RETROFITTING AN RV WITH
A HOT-WATER HEATING SYSTEM
- SYSTEM IS LESS EXPENSIVE THAN CONVENTIONAL HEATING
- USES HALF THE GAS OF A CONVENTIONAL FORCED-AIR FURNACE
- USES ONLY A SMALL AMOUNT OF D.C. ELECTRICITY,
MAKING THE USE OF SOLAR PANELS POSSIBLE
- MORE EVEN, SATISFYING HEAT
- QUIET OPERATION
"A Simple RV Hydronic Heating System"
(Published in: Home
Vol. 26, pages 53-56, January, 1992)
Conventional forced-air heating systems have fixed heat output and are either on or off. Hot water heating offers gentle, continuous heat and adjustable heat output. A simple hydronic system using a hot water circulating pump, large fan, and automotive type heater core transformed a poorly heated 24 foot travel trailer into a comfortable, evenly heated home. The system was a big improvement in efficiency, economy, quietness, and comfort over the propane forced-air furnace it replaced. The project served as a valuable learning experience and a prototype for larger applications.
What ever happened to good, old-fashioned hot water heating systems? I used to love to visit my grandmother in Ohio because her house with its hot water circulating pipes in the floors was the warmest and coziest place in town. I decided to try and install a hydronic heating system in my (not so warm) 24 foot travel trailer home. I could scarcely find any books or articles on the subject or anyone who knew much about it. My truck's heating system is hydronic and works very well. Why not use this as my model? The result was a system which keeps my trailer as warm and cozy as Granny's house. The system is at least 50% more efficient and economical than conventional heating systems. This hydronic system amazed my skeptical friends and the local RV repairman who were all quite convinced such a "primitive" system could not possibly work.
The concept of my hydronic heating system is simple. Establish a closed loop in my existing hot water system: a standard RV propane water heater with a six gallon storage tank. Circulate the hot water through a standard automotive heater core. Blow the heat from the core out into the room by placing a fan behind the core. Enclose the fan and core in a box to minimize heat loss and direct all air from the fan through the core. Locate a high efficiency fan and circulating pump to minimize Amperes consumed in my modest 12 Volt, two PV panel electrical system.
While the fan, pump, and heater core were purchased new, somewhat less efficient and/or used items could have been obtained at less expense, or recycled items used. The Hartel pump has three advantages over other pumps on the market: (1) high efficiency with very low 0.7 Amp draw, (2) remarkably quiet operation (unlike many other circulating pumps); and (3) no radio interference due to its brushless design.
The fan used in this system is also remarkably efficient and quiet. This is due to the fact that it utilizes a Clifton industrial motor which is a 24 Volt unit running at 12 Volts and therefore half speed at only 750 rpm. The fan has large, very light weight 16 inch aluminum blades. This fan moves an amazing 1,000 cubic feet per minute (CFM) of air using only 1.1 Amps. A standard RV fan draws about 1.4 Amps and moves only 350 CFM.
Putting It Together:
Initially, I used a small RV fan placed behind the heater core and did not enclose them in a box. This arrangement was sufficient to heat the trailer, but high water temperature was required as well as running the fan at full speed. Utilizing the larger fan and enclosing the fan and core in a box dramatically increased efficiency. Much lower water temperature and fan speeds were needed. A small door was cut in one side of the box so that the position of the heater core could be adjusted and hose clamps tightened. A large air in-take hole was cut in the rear of the box. I found it very important to cut the hole for the heater core exactly to size to avoid air leakage and loss of efficiency.
I tried connecting the pump on either side of the heater core in the closed loop. Both positions seemed to work equally well to circulate the water through the core. However, since my water system is not pressurized or connected to any external system, the water storage tank does not fill to the top where the hot water outlet is located. With the pump sucking air instead of hot water, the system didn't work at all. I turned the pump around and reversed the direction of flow in the loop. With the pump now pulling hot water from the bottom of the tank and returning the cooler/used water to the top of the tank, I was in business.
To complete the system, a 12 Volt dimmer control ("conserve switch") was wired to the fan. Being able to adjust the speed of the fan precisely is more than just a luxury since it enables you to control the exact amount of heat convected into the room. The conserve switch is a sophisticated, solid-state device rather than just a few resistors which dissipate and waste the energy not used by the fan as heat. With the fan running most of the time at half speed or less, the dimmer control saves better than half an Ampere per hour. This adds up to a significant savings over time, particularly in a small PV system.
Noel and Barbara Kirby in their book "RVers Guide to Solar Battery Charging" observe that "the typical RV furnace is an abysmal creation that will use all your propane in a weekend and kill your battery overnight." They are dead right. That is just what happened before installing the new hydronic system.
While I live in the Anza-Borrego Desert near Borrego Springs, California where winter temperatures rarely go below the mid-30s. The area has frequent high winds at night. Wind-chill temperatures can be very low and provide a good test of any heating system. Using high fan speed and water temperature, the new system passed the test with flying colors. There were a few memorable windy nights when I felt the trailer was making an unscheduled trip to Arizona, but I am happy to report that at least the journey was a warm one.
In round figures, the hydronic system uses at least 50% less electricity and propane than the forced-air furnace. Both systems use propane as heat sources, but the burner in the furnace is larger and consumes about twice as much propane per hour. Both system use electric power, the hydronic for its pump and fan, the furnace for its fan.
