How can I cure my gravity circulation problem?

[John.g]

Yes, I suppose one can put it that way, for any given installation the only real variable is the flow temperature, with a cold cylinder, the temperature/density differences will be at their greatest so the driving force will be at its greatest = greater circulation & greater energy to heat the water, as the water temp increases then the coil deltaT decreases so the flow rate decreases and energy to water decreases until finally almost no circulation.

 

[SJB060685]

Top man. Perfect explanation again.

 

[John.g]

Vertical about 2.7m & horizontal about 2.6m. Both as well insulated as they have been for 50 years.

Despite being a returning member it appears to take about 6 hours for my messages to be approved and displayed.

Is there any way you can view/photo the coil to see if the coils are "flat" and not sagging in places, not easy if you only have 1 ins tappings but you might be able to obtain/borrow a borascope somewhere which would tell you a lot.

 

[Neville Hillyer]

Is there any way you can view/photo the coil to see if the coils are "flat" and not sagging in places, not easy if you only have 1 ins tappings but you might be able to obtain/borrow a borascope somewhere which would tell you a lot.

My first post here said "a coil which probably does not go consistently down all the way" but perhaps I should have said 'unlike the self-supporting coil in the cylinder it replaced the flimsy corrugated coil is supported in various places but droops slightly between the supports'.

I no longer have to wait for my messages to be to be approved and displayed.

 

[John.g]

I would provide this as proof of non performance to your supplier and ask them to show you a performing gravity fed coil of the same type somewhere near you.
It would be nice if a photo was available.

 

[Neville Hillyer]

I would provide this as proof of non performance to your supplier and ask them to show you a performing gravity fed coil of the same type somewhere near you.
It would be nice if a photo was available.

An interesting approach. I may end up doing this.

I discovered the slightly drooping coil just after delivery. As you indicated it is not easy to see through a small hole especially with light reflecting from internal surfaces. Even if a photo is possible it would probably have to be done with the cylinder removed from the airing cupboard - a long, but not impossible, task.
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I do not have a drawing of my system but will try and produce one later. Apart from changes to accommodate the new pipe connection locations everything is the same as it has been for the last 50 years.

I have attached the promised schematic. I do not know exactly what is between the ceiling (green) and the floor (brown). This 2D drawing is a reasonable guide to total pipe lengths especially between boiler and cylinder.

 

[Gasmk1]

Have you checked the cold feed coul be partially blocked stopping the Gravity

 

[Brambles]

Neville,

the person we use for advice on cylinder design for gravity flow Aga / Reyburn oil and solid fuel hot water systems is Dr Angelo Goonesekera at Sheffield University.

Looking over recent designs, the coil bore internal diameter is specified as a minimum of 28mm and the vertical differential between the inlet and the outlets of the coil is significantly larger than for conventional vented cylinders.

Perhaps worth comparing the dimensions of your original cylinder against your new cylinder.

 

[SJB060685]

Neville,

the person we use for advice on cylinder design for gravity flow Aga / Reyburn oil and solid fuel hot water systems is Dr Angelo Goonesekera at Sheffield University.

Looking over recent designs, the coil bore internal diameter is specified as a minimum of 28mm and the vertical differential between the inlet and the outlets of the coil is significantly larger than for conventional vented cylinders.

Perhaps worth comparing the dimensions of your original cylinder against your new cylinder.

You're one of a few I consider guru's. Its good to see even you seek help from others at times

 

[Neville Hillyer]

Have you checked the cold feed coul be partially blocked stopping the Gravity

I can flush all pipe sections.
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Neville,

the person we use for advice on cylinder design for gravity flow Aga / Reyburn oil and solid fuel hot water systems is Dr Angelo Goonesekera at Sheffield University.

Looking over recent designs, the coil bore internal diameter is specified as a minimum of 28mm and the vertical differential between the inlet and the outlets of the coil is significantly larger than for conventional vented cylinders.

Perhaps worth comparing the dimensions of your original cylinder against your new cylinder.

Thanks for the name - it could be very useful.

This is confusing. 1 inch (25 mm) often has an OD of 28 mm.

 

[John.g]

Neville,
I would give very serious thoughts to installing a circ pump (even just temporarily) on the cylinder coil where it returns into the boiler. You will then be more or less certain of proving that the fault is with the coil design, even if you just power it up while the boiler is firing to do so. If contemplating this, I would suggest a Wilo Yonos Pico 6M (or 5M, but a bit of a odd ball) simply because you can set the head on this down to as low as 0.1M (I will check my own later to see), It is not ultra expensive (~ £125) to prove your point.

