Floor Vent Research For The Small RV
Boondocking is getting more popular as a cheap alternative for the full-time RV’er. For those of us that can handle the lack of air-conditioning, yet cannot stand noisy generators, many backcountry destinations are waiting to be discovered. Despite that Lithium battery technology may provide limited or full A/C-service in the future, currently temperature regulation in recreational vehicles is limited at best.
Without the utilities offered by full-service campgrounds, all RV’s heavily rely on insulation and ventilation to control interior comfort. Especially while visiting the southern parts of the USA during the hot summer months, any additional level of temperature control is cherished, even by the most seasoned camper.
A little while back, during my fact-finding mission for the new Cargo Van conversion, I stumbled on several resources, who were using floor vents to improve passive cooling through natural convection. Theoretically, the lower location of the floor vent (in comparison to an open side window) improves convection by extending the distance between cool air entry and hot air exit, while utilizing an untapped source of relative cool air supply, located below the vehicle. This area of perpetual shade is significantly cooler then its surroundings and has the potential to enhance the interior livability of smaller RV’s.
To find out whether the inherent temperature variations are sufficient enough to support further exploration of these floor vents, I took it upon myself to record actual temperature readings around and under my current conversion van.
Clearly not under scientific conditions, these readings have some limitations:
- Readings are restricted to my Florida location and taken mid July 2015.
- Inside measurements taken with closed roof vent and windows, which is not entirely practical.
- Outside window readings, are exposed to solar radiation.
- Absolute numbers may not be as significant as the differences between them.
To understand the impact of daily temperature fluctuations, readings are taken early morning (8:00 AM – when temps are are their lowest, following the overnight cooling off period), mid-afternoon (4:00 PM – hottest time of day) and early evening (7:00 PM – when the cooling of has started, but with the heat still lingering). These readings are also done with the van fully exposed to the sun, simulating the mostly unshaded campsites preferred by boondockers, who try to extend their stays by means of solar panels.
|DAILY TEMPERATURE FLUCTUATIONS*|
|under floor||outside shade||outside sun||inside|
With these readings as rough indicators, we can quickly discard the 8:00 AM temperatures. At this time of day, the coolness of the night still lingers and both interior and exterior temps are still in the ‘comfortable’ 80’s.
Most interesting are the daytime numbers, culminating at 4:00 PM. Exterior temps vary between 95°F in the shade and 105°F at the window level in the sun, where normally the air would enter the vehicle. Realizing that incoming air at the side windows should preferably enter on the shadow side, I took some extra temperature readings at high noon at the windows on both shaded and sunny side of the vehicle. A 7 degree Fahrenheit difference was noted and it is safe to assume a relative 100°F at window level.
|under floor||at window||inside|
This 9°F (5°C) increase in temperature difference (∆T – see explanation below) during most of the day, will enhance convection flow substantially within the RV in addition to forced flow induced by the roof vent.
The readings at 7:00 PM indicate an expected quick downturn of all recorded temperatures, making the temperature difference between window and floor vent opening less of an issue during the evening hours.
|under floor||outside shade||outside sun||inside|
It is safe to say, that the 9°F variation is enough reason for me to continue this research and in the coming weeks or months, I will revisit this floor vent issue with the new Cargo van, taking new readings and comparing them after installation of the actual roof vent, to simulate real life circumstances.
With my focus on the 2016 Ford Transit equipped with ‘All-Around Windows’, there are limited opportunities to open these windows for ventilation, making incorporated floor vents a more important part of the conversion.
There is another, possible advantage for an additional floor vent located at the fridge. If you use a 12V only (Danfoss) refrigerator, you know that the approximate 4Amps/Hr usage, lays a heavy demand on your energy production. To lower its impact, additional 2” insulation on all sides and installation of a small computer fan, behind the appliance, may reduce that by a factor 2. If the fan could blow air from a separate floor vent, to improve the overall efficiency of the fridge, shorter running times can be expected.
It’s early December 2015 now and the new Ford Transit van is parked in the driveway. To verify my earlier temperature readings, I found the following results.
With an exterior and under floor differential of 16°F on a balmy December day, I have to conclude that they are in line with the earlier results.
Calculating The Cooling Rate of Natural Convection
Natural convection is a type of heat transport, in which the air movement is generated by density differences in the air occurring due to temperature gradients. In natural convection, hot air is less dense and rises. The surrounding, cooler air then moves to replace it. This cooler air is then heated and the process continues, forming a convection current. The larger the temperature difference, the more likely and/or more rapid the natural convection will be. Combined forced convection and natural convection occurs when natural convection and forced convection mechanisms act together to transfer heat.
There are three major sources of unwanted summer heat:
- Direct solar impacts on the van and through windows and roof vents.
- Heat transfer and infiltration of exterior high temperatures, through the skin of the vehicle.
- The internal heat produced by appliances, equipment and passengers.
The convective method draws in cooler air located under the vehicle to drive out the warm air.
Natural convection can be used to ventilate and cool a van as long as the outdoor air is cooler than the indoor air at the ceiling of the van. Since warm air rises, vents located at high points in the interior will allow warm air to escape while cooler outdoor air flows in through low vents to replace it. The coolest air around a van is usually found below the vehicle, especially because this area is well shaded. Cool air intake vents are best located at floor level along the center line of the vehicle. The greater the height difference between the roof and floor vents and the greater the distance between vents, the faster the flow of natural convection and the more heat mitigation can occur.
There are two basic ways to enhance the convective cooling rate:
- Increase the volume of air escaping per minute.
- Bring in cooler air.
If ∆T is the temperature difference between exiting indoor air and incoming outdoor air, the overall cooling rate in BTU’s per hour is given by the simple equation:
Cooling rate = 1.08 x V x ∆T
where V is the volume of air escaping in cubic feet per minute. The following table contains sample values of the cooling rate for selected air flow rates and temperature differences.
Temperature Difference – ∆T Air Flow – V 5°F 10°F 15°F 20°F 100 cfm 0.5 1.1 1.6 2.2 500 cfm 2.7 5.4 8.1 10.8 1000 cfm 5.4 10.8 16.2 21.6 2000 cfm 10.8 21.6 32.4 43.2
For example, with the aid of a standard Fantastic Fan, air flow rates can reach between 478 and 920 cfm. If incoming air is 10°F cooler than the indoor air, the overall cooling rate will be about 2,500 to 10,000 BTU’s per hour.
Fantastic Fan 478-920 cfm:
@ 10°F: Cooling rate = 1.08 x 478 x 5.1 = Cooling rate 2632 BTU/Hr.
@ 10°F: Cooling rate = 1.08 x 920 x 9.9 = Cooling rate 9836 BTU/Hr.