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

 

[John.g]

Corrugated coils are only recommended I think for pumped systems, it's probably a longer coil with a smaller bore to promote turbulence and the corrugations may trap air.
You could try (if not already done) installing a air vent very close to the coil entry at the top of the cylinder and also fill the coil very slowly. Other than that, a fully pumped system might be the only answer.

Maybe also just try to get the circulation going by starting off with a cylinder full of cold water, it just might help to promote circulation and shift any air.

 

[SimonG]

Requires specific cylinder with full bore coil. If it's a standard boiler and not solid fuel then the system should have been updated to fully pumped to comply with part L building regs.

 

[Neville Hillyer]

Corrugated coils are only recommended I think for pumped systems, it's probably a longer coil with a smaller bore to promote turbulence and the corrugations may trap air.
You could try (if not already done) installing a air vent very close to the coil entry at the top of the cylinder and also fill the coil very slowly. Other than that, a fully pumped system might be the only answer.

Maybe also just try to get the circulation going by starting off with a cylinder full of cold water, it just might help to promote circulation and shift any air.

Thanks for your helpful reply.

In my house the original 50 year old boiler and a large gravity primary cylinder have worked very well and I would prefer not to change it to fully pumped.

I made it very clear that I was ordering a gravity primary cylinder to replace my Hercal and was surprised that the one made for me had a long corrugated coil. I would be very interested if anybody could point me to something which documents that 'corrugated coils are only recommended for pumped systems'.

The cylinder primary feed has always been connected by a relatively short horizontal pipe to a tee in the hot feed from the boiler which vents to the header tank in the roof above the cylinder. The cylinder always starts from cold as there is no other form of water heating.

 

[John.g]

Did Telford's have any comment to make in view that you made it very clear that you required a gravity flow system?.
Have you any schematic of your system?, circulating head & horizontal distances etc., it is strange that you can't achieve some flow/performance even if it is reduced.
[automerge]1589379874[/automerge]
Should also have asked if you slackened the top connection to the coil to expel any air until water exits.

 

[Gasmk1]

Gravity coil
When ordering your Gledhill cylinder, you would need
to specify if it requires a gravity coil. This is where the circulation of the hot water circulation is attained by using the natural convection current rather than by the use of a circulation pump to ‘force’ the water around the system. The main difference in the cylinder design between
a pumped and gravity circulation system is that the connections/coils are larger in the gravity systems to provide less resistance and increase the flow rate.
Grade 1
(up to 2.5 bar)
Grade 2
(up to 1.5 bar)
Grade 3
(up to 1.0 bar)

from gledhills site

Indirect Vented - https://www.telford-group.com/product/indirect-vented you may need to contact them ahagin to check they sent right cylinder

 

[Neville Hillyer]

Did Telford's have any comment to make in view that you made it very clear that you required a gravity flow system?.
Have you any schematic of your system?, circulating head & horizontal distances etc., it is strange that you can't achieve some flow/performance even if it is reduced.
[automerge]1589379874[/automerge]
Should also have asked if you slackened the top connection to the coil to expel any air until water exits.

This has been a long saga over more than a year.

My order was with PlumbNation because Telford refused to take an order from a retail customer. Although I have had several exchanges with Telford's technical and senior staff and various people at PlumbNation I am at a loss to know how to resolve this. My supplier was PlumbNation but they forward everything to Telford who appear not to want to accept that they should not have used a corrugated coil. Please see Telford's drawing attached which was produced from my specification. At no point before delivery was I told about the use of a corrugated coil. Telford are aware the order was for gravity primary and have confirmed this more than once.

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.

There is no hot flow through the tank although the parallel bathroom radiator gets hot as usual. As previously stated I can flush water through every section of pipe including the cylinder coil.

I have tried slackened the top connection etc but it did not alter the situation.

 

[Brambles]

Neville,

That tank configuration with a 25mm coil should work on a gravity system. Are you concerned that it is not fitted with a 25mm coil? I would not be distracted by the corrugated coil issue - that may imct performance, but at 25mm it won’t stop it from working.

You have no doubt extensively “trouble shot“ the system. My advice would be to thoroughly check that the flow route from the boiler is running slightly up hill, that it has air vents where needed and that it enters the tank at the lowest connection to the coil ( I have seen gravity tanks plumbed in as pumped tanks). Check and test any non return valves on the system. Look for any other valves in the system and test them. Check that the header is functioning and that the feed pipe is clear of debris / gunge If the system is correctly installed the culprit is probably air - that can be both difficult to find and hard to remove. Flushing through with mains pressure water sometimes does the trick.

There is probably nothing in the above that you have not already tried

The root cause will be close to the area where the flow from the boiler is cool - shortly after the boiler is first fired. That is the point where the heat transfer is changing from convection to conduction.

Water takes the route of least resistance. However, be careful not to fully restrict the by pass loop - but ensure that it is not starving the tank.

Good luck

 

[John.g]

Agree with most of above apart from "that it enters the tank at the lowest connection to the coil", but I think Neville's logic is that it worked perfectly for years with a non corrugated coil even though its difficult to see why there isn't some circulation, hot water must be getting fairly close to the cylinder coil entry as a parallel rad, presumably also gravity fed, works normally, one feels that if he could get the circulation going by hook or by crook that the problem might disappear. I remember a old direct gravity fed cylinder which was located just above the open fire in my patents house, you just gave the cylinder a belt of you fist to get the circulation going and away you went.

