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Jenwil

Backboiler question

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19 minutes ago, rusty69 said:

Ours are diagonal. But, I see what you mean about connecting them on one side,but may be detrimental to the flow through rad and hence heat?

If it was then I can't see how a gravity system would work at all in the first place. Seems to me domestic rads heat evenly along the top even with fairly little flow though them.

But if people prefer one way instead of the other that's fine by me... :)

How did you get on with piping up your rads into the system?

Edited by smileypete

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14 hours ago, Jenwil said:

I'm not planning on having any soldered joins just compression fixings so hopefully that will give some flexibility.

I saw what you did there. 

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A gravity system doesn't need the slope in the flow and return pipes.

The system relies on the water in the radiators to cool and become more dense and fall.

The slopes will have no effect unless there is a substantial temperature difference from one end to the other.

If anything the upwards slope of the flow will actually make things worse as the water starts to cool and present a small back pressure.

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On ‎05‎/‎12‎/‎2017 at 12:40, Jenwil said:

Hot pipes running through cupboards seems a bit strange no?

Hot pipes running through cupboards is Brill. stops cold and damp etc when run under beds, wardrobe spaces etc etc.

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On 07/12/2017 at 19:49, smileypete said:

If it was then I can't see how a gravity system would work at all in the first place. Seems to me domestic rads heat evenly along the top even with fairly little flow though them.

 

Gravity works fine and only a few decades ago was the normal way of heating domestic hot water.

The important thing with radiators is to feed the hot water to the top and have the return water from the bottom. The hot water will fall as it cools.

But you're right in that you need a temperature gradient from top to bottom of the radiator for a gravity system to work.

 

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Diagonal piped radiators produce the most heat.   The technical term is TBOE (Top-Bottom-Opposite Entry)  and is the connection system which gives the rated heat output of the radiator at the rated flow temperature.   Usually 180F so not much like a typical boat set-up!.

In a boat it also gives the shortest 15mm pipe runs in a gravity circulated  high level flow and low level return set-up.

 

N

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3 minutes ago, BEngo said:

Diagonal piped radiators produce the most heat.   The technical term is TBOE (Top-Bottom-Opposite Entry)  and is the connection system which gives the rated heat output of the radiator at the rated flow temperature.   Usually 180F so not much like a typical boat set-up!.

In a boat it also gives the shortest 15mm pipe runs in a gravity circulated  high level flow and low level return set-up.

 

Top and bottom entry are essential for gravity circulation.

One member alluded to the difficulty if a radiator needs to the changed when going for this format, where same side entry would make a radiator change easier.

That is assuming each manufacturer makes slightly different sized radiators, and changes them over time!

 

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On 10/12/2017 at 12:25, Mikexx said:

A gravity system doesn't need the slope in the flow and return pipes.

The system relies on the water in the radiators to cool and become more dense and fall.

The slopes will have no effect unless there is a substantial temperature difference from one end to the other.

If anything the upwards slope of the flow will actually make things worse as the water starts to cool and present a small back pressure.

 

I rather disagree with this first bit.

A slope in the pipes in needed to ensure they properly fill with water during filling and to ensure no air locks occur. The motive force causing the gravity circulation is microscopic and will not shift at air lock like a pump will. 

There MUST be a method at the high end of each near-horizontal pipe run for air to escape. This is usually from the open vent pipe, and may be via the top and bottom connections to a radiator or via the back boiler or with a manual or automatic air vent. 

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12 minutes ago, Mikexx said:

 

Top and bottom entry are essential for gravity circulation.

One member alluded to the difficulty if a radiator needs to the changed when going for this format, where same side entry would make a radiator change easier.

That is assuming each manufacturer makes slightly different sized radiators, and changes them over time!

 

The perhaps you can explain why when I had a gravity system on an Ellis boiler it worked very well with just bottom connections - much like single pipe domestic central heating systems. Before you say it, no my system did not have those venturi T joints on the radiator outlet into the main 28mm pipe.

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19 minutes ago, Mike the Boilerman said:

 

I rather disagree with this first bit.

A slope in the pipes in needed to ensure they properly fill with water during filling and to ensure no air locks occur. The motive force causing the gravity circulation is microscopic and will not shift at air lock like a pump will. 

There MUST be a method at the high end of each near-horizontal pipe run for air to escape. This is usually from the open vent pipe, and may be via the top and bottom connections to a radiator or via the back boiler or with a manual or automatic air vent. 

