The Hydro-Hot system uses heat transfer fluid HTF (propylene-glycol solution heated to 190F) stored in a tank to impart heat energy to water through a heat exchanger. As cold water enters the heat exchanger it cools the heat transfer fluid which creates cold spots or a temperature gradient within the tank. These cold spots or gradient prevent further heating of the water. A stir pump has been included which removes HTF from the bottom of the tank and injects it in the top. This "stirs" the HTF which reduces cold spots and gradients thus allowing more heat energy to be transferred to the water. At the tap, it avoids annoying losses in hot water.

The stir pump automatically cycles when the burner comes on and/or demand for hot water is continuous. Our Hydro-Hot system is a model HHE-200-09F circa 2005. Some time ago, I noticed a delayed start on the burner when using hot water and hot water running cold in just a few minutes. This prevented using the shower. Laundry, dish washing and sink activities would often be conducted with cool water. Having enough of this inconvenience, I decided to investigate which revealed a failed stir pump.


12 VDC Stir pump made by GRI. Pump has been taken apart to show internals.


The impeller's plastic broke down in the presence of propylene-glycol in several places.

There's a kit available for between $350 - $375 which includes a new Cooper Standard pump, hoses, restraints, cable and bracket. The Cooper Standard pump appears to have greater pumping capacity which is good. I can't say how well its plastic parts will endure repeated exposure to hot propylene-glycol. Time will tell on this.

The stir pump is located in the back right corner of the Hydro-Hot unit. You'll need to remove the Webasto burner assembly to get at the pump. Before removing the pump, turn off the Hydro-Hot, let it cool off and drain out the heat transfer fluid. The pump's bracket is riveted in with aluminum pop rivets. It was easier to remove three screws from the pump's housing to remove the pump from the bracket. Needle nosed pliers were used to remove the 4 tension clamps on the hoses. Wire spade connectors to the motor parted easily. Luckily hoses came off the tank barb fittings with relative ease. Once you remove the pump with hoses, you can access the pop rivets. Drill them out using a 3/16" bit. It's a bit of a pain in the ass to keep the drill centered on the rivets but sooner or later they drill out. Carefully clean up the metal filings.

Follow the instructions that came with the kit to install the new pump in your unit. In my case, the bracket wasn't the correct one. And the tension clamps for the tank barb fittings were almost impossible to deal with. I used the old ones over again for these connections. As you will discover, getting back in there is a bit awkward. You'll need to cut the new hoses to their proper length. Make sure the outlet hose bends without any crimps. The intake hose is almost a straight shot into the tank. I put the hoses on the pump first and secured them. Remember to put the tank side clamps on before sliding the hoses over the barb fittings. Secure clamps with needle nosed pliers. Make sure it's all tight. Reconnect power to the motor using the supplied short cable. Not sure what advice to give on the pump's mounting bracket as mine didn't fit.


The GRI pump includes a magnetic drive.


The pump housing was in good shape.


Motor and impeller showing magnetic couplings.

The new stir pump has solved the problem of quickly running out of hot water. During showers a small loss of hot water still occurs but is easily compensated for at the faucet. Perhaps some tuning and/or adjusting of the anti-scald valve might improve things. Overall I'd say the Hydro-Hot is working now within design parameters. Nothing is absolutely perfect, however. Showers are good as are all other activities that require uninterrupted hot water. Problem solved.

I think the OEM stir pump should have been made to withstand the fluid it was expected to pump. Perhaps Vehicle System chose an incorrect part for the application. Who knows at this point. As I've said before, I don't think expensive parts should fail as quickly as they do. In addition to a loss of one impeller blade the guide shaft came loose which caused a complete loss of pumping action. The impeller just crumbles when you subject it to mechanical force. I think a cast metal would be a better material for this application. If that's not possible, pick the right plastic then. Overall this repair wasn't difficult and could be done by anyone that's handy. The replacement pump seems to have better capacity and that's a good thing. Truth be known, I'm off to take a nice hot shower right now!

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