Heat exchange coil leaking into heat store

[shellards]

I have an Albion Mainsflow heatstore which provides hot water indirectly via a coil running through the store. Water is drawn directly from the store for under floor heating. In recent months I have noticed water coming from the over flow of the expansion/feed tank located above the heat store. Initially the flow seemed to be related to the cycling of the boiler and I thought the immersion thermostat controlling the maximum temperature of the water in the tank was to blame. However, tests suggested this was not faulty.

In recent days the flow of water has become continuous, albeit only a drip when the boiler is not cycling. I have checked the ballcock controlling the flow of water into the tank and this seems to be shutting off the water OK. Reluctantly and by various tests I have concluded that water is entering the heatstore from the coil which passes through it. My questions are as follows: Are there other possible explanations? Is a repair possible or must I replace the tank? As this problem has been developing for a period during which I think I have misidiagnosed the problem, how quickly must I repair the problem before irreparable damage is done to the system? As the central heating is underfloor, pipe work is largely plastic. Other pipework in the system including feed and return to the Grant boiler is also plastic. I assume corrosion must have taken place in the copper/steel elements of the system (e.g. the small expansion vessel connected to the heat exchange coil.) Finally, my question is, why has this happened and how may it be prevented in future?

Though I am not an expert, I installed the system as part of a self build project according to a design I bought in. It has worked very well indeed continues to do so apart from the problem described.

 

[Agile]

Anything can has happened although an internal leak is unlikely.

Before you prematurely rush into anything, turn off the water supply to the float valve and monitor the level in the F&E tank for a few days.

It could just be the level is set too high!

Tony

 

[shellards]

Thanks for your interest Tony. I have already tied up the float valve (having drained most of the water from the tank manually.). There was quite clearly not even the smallest drip entering the tank in this way. (The side of the tank below the float dried off. There were no drips onto the surface of the water in the tank.) I had whilst doing this turned off the boiler so that expansion would not raise the level of water in the tank. (Indeed, cooling of the water in the heat store might have been expected to lower it.)The level in the tank continued to rise - about a centimetre in a couple of hours. Following this, I turned off the supply to the heat exchange coil, (and turned on an upstairs hot tap to depressurise it -i.e. short burst of water followed by no flow from the tap.) After a couple of hours there was no visible rise in the level in the tank, hence my reluctant conclusion that there must be a leak in the coil.

Further comments welcome. Also, answers to my original questions.
Thanks again. Stephen.

 

[oilman]

Are there other possible explanations?

Unlikely

Is a repair possible or must I replace the tank?

Probably a replacement is needed, but try speaking to Albion.

As this problem has been developing for a period during which I think I have misidiagnosed the problem, how quickly must I repair the problem before irreparable damage is done to the system?

It is unlikely irreparable damage will be done, unless the cylinder water is pumped around the heating circuit. Is there a heat exchanger for the heating circuit? This would mean the cylinder has 3 coils.

As the central heating is underfloor, pipe work is largely plastic. Other pipework in the system including feed and return to the Grant boiler is also plastic. I assume corrosion must have taken place in the copper/steel elements of the system (e.g. the small expansion vessel connected to the heat exchange coil.) Finally, my question is, why has this happened and how may it be prevented in future?

Again, Albion would be the best source of help.

 

[shellards]

It is unlikely irreparable damage will be done, unless the cylinder water is pumped around the heating circuit. Is there a heat exchanger for the heating circuit? This would mean the cylinder has 3 coils.

Thanks Oilman for your clear commentary. The feed to the underfloor pipes from the heatstore is direct. A mixer valve combines returning water to the correct temperature for the return. Therefore there is no heat exchanger for this part of the system. I had assumed a single heat exchanger provided the domestic hot water. I am puzzled by your suggestion that there might be three heat exchangers?

The central heating, by the way, appears to be functioning fine. Are you suggesting that I should stop using it immediately lest it become blocked or in some other way damaged? It's not the best time of year for this, but if it must be done I can isolate the heating. I had hoped it would be possible to limp on until a replacement tank was ordered up or a repair effected if that is possible. I will try to contact Albion, though doubt I will get much action from them in the next day or two

 

[Agile]

One thing you do not seem to have told us is how old the cylinder is?

Also what water treatment has been done to the system water?

Finally I assume your mains water supply is not "agressive" ?

Its just possible the cylinder is still under some kind of warranty!

Tony

 

[shellards]

One thing you do not seem to have told us is how old the cylinder is?

The cylinder is about 4 years old. I imagine all warranty is long out of date.

Also what water treatment has been done to the system water?

I added an inhibitor at a fairly early stage. I don't remember the name or precise quantity, though I did add what seemed to be a reasonable dose. I did not however do exact calculations on the quantity of water in the system. I assumed that the quantities would be similar to a radiator system of similar size and took advice from my plumbers merchants - perhaps not wise in retrospect, though I would not wish to fault him as he's a good bloke. Of course, given that I now believe the problem has been ongoing, the inhibitor will have been diluted to zero long ago, I expect.