Most boats have a bow higher than the stern, so there is an inherent small slope. If a vent is placed at the most forward part of the flow pipework, then that should be sufficient.

Certainly agree that airlocks are the bane of gravity systems.

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16 minutes ago, Tony Brooks said:

The perhaps you can explain why when I had a gravity system on an Ellis boiler it worked very well with just bottom connections - much like single pipe domestic central heating systems. Before you say it, no my system did not have those venturi T joints on the radiator outlet into the main 28mm pipe.

I don't know, the only possibility I can suggest is whether the flow was created by gravity circulation from a calorifier at the end of the flow and return pipes?

The system doesn't sound optimal and single pipe is now rarely used in domestic installs. I'm not sure if the regs still allow it?

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On 10/12/2017 at 13:07, Tony Brooks said:

The perhaps you can explain why when I had a gravity system on an Ellis boiler it worked very well with just bottom connections - much like single pipe domestic central heating systems. Before you say it, no my system did not have those venturi T joints on the radiator outlet into the main 28mm pipe.

 

I'd say the answer lies in the exact design of the rest of the system. The water only needs to stratify across the whole system and this can happen in some systems that appear surprisingly gravity-hostile. I attended a flat with a thermal store boiler on Friday that had a microbore pumped CH system installed for example. The flow and return pipe both rose vertically to the microbore manifolds above the ceiling, then 10mm pairs of pipes went off, falling to each rad on the same floor as the boiler. The pesky system still heats the rads when the CH pump is OFF by gravity circulation. A spring loaded NRV was required to stop it. 

On the other hand, sometimes apparently well designed systems refuse to circulate. I think by designing a system well you just make it ever more likely it will work properly. Each compromise introduced means you rely more on luck.

Edited by Mike the Boilerman

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I would suggest that as the boiler was at the stern and the boat sits fairly bow high the main pipe system was very good for gravity circulation.  I have no idea how a calorifier can cause gravity circulation in any way better than a boiler or stove and in fact the calorifier should have had worse circulation normally because the pipe to it ran down hill slightly.

As Mike's example shows you can get gravity circulation under apparently  impossible conditions but I do not think two bottom connections is an impossible situation. The whole system will be cold, as the water heats up it starts to circulate and although (I understand) it tends to convect within the height of the pipe there will be warm/hot and less dense water in the pipe than the rad so water rises into and to the top of the rad. It then spreads across the top of the rad and starts to cool, dropping as it doe so exiting from the other lower connection.

I am merely questioning the assertion that " Top and bottom entry are essential for gravity circulation."  I know they are not from personal experience of both my boat and my previous domestic system.

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2 hours ago, Mikexx said:

A gravity system doesn't need the slope in the flow and return pipes.

The system relies on the water in the radiators to cool and become more dense and fall.

The slopes will have no effect unless there is a substantial temperature difference from one end to the other.

If anything the upwards slope of the flow will actually make things worse as the water starts to cool and present a small back pressure.

I kinda covered that in post #43, trapped air will kill gravity circulation stone dead so a 'self bleeding' system is very useful.

2 hours ago, BEngo said:

Diagonal piped radiators produce the most heat.   The technical term is TBOE (Top-Bottom-Opposite Entry)  and is the connection system which gives the rated heat output of the radiator at the rated flow temperature.   Usually 180F so not much like a typical boat set-up!.

In a boat it also gives the shortest 15mm pipe runs in a gravity circulated  high level flow and low level return set-up.

I don't think so really, a domestic system has bottom entry and exit, usually on startup the hot water evenly fills the top of the rad and then spreads evenly down.

2 hours ago, Tony Brooks said:

The perhaps you can explain why when I had a gravity system on an Ellis boiler it worked very well with just bottom connections - much like single pipe domestic central heating systems. Before you say it, no my system did not have those venturi T joints on the radiator outlet into the main 28mm pipe.

I expect the top backboiler exit was above the bottom rad tapping or at least close to it. Trouble is only say 2% of boat backboiler systems are like that, the rest follow the usual layout with stove at front. What works very well for your sample of 1 will not work for the other 98%. :unsure:

TLDR; I posted a sample layout, if people want to do it differently, that's absolutely fine by me. :) But if it underperforms then what can I do, I tried my best... :mellow:

ETA: probably well worth doing a diagram anyway, it's likely to be a fraction of the time spend on installing and will help identify booboos. Even a sketch on paper is much better than nothing...

Edited by smileypete

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