Water heater – revisited

9 June 2021 – Water heater Revisited

I needed to come back to do some work on my water heater. Let me explain.

We have a 6 gallon Kuuma water heater on the boat. This water heater has two methods of heating the water, an electric element (controlled by a thermostat) and an engine coolant recirculating loop. When the Volvo engine is running, the coolant is circulated through the water heater, and makes the domestic water in the tank very hot. Even 24 hours later, it’s still hot.

The second method of heating, the electrical element – is the subject of this post. This Kuuma water heater comes standard with a 1500W element. It is also possible to buy a 1200W element or even a 900W element. When docked, it shouldn’t make much difference. A higher power element will consume more electricity, but get hot water faster. Some years ago, I discovered a dual element sold by SV Hotwire that enables boaters to get either AC or DC current into the element, using a dual element: 300W DC or 500W AC.

I hooked this all up several months ago, but unfortunately, one morning I mistakenly left BOTH the AC and DC heating elements in the ON position for about 30 minutes – and sadly, that was enough to burn out the element.

I sourced a replacement dual element from SV Hotwire again, and this time John supplied me with a small timer circuit for control. I used a SPDT 80A relay (sourced from AliExpress) and made up this circuit diagram. I tried a 30A relay, which should have been sufficient since the current for the water heater is about 23A — but it just got too hot with the continuous current. So, I upsized to an 80A relay, with larger connectors and wire – which is more suited to the continuous current. I have considered different methods of controlling this hot water circuit (temperature, battery SOC, battery voltage, time) and eventually decided that a simple timer circuit is best. This way, I can simply set the time (in minutes) at the moment I turn it on. It will automatically shut off when the allotted time is reached. If we have to leave the boat to go shopping, the circuit will shut down on its own. This is the circuit diagram I made up.

This is a photo of the finished product. I considered putting it all in a small box, but in the end, I don’t mind the way it looks. It’s easy to operate, just set the time and push the ON button

One more boat job taken care of.

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