Back boiler - more trouble than it's worth?

[Pete & Helen]

Calorifiers are pressure vessels made of non-corrosive materials that hopefully have been tested to pressures well above 1.3 bar. The difference is that you have no knowledge that your corrosion prone backboiler has been tested to such pressures, so how do you know it's not going to crack or burst below the set pressure of your PRV?

 

The calorifier only has to take the pressure of your water pump, its the internal coil that has to take the pressure of the boiler water

[Stilllearning]

The only backboiler systems I know of are not pressurised. On each boat I have had they were/are open systems with a header tank at the highest point of the system. Here in France we are about to have a stove with back boiler fitted, running the house central heating. It will be an open system with header tank up on the top floor.

On a boat it is the best way to heat the living space IMO, and it runs at atmospheric pressure, about 14 psi.

[John V]

Further to my last post a quick browse of the AGA web site gives a max boiler pressure of 25.55 psi

[John V]

Calorifiers are pressure vessels made of non-corrosive materials that hopefully have been tested to pressures well above 1.3 bar. The difference is that you have no knowledge that your corrosion prone backboiler has been tested to such pressures, so how do you know it's not going to crack or burst below the set pressure of your PRV?

 

If the heating coil cracks or bursts it would be a miracle as the water pressure in the tank if you are using say a Parmax water pump would be 25 psi if the small pump or 40 psi if the big ones and the pressure inside the heating coils would be approx 15 psi

Edited July 1, 2014 by John V

[John V]

I seem to be having trouble with editing that post......

I was trying to add....The point that seems to being missed is that in an un-pressurised system the water pressure in the back boiler in a house with a loft mounted header tank (normal system) only 10 metres above the stove would be.......14 psi this is the same as the pressure in the back boiler of my pressurised system the loading on the components and therefore the risk of failure is identical.

 

If the tank should "burst" (and that is an emotive word that is not really applicable in this context) if the boiler should leak then the resulting flood would be worse with an un-pressurised system due to the greater quantity of water in the header tank

[Ian Fossett]

an extract from the Solid Fuel Association's advice on stoves linked to central heating systems

 

Pipework Design

The output from a solid fuel stove can be

turned down but not turned off. Therefore,

there are two essential rules to be observed

when designing the system.

1. The system must have an unrestricted open

vent to atmosphere.

2. There must be scope for the dissipation of

heat from the appliance in the event of a

pump or power failure. A heat leak radiator

should be provided sized to the turn- down

output of the appliance (usually 25% of

maximum output).

The stove primary pipework should always be

designed so that in the event of a pump

failure, heat can circulate by gravity

(thermosiphon) to the store or to the heat

leak radiator.

Where the primaries are pumped, a bypass

should be fitted to the pump to facilitate an

unrestricted cold feed to the appliance

(10mm diameter).

Important note: A stove should never be

connected to an unvented cylinder unless two

tiers of safety can be provided in accordance

with Part G of the Building Regulations.

Guidance should be sought from the

manufacturer of the system

[John V]

 

an extract from the Solid Fuel Association's advice on stoves linked to central heating systems

 

Pipework Design

The output from a solid fuel stove can be

turned down but not turned off. Therefore,

there are two essential rules to be observed

when designing the system.

1. The system must have an unrestricted open

vent to atmosphere.

This is negated by the last paragraph

2. There must be scope for the dissipation of

heat from the appliance in the event of a

pump or power failure. A heat leak radiator

should be provided sized to the turn- down

output of the appliance (usually 25% of

maximum output).

The stove primary pipework should always be

designed so that in the event of a pump

failure, heat can circulate by gravity

(thermosiphon) to the store or to the heat

leak radiator.

Where the primaries are pumped, a bypass

should be fitted to the pump to facilitate an

unrestricted cold feed to the appliance

(10mm diameter).

This has nothing to do with closed/open systems it concerns design of pumped as opposed to gravity systems

Important note: A stove should never be

connected to an unvented cylinder unless two

tiers of safety can be provided in accordance

with Part G of the Building Regulations.

Guidance should be sought from the

manufacturer of the system

nobody in there right mind would only have one prv in a sealed system

 

[Tony Brooks]

That's the second time you've posted that link, are you sure you've no connection?

 

My thoughts on the first post.

