Is a backboiler a good idea?

[Gi and Barnacle]

Hello Canal Forum Folks

 

This is my first post so, first off, hello. Let me introduce my boat. She's called Barnacle and is an aluminium hulled 28 foot former liveboat that runs off a solar and wind powered electric engine. I wonder if any of you folks could give me some advice about whether to install a back-boiler or not.

 

I have a both a woodstove and a Mikuni diesel heater but, annoyingly the Mikuni only works when its sunny or windy. (When there is enough renewable energy to provide the battery power the Mikuni starter motor requires). In the winter my woodburner is often my only source of heat. I need to replace my woodburner because it has cracks in and I was wondering about replacing it with a woodburner with a back-boiler so that I would get hot water as well as heat in the winter. The backboiler would presumably be fitted with a 12v pump to feed water into my calorifier. I've asked every boat owner I know and I've got a lot of conflicting advice. Some say that back-boilers are troublesome and dangerous, others love them. Do you have any advice? I am particularly concerned about the safety aspect. My 12v batteries are often rather low on power in the darkest months of winter. The pump that pumps water from the back-boiler to the calorifier would need to rely on these batteries. The stove is in the lowest part of the boat so there is no possibility of installing a gravity feed radiator as a heat sink if the pump should fail. Any thoughts would be gratefully received

 

All the best

 

Gi (and Barnacle)

[Captain Fizz]

I think you would struggle to get the required heat loss to stop the system "kettling" and as you rightly say, without the pump running you would definitely be in trouble.

It isn't a problem when a stove is shut down low but when you need good output for heating, it just overheats the water.

[MoominPapa]

The stove is in the lowest part of the boat so there is no possibility of installing a gravity feed radiator as a heat sink if the pump should fail.

That's the best place for it. A gravity circuit works best with the stove low down and the heat-sink higher up.

 

If it were me, I'd go for it if I could get a gravity-loop radiator into the circuit, even if it needed an electric pump to get circulation to the calorifier. Use an unsealed header tank feeding the lowest point on the system, and an expansion pipe from the highest point, rising continuously from the boiler and looped over into the top of the expansion tank.

 

MP.

[nicknorman]

Some people seem to get by happily with just gravity circulation through radiators, but it requires careful installation with wide bore pipe and no sharp corners.

 

If I were doing it (and I might in the future, because our boat is toasty at the front and freezing the back unless I run the radiators from Mikuni or engine) I would design for modest gravity circulation but supplement it with a centrifugal pump. In particular I think a pump is needed to get circulation through a calorifier for hot water. Centrifugal pumps (which represent the majority in this sort of application) don't block the flow too much if they stop running.

 

Then I would just be careful to have the stove turned down very low if I left the boat, so it could dissipate the heat by gravity circulation if the pump should stop. When on the boat (and awake) I could turn the stove up if necessary and rely on a pump to circulate the heat to the calorifier.

 

As you probably know, the potential danger with a back boiler is to overheat the water, boil the contents of the back boiler away so there is just very hot gas in it, which allows it to get very very hot, then a bit of a gurgle and a pulse of water gets back to the boiler and suffers explosive boiling which can rupture the boiler and spread shrapnel. That's a pretty worst case scenario though, and would only happen if the stove was left running fast.

 

Edit: MP beat me to it but at least we are saying the same thing. Unusual on this forum!

Edited October 9, 2013 by nicknorman

[bizzard]

Hello Canal Forum Folks

 

This is my first post so, first off, hello. Let me introduce my boat. She's called Barnacle and is an aluminium hulled 28 foot former liveboat that runs off a solar and wind powered electric engine. I wonder if any of you folks could give me some advice about whether to install a back-boiler or not.

 

I have a both a woodstove and a Mikuni diesel heater but, annoyingly the Mikuni only works when its sunny or windy. (When there is enough renewable energy to provide the battery power the Mikuni starter motor requires). In the winter my woodburner is often my only source of heat. I need to replace my woodburner because it has cracks in and I was wondering about replacing it with a woodburner with a back-boiler so that I would get hot water as well as heat in the winter. The backboiler would presumably be fitted with a 12v pump to feed water into my calorifier. I've asked every boat owner I know and I've got a lot of conflicting advice. Some say that back-boilers are troublesome and dangerous, others love them. Do you have any advice? I am particularly concerned about the safety aspect. My 12v batteries are often rather low on power in the darkest months of winter. The pump that pumps water from the back-boiler to the calorifier would need to rely on these batteries. The stove is in the lowest part of the boat so there is no possibility of installing a gravity feed radiator as a heat sink if the pump should fail. Any thoughts would be gratefully received

 

All the best

 

Gi (and Barnacle)

Ahoy Gi and Barnacle. If you want to burn wood at a really high heat output rate a proper wood burning stove is the thing. But they are the long coffin shaped things with the flu take of at the back of an elongated end with the door at the other, rather ugly and not the ideal shape to fit in a boat unless its fitted sideways. The squat type they call multifuel stoves are ok for burning wood though but have not the heat output power of the proper wood burner already described. Because of your doubtful electricity supply to run a pump I would use your calorifier as the initial heat sink, mounted as high and if possible higher than the stove, plumb it in with as large a bore pipes as possible with gentle bends and or with large radius elbows so that it thermo-syphons properly. Radiators can be T'd off this primary circuit also ideally in thermo-syphon fashion but could be pumped if need be. Most small back boiled stoves have the boiler stuffed in them with no proper room for the flames and heat to lick around them properly or to clear soot from around and on top of them. Morso Squirrel is a classic example of this. Most small back boilered stoves need their body to be a few more inches taller in my opinion to give adequate room around the boiler and a deeper ash pan area. Most multi fuel stoves are bad in this respect and have a mean little shallow ash pan area and door.

[smileypete]

With a small boat I wouldn't have though there's much call for a back boiler, unless used to heat water as well.

 

Do you have gas? The cost of a decent back boiler setup could probably buy a lot of cylinders of calor gas, enough to keep a Morco water heater running for a loooooong long long time

 

cheers, Pete.

~smpt~

Edited October 9, 2013 by smileypete

[SamKingfisher]

I would advise against. The boat is too small to require radiators and a calorifier is not sufficient to stop kettling.

 

Do what Pete says unless you have a gas free boat.

 

I thought about having a very small purpose built second stove in the engine room purely for water heating but in the end I couldn't be arsed also it is probably a BSS fail.

[nicknorman]

Ah I hadn't clocked the length of the boat, yes I agree its too small to warrant a back boiler.

[smileypete]

I have a both a woodstove and a Mikuni diesel heater but, annoyingly the Mikuni only works when its sunny or windy. (When there is enough renewable energy to provide the battery power the Mikuni starter motor requires). In the winter my woodburner is often my only source of heat.

 

If you're that short of power in winter, maybe a reasonably cheap genny/charger combination is worth considering, if so do also look into safe and secure storage for the genny and it's fuel.

 

cheers, Pete.

~smpt~