How can I cure my gravity circulation problem?

[Neville Hillyer]

I am a retired Chartered Electrical Engineer and am looking for information about the necessary conditions for gravity primary circulation in a 28mm domestic indirect vented hot water circuit.

Until recently I have fitted Hercal cylinders in my house and they always worked without any difficulty. I have now replaced the old copper Hercal with a stainless Telford cylinder but cannot get any primary circulation.

I have flushed all sections of pipework and tried bringing the boiler up to boiling point but there is still no hot flow down the coil. I can only assume that there is an air lock resulting from a coil which probably does not go consistently down all the way. It may not be helped by the non-smooth very flexible pipe. Perhaps a corrugated surface is not particularly good at promoting downward flow of water combined with chalk and cast iron particles whilst encouraging upward flow of trapped air.

I would be grateful for any advice or pointers to sources of information about this.

Neville

 

[Brambles]

John G,

I fully agree. The calculation may reveal the cause, but it contributes very little to delivering hot water !

 

[John.g]

I don't think the calculation reveals the cause, gravity fed systems have been delivering hot water for hundreds of years?. Pumping the above system may and probably will give hot water but a gravity system was specified.

 

[SJB060685]

Yes Brambles, that was a typo error.

 

[Brambles]

It’s not a typo - the calculation is showing that the coil inside diameter needs to be around 28mm for the gravity and temperature generated pressure differential to be greater than the Installed pipe (and coil) resistance.

 

[SJB060685]

It’s not a typo - the calculation is showing that the coil inside diameter needs to be around 28mm for the gravity and temperature generated pressure differential to be greater than the Installed pipe (and coil) resistance.

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.

 

[John.g]

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.

 

[Neville Hillyer]

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.

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.

 

[SJB060685]

My experience is that large old vented systems can slowly suck in small quantities of air at any time.

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.

 

[John.g]

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.

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.

 

[Neville Hillyer]

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.

 

[Brambles]

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.

My apologies and again to get to 20 characters!
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Brambles, can you please give me your calculated flow and resistance for the installed coil, preferably with the resistance in M.
[automerge]1589800269[/automerge]
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.

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

 

[John.g]

Still, one would expect some flow through that coil, corrugated or not, with reduced output, but not zero, it will be very interesting to see Neville's results.

 

[Brambles]

John,

Yes, I fully agree - I have learn’t quite a lot from this exercise!

 

[Gasmk1]

I am still going for partially blocked cold feed where the cylinder is if his drawing is correct

 

[SJB060685]

John,

Yes, I fully agree - I have learn’t quite a lot from this exercise!

I've learned a lot as well. Up until this dilemma ive never needed to do these calculations.