Indirect Hot Water Cylinder - possible fault?

[practicalsolutions]

Greetings DIYers,

I have a question concerning an indirect hot water cylinder which is about 30 years old. It's connected to an open vented system with tanks in the roof and 9 radiators.

We've just had a new Gas Boiler fitted and everything seems to be working fine except that the hot water seems to take too long to heat (from cold).

When the engineer fitted the Boiler he used a PowerFlush machine and cleaned out a load of sludge from the system.

I'm wondering if its possible for the heat exchanger pipes INSIDE the cylinder to be 'furred' up a bit, and thus significantly reducing the efficiency of the heat transfer process to produce hot water in the taps.

Is this possible please? Have you ever heard of such a problem?

Thanks in advance and best regards,

 

[Dan Robinson]

The domestic side of the coil could be heavily scaled up depending on where you are in the country.

Either that or the bypass is poorly adjusted.

Or most likely there isn't a problem and you're just not getting the performance you were expecting.

 

[practicalsolutions]

Crikey Dan, that was quick! Many thanks.

Good point - scaling on the outside of the indirect coil is something that I hadn't considered. Presumably this can reduce the heat transfer process. Location is Somerset (Wessex Water) - I don't know what type of water is prevalent here?

How can I know if the bypass is poorly adjusted please? If it was incorrect, wouldn't the boiler cut off too early (which isn't happening), bearing in mind I've got the central heating switched off.

What is a typical heating time from cold - two hours seems rather too long for a brand new Vaillant Eco-tec 418 Boiler to heat a small hot water cylinder doesn't it.

Thanks in advance for any further advice.

 

[Charnwood]

Is there a reason you stayed with the open-vented system and didn't have a combination boiler fitted?

Is the boiler rated high enough (18kw/61,500 btu) for your 9 rads plus DHW?

 

[practicalsolutions]

Hi Charnwood,

Thanks for your message.

Yes, open vented is a tried and tested system and when I researched the issue over several months, it became clear to me that a Combination Boiler was definitely not what I was seeking. Can't remember all the pros and cons now but it was a no-brainer as far as I'm concerned. Each to his own of course and I'm not condemning Combi-Boilers per se.

Yes, I carefully reviewed the rating of the Boiler too. Three completely independent Engineers came up with the same suggestion, plus two alternative brands with very similar outputs. The new Boiler is rated higher than the old one and seems more than enough to heat all 9 radiators very quickly and efficiently. I also looked at higher output Boilers (personally I always tend to over-engineer things) but found that if you overdo things you end up with significant less efficiently for no gain.

In summary, the Boiler capacity is more than adequate but I'm convinced that there is something not working effectively in terms of the heat exchanger within the cistern. The system water enters the cistern very hot (ie. direct from the Boiler) and seems to me to be far too hot when leaves again via the exit pipe. Very little heat appears to be being transferred which is why it takes so long to heat the domestic water.

Thanks again for your time.

 

[Dan Robinson]

As I mentioned last night in the closed part of the forum, combination boilers are not the be all and end all of boilers. Out of the current active installs at the moment only a couple are combis.


One thing I forgot to ask is the actual reheat time.

On a 30 year old cylinder that could easily be an hour or more.

Especially if it's scaled up. Which is unlikely for you.

http://www.wessexwater.co.uk/customers/water-quality/waterhardness.aspx?pc=TA5 1UD

 

[practicalsolutions]

Thanks again Dan,

Reading comparison reviews and forums, I came to the conclusion that there were no benefits (for us at least) to change to a Combi but lots of potential risks, eg. unreliability. The original system had been working fine for 29 years but our old Potterton really had come to the end of its days!

The reheat time is nearer two hours, which seems too long to me. And please note my comment (above) concerning the temperature of the input and output system water pipes adjacent to the cistern.

If the cistern water was cold, and the indirect transfer system was working well, would you expect the temperature of the system water exiting the cistern to be significantly cooler that than entering it from the boiler?

 

[Dan Robinson]

If heat is entering the coil from the boiler, then coming out of the coil at a similar temperature, then there is a heat transfer issue inside the cylinder/cylinder coil.

If ti is 30 years old it is probably worth investing in a newer one anyway - reheat time would be around 20 minutes.

 

[practicalsolutions]

Dear Dan,

Many thanks - you've confirmed my suspicions.

Even though the premises is located in a 'Moderately Soft' water area (many thanks for the weblink), I guess the indirect pipes could still have picked up some residue on the domestic side as you suggested over the last 29 years, and there may also be some 'furring' on the inside of the heat transfer pipes. One or both of these issues seem to be causing the problem.

I'm minded to change the cistern myself - there's plenty of access and the new units appear to have connections in virtually the same places so hopefully minimal pipework should be involved. However, I've got some other major projects on the go for the foreseeable future, so I'm going to have to live with the inefficiency and hope that the cistern will hold out for another 9 - 12 months.

Thanks again and best regards,

 

[Dan Robinson]

To gauge how bad your area is look in the kettle.

How often do you/the missu descale or replace the kettle in your house?


4 years we have lived with a softener and I am still trying to convince the missus to fill from the hot tap (softened) rather than the cold (blo0dy hard).

She won't be told.

But still I don't have to descale the thing every 4 weeks.

 

[Dan Robinson]

These are the results for my postcode:

To help set your domestic appliances, the water hardness in different units is:
161 mg/l (or parts per million) :Calcium Carbonate
64.4 mg/l (or parts per million) :Calcium
11.206 °C egrees Clark
16.1 °F egrees French
9.145 °dH egrees German
1.61 mmol/l :Millimoles

The ppm figure is wrong by some way - I have my softner set to 210.


Your figures may be off too.

 

[polesapart]

If flow and return at cylinder similar temps (start from cold) then maybe poor heat transferance at cylinder coil and replacement indicated.

If boiler flow and return hot but flow and return markedly different at cylinder (DHW only selected at programmer) then suspect circulation issues.

Personally I have never seen an externally calcified cylinder coil in the Wessex Water area (I'm based in Wellington, Somerset).

Replacing a 30 year old cylinder would do no harm, but my bet is on an incorrectly adjusted bypass.

 

[D_Hailsham]

I have a question concerning an indirect hot water cylinder which is about 30 years old.

What insulation is there round the cylinder? If none or just a loose red jacket, you should change it for a modern one with integral insulation. Not only will the cylinder heat up faster, it will also retain its heat for much longer.

 

[Agile]

If the water in and out of the heat coil is hot then its not a bypass or flow problem!

Its just poor thermal transfer in the cylinder.

Not helped by the likely fact the old boiler was running at a flow of 80 C and probably the new one at 75 C.

I am very surprised the boiler installer is not being involved with these questions!

Also surprised that he did not advise a new cylinder as we are supposed to do!

Tony