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Kitten Sniffer

Gravity back boiler with pumped circuit

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Hi guys, I'm fitting a new burner with back boiler to the boat. The previous system was wholly pumped, but I want the primary circuit to be gravity fed and then the rads and calorifier to be pumped. Do I need a feed and expansion set up or will a simple one pipe header be sufficient? Any thoughts would be greatly appreciated.

 

Note- I didn't draw it but was planning a cold feed to the header with a copper ball valve version of a cistern filler.

IMG_20190913_135336_hdr.jpg

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The connection to the header tank must be at the highest point of the gravity circuit. If you are going to use a single feed/expansion pipe I sugest it should be at least 22m diameter to allow any steam to flow up while water flows down if the boiler should boil.

If the layout permits it would be better to put the calorifier on the gravity primary circuit, rather than the pumped secondary.

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Personally I would connect the header tank top the lowest point in the circuit and then provide a vent pipe from the highest. In that way I think you minimise the danger of airlocks. I would not rely upon the header tank pipe as the vent in case of the stove running away and boiling its water because there would be more water around  to be sprayed all over the interior.

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I faced a similar issue. What troubled me most was how to ensure that the pumped water didn't just find the route of least resistance and circulate around the heat sink rad. I put in a clack valve reasoning that the heat sink was just a failsafe and in an emergency the flow would force the water past it. Not sure if that is the case as unfortunately, it has not been used yet as I've been suffering with chronic joint pain for a while so everything has been on hold. I was also advised via this forum to use a vehicle expansion tank as the header and not bother with the ball cock, instead just monitor it and top up as necessary. 

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What output is your backboiler, and how big is the circuit? I think your problem is more likely to be controlling the pump so that you don't circulate cold water. I wouldn't think you need a heat leak radiator unless you have one very big fire.

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29 minutes ago, stegra said:

I included the heat sink radiator purely as a safety back up in case the pump failed. 

If it’s a centrifugal pump, water can flow through it when it’s not running.

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David- I was planning on using 28mm to the header to avoid any restrictions. And, I intend on repurposing an old copper sink as the header...

Tony- would you still have concerns with a shared vent and feed at 28mm? And if not, is the pipework to the header best on flow (highest) or return (lowest) and if separate feed an return indicated what bore will suit- as 28mm is £24 a length, I'd prefer to use only where necessary...

Stegra- the pump will be on a tee and will only have chance to affect the circuit flow where it rejoins the return and the injector tee deals with flow here.

Ex Brummie- the boiler is 2kw and the pump will be switched via a thermostat so won't run if the system is cold (I'll set it for 55-65C) the heat leak rad will be 10% of boiler output and if my grasp of fluid dynamics/path of least resistance is correct, then it will stay cool while the pump runs, stop boom times if the pump fails and allow the pump to switch off as the burner cools but maintain circulation.

Chewbacca- the pump is a cute low power circulation thing, though I doubt any of the pumped circuit will work via gravity as the bore is 15mm, convoluted and designed only for pumping

Edited by Kitten Sniffer
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1 hour ago, Kitten Sniffer said:

David- I was planning on using 28mm to the header to avoid any restrictions. And, I intend on repurposing an old copper sink as the header...

Tony- would you still have concerns with a shared vent and feed at 28mm? And if not, is the pipework to the header best on flow (highest) or return (lowest) and if separate feed an return indicated what bore will suit- as 28mm is £24 a length, I'd prefer to use only where necessary...

Stegra- the pump will be on a tee and will only have chance to affect the circuit flow where it rejoins the return and the injector tee deals with flow here.

Ex Brummie- the boiler is 2kw and the pump will be switched via a thermostat so won't run if the system is cold (I'll set it for 55-65C) the heat leak rad will be 10% of boiler output and if my grasp of fluid dynamics/path of least resistance is correct, then it will stay cool while the pump runs, stop boom times if the pump fails and allow the pump to switch off as the burner cools but maintain circulation.

Chewbacca- the pump is a cute low power circulation thing, though I doubt any of the pumped circuit will work via gravity as the bore is 15mm, convoluted and designed only for pumping

 

I have no intention of giving a definitive answer your question about 28mm pipe, you may or may not be correct in that it will both fill and vent air while filling but I would not bank on it. 15mm pipe should be perfectly adequate for the feed and although I would prefer 22mm for the vent I am sure many boats run perfectly satisfactorily with a 15mm vent pipe.  It is your boat so your choice but I think trying to minimise the likelihood of getting air locks when filling is a good policy.