 

[Neville Hillyer]

Neville,

That tank configuration with a 25mm coil should work on a gravity system. Are you concerned that it is not fitted with a 25mm coil? I would not be distracted by the corrugated coil issue - that may imct performance, but at 25mm it won’t stop it from working.

You have no doubt extensively “trouble shot“ the system. My advice would be to thoroughly check that the flow route from the boiler is running slightly up hill, that it has air vents where needed and that it enters the tank at the lowest connection to the coil ( I have seen gravity tanks plumbed in as pumped tanks). Check and test any non return valves on the system. Look for any other valves in the system and test them. Check that the header is functioning and that the feed pipe is clear of debris / gunge If the system is correctly installed the culprit is probably air - that can be both difficult to find and hard to remove. Flushing through with mains pressure water sometimes does the trick.

There is probably nothing in the above that you have not already tried

The root cause will be close to the area where the flow from the boiler is cool - shortly after the boiler is first fired. That is the point where the heat transfer is changing from convection to conduction.

Water takes the route of least resistance. However, be careful not to fully restrict the by pass loop - but ensure that it is not starving the tank.

Good luck

Thanks for your remarks about the coil. I have no previous experience of corrugated coils in heating systems but have seen some posts about them causing my type of issue even at larger sizes.

There is a hidden 'horizontal' section between the kitchen ceiling and the airing cupboard floor. This has always been there and it has not caused any problems in the past. After entering the airing cupboard the flow pipe is vertical to the header vent.

I am concerned about your remark about the pipe from the boiler entering the lower connection on the cylinder. This is contrary to my training and experience. My hot flow is from the top of the boiler to the top of the cylinder coil. Hence cooling in the cylinder assists circulation.

I have tried all settings of the bathroom radiator valve.

 

[Brambles]

From an efficiency and design perspective, I fully agree. However, you are not, or don’t appear to be, getting convection to the top of the coil ( thereafter the flow cools and drops drawing hotter water behind it).

If you have a weak thermosyphon, you keep the hot flow running slightly uphill for as long as possible. Entering the coil at the bottom is less efficient, but the flow will still rise (slowly) through conduction.

The other potential cause may be debris that has entered the pipework during the cylinder change.

It you are feeling adventurous it may be worth fitting two three way tees on the flow and return at the boiler, to allow you to isolate the boiler and test / flush the rest of the system with mains pressure water to prove that you do indeed have an unencumbered flow and no air locks.

 

[John.g]

What is the circulating height (roughly), the distance between the centre lines of the boiler flow&return and the cylinder coil flow&return? and the length of that horizontal section on the flow and ditto on the return if any horizontal section fitted?.
My (old) calculations show that the circulating head/meter of circulating height is ~ 0.018M (0.7 inches) for 65C/20C flow/return and ~ 0.008M (0.3 inches) for 65C/50C flow/return, not a lot if the calcs are anyway accurate. its also vital that especially the flow line is properly insulated to give the difference in densities.

 

[Neville Hillyer]

What is the circulating height (roughly), the distance between the centre lines of the boiler flow&return and the cylinder coil flow&return? and the length of that horizontal section on the flow and ditto on the return if any horizontal section fitted?.
My (old) calculations show that the circulating head/meter of circulating height is ~ 0.018M (0.7 inches) for 65C/20C flow/return and ~ 0.008M (0.3 inches) for 65C/50C flow/return, not a lot if the calcs are anyway accurate. its also vital that especially the flow line is properly insulated to give the difference in densities.

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.

 

[Brambles]

John G raises a good point on design philosophy to help think through the issues.

 

[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.

This elevation of 2.7M should theoretically give one to two inches of circulating head, it would be interesting to see what flow rate the coil would give at this differential head, Telford might have some flow coefficient for this coil. The 2.6M horizontal run, especially if both ways, might seem excessive but I don't know if there is a maximum vertical to horizontal ratio for gravity circulation.

 

[SJB060685]

What is the circulating height (roughly), the distance between the centre lines of the boiler flow&return and the cylinder coil flow&return? and the length of that horizontal section on the flow and ditto on the return if any horizontal section fitted?.
My (old) calculations show that the circulating head/meter of circulating height is ~ 0.018M (0.7 inches) for 65C/20C flow/return and ~ 0.008M (0.3 inches) for 65C/50C flow/return, not a lot if the calcs are anyway accurate. its also vital that especially the flow line is properly insulated to give the difference in densities.

John any chance you could upload another excel sheet of these calculations? Honestly this is not something I know

 

[John.g]

Here you are SJB, you can input your own numbers.

 

[SJB060685]

Thanks again John, I do admire your knowledge base.
So stupid question. The greater the circulating force in metres or inches means the greater the thermosiphon affect correct?

 

[John.g]

It's the thermosyphon effect that creates the driving force, this in turn is influenced by the flow temperature, the circulating height, the HW cylinder temperature and the coil resistance to fllow...also pipe resistance & horizontal run. Probably other factors as well that I havn't mentioned.

 

[SJB060685]

It's the thermosyphon effect that creates the driving force, this in turn is influenced by the flow temperature, the circulating height, the HW cylinder temperature and the coil resistance to fllow...also pipe resistance & horizontal run. Probably other factors as well that I havn't mentioned.

Ok. So the larger the driving force or circulating force the better?

 

[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.