the living area and is nice and toasty when we get up and sit for our coffee.

Steve

Another option- if you have a ducted a/c in the overheated area, run it on low fan only. That will help distribute the heat.

WEB