 

I had hoped that the forumites would discuss the design linked to and its application to boats. I note we still have no electrical consumption data.

[John V]

Just been having another dig round stove sites .....new back boiler for Rayburn 400 series is rated at 65 psi

 

I have also been digging around for HSE information and so far the only notes I have found have concerned back boilers that have been taken out of service and have been left partly or almost empty and then sealed..

 

I have yet to hear an explanation of how a pressurised system is quote "very dangerous, even more so on a boat"

[Tom llewellyn]

I have a gravity fed back boiler o my boat.... problem with it tonight.... the water is suddenly dropping in the high up water container and then its bubbling and rising up and spilling over and flooding the cupbard where it sits... anyone any idea what is going on? Up and down up and down 

[BEngo]

It is boiling.  The bubbling and rising is caused by the steam generated.

Turn the fire down, or out  and investigate why the gravity circulation had stopped.  Has someone turned off a radiator or closed a valve that should be open?

N

[LadyG]

My back boiler is a great thing, it's gravity fed, and has some fat copper pipes which send heat up through the dinette, the bathroom, and the bedroom, and the wardrobe, so 

 I have an amazingly warm and dry boat, absolutey no black mould, I shift my laundry and clothes about every so often, they are always dry. I know nothing about installation, and less about presurised / pumps, there is a pressure relief valve at the stove, it is quiescent.

I think my system is perfect for me, stove is central, Webasto kicks in if it dies down.

Edited November 30, 2020 by LadyG

[doratheexplorer]

I thought long and hard about upgrading my Squirrel to have a back boiler.  The stove currently heats my boat fine, so the reason for me would be for the hot water.   I'm  sure if I had it installed, I'd love it while it worked ok, but the issue for me is when something goes wrong.  I don't think I could ever sleep soundly with a system in which water could be boiling in pipes and with the chance that the pressue build up wouldn't be able to escape.  With a pump system, the pump could fail at any time, and it's depleting batteries.  With a gravity system like Tom has described above, if something goes wrong, the gravity circulation slows or stops and the water starts boiling....  the consequences could be very bad indeed. 

 

I'd rather find other ways of heating my water.

[Tracy D'arth]

2 minutes ago, doratheexplorer said:

I thought long and hard about upgrading my Squirrel to have a back boiler.  The stove currently heats my boat fine, so the reason for me would be for the hot water.   I'm  sure if I had it installed, I'd love it while it worked ok, but the issue for me is when something goes wrong.  I don't think I could ever sleep soundly with a system in which water could be boiling in pipes and with the chance that the pressue build up wouldn't be able to escape.  With a pump system, the pump could fail at any time, and it's depleting batteries.  With a gravity system like Tom has described above, if something goes wrong, the gravity circulation slows or stops and the water starts boiling....  the consequences could be very bad indeed. 

 

I'd rather find other ways of heating my water.

There is very little chance of a gravity system going wrong. With an open expansion tank its difficult to imagine how any pressure would build up.

As for fully pumped systems, I believe that they should never be installed with solid fuel on a boat.

 

As for sealed pressurised pumped solid fuel systems I am certain that they should not be allowed on a boat.

[doratheexplorer]

10 minutes ago, Tracy D'arth said:

There is very little chance of a gravity system going wrong. With an open expansion tank its difficult to imagine how any pressure would build up.

As for fully pumped systems, I believe that they should never be installed with solid fuel on a boat.

 

As for sealed pressurised pumped solid fuel systems I am certain that they should not be allowed on a boat.

I think you're right.  But I would still worry about it.  So I prefer not to have one. 

[LadyG]

22 minutes ago, doratheexplorer said:

I thought long and hard about upgrading my Squirrel to have a back boiler.  The stove currently heats my boat fine, so the reason for me would be for the hot water.   I'm  sure if I had it installed, I'd love it while it worked ok, but the issue for me is when something goes wrong.  I don't think I could ever sleep soundly with a system in which water could be boiling in pipes and with the chance that the pressue build up wouldn't be able to escape.  With a pump system, the pump could fail at any time, and it's depleting batteries.  With a gravity system like Tom has described above, if something goes wrong, the gravity circulation slows or stops and the water starts boiling....  the consequences could be very bad indeed. The pipes are quite wide bore , copper, none are in 'a vulnerable location', they just seem to work with no leaks.

