Installing a Holding Tank Monitor

How well I remember the first (and only) day I overfilled the holding tank. The good news was that nobody was standing beside the boat when the noxious fluid came shooting out of the vent.   After cleaning and deodorizing the dock, I knew I had to prevent such an occurrence from ever happening again.  

My chance came when remodeling the head was next on my project list. I stripped down the head to the bare walls, removed all the old hoses and globe valves and removed the holding tank for steam cleaning. The rebuilding process would include all cabinetry, an additional overhead hatch, new hoses, a new Y-valve, vented loops, a macerator pump, vent line charcoal filter and, of course, a tank monitoring system.  

One tip for you if you ever decide to do this to your boat: adult diapers. Seriously, I bought several packages of adults diapers for use when removing the old plumbing and holding tank because they are small enough to fit the area and they absorb all the spillage. I jammed them around and under each of the hoses before I removed anything. Anything that leaked out leaked onto the diapers, which I then just threw away.  

Back at the project at hand, I found several manufacturers who could supply tank-monitoring systems. Most of these were dual-purpose units that could monitor either fresh-water or black-water tanks. I chose a Tankwatch 4 (R) unit from Sealand Technology for no other reason than I was also using their sanitation hose in my head project.  

The tank monitoring system consists of two main parts. The first is a sending unit that threads into a standard 3-inch NPT cleanout port flange. Most holding tanks are equipped with such a cleanout port, while many water tanks are not. The manufacturer can provide flanges for installation in tanks that don’t have the port already in place. These sending units also have a fitting that will accept a 5/8” vent hose. If your holding tank isn’t vented or is vented through the existing cleanout port, you can use the one in the sending unit.  

These sending units come with either of two lengths of probes, 22-inches or 45-inches long. The probes are polyethylene tubes that carry the float switches and are cut to match the depth of the tank. Each sender has three tubes, one for full, half-full and empty. By the way, a very complete set of instructions is also supplied; so don’t worry about the installation process.   The second part is the monitoring panel. This panel has a series of LED lights to indicate the status of the tank. For a holding tank, red indicates full, amber is one half full and green indicates the tank is empty. The light sequence is reversed if the monitor is installed in a water tank. The control panel comes with two different sets of lens inserts, one for each type of use.  

In addition to the tank monitor kit, you’ll need electrical wire and quick disconnect terminals to complete the installation. Before doing any actual installation you’ll need to find a location for the control panel, find a path for the wiring from the sending unit to the control panel and a source of 12 volt DC power. I placed the control panel in the head, in plain view of anyone using the facilities.  

The first step in the installation process is preparing the sending unit. The probe tubes containing the float switches need to be cut to the proper lengths to match the depth of your tank. This trimming process matches the proper probe lengths to your individual tank. The only caveat here is to make sure you don’t cut the switch wires when you cut the tubes. Once the tubes are cut to length, they are locked in place by tightening up a compression nut.  

The floats need to be configured for the type of tank in which the monitor is being installed. Again, very complete instructions are provided.   The next step is to cut a hole for the control panel. A full size template is provided in the instructions. The hole is 3-7/16” wide and 2-7/15” high. The panel is held in place with 4 #6 x 1” oval head screws. I installed mine in the paneling above the sink in the head. The paneling was 1/2” MDO plywood covered with almond laminate and the black control panel fit perfectly and didn’t look out of place.  

I installed a short terminal strip behind the paneling as a convenient place to terminate all the wiring. I used quick disconnect terminals at the sender end of the wiring. All of the wiring from the sending unit to the terminal block was contained in black vinyl spiral wrap and wire clips every 18”.   Select the correct wiring diagram for the type of tank (fresh water or black water) and connect the wiring accordingly. Apply power and move the floats up and down by hand to make sure you have made the right connections (don’t ask how I know this step is important).  

I then disconnected the wiring at the sending unit and tightened the sending unit in the clean out port. I made sure I remembered to install the O-ring seal. After reconnecting the wires I was in business. I hooked up my tank monitor so that it was powered up any time the main battery switch was on. The unit only draws .017 amps, so battery life wasn’t an issue.   If you have an electric head or a LectraSan, Sealand can provide an optional relay to disable the power to the head when the tank is full. Another option is a “Do Not Flush” indicator light. This is usually installed in the bulkhead behind the commode, presumably where the person using the facility can see it. This obviously lights up when the tank monitor senses a full tank.  

I’ve had the tank monitor installed for over three years now and haven’t overfilled the holding tank once in that time, for which my dock mates are eternally grateful.