New hot-water cylinder after three years??

[Moonraker]

It's three years since I posted in these forums. (Then I was agonising how best to clean out 40-year-old pipework prior to having a new boiler fitted - all went well, with an excellent father & son removing the rads, hosing out a great deal of sludge, then refitting with thermostatic valves).

This time, I'm enquiring on behalf of a Polish friend who three years ago moved into a brand-new house near Stansted Airport. Her water is taking longer and longer to heat up. A British Gas engineer said the cylinder is good quality, but the system needs a power flush. (I've seen the thread expressing scepticism about BG's enthusiasm for this process.)

An independent engineer says that the cylinder is poor quality and needs to be replaced - at a cost of £3000

My first thought was that the immersion element was lime-scaled and needed replacing, but that wouldn't explain slow heating with the gas boiler.

Given that I know next to nothing and that my friend and her husband know even less, I wouldn't want to trouble you to enter into too many (indeed any) theories and suggestions. My only question is this: would a hot-water cylinder really need replacing after just three years? (My first one lasted 20 years.)

 

[Best]

There might be a problem with a zone valve blocked somehow, or perhaps the gas boiler is a system boiler and the system pressure is too low, but I assume the BG engineer would have checked that.

Depends on what type and make their cylinder is. A copper vented cylinder is a lot cheaper than a stainless steel unvented unit. But neither should be £3000 to replace!!

 

[townfanjon]

If its a stainless cylinder , either unvented or vented it should be under warranty.
some copper cylinder have a very small warranty

 

[Jones82]

Is the cylinder indirect or direct? (heated by gas or electricity) it could be incorrectly installed, I'd get at least another two quotes.

I'd be leaning towards poor installation if its failed after only 3 years.

You could flush the coil with mains pressure from a filling loop then fit a magnetic filter. Surely it can't need a power flush after only 3 years? Do the radiators get hot quickly and thoroughly or same story with them?

 

[Moonraker]

Thanks for these prompt replies. My friends have always complained about low pressure.

Regarding Jones82's questions, I'm relying on my Polish friend's attempts to explain the situation to me and though her English is good she's not at all technical. (Her name, by the way, is Aga , short for Agnieszka.) I fear that already I may be confusing her and her French husband with my attempts to help, though at least my suspicions about the need for, and costs of, a new cylinder are being confirmed.

Her next step is to consult a Polish plumber in her village, who hopefully will be able to discuss the matter in her language. (Twice I've lunched with her when she's run into problems explaining in English what she wants from a Polish waiter - why not discuss in your own language, I suggested .)

 

[Chuck]

I believe that Stanstead is a reasonably hard water area but three years is much shorter than I'd expect for limescale to cause a noticeable problem.

Similarly, unless there is no inhibitor in the system, which would leave the blame with the builder IMO, it is unlikely to be sludge requiring a 'power-flush'. Also, sludge usually affects the radiators well before it affects the HW.

More likely for a system of that age is a faulty three-port valve or an incorrectly set programmer that is not heating the tank for long enough at the right time of day.

A competent heating engineer (go for a small independent firm with a good reputation who has been in business for a long time) will have no difficulty distinguishing between these possibilities and will actually check their diagnosis using tests rather than guesswork before demanding hundreds/thousands of pounds for a cure.

An electric immersion heater, typically 3kW, will take a long time to heat a tank of water. It depends on the size of the tank, obviously, but several hours certainly and possibly longer. Contrary to popular belief, limescale does not change the heating rate much because the power depends on the element's electric resistance, which is not directly affected by limescale. What the limescale does is reduce the operating life of the element by causing it to have to run at a higher temperature in order to push the 3kW through the limescale.

 

[Tea-Break]

Looking back through the posts, I see it’s a gas boiler. This must therefore be an indirect cylinder.
Although being only three years old I would guess it is also an Unvented Cylinder.
Reasons for the poor heat recovery time. Firstly I doubt the system requires a ‘power flush’. An application of a chemical cleaner could be applied as a low cost precaution.
Secondly as already mentioned (and most likely) is a problem with the motorised valve. Easy to check.
Finally a heavy buildup of lime scale on the cylinder heat exchange (coil) would slow the heat recovery period. If they are referring to the immersion heaters. The build up of scale deposits on the element has an insulating effect and usually results
in the element overheating/failing prematurely.
Advise them to seek a member of the CIPHE ( look at web site).

 

[Welder]

Contrary to popular belief, limescale does not change the heating rate much because the power depends on the element's electric resistance, which is not directly affected by limescale. What the limescale does is reduce the operating life of the element by causing it to have to run at a higher temperature in order to push the 3kW through the limescale.

The limescale inhibits (reduces) heat transfer Chuck which means standing losses increase - as well as the potential for overheating.
By inhibiting energy transfer you increase the time taken for it to reach temp plus you increase standing losses (wasted energy).

 

[Chuck]

The limescale inhibits (reduces) heat transfer Chuck which means standing losses increase - as well as the potential for overheating.
By inhibiting energy transfer you increase the time taken for it to reach temp plus you increase standing losses (wasted energy).

The limescale increases the thermal resistance between the element and the water. This causes the element to run at a higher internal temperature to compensate because it still needs to dissipate the electrical input power somehow. The higher element temperature also increases the heat transfer along the element to the flange but this is a very small proportion of the total and, in any case, heating the cylinder casing via the flange will still transfer most of this heat to the water in the cylinder.

The effect I can't immediately quantify witout doing some digging is the impact that running at a higher temperature has on the electrical resistance R and hence the element power, which is V^2/R. From memory, the increase in electrical resistance between an element at room temperature and in normal operation at 240V is about 10% so I doubt the change is much greater than that but if you have any actual measurements I'd be interested to see them.

 

[Welder]

The limescale increases the thermal resistance between the element and the water. This causes the element to run at a higher internal temperature to compensate because it still needs to dissipate the electrical input power somehow. The higher element temperature also increases the heat transfer along the element to the flange but this is a very small proportion of the total and, in any case, heating the cylinder casing via the flange will still transfer most of this heat to the water in the cylinder.

The effect I can't immediately quantify witout doing some digging is the impact that running at a higher temperature has on the electrical resistance R and hence the element power, which is V^2/R. From memory, the increase in electrical resistance between an element at room temperature and in normal operation at 240V is about 10% so I doubt the change is much greater than that but if you have any actual measurements I'd be interested to see them.

We are in violent agreement

 

[Best]

I have seen motorised valves intermittently working and also preventing the hot water to be fully heated.

 

[Moonraker]

Um ah. It appears that the second plumber didn't tighten something on the "hot tank" (perhaps he checked the immersion element?) resulting a leak down a wall. The third (Polish) plumber:

"came to similar conclusion as others. There must be something inside the cylinder that might fell off and got stuck somewhere or it’s the lime scale. He advised to contact [housing-site] developer ( so we did - but he said that’s not his business and we need to contact the cylinder manufacturer ). He sees two solutions: To take the cylinder out and clean it then install the water softener or buy the new one with new water softener."

Of course contacting the manufacturer is a waste of time - it will obviously (and reasonably) point to the installer - and I see from other threads that blockages in pipes can cause problems to a cylinder's efficiency.

I think we've taken this far as we can, and I'm grateful for your input.

Now as it happens, another Polish friend has just been gifted a house in East Kent. A new boiler was installed three years ago and for most of that time the house has not been lived in. Now Evey has been quoted £500 for repairs to the boiler, which broke down shortly after she moved in ...

Seriously, no need to reply; I'm just noting the minor coincidence.