Everything about Gravity fed systems for heating with a backboiler

[mascip]

Hi, here is all the information that I could find on this subject:

 

Gravity fed systems for heating with a backboiler

 

- the Intro section says enough that you can get a rough idea of what you must do (Added on the 18/09/2015)

- Key principles is enough to get you started

- and in Design there is more information for geeks who like to understand it all, or for people who need to debug

 

I am designing a simple system which does not include a calorifier, so I have not included any information on this subject in this Summary. If you would like to add it, let me know and I will let you modify the text.

 

 

There are still some questions that have not been answered. I have copied all of them here, below.

If you have a short and clear answer to any of these questions, please post it here and I will add it to this summary. I might start discussion threads for some of these questions, if it turns out that the answer is complex and needs further discussion, or if no-one answers the question in the next few days.

 

 

Important questions:

Slope:

- Why is the return pipe often straight-ish? Is the slope on the top pipe more important than the slope on the return? If yes, why?

Pipes:

- do you anneal copper pipes yourself, or do you buy them annealed?

- could I use plastic hose to connect both the top pipe and return pipe to the radiators? That would save a lot of copper pipe bending. What would be the drawback, apart from appearance? The hose would have to tolerate boiling water temperature, too...

Header tank:

- is it a good/bad idea to have the top of the header tank outside of the boat?

Radiators:

- how can I know whether a radiator has a suitable bore, or whether it will cause too much friction?

Coolant:

- can you use tap water, or should you use distilled water? Why?

Fiiling ratio:

- how much water / coolant do you put in your system? (read the relevant part of the document, for more info)

Geek questions:

Thermostat:

- is a thermostat only useful if you have a pump, or also for gravity-fed systems? (to prevent the water from flowing below 30C, for example) And where would you put it? On the return, on the top pipe, or on both?

Pump: Apparently a centrifugal central heating pump would allow water to circulate via gravity when it's not running. Is that true? How would you choose such a pump? (how much power, etc)

Anti-freeze:

- Why do you need to premix anti-freeze and water before? (what difference does it make?)

- How much anti-freeze do you put, with water? 50%? But Anti-freeze does not flow as well as water, and does not transfer heat as well, so less anti-freeze might be better? Is 30% alright in the winter?

Pipe size:

- Why do people recommend a 22mm pipe to the header tank, instead of a 28mm pipe which would make it easier for hot water to escape?

- Is it better to have 28 to 15mm adapters on the rads, or to have 15mm pipes Tee-ing to the rads? The second options enables to have a tighter bend, which causes friction, but enables the radiators to be higher.

Misc:

- SmileyPete said this in an old post, and I don't understand the word "lagged" in this context, any idea? Here is the text:

"a good way of doing it is to have an adequately sized gravity 'heat dump' rad(s) near or next to the stove, then more distant rads and/or calorifier on a pumped circuit connected by 15mm pipes, ideally lagged. That way the gravity rads can be shut off and all the heat directed to the furthest rad."

------------

I wrote this by reading, organizing and summing up all of the relevant information discussions threads that are mentioned in the Index of common topics on this forum, plus a few other sources of information on internet.

Edited September 19, 2015 by mascip

[talisman]

Wow. Need to read a couple of times lol.

[mascip]

I'm happy to improve the text if some parts are unclear or ambiguous, just let me know.

[koukouvagia]

I'm glad I didn't have this list before I fitted central heating to our two boats. I'd be still figuring everything out!

[MtB]

My eyes glazed over by the time I was 20% in.

 

Most of what I read seemed correct, if blindingly obvious, but then I 'd expect to have a better grasp of the subject than a layman given my occupation.

 

So overall, it looks to me as though it's probably a really useful compilation for someone designing and fitting a natural convection heating system. I'll try to read the whole of it later and if there is anything I think could do with amending or clarifying, I'll post again.

[Bee]

Cor, that's a lot of work. Best thing is with a gravity system is that its simple (!) Start low, finish high, then return to bottom of boiler. Take expansion/header tank from highest point of hot water rise or the convection gets confused and gets stuck or surges or odd things happen. Difficult to achieve constant slopes and tidy pipework but it is a boat and not a Barratt home so a few pipes really don't matter. Its 'fit and forget' when it all works.

[OldPeculier]

I think you have covered most points there.

 

I finished and commissioned my heating system a few days ago and I would agree that its best kept as simple as possible.

 

Anyone have any recommendations as to what to fill it with? I have put water in for the moment - just in case it leaked, but will need to put some additive in next time I go to the boat.