 

I bet a stove run away will require more than a 10% heat sink - runaway stove have been known to melt the glass and fire bars.

Edited by Tony Brooks

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2kw is barely more than kettle sized. The amount of water you are talking about is never going to cause any problem, and you would be amazed at how well thermo syphoning will work on 15 mm pipework.

For 30 years, I have run a 4kw back boiler on an open system with a 15mm feed/vent from a header tank, a 3 kw radiator and a thermostatically controlled pump, very small, which I only switch on for the start up, maybe 2 hours. Once the system has warmed up, the 15 mm circuit works well on gravity. If I leave the pump on, it runs for about 40 seconds in 4 minutes.

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Thanks Tony, I take full responsibility for my installation and will consult as widely as possible for info.  My first gravity system. Separate vent and feed seems more sensible than hoping a large enough bore would allow for both. Seems wise with two pipes to have the feed on the return and vent at the top of the flow. 

 

Thats good to know Brummie, but as I'm taking an existing pumped system and only only adding enough gravity to prevent any boiling if the pump dies and to increase pump efficiency when burner is cooling. I prefer belt and braces as well so some chunky pipework is a must.

 

What's everyone's thoughts on materials and fittings? I know that plastic is ok after 1m of the boiler (seems a little close to me as barrier pipe is rated for 85C and melts at 120...) But it's pretty ugly. 

I like copper -and 28 over 22mm seems a no brainer. But, all the gravity systems I've seen have been compression so I assumed it was regs- not so. I'm going with solder as I'm happy with a torch and prefer the look. Gonna make a year leak radiator towel rail from polished and lacquered 28mm

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Plastic also expands and sags a lot on central heating applications. If I were doing my Alde system again it would all be in copper, it looks much better if exposed - it would have saved a lot of boxing in.

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13 hours ago, Tony Brooks said:

I have no intention of giving a definitive answer your question about 28mm pipe

 

I can perhaps help there. 

 

In the world of domestic central heating, 28mm is considered to be the smallest diameter pipe that will allow water to flow one way and air/steam the other, simultaneously.

 

So when installing a header tank using a single combined cold feed and vent pipe, 28mm is the smallest it should be.

 

This is also the reason all thermosyphon systems should be piped up in 28mm. 

 

 

Edited by Mike the Boilerman

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1 hour ago, Mike the Boilerman said:

 

I can perhaps help there. 

 

In the world of domestic central heating, 28mm is considered to be the smallest diameter pipe that will allow water to flow one way and air/steam the other, simultaneously.

 

So when installing a header tank using a single combined cold feed and vent pipe, 28mm is the smallest it should be.

 

This is also the reason all thermosyphon systems should be piped up in 28mm. 

 

 

My thoughts were more to do with trapping air in the system. Of course if the system is designed so there is oly one low and only one high point then the chances of trapping air will be reduced but with that pumped part I think there is still a possibility. By filing at the lowest point and venting from the highest I think you minimise that chance because as the liquid rises in the system air can still be driven to the highest point by all the paths through the circuits. I do knowledge air will still be trapped in radiators with bottom entry and exit but those can be bled easily enough..

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14 hours ago, Kitten Sniffer said:

Thanks Tony, I take full responsibility for my installation and will consult as widely as possible for info.  My first gravity system. Separate vent and feed seems more sensible than hoping a large enough bore would allow for both. Seems wise with two pipes to have the feed on the return and vent at the top of the flow. 

 

Thats good to know Brummie, but as I'm taking an existing pumped system and only only adding enough gravity to prevent any boiling if the pump dies and to increase pump efficiency when burner is cooling. I prefer belt and braces as well so some chunky pipework is a must.

 

What's everyone's thoughts on materials and fittings? I know that plastic is ok after 1m of the boiler (seems a little close to me as barrier pipe is rated for 85C and melts at 120...) But it's pretty ugly. 

I like copper -and 28 over 22mm seems a no brainer. But, all the gravity systems I've seen have been compression so I assumed it was regs- not so. I'm going with solder as I'm happy with a torch and prefer the look. Gonna make a year leak radiator towel rail from polished and lacquered 28mm

I made a towel rail but painted it with white radiator paint

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Thanks Mike, good to know, but belt and braces will see me fit separate feed and vent in 22mm and have piece of mind in the system. And again, thanks Tony, opposed feed and vent on the flow and return seems the best approach. To work! ...as soon as the rest of the order arrives... 

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2 hours ago, Kitten Sniffer said:

, opposed feed and vent on the flow and return seems the best approach.  