Finally I assume your mains water supply is not "agressive" ?

I assume this also. This is a soft water area. Also, when talking to local people with plumbing knowledge about adding inhibitors to the system, they were very relaxed about this saying there was no need to rush as very little damage would occur in the short term.

 

[Agile]

There may lie the cause or accelerating factor.

Most heating systems have a water content of about 70-100 litres.

Your system has that PLUS the content of your thermal store of maybe 200 litres making the total content perhaps 300 litres?

The dosage should have been 3-4 cans of X100 or whatever inhibitor you chose.

Some cylinders have a three or five year warranty, worth checking! Some manufacturers MAY even cover something that has failed outside their warranty period because they are concerned about their reputation. Unless there is something you have done wrong this should NOT have happenned.

Soft water is the worst! It can be acidic like parts of Devon and Cornwall and eat away copper!

Talking to "plumbers" is very hit and miss! Many like to talk a lot based on little real knowledge of rarities like thermal stores.

Try talking to the Albion technical help and get the name of the person you speak to! You may need a brummie/scottish language conversion course first.

Tony

 

[shellards]

Try talking to the Albion technical help and get the name of the person you speak to!

There does seem to be an albion website but absolutely no contact information or technical helpline offered. I do have head office telephone numbers from literature I have on file for West Midlands, Manchester and Lincolnshire, but I am not bothering with these today. An email address would have been helpful, but that doesn't seem to be an option.

Thanks for general advice. I think your comments about the quantity of water in the system are sound. I had a nagging concern about this and should have acted on it. The system designer (also very good in all other respects) did not specify on the question of an inhibitor, and given your own off the cuff prescription, this seems surprising. Perhaps I am just trying to lay the blame off elsewhere!

 

[oilman]

I suspect you have acidic water, do you know the pH? When you replace the tank you should add sodium carbonate to the water to give a pH of near 8.5. This will give the least corrosive conditions for steel/copper combinations which are the biggest problem. Do not rely solely on inhibitor to give protection. This is a minefield subject and there are PhD chemists who have lonnnngggg discussions about it. You should check the pH every year and adjust as necessary, and also check the inhibitor concentration, though adding 10% of the initial dose each year is an alternative.

 

[shellards]

What is the simplest way to determine the PH of the supply? Indicator paper, I suppose, (where can I get some?) or perhaps the water company should be able to tell me this? By the way, I would value your comments on the need to isolate the Central Heating circuit in the present circumstances and until the matter is resolved. Thanks for your continuing interest.

 

[masona]

where can I get some?

Hot tub, swimming pool shop, not sure if plumbing merchants sell them though, here one of them.

I get mine from http://www.poolstore.co.uk/

 

[oilman]

You could isolate the potable water from the thermal store by connecting the water mains water to a plate heat exchanger as used in combi boilers, and then connecting the primary side of the plate heat exchanger via a circulating pump to the thermal store. You will have a much lower flow to taps because the pipes to the plateheat ex are usually 15mm.

You need to establish if your problem is acidic corrosion before you replace the store. It may be better use a stainless steel model, Gledhill produce one if Albion don't.

 

[shellards]

Since the inhibitors in the cylinder will now be diluted and the pressure of water in the hotwater coil is greater than that in the cylinder (pressure in coil - 2 bar) I assume that there is very little risk of water passing in significant quantity from the tank to the coil. Added to this, the water in the coil will exit via the hot water taps and while I dare say there is a chance of diffusion back into the cold water flow, taking all of the factors together, I was seeing no great risk to my health. I do realise that legionella can develop within hotwater tanks. Are you suggesting there is significant risk of a) legionella or other pollutants accumulating in the tank and b) cross infection of the domestic supply, given the various factors I have outlined. I am not familiar with the plate exchanger concept. This seems quite involved for what I assume would be a temporary arrangement.

Thanks to masona for suggestions as regards ph testing. I have in the meantime rung my water company who assure me that if I ring again on Monday morning they will be able to tell me the ph of my supply on the basis of my post code. This service is not available at the weekend.

Oilman says:

You need to establish if your problem is acidic corrosion before you replace the store. It may be better use a stainless steel model, Gledhill produce one if Albion don't.

As you can see, I am working on the question of acidity. I am initially unenthusiastic about the suggestion of a Gledhill cylinder, given that the connections will certainly be different and this will complicate the job of replacement. The space around the cylinder is quite tight. Still, thanks for your suggestion; I am certainly bearing it in mind.[/b]

 

[ollski]

What is the simplest way to determine the PH of the supply? Indicator paper, I suppose, (where can I get some?) or perhaps the water company should be able to tell me this? By the way, I would value your comments on the need to isolate the Central Heating circuit in the present circumstances and until the matter is resolved. Thanks for your continuing interest.

Ph testers will be available off the shelf at your local tropical fish store.