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dmac · 05-11-2005, 10:04 AM

On cool/cold nights when the trailer is closed-up we get a lot of condensation on the windows. One answer is to open a window or a vent, but that lets the heat out.

Our house has a device that brings cold, dry, outside air into the house, and blows warm, moist indoor air outside. It uses an air-to-air heat exchanger to transfer some of the heat energy from the outgoing air to warm up the incoming air. Does anyone know of a similar but much smaller 12 volt product for use in RV's? If not, is there a market for one?

Inland RV Center, In · 05-11-2005, 11:36 AM

dmac

Dan.

Some owners, use the AC in conjunction with the furnace.

The trick is to set the two thermostats so the furnace and AC don't fight each other.

Once those settings have been found, plenty of heat, with low humidity, will make you very comfortable.

Andy

dmac · 05-11-2005, 11:38 AM

Having the AC cooling on a cold night while the furnace is heating seems a bit wastefull, and noisy. In my trailer a single thermostat operates both the AC and furnace, can't run both simultaniously. Also, what if you are operating in the 12 volt world?

Inland RV Center, In · 05-11-2005, 11:47 AM

dmac

Dan.

Special circumstances call for special methods.

When it comes to a persons comfort level, we all do things that are wasteful.
But if that makes us happy, then go for it.

Bring along a generator.

Your furnace should have a separate thermostat from the AC.

If not, they can be changed, so that they are.

Andy

joatmon · 05-11-2005, 11:51 AM

Quote:

Originally Posted by Inland RV Center, In

Some owners, use the AC in conjunction with the furnace...

Doggone it! I thought I was being original when I started doing that!

It works quite well although it appears to annoy my wife with all the airflow.

markdoane · 05-11-2005, 11:54 AM

Dan,

I couldn't find any ready-built, air-to-air exchangers with 12V fans that are smaller than 18" x 18" x 24".

What type furnace do you have? If you have a forced air system, you could add a bigger fresh air intake damper to draw in more fresh air, and immediately run it through the furnace. And add a small exhaust vent with a small muffin fan to get rid of part of the wet air.

Silvertwinkie · 05-11-2005, 02:42 PM

Well, as I see it there are two options...stop breathing or deal with it....me I'm in the same boat as Dan and my choice was to live with it since I too can't have my A/C and furnace on at the same time...and Dan's right, it would be very noisy.

Inland RV Center, In · 05-11-2005, 03:45 PM

A glass of ice water, at a comfortable room temperature, will sweat on the outside of the glass.

Your Airstream trailer is the reverse. Cold outside, but warm inside. Therefore the windows will sweat on the inside.

Airstream attempted years ago to try and solve the problem with the double pane windows. Unfortunately, the double pane windows caused more problems than they solved and were discontinued.

Camping usually means we cannot have all the comforts of home. So as silvertwinkie said, "live with it", or "without it", as the case may be.

But you certainly can add it to one of the reasons why some wives won't go camping in the cold winter months. So guys, you will have to love with that, too.

Andy

j54mark · 05-11-2005, 04:00 PM

We camp in some moderately cold weather - down to 30 or so, and find that leaving the bathroom vent cracked just open keeps condensation to a minimum without allowing too much warm air to escape. Of course, our coach is just 25', and there are only the two of us.

Mark

markdoane · 05-11-2005, 04:26 PM

If you buy some of those little dessicant sachets, they will keep the humidity down and reduce frosting of the windows. As a side benefit, as the dessicant absorbs water, they give off heat.

According to my calculations, you would need about 45,600 packets to be effective. Might be a storage problem. So I guess we'll have to love with that too!

My other idea to reduce the amount of respired moisture is to breathe into a plastic garbage bag. Every fifteen minutes, you tie the bag and place it outside the front door. In the morning all of your respired moisture will have frozen into ice, so you can dump it and reuse the bags.

Silvertwinkie · 05-11-2005, 08:42 PM

Quote:

Originally Posted by Inland RV Center, In

But you certainly can add it to one of the reasons why some wives won't go camping in the cold winter months. So guys, you will have to love with that, too.

Andy

I don't see that as a problem.

Inland RV Center, In · 05-12-2005, 09:44 AM

Typos typos.

My last sentence in the above post, should have said "guys you will have to "live" with that, too".

Andy

dmac · 05-12-2005, 09:50 AM

Back to my original question... I am thinking about:

- Buy a spare air-to-air heat exchanger core
- Cut the core on a bandsaw into smaller cubes, about 4" X 4"
- Attach two 12V computer muffin fans to a cube (one for intake, one for exhaust)
- Plumb it to provide the air exchange (for a test I could leave a window open, make a cardboard filler to block the open window with openings for intake and exhaust)
- And see if/how it works!

The trailer has a volume of about 1500 cubic ft. Using 20 cfm fans, minus 50% due to resistance, would completely change the air every 2.5 hours. Some housing codes specify .35 air changes per hour, or about 3 hours - so we are in the ballpark! 60 - 80% of the heat energy from the outgoing air is transferred to the incoming air. Two fans at .2 amps would consume 3.2 amps of power over 8 hours. On paper this all seems reasonable...

Perhaps it's an exercise in futility, as no one on the list seems too concerned about the condensation in their trailers. But it will keep me out of the bars for a few days

Silvertwinkie · 05-12-2005, 03:36 PM

Quote:

Originally Posted by dmac

But it will keep me out of the bars for a few days

LOL!!!

markdoane · 05-12-2005, 03:48 PM

Dan,

I think you have the air changes done pretty well, but I don't know if you will get the 60-80% recovery in a 4" cube. It just seems a little ambitious for such a small (shortpath) unit.

But it's worth a shot, and I applaud your effort! At the very least, you will have a way to control the air changes that is better than just cranking open a window and cracking open a vent.

dmac · 05-12-2005, 04:18 PM

I'm hoping that the short path through the heat exchanger is compensated by the slow movement of air through it (~10 cfm?) to provide efficiency similar to the full sized units. I don't know how the efficiency rating is calculated, but I assume the inputs to the calculation are the inside and outside ambient temperatures, and the temp of the air blowing into the trailer.

markdoane · 05-12-2005, 04:28 PM

I agree that the efficiency is probably based on the inside/outside temperatures.
I'm a little worried about the slow air movement though. While the 'transit time' through the unit may be the same, I think slowing down the flow rate will reduce the heat transfer rate significantly.

To get good heat transfer you need to have a high Reynolds number (very turbulent flow), and I think you may be in more a laminar flow regime.

BUT- I encourage you to try it anyway, I would love to be proven wrong. Do you want to split the cost of the heat exchanger? I'll go halfsies.

RivetED · 05-12-2005, 09:33 PM

Quote:

Originally Posted by markdoane

...To get good heat transfer you need to have a high Reynolds number (very turbulent flow), and I think you may be in more a laminar flow regime.

Looks like we have a displaced high-performance sailor!!!

Perhaps some 'tell-tales' along the inner walls will aid in tuning your vents!!!!

mswartz · 11-27-2005, 06:45 PM

DMac or others-
Has anyone worked out the thermodynamics of an air-to-air heat exchanger to fit an Airstream? And has anyone tried using one?

dmac · 11-28-2005, 07:45 AM

I have not been able to find an air-to-air heat exchanger for RV's, although it seems like there might be a market opportunity. I have not built one yet, due to my other time commitments.