I have an Eberspacher (Espar) Hydronic which is fairly troublesome.
There is a fair amount of maintenance you can do yourself, like clean or change the glow plugs, decarbonize the combustion chamber, and diagnose the thing, even, if you have the more elaborate control panel
which gives diagnostic codes.
After four years you are due for a service
, so I would just call in the authorized service center. By this time you will surely need decarbonization and a glow plug, at the very least, and I think a thorough examination by a trained tech will be valuable enough to be worth not trying to do it yourself.
The fact that you used it only sporadically will not mean it requires less service. On the contrary, according to what all the techs who have worked on mine have told me, they carbon up faster if they are run irregularly and especially if they are run for short periods.
I just had mine done after about three years since the last one. The tech insisted on taking it out of the boat and putting it on his bench. It wasn't all that expensive, and now I know that everything has been gone through. I would not have understood myself what I was looking at, if I had tried to do it myself.
The first time I had it worked on the entire control board had to be replaced, so the problems aren't always obvious mechanical things.
For many boats, there just isn't a reasonable alternative way of heating
the whole boat. But I'm afraid you are somewhat dependent on professional support, with these heaters. Not like one of those drip-feed bulkhead stoves which you can fix with a bent nail and a pair of pliers.