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Hydronic RV Heating System by Jim Phypers

PV Modules provide electric power for pump and fan

PV Modules provide electric power for pump and fan

Car Heater Core

position of the heater core could be adjusted and hose clamps tightened. A large air intake 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. Big Advantages

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.

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.

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

Hydronin Heating Advantages 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-a total of only 1.5 Amp-hrs. The furnace's fan, which draws 6.5 Amps, runs only 25 minutes, drawing a total of 2.8 Amp-hrs. for the one hour test period.

Two Complications

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°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

Hydronin Heating Advantages

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