Eleven years of light use on my Espar Hydronic D5W diesel heater have been overwhelmingly good. The boiler itself has been trouble-free. When I had it checked out on the bench in 2015 the hours were so low that the technician advised me not to have it cracked open yet.
The reason I had it out for a check-up was that while we were still in the yard in the spring of 2015 it refused to start after a long time sitting and not being run during the 2014-15 layup. The technician found no faults with the boiler, but said that the boiler’s memory reported that the fuel pump was not delivering fuel. I could confirm that because the usual clicking of the pump was not present during my start attempts. The fuel pump problem was due to strain on the pump power wire resulting in a pin in the plug retracting. I re-seated the pin and then lengthened and re-routed the wire, which runs across the back of the space under the cockpit. I'd been working a lot in there and put some strain on it.
During the 2015 cruising season we got out for two shake-down cruises of less than a week each and then, late in the season, for 3 straight weeks in the San Juan Islands. We ran the heater regularly in the evening and during many of our navy-style showers. We have a simple thermostat controller from Espar which has a dial to set the temperature. It has the system power switch, the temperature dial, a power LED (red) and a heater-on status LED (green).
What we noticed during the season was that the green LED and the heater fans would take 30 seconds or longer to come on after the dial was turned up to start the heater. Until this point, the green light and fans would come on immediately with the click of the little relay and the start sequence would begin a few seconds later. The heater always started after the delay and we left well-enough alone.
Well-enough was no longer good enough this fall. We missed the cruising season fixing the engine, but I was still running the heater every 4-6 weeks. A couple weeks ago the symptoms got worse. No green LED, no fans and no heater starting even after many minutes of waiting and power cycling. So, I removed the four wires from the thermostat control and took it off the bulkhead to start the troubleshooting process. The little relay was located there and it seemed like an easy place to start.
I hooked some 12VDC power to the two terminals (1, 4) on my workbench and worked the temperature setting dial and found that the green LED did not illuminate despite the clicking of the relay. Checking the resistance across the relay contacts showed a 500 ohm resistance, not the zero ohms I expected. I traced the circuit and discovered that the green LED got its power through the relay contacts. The green and white wires connected to the contacts were used to trigger other relays on the heater panel in the lazarette. At this point I started looking for a direct replacement for the OKO K6B12 SPDT relay. I found the specs online easily and the contacts were rated at 12 amps! Pretty big current for a tiny little relay. I found a couple places in the UK that could sell me the exact relay for a couple pounds each. I set those aside and began looking for a local source by telephone and by checking websites. Eventually I found that Fry’s Electronics had an NTE R46-5D12-12 relay that looked exactly the same on the outside and had the same 12 amp contacts and 400 ohm coil as the OKO. For $3.79 each I bought two of them.
Before removing the old relay, I decided to test the heater to see if there were any other problems getting the heater to run. I thought about it and decided to use a cutting wheel on my Dremel to carefully cut off the top of the relay to see if I could clean the contacts. I was successful getting the top cut free and didn’t damage the internals. I was able to clean the contacts with a piece of sandpaper and got the resistance across the contacts back to zero. There was no pitting that I can see, so maybe they just got a little dirty from making and breaking for 11 years? The relay is potted on the bottom, so it is a sealed unit. Stuff gets old I guess.
Back at the boat I connected the four wires with an ammeter inline with the relay contacts to measure the current through them. The heater started just fine, with the fans and green LED coming on immediately. The initial current was 2.9 amps and after about 20 minutes I noticed the current came down to 2.5 amps. The 12 amp rating on the contacts is certainly needed in this circuit. So we ran the heater for a good half hour and got all the moisture out of the exhaust. Heated up the interior of the boat nicely. After some more ruminating on this I decided to put the covers back on the thermostat control and use the heater with the exposed relay contacts for awhile. If they last for years, that’s great. It will be easy to shine up the contacts again if they do get dirty. I will carry a spare relay and can replace the it if the time between failures is short.
I'm not recommending anyone cut off the tops of miniature relays. With the replacements in hand, I decided to try to see exactly what the condition of the relay contacts were before taking up the soldering iron.