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- Thread starter Neville Hillyer
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Discuss How can I cure my gravity circulation problem? in the Central Heating Forum area at Plumbers Forums
I get the gist of what is being discussed above but it must be remembered that the gravity driving force available is only determined by the flow/return temps and the head available.
If one realistically accepts a max head of 2.5M and a flow/return temp 75/15C with a cold cylinder and 75/60C when hot, then the max circulating force is 0.061M/2.42ins and the min is 0.021M/0.84ins. A pumped system with a differential head of 3M will give a flow rate factor of X12 in the first case and X7 in the second case and that is one and the main reason that pumped systems are now almost universal. I do have a vague memory (50 years) of some gravity driven heat exchanger with a huge finned coil.
If one realistically accepts a max head of 2.5M and a flow/return temp 75/15C with a cold cylinder and 75/60C when hot, then the max circulating force is 0.061M/2.42ins and the min is 0.021M/0.84ins. A pumped system with a differential head of 3M will give a flow rate factor of X12 in the first case and X7 in the second case and that is one and the main reason that pumped systems are now almost universal. I do have a vague memory (50 years) of some gravity driven heat exchanger with a huge finned coil.
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No you misunderstood me. I said that was a typo to your reply where I mistakenly said 100 n/m2 equates to 1 bar. As I said above 1 bar = 100,000 Pa or should have read 100,000 n/m2. It was late and I missed out some noughts.
Brambles, can you please give me your calculated flow and resistance for the installed coil, preferably with the resistance in M.
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My own basic calcs would indicate that 5.8M of 22mm ID pipe should flow ~ 7.5 LPM @ 0.061M head which should satisfy the requirements except that the corrugations are having a huge effect but even if they do then there should be some reduced level of performance.
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My own basic calcs would indicate that 5.8M of 22mm ID pipe should flow ~ 7.5 LPM @ 0.061M head which should satisfy the requirements except that the corrugations are having a huge effect but even if they do then there should be some reduced level of performance.
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I would prefer a better source than Amazon but it appears that corrugated pipe is specified in the old British way ie by internal diameter. The following is an example from: https://www.amazon.co.uk/Corrugated-Stainless-Steel-Pipe-DN25/dp/B07B2JVJFX
Nominal width: DN25
Wall thickness: 0.20 mm
Inner diameter: 25.5 mm
Outer diameter: 31.8 mm
Bending radius: 39 mm
Operating pressure: 10 bar
This appears to be compatible with my specification to Telford and my 1 inch (28mm) pipes. What was not clear to me was the use of corrugated pipes to replace my original standard coil.
I had hoped that somebody would say they had some experience of the use of corrugated coils in a gravity system.
I am trying to compile a full list of possible reasons for the replacement cylinder not getting any heat. Here is my list so far:
1 - Partial or total blockage which allows full heat to bathroom radiator but no heat to cylinder.
2 - Incorrect or imprudent connections to cylinder.
3 - Partial blockage inside cylinder which still allows it to be flushed.
4 - New cylinder fails to self-clear air locks like all earlier cylinders did.
5 - Corrugations inhibit upward air flow in the face of static or slow downward water flow.
6 - Slight slopes on 'horizontal' pipes more critical than previously.
I suspect that if the cylinder fails to self-clear air locks then it will be a continuing source of trouble in the future. My experience is that large old vented systems can slowly suck in small quantities of air at any time.
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In an ideal system any air or dissolved oxygen within the system is usually expelled to minimal amounts within a few months, thereafter any remaining is only enough to slightly corrode any ferrous materials. Air should ideally not be drawn in, unless you drain down for maintenance or a pump is sucking it in somewhere.
You say that you flushed all sections of the pipework, if this didn't include the coil then you should do this. If the coil is clear then IMO, your options are to either fit a cylinder/coil like your previous one or at least one with a self supporting non corrugated coil, the other option is one that I suggested before, install a pump (like the one I suggested) on the flow side, this pump cannot, as I thought, be set down to 0.1M head but can be set to a PP (proportional pressure) setting of 0.5M, at this setting if still no flow (for whatever reason) it will ramp down to a minimum of 0.25M at zero flow which can be read off in the form of the power consumption, this, again IMO, may give you a big stick to beat some body with and, at the very worst, provide you with a spare circulating pump.
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Came across a few readings that a neighbour gave me a few years ago from his standard hot water cylinder which he converted to fully pumped from gravity when he changed his boiler. If his readings were accurate then he seems to have been getting ~ 1.5 LPM at 50C continuously in gravity mode only with 19mm ID flow&return.
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Many thanks to all who have contributed to this thread and given such helpful advice.
Having considered all the evidence and advice my view is that it may be most profitable to initially investigate a permanent 15mm link between the mains and the cold feed from the small header tank. With suitably placed isolation valves this could provide mains flushing as and when required.
I plan to:
1 - Isolate the bathroom radiator, cylinder and pump to flush around the gravity circuit.
2 - Open the cylinder coil valves and hope to get a sufficient reverse flush up the coil to remove any air locks.
Since I will be doing this while positioned near the small header tank I will be able to see:
1 - Any debris.
