A couple of posts back I wrote about replacing the anode in an on board hot water heater. I seem to have a struck a chord and as I thought many correspondents did not even know that they had an anode in their water heater. Interestingly many water heater manufacturers also keep fairly quiet about the need for replacement anodes on a fairly regular basis presumably because they can make more money selling water heaters than anodes. Unlike the anodes in engine heat exchangers, oil coolers and so forth the anodes in water heaters are magnesium not zinc. There is a lot of chatter on some of the internet forums where 'experts' are telling folks to use zinc which is incorrect. Every water heater that I have ever worked on has a aluminum as a common ingredient on there insides and zinc is too close to aluminum on the galvanic scale to do any good, magnesium is farther down the scale and is less noble than the aluminum which it protects.
Many boats are fitted with six gallon water heaters from the likes of Seaward (shown above) or Force Ten and so on. These compact oblong water heaters do sterling service but DO NOT come supplied with a sacrificial anode as standard so from the very moment that they are installed and filled with water they start to corrode from the insides out. The first thing to pack up is often the heating element and this is often the only indication that something amiss to the boat owner and a new water heater is soon required thereafter. Getting the heater out can be a real bear; often installed in the engine room there is frequently little space to spare. Thankfully there is an answer and that is that you can save yourself a big headache and a ton of cash by installing an anode. The six gallon heaters mentioned above have a drain valve which is great when the time comes to winterize the boat but this can be removed and an anode screwed in in it's place, literally a five minute job. The part number for this is 74556, a 4 inch anode which costs $20 or less from one of the many online retailers – far cheaper and a lot less hassle than having to change the entire water heater. The other benefit of this is that with the anode in place of the tap the anode has to be removed to drain the tank so you can see at a glance how much is eroded and replace it if it is more than 50% gone.
While on the subject of anode protection I changed the pencil anode in the engine heat exchanger last weekend and after 100 hours of running you it's not hard to tell the old from the new replacement. This anode cost me $16 from Boatzincs who offer a vast array of zincs to suit almost any application. Cheap insurance I'd say and if you thought I was whining on a bit about the cost of a replacement water heater I would hate to even contemplate the cost of replacement engine parts if replacing this anode were overlooked.