Our bus has a Hydro-Hot system (HHE-200-09F) which routinely develops trouble. For example, I'm on my 3rd low tank level switch which seems a bit odd. In this write up, I'll present my repair experience which I hope will help others avoid the proverbial cold shower.

The tank level switch simply shuts down the unit if the heat transfer fluid falls below a certain level. An indicator light comes on when this happens. The switch has a float that moves a magnet to and from a proximity element located in the barrel of the switch. An open contact occurs when the level is low. In a pinch the switch can be jumped out but this circumvents a safety device which is dangerous. I don't recommend this as these things have a way of being forgotten and becoming inadvertently permanent.

If your switch is bad you'll need a 7/8" deep socket to remove it. It's located on the right side of the unit. Sort of front top. Look for two yellow wires coming from the body.


Hydro-Hot system in 2006 Allure 430.

Before removing the switch, you'll need to drain out the fluid. Vehicle Systems used cheap plastic fittings on the drain connection so be careful as they break. It's a good idea to replace all the plastic junk with copper fittings while the tank is empty. Chances are good you (or someone else) will need to drain the system again for another repair. I did this and installed a small ball valve to make draining easy. I keep a plastic hose inside the bay which I use for draining. Turn off the unit and run hot water until cool to bring down the fluid temperature a little bit. A clean 5 gallon bucket makes a good container for the fluid.

The level switch is reasonably easy to get at and a deep socket allows room for the wires. I had to cut off the crimp terminals so everything would fit inside the socket. Some people have a special socket with a slit in the side for the wires. Unscrew the old switch and carefully remove it. Spin it around a little if it hangs up in the hole during removal.

When installing the new switch, you'll need to mark the socket at the "NO" stamped on the fitting which needs to face up. If the switch isn't put back properly the float won't work as expected. Wrap the threads with teflon tape to avoid leaks and tighten to the proper orientation. Hook up the wires as required and put back the drained fluid. As you fill, you'll see the low level indicator go out. If this doesn't happen, you screwed something up. Check switch position, wiring etc. Correct any mistakes.

The switch fails because the plastic float stem eventually breaks down after being submerged in propylene-glycol at 190F for several years. Specifically the magnet breaks off.


Broken tank level switch.

As shown in the photo above, the plastic stem where the magnet is molded in is the weak area. I gather the plastic is thin around the magnet which weakens it. The magnet piece just sinks and magnetizes itself to the bottom of the tank. You just hope it doesn't clog one of the pump inlets.


Close up of broken stem. Where did the magnet go?

This is one of those repairs that doesn't really fix the problem it just delays the next repair. I've saved my old switch and will experiment with building a small float assembly from copper tubing using solder. The goal is to make a float that's tight and impervious to propylene-glycol. I'll post a write-up when I have something that works.

Slide show HERE. (2 frames)
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