The electric portion of my HHE-500-07M Hydro-Hot quit. The shop manual, a voltmeter and 20 minutes trouble shooting led to the conclusion that there was no 120 V power to the heating element. To get to the element, power has to go through a relay and then a high temperature limit switch. The relay is operates from 12 volts that comes from the main control board. With the Hydo-Hot electric switch on and diesel off, the appropriate light was shining on the circuit board and I could trace 120 V to the relay.
That meant the high temp limit thermostat was suspect. I disconnected the wires and the continuity tester would not beep with the probes on the exposed thermostat contacts. I made a temporary jumper to go around the thermostat and the normal amp load showed up on the Aladdin confirming the thermostat was not functioning.
Looking at the wiring, it seemed that to power a 1650 watt element, the wires and thermostat were a little small.
I posted a note on the rvhydronicheaterrepair.com forum. The owner/moderator, Rodger Berke, quickly responded with a part number and strong suggestion for a $114 upgrade kit.
The following Monday, I got the kit with an electrical box extension, a new relay, a new thermostat, wires, screws and wire nuts. The new parts were significantly more heavy duty than the original.
Marty S volunteered to supervise the repair. The only hard part is that all of this is in a tight place. I got all the wires out of the way. A 5/8" socket on an extension quickly removed the old thermostat which then fell apart, except for the brass base. The thermostat comes out of a threaded socket welded to the outside of the tank. No fluid comes out of this hole. Marty had a flexible claw tool (like you would pick up bolt you dropped) that grabbed the new thermostat and allowed me to get it started. Then Marty, with smaller hands than I have, screwed it in to "thumb tight". We took turns putting the wires back. The new relay had screw terminals and the old one had spade terminals. For the twelve volt, we decided not to cut the ends off of the already short wires. Instead Marty put male spade terminals on short wires he made up.
In hindsight, it was all pretty easy and maybe not worth posting. But, the original parts are so cheezy in comparison to the replacement kit, I thought it was worth "spreading the word". With it falling into pieces on removal I doubt it was functioning as a safety device at all -- even if it had been passing the current to the element. If you have the same Hydro-Hot I have, you will have replaced or are going to replace this part. Beaver put a 20 amp breaker feeding 10 gauge wire to a 1650 watt element. The six inches, or so, going to this thermostat looks like 14 gauge. The old thermostat had a crummy little plastic reset button that melted. The old relay is a third the size of the new one. This upgrade kit would not have been assembled if the first set up was adequate.
I suggest you ensure the thermostat is at least inspected on your next service. Maybe you should even have it preemptively replaced. If you have the original still installed, at least consider carrying the upgrade kit in your spares box.
Stay warm!