On your present System: Does the boiler just fire up when hot water only is required and if so what stops the thermosyphon effect on the upper rads on the pumped circuit?, is there a anti syphon valve fitted?.

 

[Neville Hillyer]

Neville,
I would give very serious thoughts to installing a circ pump (even just temporarily) on the cylinder coil where it returns into the boiler. You will then be more or less certain of proving that the fault is with the coil design, even if you just power it up while the boiler is firing to do so. If contemplating this, I would suggest a Wilo Yonos Pico 6M (or 5M, but a bit of a odd ball) simply because you can set the head on this down to as low as 0.1M (I will check my own later to see), It is not ultra expensive (~ £125) to prove your point.

On your present System: Does the boiler just fire up when hot water only is required and if so what stops the thermosyphon effect on the upper rads on the pumped circuit?, is there a anti syphon valve fitted?.

Yes the boiler is called by the boiler thermostat when central heating is not called. No anti-syphon valve that I am aware of. I assume the stationary pump prevents thermosyphon to all radiators except bathroom.

 

[John.g]

A pump in clean condition will give virtually no resistance to flow, some zoned systems use pumps for zoning and all have anti syphon valves fitted which are spring loaded NR valves that require a slight pressure to open, unwanted thermosyphon circulation is often a indication of a seized valve so a bit strange that there is no thermosyhon effect in these rads as well as the bathroom one.
I presume that you have proven that there is no build up of sludge in the boiler by flushing through, is it gas or oil fired?. Oil fired boilers have virtually no resistance but the newer gas fired boiler Hxs do but I would think that the older type have some form of cast iron Hx which probably also has a very low resistance.

 

[Neville Hillyer]

A pump in clean condition will give virtually no resistance to flow, some zoned systems use pumps for zoning and all have anti syphon valves fitted which are spring loaded NR valves that require a slight pressure to open, unwanted thermosyphon circulation is often a indication of a seized valve so a bit strange that there is no thermosyhon effect in these rads as well as the bathroom one.
I presume that you have proven that there is no build up of sludge in the boiler by flushing through, is it gas or oil fired?. Oil fired boilers have virtually no resistance but the newer gas fired boiler Hxs do but I would think that the older type have some form of cast iron Hx which probably also has a very low resistance.

It is the original 50 year old pump so it may have a valve. Thermosyphon works in the bathroom radiator whenever the boiler is on which probably implies there are no blocked pipes. The gas boiler in a 100,000 Btu/hr cast iron Ideal Standard stopped down to 80,000 Btu/hr. At about 10 years old I had to clear cast iron sludge from the pipe under the first ground floor radiator but since then it has not been a significant problem. At about 10 year intervals, while replacing cylinders, I thoroughly flush and add new Fernox corrosion inhibitor.

 

[John.g]

Well, looks like a impasse if Telford claim that "their" coil is gravity suitable and you are reasonably/quite certain that it is not and you will have to pursue some option either legally or otherwise to get hot water.

 

[SimonG]

40 posts in now. Easier to pump it.

 

[Brambles]

This is confusing. 1 inch (25 mm) often has an OD of 28 mm.

The two designs I looked at, that were prepared for an indirect gravity hot water cylinder for solid fuel Rayburn ovens. They both had 28mm internal diameter coils specified extending over the lower two thirds of the cylinder.

 

[gmartine]

I only read the OP's first post and not much else, would the fact that the new cylinder is made from stainless steel rather than copper have a stalling effect being experienced because of the less efficient thermal transfer or are the coils made from the same material?

 

[Neville Hillyer]

I only read the OP's first post and not much else, would the fact that the new cylinder is made from stainless steel rather than copper have a stalling effect being experienced because of the less efficient thermal transfer or are the coils made from the same material?

The earlier copper cylinders were 100% copper and the new cylinder is 100% stainless steel. If you view conductivity on a log scale between rubber and metal, glass is somewhere in the middle and all metals are very close together at the end. Any difference between copper and steel conductivity is negligible in this application.

 

[John.g]

Above posts are interesting but while copper should give a faster warm up than stainless for the same coil area, using stainless should not cause stalling.

Is there any way that the coil has been installed somewhat like the attached even though the flow/return connections are shown at 600mm apart in your drawing.

Also, if you don't me asking, what do you intend to do to resolve your problem as you have lived with it for a year or so?.