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hot water system

i am starting a self fit out of a 32ft steel hulled boat. the boat doesnt have an in board engine but i would still like to use a calorifier linked to a backburner on the solid fuel stove. to me it makes sense using the excess heat this way,but i would like a back up source of hot water for times when the woodburner is not in use or just to bring the preheated water from the calorifier to a comfortable showering temperature. the ideal solution would be, i think, an instant gas hot water heater controlled by a thermostat which would kick in whenever the water wasnt at the desired temperature. the only water heaters i can find simply raise the incoming water temp by 25c or so with no control over this. i can imagine a problem with dangerously hot water coming out of the taps.
do you know of an instant hot water heater which can be used like this or have any other suggestions for my hot water needs. the boat is not really long enough to warrent having full central heating, and energy efficiency is very important to me, thanks theo.

Asked by: Theo  | 11.29am, Saturday 12 February

WW says:

Firstly, the boat safety certificate strictly forbids linking a calorifier to any instant gas water heater, as you rightly point out, it could lead to boiling/scalding water coming out of the tap. There is no system that you could use to boost the water currently available.
You could fit an either/or system, where the water is drawn from either the calorifier, or through a gas water heater (properly installed- again, your choice of fitting new ones is limited, but possible if you follow good advice in the BSS guide). The water cannot be allowed to flow through both.
Another way might be to use one of the gas storage boilers, such as used on caravans, etc. Truma make a "boatboiler" which is a 10 or 14 litre container, which is heated by a small gas burner, giving a tank of water at 70 deg C, in about 50-70 min- and is very efficient (much more so than an instantaneous unit). Often installed on cruisers as the hot water system, they are quite tried and tested- they also have a 230V 850W heater input as well, if required for faster heating, or for constant heat overnight connected to a shoreline.
Adding a calorifier to a back boiler is a good idea, though you might want to add a small radiator or finnrad on the downstream flow side, as a extra heat sink- althoug a back boiler might give 1 to 3KW of heat output to a back boiler, once the water tank is hot, the water in the circuit could possibly boil if the energy cannot be dissipated. the circuit should work well by gravity circulation, as long as you get a decent rise on the "up" side from the heater, for at least half the run of piping- we have done articles before on gravity run central heating systems. You will also need a good sized expansion tank, around 10% of the total volume of the system, with an air release.
One final note- if you have stored hot water, it is essential that it is frequently raised to over 63 deg C (preferably 70 deg C) to avoid any potential problems with microbial contamination.
Hope that helps- let us know if you want more detail or more specifics.

Mark Langley  | 10.20PM, Saturday 12 February

Readers say:

thanks very much for the informative answer, much appreciated.

Theo  | 8.29AM, Monday 14 February

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