Gravity fed central heating

[Tomsky]

Hi

 

We are having a boat built later this year, and want a sf stove with back boiler to run 2-3 radiators and for hot water. I have seen references to 'gravity fed' systems, but am wondering how this works (boats are quite flat!).

 

Are there limitations to how well this works - eg length of pipe runs, positioning of rads/calorifier etc? (Boat will be 62').

 

Do people use other (pumped?) systems with sf back boilers, and what are the advantages and disadvantages?

 

Thanks for your help.

[Tony Brooks]

This refers to how the hot water is circulated and it requires large bore pipes to be really effective. It's main advantage is that no electricity is required to make the system work. IT also requires all the radiators etc. to be on the same side of the boat. I would suggest that you get hold of Waterways World New Narrow Boat Builders Book (I think that title is correct) which explains it all.

 

Because of the cheapness of smaller bore plastic pipes and the fact they are easier to hide I suspect the majority of boats mow use a pumped system but it carries an electrical overhead that many live-aboards may have difficulty in meeting. The best pump is probably made by Bolin but it costs over £100.

 

I would say if cost and aesthetics are not limiting factors a well designed and installed gravity system is less problem than a pumped system. However if you have lots of solar or run for many hours a day so electricity is not an issue there is little to choose between the two systems.

[Tomsky]

Thanks Tony.

 

Do you mean that the stove, calorifiers and all the radiators have to be on the same side of the boat?

[Tony Brooks]

Thanks Tony.

 

Do you mean that the stove, calorifiers and all the radiators have to be on the same side of the boat?

 

 

In effect yes because you take the hot (top) outlet from the stove boiler upwards to either just under the gunwales or to almost roof height. The vertical section allows the hot water to rise up the pipe and draw cool water into the boiler. The pipe then runs forwards (usually, as boats tend to trim down by the stern) with the return to the stove almost at floor level. The radiators either plumb into the return pipe as per single pipe central heating systems or between the upper and lower pipe. It is very unlikely that running spur pipes up or down to cross the boat will allow gravity circulation, but nothing is certain.

[blackrose]

Thanks Tony.

 

Do you mean that the stove, calorifiers and all the radiators have to be on the same side of the boat?

 

Getting a calorifier to heat on a thermosyphoning (convection/gravity pumpless) system takes some careful design. It can be done, but the restrictions of narrow coils in most calorifiers a a major obstacle so you should find a calorifier with wide bore coils.

 

I have a backboiler on my stove which feeds two radiators without a pump, but not my calorifier because it's too far away. The main run is 28mm diameter copper pipe and this has to run right around the rads - out over the top of the rads and back under the bottom. Where the pipe goes around a 90 degree bend I have use 2 x 135 degree fittings instead to facilitate flow. I don't really know how much difference this makes but just bear in mind that you're trying to get the hot water to flow around the system by itself so anything you can design into the system such as wide bore pipes should help. Connections from the main run to the rads is done in 15mm copper.

 

On my unpressurised system the main run rises from the stove to under the gunwhales and the header tank is Td off to a higher point inside a cupboard in the bathroom. If you T off at the highest point the system will be self-bleeding. I haven't found much heat loss with this system.

 

Because the copper pipes are often exposed, many people make a feature of them in the boat. Even with 28mm dia pipe there are limitations to what a thermosyphoning system can do - mainly in terms of the length of the main runs.

 

With a pumped system of course you don't have to bother about all that - 15mm flow and return pipes on the floor going to the rads and calorifier is all you need to do.

Edited July 17, 2011 by blackrose

[Gibbo]

Getting a calorifier to heat on a thermosyphoning (convection/gravity pumpless) system takes some careful design.

 

It does indeed. And yet we're always hearing of people having to stick non return valvs in the calorifier feed from the engine to stop it happening because they've inadvertently made a thermosyphon by poor design!

[blackrose]

It does indeed. And yet we're always hearing of people having to stick non return valvs in the calorifier feed from the engine to stop it happening because they've inadvertently made a thermosyphon by poor design!

 

Yes, that's true, but the engine is often in close proximity to the calorifier so I guess it can happen more easily.

 

I wonder if the engine being a big heat sink has anything to do with it? Cold engine draws heat from the warm pipes by thermal conduction and initiates the thermosyphoning process?

Edited July 17, 2011 by blackrose

[Smelly]

We've a gravity fed system that works a treat. The fire is in the middle of the boat with. calorifier pretty much next to the fire and a pipe run to forward rads from there. The saloon's rads tee off down the boat but they work despite a a slight descent with the trim.

 

There's a pretty big rise on the return pipe from for'd that a plumber told me breaches the law of physics but it works; although the forward rads take a while to get hot.

 

I.suspect the magic is as much about the pressure generated in the BB as the.convection carrying it round the system; from cold we.hear the BB fizz then stop and then the for'd.rads, with the rising return have suddenly got hot.

 

Even with a pumped system I'd recommend a gravity fed rad or two; if the batteries fail, as is so often the case, you can still light the fire.

[Tomsky]

In effect yes because you take the hot (top) outlet from the stove boiler upwards to either just under the gunwales or to almost roof height. The vertical section allows the hot water to rise up the pipe and draw cool water into the boiler. The pipe then runs forwards (usually, as boats tend to trim down by the stern) with the return to the stove almost at floor level. The radiators either plumb into the return pipe as per single pipe central heating systems or between the upper and lower pipe. It is very unlikely that running spur pipes up or down to cross the boat will allow gravity circulation, but nothing is certain.

 

I have managed to come up with a plan that has rads on the same side as the sf stove (I have decided not to include the calorifier in the gravity system). The only problem is that the stove will be at the front of the boat, so the pipes will run backwards rather than forwards (2-3 rads , furthest being about 30ft from stove). Will this still work?

[koukouvagia]

The only problem is that the stove will be at the front of the boat, so the pipes will run backwards rather than forwards (2-3 rads , furthest being about 30ft from stove). Will this still work?

 

I've had a gravity system working using 15mm pipes – mainly to avoid the constriction in the calorifier coils as Blackrose has mentioned. It consisted of a Refleks, three radiators and a calorifier.

 

It functioned well, but I found that it worked better if I by-passed the calorifer until the radiators were nicely hot. The important thing is the angle that the flow pipe is set at. This needs to rise 5cm for every 100cm. This was fine when the fire was at the back of the boat, but after a refit, where the fire was at the front, I didn't have enough room under the gunwales for the pipework, so I fitted a pump and had the pipes running at skirting board level – much simpler.

[Speedwheel]

IT also requires all the radiators etc. to be on the same side of the boat.

 

Not necessarily. I've had a boat which had a circular system that runs right around the boat and I know of at least two others with the same type of thing.