The Dark Art of Open Vented Gravity Flow Systems

[roamer808]

Anyone out there have trouble with a boiler stove gravity circuit?
Trying to pin down my little nagging problem - little to no heat rising up the gravity flow pipe. I've read dozens of pages on the subject - and of course no two systems are the same or reflect the same problems so I am relying on the hard facts of plumbing/engineering rules.

The stove is 14kw (9.5kw to water). The pipes are all done in 1" coming off the stove at the top left for the flow and bottom right for the return, as per the makers spec. The gravity is horizontal for 6' and then rises vertically for 6' into the ceiling joist void, where it is horizontal but rises about 5" over 8' to rise dead vertical again to the cylinder. Pipes are lagged in the ceiling. Cylinder is 240L triple coil. Gravity is to the secondary coil, Primary circuit is the pumped heating. Thermostat is on the Primary return, set to 50degC. Open vent is on the Primary. Primary circuit has an autoair vent and I bled the gravity for air also.
No suggestions of pumps please - we all know what gravity/pumped by definition means.
Any pointers welcome.

Untitled

• roamer808
• 26 Mar 2011
• 1

Untitled

• roamer808
• 26 Mar 2011
• 1

 

[45yearsagasman]

Without seeing a picture of your installation I would suggest that there is too much horizontal pipe to begin with.The gravity circulation stands little chance of getting started if the 1st six foot section is horizontal.That also applies to the return as well.

 

[moggridgeplumbing]

If you are sure there is no air trapped in the gravity side, then the problem I would be looking at would be the initial 6' horizontal run (without a rise?). Not only has this length of pipe needs to heat before the vertical run to accelerate the gravity circulation, because of the slow circulation the horizontal pipe could be building up with sludge, restricting the flow further. A flush of the system maybe in order.

 

[roamer808]

Would putting a vent on the gravity help circulation?
At present if I turn the thermostat on the primary way up, the system just kettles. Everything else is flying - house is roasting and the cylinder is scalding, but all with the primary. I won't go into the details of the Systemlink zone controller because it will distract the post topic. The gravity should work regardless of anything else installed. Entire system is in a month. Flushed it twice after all fitting was finished.

 

[petit_pablo]

To much horizontal pipework and restrictive 90deg compression elbows wot help.

 

[Agile]

Presumably this is all your own DIY which you did without consulting any experts.

If you had then I am sure they would have told you that to get gravity flow initiated the flow output from the boiler should rise upwards for at least two metres immediately above the boiler.

Its not a dark art but just a situation which all well experienced heating engineers will be aware of.

Tony

 

[PEDANTICVINDICTIVEMAN]

Pump it.

 

[The Baker]

Is there any chance of a picture showing the Stove, connections, and initial few feet of piping leading to the risers?

In an attempt to sort out your problem may I suggest you apply your blow lamp to the flow pipe where it rises after the initial horizontal run.

Do this when you fire up the Stove and it may enable Thermo siphoning to start.

Once started it may carry on working until the flow and return pipes reach equilibrium.

Tim

PS don’t be put off, life is one long interesting lesson.


http://en.wikipedia.org/wiki/Thermosiphon

 

[mysteryman]

I agree. It looks like an airlock, and the real answer is to re-arrange the pipe runs. If you can let it boil momentarily, that might clear it, but it is a dangerous game, so be careful. 'Horizontal' pipes on this layout should always have a gentle rise away from the boiler, with the vertical parts of the run nearest the boiler, as Agile says.

 

[moggridgeplumbing]

Now you have added the pics, I revise my diagnosis. Definitely the horizontals - too horizontal. Even if they had been installed with an upward slope on the flow, gravitation might have a chance.

If you can re-pipe to get that initial vertical run do, otherwise pump.

Not repeating you mysteryman just seems we where typing reply at same time.

 

[roamer808]

Knew it was something small and silly. Yes I am a DIY'er, but a seasoned one. 20 years a chippy and I have done a lot of plumbing work.
Asked several plumbers to install my system - anything apart from a sealed gas system boiler setup scared them. Anything involving 'gravity' is an art which few plumbers are comfortable with. Solid fuel systems are very rare in Dublin town - the gas company has done a great job of drilling 'clean??' instant energy into the masses.
So I spent the last 6 months learning about Bi-valent systems and most of the associated works. Obviously my greeness caught up with me. Apart from the gravity error I am chuffed with the system. Completely piped the house, put in a triple coil cylinder, all the rads (balanced), tanks, expansion right up to the ridge of the roof, sinks, toilets, 3.5 bar pump to a rainwater head shower. The whole lot is singing along.

Changing the pipes - no can do. All finished and plastered. Now that I know, given the ratio of horizontal pipe to vertical pipe, I would have my doubts if altering the pipes where I can would drastically improve the syphon effect.
Pumping the gravity is not the end of the world - I just really want to know why it is not thermosyphoning on its own. I'm not one for making do with a situation - a dangerous way to approach 14kw boiler stove.

In the event of a power failure, the stove is very controlable. Shut the air vents and it dies down immediately. The 1" vent pipe rises 10 feet above the top of the cylinder. We don't make a habit of leaving the stove blazing and going out. Everyone has been taught what to do if the system kettles or if there is a power cut. So the safety angle is covered.

To you wiser guys out there. The apprentice reads the book on how it's done, the master will tell you why it's done. Nothing substitutes time served experience, for that I am grateful for your help.

 

[picasso]

The coil in your cylinder is for pumped systems only, probably too restrictive for gravity feed.

 

[JohnD]

No suggestions of pumps please

It's time you moved into the 1980's

 

[bengasman]

The tag of this thread is wrong, it should read: the dark ages of gravity systems.

 

[Weedrip]

You do not mention if there is any form of heat sink connected to the primary gravity circuit.

You mention that you vented the gravity circuit. You also ask “Would putting a vent on the gravity help circulation?”

Is it reasonable to infer that this section of the system is not served by a permanently open vent?

Presumably you have also a connection from the stove to the heating system via the systemlink unit?

“At present if I turn the thermostat on the primary way up, the system just kettles. Everything else is flying - house is roasting and the cylinder is scalding”

Has the injector tee between the heating return and the primary return been fitted?

Does the gravity circuit have a high limit stat to kick on the heating pump as an overrun stat?

Sorry for the questions but you don’t mention the above and to understand what you have done with the system if feel it is important to ask before commenting further.