If you do that, I think you will get pumping over when the pump runs.

Common on domestic systems with fully pumped used to be vent first on the flow and return 4" away also on the flow, no pumping over. You could of course go sealed system with an expansion vessel and pressure gauge.

Edited by Boater Sam
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Ah! The plot thickens... Could you explain pumping over? I have an idea, but a little knowledge is a dangerous thing. Here is a sketch for the full system as planned, happy to consider revisions based on real-world fluid dynamics.

IMG_20190915_004658.jpg

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Pumping over, a circulation in the header tank from vent to fill causing fresh oxygen to go into the water.

In your diagram you are more at risk of drawing air down the  vent because the boiler has some resistance to flow therefore you will have a negative pressure on the vent.

 

If you feel you must have a seperate vent and fill, connect the fill to the flow pipe immediately after the vent connection on the high side before where you have your pipe stat, the pressure on the vent and fill are then the same so no circulation or air draw.

 

With only 2kw boiler, I would not worry about having separate pipes, just have a vent, most I see are only 15mm too. Forget the ball fill valve, it will never do much, just manually half fill the tank and check it now and then,

 

The header tank needs to be higher than the highest bleed point of course.

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1 hour ago, Boater Sam said:

Pumping over, a circulation in the header tank from vent to fill causing fresh oxygen to go into the water.

In your diagram you are more at risk of drawing air down the  vent because the boiler has some resistance to flow therefore you will have a negative pressure on the vent.

 

If you feel you must have a seperate vent and fill, connect the fill to the flow pipe immediately after the vent connection on the high side before where you have your pipe stat, the pressure on the vent and fill are then the same so no circulation or air draw.

 

With only 2kw boiler, I would not worry about having separate pipes, just have a vent, most I see are only 15mm too. Forget the ball fill valve, it will never do much, just manually half fill the tank and check it now and then,

 

The header tank needs to be higher than the highest bleed point of course.

 

For that to happen the ;liquid will need to be compressible otherwise the liquid remove from one side of the pump will push exactly the same volume back to the other side so the question is just how compressible is an antifreeze-water mixture. I was taught it was virtually incompressible which is why boiler pressure tests are done hydraulically. If there is air in the system then I accept you might get the effect referred to but removing the air will remove the problem.  At one time the vast majority of house systems had pumps and a separate fill and vent pipe, in fact mine still does.

 

I am also interested in how  aback boiler has any more resistance to flow than a length of 22mm pipe. I would agree if it were a gas or diesel boiler using a heat exchanger with many small  galleries but not a back boiler that is not much more than a metal box.

  • Greenie 2

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Thanks guys, all good points. To clarify, the diagram was about showing the system it is neither to scale not showing elevation. The header is to be the highest point in total with the heat leak being the highest point in the circuit. Just need to find a 28mm like bender now...

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2 hours ago, Kitten Sniffer said:

Just need to find a 28mm like bender now...

Assuming (always dangerous!) that means a 28mm pipe bender, what I did was use as many 45 degree elbows as possible to give the least disturbed flow in 28mm copper I could manage.  There are a few pipes at visually untidy angles, but my thermosyphon works very well, and the additional pumped 15mm circuit to the calorifier works too.  

 

It's not the flow circuit I would have designed from scratch, but it's the best I can do on my boat without a total refit and moving windows and hatches.  The backboiler heats the heads and the rear cabin (bathroom and bedroom for the heathens!) without any electrical power, and with added power I get hot water to the bath and the sink too.

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5 minutes ago, TheBiscuits said:

Assuming (always dangerous!) that means a 28mm pipe bender, what I did was use as many 45 degree elbows as possible to give the least disturbed flow in 28mm copper I could manage.

 

Yes I agree, two 45s are almost as good as a pulled 90 degree bend. 

 

You can also still buy 90 degree swept bends in 28mm soldered format which are even better.

 

 

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16 hours ago, Mike the Boilerman said:

 

Yes I agree, two 45s are almost as good as a pulled 90 degree bend. 

 

You can also still buy 90 degree swept bends in 28mm soldered format which are even better.

 

 

Endfeed 28mm Swept Elbow 90 - BS EN 1254 pt 1

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37 minutes ago, ditchcrawler said:

Endfeed 28mm Swept Elbow 90 - BS EN 1254 pt 1

It’s called a “28mm end feed street elbow” if anyone wants to buy one.  The male end is pipe diameter, so will need a fitting to connect to a pipe.

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