2 - Air - this will require a short temporary hose or pipe to take the vent below water level.
The above may take a while but I plan to keep this thread updated.
Having considered all the evidence and advice my view is that it may be most profitable to initially investigate a permanent 15mm link between the mains and the cold feed from the small header tank. With suitably placed isolation valves this could provide mains flushing as and when required.
I plan to:
1 - Isolate the bathroom radiator, cylinder and pump to flush around the gravity circuit.
2 - Open the cylinder coil valves and hope to get a sufficient reverse flush up the coil to remove any air locks.
Since I will be doing this while positioned near the small header tank I will be able to see:
1 - Any debris.
2 - Air - this will require a short temporary hose or pipe to take the vent below water level.
The above may take a while but I plan to keep this thread updated.
My apologies and again to get to 20 characters!
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Ok, so for 22mm ID at 7.5lpm, that will equate to a velocity of around 0.33m/second.
If you run the two equations and consider the coil length is 8m. The minimum head required for the thermosyphon to operate at that velocity is 2.4 inches of water or 0.061m of water.
This may be on the low side depending on the friction factor if the pipe coil is corrugated on the inside.
The above only calculates the force required to drive the flow at 7.5lpm through the coil - it does not allow for the interconnecting pipework.
As stated earlier, for a 28mm dia coil at the same velocity, the force required is significantly lower
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I've learned a lot as well. Up until this dilemma ive never needed to do these calculations.
You never should need to - it is a theoretical exercise anyway unless you know the friction factor of the coil.
My view is that identifying the cause will not change the most cost effective remedy of converting to a pumped system.
With respect to Telford, I have always found them very responsive and quick to offer a substantial discount against the delivered item that I was unhappy with ( rather than to collect and replace).
My view is that identifying the cause will not change the most cost effective remedy of converting to a pumped system.
With respect to Telford, I have always found them very responsive and quick to offer a substantial discount against the delivered item that I was unhappy with ( rather than to collect and replace).
Came across this re friction loss in corrugated pipes, the DN25 is nearest to the one above, a flow rate of 10LPM = 0.02M/M loss so the 5.8 coil should require 0.116M head. If the flow rate was 2 LPM (due to Thermosyphon) then the coil loss would only be 0.00464M and even allowing for the added pipeline losses there should be no reason IMO why the higher corrugated pipe losses should be a problem.
I read somewhere that clearing air from corrugated pipes requires a velocity of > 0.5 mps which will never be achieved with a gravity flow and this bore piping so hopefully your air purging etc will get rid of any air as the flow rate has to build from zero in this gravity system and any trapped air won't help.
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www.easyflex.co.za
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Came across this re friction loss in corrugated pipes, the DN25 is nearest to the one above, a flow rate of 10LPM = 0.02M/M loss so the 5.8 coil should require 0.116M head. If the flow rate was 2 LPM (due to Thermosyphon) then the coil loss would only be 0.00464M and even allowing for the added pipeline losses there should be no reason IMO why the higher corrugated pipe losses should be a problem.
I read somewhere that clearing air from corrugated pipes requires a velocity of > 0.5 mps which will never be achieved with a gravity flow and this bore piping so hopefully your air purging etc will get rid of any air as the flow rate has to build from zero in this gravity system and any trapped air won't help.
Technical Information | Easyflex
It would be helpful if you could find the reference for "clearing air from corrugated pipes requires a velocity of > 0.5 mps" especially as Telford advertise :
"Telford Stainless Vented Cylinders
Suitable replacement for copper vented
Telford Copper & Stainless Cylinders Ltd offer a stainless steel version of the conventional open-vented cylinder, traditionally made from copper. Made in the same production line and from the same material as our Unvented range, we can offer a cost effective alternative to copper vented cylinders that are manufactured to the same high standards as our unvented range."
The above words appear in their earlier publications but at the time of writing it is at the top of page 7 of the first download at: Telford Copper & Stainless Cylinders Brochure | Tristor, Tornado & Tempest Installation Guides - https://www.telford-group.com/downloads/general-downloads/
Before I placed my order I wrote to Telford and said: "I wish to replace a 20 year old 1200x450 Hercal hot water cylinder with the minimum of plumbing alterations." My initial email to Telford included a full specification and drawing of the cylinder I wished to replace.
They did warn me that most pipe connections would be in different locations to the Hercal but they did not warn me that they would supply a corrugated coil which would not self-clear air locks.
The delivered cylinder came with a 5 year printed guarantee and despite several protests they have still not replaced it with their widely advertised 10 year guarantee.
It is unfortunate that an established, well respected company, allowed this order to go so badly wrong and have done little, so far, to put it right.
Sorry Neville, but I just can't locate that link just now but it certainly stated a minimum velocity which I think was 0.5 mps so common sense would indicate that corrugated piping isn't compatible with large bore piping and low flow rates which are common to gravity flow systems, IMO.
Its strange that Telford can't/won't provide "evidence" of a cylinder similar to yours with a corrugated coil running on gravity circulation.
Its strange that Telford can't/won't provide "evidence" of a cylinder similar to yours with a corrugated coil running on gravity circulation.
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