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Willis / Belfast / External immersion heater


Onewheeler

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Wondered if anyone has experience of using an external immersion with a marine calorifier. It would be nice to use shore power, and easier to plumb on our boat than installing an Essex flange [note to moderators: that's only rude if you have a filthy mind] on the tank. Is convective circulation likely to be sufficient or would a pump be needed? Thinking of using a 1 kW heater as our shore power capacity is only 6 A. It would take a few hours to heat up a respectable volume of water but fine if on a timer for the morning shower.

Martin/

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Perfectly normal setup on many Boats, in fact you can't buy a Surecal or any other major manufacturer's model without an immersion element any longer.

Edited by NMEA
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Much will depend on the heat absorption capacity of the calorifier coil.  Ideally it needs to be greater than the power of the immersion element so that you get reliable gravity circulation and avoid short cycling (frequent On/OFF swaps) of the thermostat which will shorten its life and the life of the element.  You will need to arrange the coil and plumbing with a rise from the willis to the cal inlet and a fall from the cal outlet to the willis, using the bare minimum number of fittings and bends.

The willis device is usually designed for a vented system so you will need to have a spare coil and fit  an immersion heater header tank  and  separate water circuit .  It is not impossible to conceive a set up where the willis shared a header tank and fluid with the engine coolant, or a central heating system, but it is important to prevent the willis heating the rads or the engine so as to make it more efficient.  Good lagging will be needed to reduce heat loss too. 

I have never seen an Essex flange (of the plumbers variety) big enough to accept an immersion heater!

Normal practice  in the old days before cylinders came with immersion bosses would be to sweat on an immersion heater boss- but I'm not sure about A) the high pressure in a boat system on a soldered boss and B ) how you would make sure the flange is in the right place so an immersion element does not contact one or other of the cal coils.

N

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I (had) fitted a Willis immersion to our domestic tank, although entrusted it to a professional plumber (with instructions on how to fit as well as the included instructions) but he chose to fit as he thought best, and I will now have to re-fit. However I think the concept of the external Willis immersion is excellent and not at all put off by an incompetent plumber. The purpose of my installation is to allow excess solar energy to heat nearly all of the DHW cylinder, rather than to top part only.

However that was on a large vertical domestic tank, although on a smaller boat tank that is possibly (probably ?) using a horizontal tank,  I think it would still work.  It actually heats the secondary water in the tank, as opposed to the water in the calorifier or primary circuit, and so needs only access to the cold in and hot out pipes. If the tank is horizontal, it would probably need a circulating pump as the head may not be enough to provide adequate circulation. No complicated flanges involved as only tee-ing into the input and output pipes is required...

Nick

 

Edited by Nickhlx
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I see no reason why this should not work although you may need a circulating pump. If you want the immersion heater to last you need an incoly element as these are better resisting chalk and corrosion,  although more expensive.

 

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15 hours ago, Nickhlx said:

<snip>  It actually heats the secondary water in the tank, as opposed to the water in the calorifier or primary circuit, and so needs only access to the cold in and hot out pipes. If the tank is horizontal, it would probably need a circulating pump as the head may not be enough to provide adequate circulation. No complicated flanges involved as only tee-ing into the input and output pipes is required...

Nick

 

The secondary water in boat calorifier is at pump cut-out pressure once the pump stops.  This is normally somewhere near 30-35 psi and may go up to the PRV setting, typically 3.5 bar (~50 psi) .  Domestic calorifiers and willis heater cylinders are not made for this sort of pressure and the Willis is likely to quickly spring a leak.  That is why I said:  "The willis device is usually designed for a vented system so you will need to have a spare coil and fit  an immersion heater header tank  and  separate water circuit . "

  It may be that the OP has a low pressure water system, possibly  with a separate pump pressure switch (to check measure the pressure in the accumulator once the pump has stopped) or that his pump cut out pressure can be adjusted to be within the SWP of the Willis unit. 

 

N

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Yes - my Willis is on the secondary circuit and is at tank pressure (about 1 bar). If not a vented system, it would be necessary to fit a pressure blow off valve to this side of the system, venting overboard, in case the thermostat failed - I have also fitted a secondary tank stat to ensure the water can never boil (tank stats can fail in the welded shut position !).  The Willis immersion itself is all brass / copper pipes so would cope with several Bar of pressure, as well as boiling water...

I am sure in my research I came across units rated to well over the 3.5 bar of most pressurised systems ( and I think mine was one of those - it is certainly robust enough, and the element is actually soldered in, as opposed to a screw in type with a fibre washer.

Nick

Edited by Nickhlx
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Yes, I was wary of the common statement that the Willis is usually used on a vented system. The one I am looking at is rated to 3 bar, which is adequately above our system primary pressure and the accumulator / expansion tank is plenty large enough to cope with heating up the water.

Still thinking about it but there don't seem to be major issues that can't be resolved.

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I guess if you have an immersion inside your DHW cylinder, it is no more hazardous having it outside - both will need a means of venting if the thermostat fails...   I was told that it is not unknown for the immersion heater thermostats to fail closed which is why I fitted a new one and also a further tank stat in series with it at the top of my DHW cylinder to further minimise the possibility of it boiling, and its also a vented system. We heat all our domestic hot water from excess solar that would otherwise need heating from the oil boiler or the grid, all managed by an "Intelligent Immersion 3" box, which proportionally diverts excess solar energy that would otherwise go out to the grid - two power showers and all hot water each day take around 8kwH per day to replenish....

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