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Neville Hillyer

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
 
J

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.
 
Last edited:
SimonG

SimonG

Plumber
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.
 
OP
Neville Hillyer

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.
 
J

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.
Post automatically merged:

Should also have asked if you slackened the top connection to the coil to expel any air until water exits.
 
Last edited:
Gasmk1

Gasmk1

Gas Engineer
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
 
OP
Neville Hillyer

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.
Post automatically merged:

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.
 

Attachments

B

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
 
J

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.
 

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