I have found that the burners in each system operate roughly the same length of time, about 25 minutes per hour during the evening with a 35 degree differential between inside and outside. This translates to 3600 BTUs per hour for the hot water system, 8200 BTUs per hour for the forced-air system. While the pump and fan run all the time in the hydronic system, they draw only 0.7 Amps and 0.8 Amps respectively under the above test conditions -- a total of only 1.5 Ampere during the hour. The blower fan for the furnace, however, which draws 6.5 Amps per hour, runs only 25 minutes, drawing a total of 2.8 Amps for the one hour test period.
No heating system relying on propane or natural gas is worth much if the main burners or pilot flame goes out at unexpected and unfortunate moments. I have found propane water heaters to be a contrary lot (at least the RV variety). No amount of cleaning, replacing the thermocouple, etc. seems to cure the tendency for their pilots to blow out. The answer to cold awakenings in the middle of the night is a $30 investment in an electronic reigniter unit. (Of course, if your water heater is a non-pilot model with an electronic ignition,you can skip this section.) These reigniter units consume only 100 milliAmps at 12 VDC and only use power while they are emitting sparks to relight the pilot. They can be hardwired into your system and forgotten. A small 2 inch x 3 inch box mounts on the inside of the hot water heater compartment with a tiny element attached to the pilot burner. Unless you don't mind lighting pilots in the dead of night at 36 degrees F., spend the $30.
If the thermostat in your water heater doesn't maintain the temperature of the water at a more or less constant level, then it will need to be replaced. Since my hot water thermostat, as well as pilot lights have it in for me, my thermostat had to be replaced. The new thermostat does not take holidays and doesn't let the water get cold before deciding to turn back on. Now the hydronic system can function as it is should.
Hydronic vs Catalytic Systems:
Some of my friends (critics) and the Kirby's themselves in their solar/RV book point out that propane catalytic heaters offer essentially the same advantages over a forced-air furnace as the hydronic system. In part, this is true. A catalytic heater is quiet in operation. It consumes less propane than a forced-air furnace, and its heat output is adjustable. The heat produced is gentle and continuous. It has one advantage over the hydronic system in that the unvented models require no fan and therefore no electricity--but see thewarning which follows. Vented catalytic heaters (the "CAT" made in Washington state) use 1/2 Amp per hour for a small fan which is designed to remove some of the exhaust fumes (6,000 BTU model).
The big disadvantage of a catalytic heater is that combustion occurs within the space being heated. Oxygen is used up and exhaust gases produced. I have used both vented and unvented models in my trailer. By the time you have opened enough windows and/or cracked enough doors, so that there is sufficient oxygen being replaced and the exhaust fumes allowed to escape, you have lost a significant proportion of your heat. This is particularly true of unvented catalytic heaters which many people regard as dangerous and which some dealers will not carry. The results of too little oxygen or too much exhaust gas in the room are obvious. I grew tired of worrying about whether I had opened enough windows. After a couple of miscalculations and getting sick, I removed the heater. In a large room with high ceilings, these problems are probably minimal. But in the confines of a 24' foot travel trailer (approximately the square footage of a 12 ft. x 12 ft. bedroom), an unvented catalytic heater was an unsafe and unwise choice for staying warm.
The vented "CAT" heater has a vacuum-like device at the top of the catalytic burner pad which sucks fumes away as they rise from the glowing pad beneath. This results in less fumes escaping into the room, but it is hardly completely effective. Combustion still occurs in the room and uses up oxygen. The need for vigilance in maintaining proper ventilation still exists. I would not recommend even a vented catalytic heater for people with respiratory and/or sinus problems. The "CAT" heater didn't last long in my trailer before I removed it.
A big plus for hydronic heating is that it works well with that wonderful non-polluting, renewable, non-fossil fuel, domestic, abundant, free energy source, our sun. No grisly, global complications are involved in using solar energy...(further and increasingly impassioned pronouncements)...Amen. There are many chilly days in winter when heat is needed; and the sun could be heating water for the hydronic system in coils on the roof. Yes, the plans are on the drawing board.
An automotive type heater core is certainly only one of several possibilities for the heat exchanger unit. For the 140 square feet of space to be heated in my trailer, it worked perfectly. For larger spaces, the coils out of an old refrigerator would seem to be a good candidate. A little imagination and recycling should create any number of workable units.
Myson, Inc. of Gateshead, England make very attractive hydronic "Classic II Fan Convector" units which are coupled with their instantaneous-demand propane water heaters. The fan convector units vary in size from 5,000 to 24,000 BTUs. They have a three speed fan and a heat exchanger made of copper tubing and aluminum fins. For larger applications, they make gas fired boilers rated at 50,000 to 80,000 BTUs to be coupled with multiple fan convector units. The Brits know how to keep warm!
Life is so much more pleasant with hot water heating. No more shouting over the roar of a blower fan. No more feeling cold when the furnace shuts off. Just the amount of heat you want when you want it. Luxury! Visitors, including a somewhat chagrined RV repairman, remark how much more comfortable and warm my trailer feels than it did before (and also how much fresher the air is than in the catalytic heater days). I am using less propane, saving money, and in a tiny way helping the planet by consuming less fossil fuel. The system is easy to install. And it is so much fun to needle my neighbors about their obsolete, old fashioned, forced-air furnaces.
Articles by Jim Phypers / Solar Haven Main Page
(Copyright 1991 by Jim Phypers. All rights reserved.)