[mascip]

I wrote this for myself, so it wasn't written as an introduction to the subject. More like an overview of all information to have in mind, when either designing or debugging.

Someone on London Boaters told me that it made them realize that heat dissipation in the pipes is the cause for their last rad to be lukewarm. So it has been useful already :-)

 

I am modifying the text right now to add an introduction. The "Design" section is only useful once you want to debug, or if you like to know everything on the subject before getting started; which is the case for me but not for everyone. So this "Intro" section should be enough for people to get started, hopefully.

 

I will post here to say when I've finished doing it. I probably need an hour or so.

[mascip]

Ok, I have done modifications:

- the Intro section says enough that people can get a rough idea of what they must do

- in Key principles they should find enough info to get started

- and in Design there is more information for geeks who like to understand it all, or for people who need to debug

 

I have removed the text from the original post, because it's much more readable in the link I think: if you only read the words in bold it gives you a good overview of the content. Plus, folding/unfolding makes for easier reading. And I can't be bothered to copy-paste it all here every time I make a change.

 

Any feedback, well appreciated. Any answers to questions, very welcome please ;-)

[mascip]

In the first post of this thread I have listed all the questions that need answers. Any help would be very appreciated. Answers will be added to the document, so everyone benefits from them.

[mascip]

I have answered one of my questions and added information about the pipe slope.

 

 

Found by searching "thermosyphon angle". The keyword "thermosyphon" returns a lot more relevant results than "gravity fed", in particular, many scientific articles.

 

 

And added a question:

- can you use tap water, or should you use distilled water? Why?

Edited September 19, 2015 by mascip

[mascip]

By reading more scientific litterature, I found that the Filling ratio is important: how much water do you put in the system?

 

If you don't put enough, then there is less contact between water and backboiler, so it's harder for the system to start.

 

If you put too much, then it creates large bubbles and film at the bottom of the backboiler, which hinders performance.

 

Has anyone here noticed a difference depending on the amount of water that you put in system?

[mascip]

I have updated the document with more information, in particular about the "equivalent length of pipe" of various fittings: bends, tees, valves, etc. Which gives you an idea of the amount of friction that you are adding in your system. Or that you could remove from your system by changing fittings.

 

I have also answered a few of my own questions, but for some reason I cannot edit my first post anymore. Any idea why?

[MtB]

By reading more scientific litterature, I found that the Filling ratio is important: how much water do you put in the system?

 

If you don't put enough, then there is less contact between water and backboiler, so it's harder for the system to start.

 

If you put too much, then it creates large bubbles and film at the bottom of the backboiler, which hinders performance.

 

Has anyone here noticed a difference depending on the amount of water that you put in system?

 

 

Dare I suggest you are 'overthinking' this?

 

You put as much water in the system as is necessary to fill it up, no more and no less.

 

I've never encountered the term 'filling ratio', but in my opinion it is meaningless drivel.

[mascip]

I'm definitely overthinking it ;-) The good thing is: I'm also getting active and making it happen on my boat. It's going to take a while though: I need to move the water tanks, remove some benches, etc.

 

No filling ratio then. Maybe it's only useful for more finicky applications. Thanks for the info :-) I'll remove it from the document.

[TheBiscuits]

Dare I suggest you are 'overthinking' this?

 

You put as much water in the system as is necessary to fill it up, no more and no less.

 

I've never encountered the term 'filling ratio', but in my opinion it is meaningless drivel.

Filling ratio is critically important in CLOSED loop thermosyphon systems.

 

You wouldn't ever want to use one of those as a backboiler loop in an enclosed space!

 

They tend to be used in things like solar hot water heating systems - although a solar PV panel running an electric pump is more efficient. There are also designs to use them as cooling systems for certain types of nuclear reactor, as a true fail-safe coolant loop.

[mascip]

Oh, that makes sense. Thank you. So I should always search "open thermosyphon" rather than simply "thermosyphon".

[springy]

 

 

Important questions:

Slope:

- Why is the return pipe often straight-ish? Is the slope on the top pipe more important than the slope on the return? If yes, why?

Header tank:

- is it a good/bad idea to have the top of the header tank outside of the boat?

Coolant:

- can you use tap water, or should you use distilled water? Why?

Geek questions:

Thermostat:

- is a thermostat only useful if you have a pump, or also for gravity-fed systems? (to prevent the water from flowing below 30C, for example) And where would you put it? On the return, on the top pipe, or on both?

Pump: Apparently a centrifugal central heating pump would allow water to circulate via gravity when it's not running. Is that true? How would you choose such a pump? (how much power, etc)

Anti-freeze:

- Why do you need to premix anti-freeze and water before? (what difference does it make?)