 

I'd rather find other ways of heating my water.

I have a pressure relief valve right at the stove, I think there is another at the calorifier, never had any action at either of these, . Never heard any boiling, 

Edited November 30, 2020 by LadyG

[Tony Brooks]

23 minutes ago, Tracy D'arth said:

There is very little chance of a gravity system going wrong. With an open expansion tank its difficult to imagine how any pressure would build up.

As for fully pumped systems, I believe that they should never be installed with solid fuel on a boat.

 

As for sealed pressurised pumped solid fuel systems I am certain that they should not be allowed on a boat.

Agreed - and I would add to your last sentence "or in a house".

 

Gravity system with nice big pipes are arguably the  safest heating system and require virtually no maintenance day to day.

[Jen-in-Wellies]

7 minutes ago, LadyG said:

I have a pressure relief valve right at the stove, I think there is another at the calorifier, never had any action at either of these, . Never heard any boiling, 

The pressure relief valve on the cauliflower is relieving pressure in the hot water inside, not the coolant circulating around the heating coil from the stove. They are separate.

[doratheexplorer]

15 minutes ago, LadyG said:

I have a pressure relief valve right at the stove, I think there is another at the calorifier, never had any action at either of these, . Never heard any boiling, 

Ah, PRVs.  Those things which never go wrong...

17 minutes ago, LadyG said:

I have a pressure relief valve right at the stove, I think there is another at the calorifier, never had any action at either of these, . Never heard any boiling, 

Wondering why you have added this sentence which makes it look like I said something which I didn't?

[Tracy D'arth]

1 minute ago, doratheexplorer said:

Ah, PRVs.  Those things which never go wrong...

 

17 minutes ago, LadyG said:

I have a pressure relief valve right at the stove, I think there is another at the calorifier, never had any action at either of these, . Never heard any boiling, 

The one right at the stove will be welded shut by now by the decomposed rubber washer. The one at the calorifier is in the hot water system not the heating circuit.

[LadyG]

13 minutes ago, doratheexplorer said:

Ah, PRVs.  Those things which never go wrong...

Is that irony? In spite of all the problems and adaptations on the boat (yes it's a boat), the plumbing seems to work, as in, no leaks.

I am not going to tempt fate by dismantling it to change a washer, if that is what they are called.

While we are on the subject of plumbing, I hear my water pump kicking in for a few secs every couple of hours, it's worse if I don't screw mixer taps down quite hard , do these ever need routine maintenance?

I am of the 'ain't broke, don't fix it" school of plumbing, engineering and on this boat, almost everything else!

I carry plenty of non specific spares, as I like to appear to "be prepared". Obviously the non specific spares never seem to actually fit anything, but that's life.....

Edited November 30, 2020 by LadyG

[doratheexplorer]

2 minutes ago, LadyG said:

Is that irony? In spite of all the problems and adaptations on the boat (yes it's a boat), the plumbing seems to work, as in no leaks.

Yes.  PRV are known to fail.  Usually by venting too often.  But they can go the other way.

[nicknorman]

On 01/07/2014 at 18:09, Tony Brooks said:

 

My thoughts on the first post.

 

I had hoped that the forumites would discuss the design linked to and its application to boats. I note we still have no electrical consumption data.

It does look quite a nice idea in some respects but the downsides seem to be overall size - pretty big, tall for a boat and thus reduced flue length and thus reduced “draw”. And electrical consumption and noise as you say.

 

Edited to add - the spec says 50w maximum. So 4A at 12v, which i suppose isn’t too different from a Mikuni /Eberspacher etc. But then these are usually considered to consume too much power to be viable for continuous usage. And of course the 50W is mains power, so you need to have an always-on inverter too, consuming more power.

Edited November 30, 2020 by nicknorman

[Ex Brummie]

On 30/06/2014 at 21:50, Pete & Helen said:

 

Do you just use your stove with no water in the back boiler now? Thought that if you did that it might melt it

why should a dry boiler melt? The rest of the stove doesn't.

[Chewbacka]

3 hours ago, doratheexplorer said:

.........  With a gravity system like Tom has described above, if something goes wrong, the gravity circulation slows or stops and the water starts boiling....  the consequences could be very bad indeed. 

 

I'd rather find other ways of heating my water.

It would be a very interesting day for everyone if gravity were to fail..........