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Old 01-12-2016, 12:58 PM   #15
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2011 Interstate Coach
Overland Park , Kansas
Join Date: Aug 2011
Posts: 2,454
Originally Posted by InterBlog View Post
It partly comes down to static vs. dynamic conditions. An Interstate set up to camp in cold, wet weather has a large temperature differential between the outside and the inside and (importantly) it tends to maintain that differential for a longer period of time (overnight) and in conditions characterized by greater air stillness (which promotes condensation) than one would experience during a drive down the road, which can really make a difference. This can be exacerbated if it's raining outside such that people climb into the Interstate already wet. The house furnace will dry out the coats and other materials, but then that water has to go somewhere. Cooking produces even more moisture. Plus the average person loses something approaching a liter of water per day through respiration and perspiration alone, so it all adds up.

The number of exchanges also affects the outcome. One of our supreme annoyances is the mechanism that MB built into the Sprinter which obligates the a/c system to switch off the recirc function every 15 minutes like clockwork. The NCV3s do it every 30 minutes. Actually turning off the Recirc could actually increase the RH depending on the exterior conditions so not buying MB's line. The owner's manual says that's a necessary humidity control measure. It might be the more adaptive option in Germany, but it's completely inappropriate on the subtropical Texas Gulf Coast. We've talked to the local MB dealer about an override, but he says it cannot easily be done. So we physically re-punch that recirc button every 15 minutes in many driving conditions.

Point being, when folks are underway, they are probably exhausting more interior air and moisture than they realize (unless they live in a place like this, in which case the design of the a/c system will sometimes result in a net increase in interior moisture). Not so much when they are parked quietly and the furnace is running.
So it's not the furnace that is causing the excess moisture and condensation. Since warm air will hold more moisture, running a heater/furnace should reduce condensation. Most, if not all, vehicles will turn on the A/C when the Defrost is selected (above about 38*) in order to dry the air hitting the cold glass. It also keeps the compressor seals lubricated.

Now if you are getting an air exchange in voids like the doors or other confined spaces that are relatively cool wrt the rest of the RV, then you could expect some condensation. In the winter when this is most likely to occur, periodically exhausting the warm, moist air and replacing w/ the cool, dry air from outside should mitigate this.

Glass half full or half empty to an engineer is simply a glass that was over designed.

PM me for sale info on my 2011. SOLD!
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Old 01-12-2016, 01:18 PM   #16
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2006 22' Interstate
League City , Texas
Join Date: Oct 2014
Posts: 1,290
Originally Posted by 73shark View Post
So it's not the furnace that is causing the excess moisture and condensation. Since warm air will hold more moisture, running a heater/furnace should reduce condensation. ....
Yes, right up until the point where that warm air comes in contact with cold surfaces, which makes it not warm anymore, and then it can no longer hold what it had been holding. And then the RV gods hear me bellow, "It's raining in my Interstate!" (in isolated spots).

I maintain a non-running but open Fantastic pretty much 24/7/365 in an attempt to minimize this effect. I have an Ultra Breeze covering it so rain can't get in. There's always a bit of air circulating up and out, but when temps are in 30's to low 40's, it's not pleasant to have a brisk flow, so I keep the place reasonably warm and condensation happens. I forgot to add wet dog to my previous list of situational aggravants. That doesn't help either.

Glad to hear they extended the NCV3s to 30 minutes on the recirc. Bet they did that because of negative user feedback on the T1Ns.

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Old 01-13-2016, 09:49 AM   #17
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2006 22' Interstate
League City , Texas
Join Date: Oct 2014
Posts: 1,290
Variation on this theme: It's raining in my fitness club. I took this window casing pic about an hour ago when the exterior temp of the building materials was probably 45 F. All that aluminum goodness sure looks spiffy, but it's not a very practical construction method for our climate - that's the type of result you get when the franchisor is in California. Rain drops keep falling on my head in southeast Texas, but it never rains in southern California. I'm never gonna stop the rain by complaining, but I can create workarounds for the smaller-scale version that occurs in the Interstate.
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