- How much anti-freeze do you put, with water? 50%? But Anti-freeze does not flow as well as water, and does not transfer heat as well, so less anti-freeze might be better? Is 30% alright in the winter?

<snip>

 

Some specific thoughts :

Slope - Practicalities - most efficient thermosyphon would have the heat sinks (radiator or calorifier) vertically

above the heat source. Not a practical option. A rising part of the system is however required, and practicalities

usually result in a gentle rise between the outlet of the heat source and inlets of the heat sinks. With the remainder

(return side) being left to fit in as best possible.

 

Header Tank :- provision must be made for the possibility of the header tank overflowing or releasing steam - putting

it outside solves some of this, however it will also act as a heat sink dumping precious cabin warming heat outside.

If using antifreeze then some arrangement should be made to catch any overflow.

 

Tap water/Distilled Water:- purity will keep the insides of the system cleaner - scale deposits on the inside

of the boiler etc, if you have a handy supply of "purer" water (for example from a dehumidifier) then its worth using.

 

Thermostat - valve NO, switch - what for ?

 

Pump - centrifugal pumps generally allow flow past the impeller, some more so than others, but they may be less

"efficient" when powered.

 

Antifreeze - How much - depends on how much frost you expect. Pre-mixing - Definitely on a pure gravity

system - neat antifreeze is denser than water and will lie on the bottom of the system, a pumped system

will probably eventually produce a thorough mixing but best mixed beforehand.

 

springy

[smileypete]

Important questions:

Slope:

- Why is the return pipe often straight-ish? Is the slope on the top pipe more important than the slope on the return? If yes, why?

 

I reckon slope is a little overrated and misunderstood, but if it helps ensure the rad is sufficently above the backboiler and airlocks are avoided, then a little 'misunderstanding' is probably not a bad thing!

 

Pipes:

- do you anneal copper pipes yourself, or do you buy them annealed?

- could I use plastic hose to connect both the top pipe and return pipe to the radiators? That would save a lot of copper pipe bending. What would be the drawback, apart from appearance? The hose would have to tolerate boiling water temperature, too...

 

Standard plumbing copper pipe will come in 'half hard' which is quite rigid, but a google search will turn up plenty of info for 'annealing copper' For the rad connections I'd just use copper, a bit more on that below...

 

To get a gentler bend in 28mm, one way is to use 2 45° bends, or, pack a section of annealed pipe with sand, fold the ends (?), then bend around something suitable. There are large radius elbows available, a bit pricey but a couple may not break the bank.

 

Header tank:

- is it a good/bad idea to have the top of the header tank outside of the boat?

 

Mainly frost/freezing considerations, an overheating back boiler and frozen header may result in a steam explosion, so far best to have inside. The sound of a 'kettling' back boiler will make you well aware before the bote turns into a sauna.

 

Radiators:

- how can I know whether a radiator has a suitable bore, or whether it will cause too much friction?

 

I'd use something like a Kudox Type 22 from Screwfix, a type 22 rad is a double rad with 1 set of fins, and a reasonable space compromise for a boat.

 

Anti-freeze:

- Why do you need to premix anti-freeze and water before? (what difference does it make?)

- How much anti-freeze do you put, with water? 50%? But Anti-freeze does not flow as well as water, and does not transfer heat as well, so less anti-freeze might be better? Is 30% alright in the winter?

As mentioned the antifreeze will stratify and prevent thermosyphoning. But I reckon even a small 12V circulation pump may fail to mix it fully, particularly if there are gravity rads too.

 

30% sounds like it'll give a reasonable level of protection for a canal boat, dunno the exact figure though, anyone know?

 

Pipe size:

- Why do people recommend a 22mm pipe to the header tank, instead of a 28mm pipe which would make it easier for hot water to escape?

- Is it better to have 28 to 15mm adapters on the rads, or to have 15mm pipes Tee-ing to the rads? The second options enables to have a tighter bend, which causes friction, but enables the radiators to be higher.

 

I s'pose 22mm header tank vent pipe would be quite big enough for a typical boat stove backboiler which is a kW or thereabouts.

 

For connecting thermosyphon rads I'd do the bend with a 22mm 'long radius street elbow' (see Ebay for examples) or 28mm elbow, then reduce just before the rad 'tapping' (connection)

 

If it's a larger thermosyphon rad say > 1kW, it may be good to use both top and both bottom radiator tappings.

