The hot water and cabin heat are designed to be flexible and configurable, but it’s leading to some complexity, which we’ll need to streamline for every day use.

The domestic hot water heater runs on 120v shore power or the generator. But, it also has a heat exchanger connected to the main engine, so any time we drive it will heat up. This is a pretty standard configuration and it’s already pretty versatile and flexible.

The hydronic system is more complex. It’s core functionality is a diesel boiler that runs on the 12v system. This heats a fluid (basically antifreeze) which is pumped to five locations around the boat. Each location has a heater core (that looks like a radiator with a fan), which is controlled by a local thermostat and fan speed switch. So, each zone has it’s own heat and the whole thing can run on the batteries. The hydronic system also has a heat exchanger connected to the main engine, so the boiler can be heated while under way without having to fire it up.

So, when we’re under way, we’ll have hot water, cabin heat, and pilothouse window defrost for “free” by capturing waste engine heat.

Now, the hydronic system also has a fresh water heat exchanger, connected to the domestic hot water loop in line with the domestic hot water heater. This is designed to be an “instant on” system that delivers hot water on demand (as long as the boiler is hot). It also adds some complications. First, when the fresh water heater is off (say we’ve been at anchor for 12 hours), the hydronic system will heat water in-line and then dump it into the domestic hot water heater (11 gallons) – which will be room temperature at that point – cooling it before it reaches the faucet. The same is true the opposite way. If it’s summer and the water in the domestic tank is hot, while the hydronic boiler is off, we’d have the opposite problem (though the hydronic tank is smaller).

Dave’s solution to this is to install a couple of in-line solenoids on the fresh water hot loop. If the domestic heater (120v) does not have power, the solenoid will close and the instant hot water from the hydronic tank will bypass it. This will be closed by default. There will be a similar solenoid on the hydronic fresh water loop which will stay open by default. Each of these can be controlled by a switch in the pilot house. The only wrinkle in this system is if the fresh water in the domestic hot water heater is hot because the engine has been on, the solenoid will be closed and we’ll need to open it manually using a switch (no sense in burning diesel if we already have 11 gallons of hot water sitting in the tank). Also, when in their non-default mode, the solenoids will draw a small amount of power.

Finally, the hydronic system also has two 1500W electric heater coils, so you can heat the water in the boiler without burning diesel. But, given these draw about 15 amps each, they’re really only an option for when we’re on 50-amp shore power or running the generator.

Oh, the hydronic system also has a “summer mode” where you can use it only as an instant on water heater. You simply close the valves on the ship-wide anti-freeze system so the boiler only heats water internally.

So, on the one hand, we’re very pleased with the options for heating the boat and unlimited hot water – in particular, the option to use engine heat, diesel, 12v power, or AC power. (We don’t want to waste any heat generated by the boat if we can redirect it to heating the cabins or the hot water.) On the other, there are so many ways to use it it will be hard to optimize. I’m working on a diagram and some simple if/then scenarios.