 

Pump: Apparently a centrifugal central heating pump would allow water to circulate via gravity when it's not running. Is that true? How would you choose such a pump? (how much power, etc)

 

For a boat stove back boiler a common pump used is the venerable £25 'solarproject' pump, some still-cheaper equivalents exist on Ebay:

 

http://shop.solarproject.co.uk/catalog/product_info.php?products_id=10

 

Misc:

- SmileyPete said this in an old post, and I don't understand the word "lagged" in this context, any idea? Here is the text:

"a good way of doing it is to have an adequately sized gravity 'heat dump' rad(s) near or next to the stove, then more distant rads and/or calorifier on a pumped circuit connected by 15mm pipes, ideally lagged. That way the gravity rads can be shut off and all the heat directed to the furthest rad."

Yeah sorry by lagging I mean pipe insulation. Depends whether heat loss from the 15mm pipes is acceptable or not, eg if they run through an uninsulated engine room, maybe better to say 'ideally insulated in some circumstances'.

 

Anyway great summary, maybe best not to wander too far into esotoric design considerations.

 

The systems that problems are reported about on here - usually on a recently purchased boat - tend to fail to meet some or all the best practices and where they do, they push the envelope tooo far.

 

Anyway I'm sure you've come across one of my sample layouts, eg:

 

 

I thought it was simple but even so it seems to put some people off from the whole idea.

 

Maybe it'd be useful to post a diagram and design details of your system for comments and feedback, plus any more questions?

 

Personally I think posting everything here with gallery pics is better than an online document somewhere, the latter may disappear if the hosting service is changed or removed. There seems to be an option for attaching documents to a post too.

 

A final note - BES Gas and Plumbing is a great online retailer for a wide variety plumbing bits, there's quite an art to figuring the best and cheapest combination of bits to use to solve a problem...

 

Anyway time for bed <yawn>...

 

cheers, Pete.

~smpt~

Edited September 24, 2015 by smileypete

[bigcol]

Hi all

 

I had a gravity system installed last year, I drained the system to fix a leak yesterday, that sorted. since that I must have a large air blockage and trying to get rid of it, by bleeding can get hot water through to bedroom by taking cold out of the bottom of rads, and the hot water replaces the cold working along to the bedroom bottom pipe. But air seams to be in the top pipe.

 

I will gladly pay someone to help and have a look if they know about central heating.

Even when I put the eberspacher or and pump on it still doesn't heat the far pipe and 2 rads.

 

I need someone who can look at the system which has worked really really well until I touched.

 

Help!!!!!!

[MtB]

Hi all

 

I had a gravity system installed last year, I drained the system to fix a leak yesterday, that sorted. since that I must have a large air blockage and trying to get rid of it, by bleeding can get hot water through to bedroom by taking cold out of the bottom of rads, and the hot water replaces the cold working along to the bedroom bottom pipe. But air seams to be in the top pipe.

 

I will gladly pay someone to help and have a look if they know about central heating.

Even when I put the eberspacher or and pump on it still doesn't heat the far pipe and 2 rads.

 

I need someone who can look at the system which has worked really really well until I touched.

 

Help!!!!!!

 

 

 

 

Eberspachers should never be connected to a gravity system.

 

I f there is a pump, it isn't a gravity system!

I suggest you contact the installer as he will know what the hell he has installed and how to make it work...

[bigcol]

The heating works well heating bathroom, wheelhouse and saloon, with or without the pump. We never have the pump on really.

Eberspacher also works well, it's just the bedroom the other end of boat that I've tried to get bled. Air seems to be top pipe not the 2 rads.

 

Col

[MtB]

As I said, get the original installer back to diagnose and trace the fault. He knows what the heck is going on.

[bigcol]

Hi Mike, I agree, but I was the one that thought I sort a small leak out on my own and drained the system,flushed through ,ran new inhibitor through all to save money.

 

Yes you must come across this all the time, people thinking they are plumbers, and then having to call a professional in to sort it.

Which is what's happened here.

, the original installer is now cruising the Thames. Not able to get hold of him.

 

Now I need a professional, other than bleeding radiators, is their a purging type system air getting riddler thingy ma jig.

[mascip]

Thanks a lot for your input, springy and smileypete! I will add it all in the document when I work on my system again. I've had other life emergencies to take care of, so will get back to it in a few weeks.

 

Then I will copy paste part of the document here in a new thread, because I cannot Edit the first message of this thread anymore.

 

 

Oh! And smileypete, I saw this diagram too, but because I am not going to connect a calorifier to my system, I didn't keep any calorifier-related information. Even with the simplest system, I manage to get into too many details

If you or anyone wants to modify the text and add things about calorifier, I can give you writing access.