Hi All,
>
> We have a rapid knocking coming from our hot-water tank. It happens
> at some strange times too - I have noticed it a few times - once late
> in the evening, once at around 0430 and this morning. On at least one
> of these occasions (0430) the hot water *should* have been off.
>
> My first assumption is that it is caused by the water boiling, and my
> first assumption would be limescale were it not for the fact that
> Edinburgh has very soft water and that the system is sealed. Any > ideas?
I know it's bad form to follow up one's own posting, but more info...
The knocking occurred at least twice last night - once around 1130, once around 0230. The hot water was turned off at the wall thermostat for the boiler.
There is, however, a switch in the hall near the boiler that says 'hot water' or similar with a red light embedded below the switch itself. Switching this off caused the knocking to tail away. Switching it back on caused the knocking to return after a few minutes. Now, that sounds like an electric element heating water to a boil to me...
Have we got electrically heated water in addition to the gas boiler? When we first moved in and switched the water on (at the boiler thermostat), we were expecting nearly instantaneous water because we believed we had a combi, but there was no hot water until we switched this 'hot water' circuit on (and then we had to wait a half hour or so). Has this been installed in the past because someone couldn't get the boiler to heat water? Most importantly, can I find out what in installed in my house without paying a fortune to get a plumber in to suck his teeth and tell me I need to spend a fortune on replacing it all?
I forgot to bring the manual for the boiler into identify it - it is a Vokera, though. I'll try to remember to do that tomorrow...
It would have been installed anyway as part of the original installation, if nothing else as a backup. Accepting that...
If you didn't get any hot water until you turned on the electric heater, possibly, or maybe the boiler was never designed/installed to.
Find out exactly what stuff is there and post again here. Follow pipes, especially around the boiler and the tank. Find out *exactly* how many pipes enter the hot water tank and where. If, for example, there is one pipe out the top and one pipe in the bottom, then the only water heating you have is electric.
As you are getting boiling, the the thermostat for the tank may be faulty. This is a relatively simple DIY task to change but involves electrical connections. If you feel you are not competent, get someone who is. It will be found under a cylindrical metal cap on the cylinder somewhere (isolate before taking the cap off). The thermostat will slide out separately and cna be replaced (with one of the same length). There is clearly nothing wrong with the element itself!
So, either:
Your only water heating is electrical by design - expensive to run, expensive to change (but, bar the thermostat possibly, it is working!)
The water heating part of the heating circuit driven by the boiler (if it exists) is not operational. Shouldn't be too expensive to fix.
Thanks Bob, I will. I didn't get a chance to do anything last night, but I will try to take a few minutes noting everything and posting it here.
Excellent, I'll give that a try. Is it possible that it is simply a matter of adjusting the thermostat, or are these set at a fixed (presumably just below boiling) level?
Also worth noting is that last night, when I drew a bath, the hot water was plenty hot enough, but definitely not scalding. I can try to get hold of a thermometer to measure the temperature at the tap...
Thanks for the advice - I'll post the info as soon as I get a chance.
First of all, thanks Bob for all the excellent information. I did some investigating last night, and...
[This is long, and my description are probably a bit cr&p...]
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that was my first thought, but then I understand that Edinburgh is a particularly soft water area, and I understnad that the cylinder is only around 3 years old, it having been installed when the loft conversion was done.
Excellent, thanks - I couldn't see that last night (the cylinder is accessed through a small cupboard door), but now I know where and what I'm looking for.
Well, following my investigations of pipe routing last night, I believe I hve worked out what's going on. I'll try to describe it as best I can, but it's a bit of a jumble...
The boiler is a Vokera Mynute 20SE and is not, as I had been told by the previous owner, a combi. It does, however, have the ability to run a hot water cylinder using either a 'Y' or 'S' configuration (I don't entirely understand these terms, but I think I have a rough idea from the manual' layout diagrams); from the piping layout it appears that our boiler is heating water using the 'S' configuration - at least, there are two motorised valves in the cupboard - with the downstairs hall radiator as the bypass. I assume from the layout diagram in the boiler manual that this means the boiler can heat the radiators or the hot water cylinder independantly.
There are just three pipes leading to the boiler - I assume these are the inflow, outflow and gas.
There appear to be three pipes leading to the cylinder, although I presume there's one more I can't see - I assume these are the mains input, hot water output and boiler system inflow/outflow.
The pipes adjacent to the hot water cylinder appear to originate from the boiler (at least, two disappear under the floor, roughly towards the boiler, and were warm when I was working last night. One (I assume boiler outflow) appears to feed the cylinder via one motorised valve, has a pipe leading off (I assume) to the bypass and feeds what appears to be the rest of the heating system via the other valve; it is also connected to the mains water via a flexible hose. The other (I assume the boiler inflow) appears to feed the cylinder and the two heating pipes. Of course, they also appear to be connected in random ways to each other, but the heating works, so I'll ignore that!
There were two hand taps in the cupboard: one is on what I assume is the boiler inflow pipe, just next to where it exits the hot water cylinder; the other was on the boiler outflow pipe, above the joint that leads to the T-joint to which both motor valves are attached. Opening each of these about a half turn appears to have given us hot water this morning despite the 'hot water' switch (electrical immersion heater) being turned off.
So I assume the electric immersion heater is a backup system to the boiler, which now *appears* to be working.
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I hope replacement won't be necessary, at least for the moment. Will the kettling cause lots of damage, or is it mostly just an auditory annoyance?
It sounds as though you have now analysed the system, at least. A few comments:
Your presumption on the pipes to the cylinder is probably correct - it is likely the cold feed to the cylinder (from a header tank in the loft/on top of the cylinder/somewhere else above it) is round the back somewhere. The pipe that comes out the top of the cylinder is the DHW (domestic hot water) take off, where the water for the hot taps comes out. The heating pipes from/to the boiler normally come in and out the side, one above the other (inside they are connected by a copper coiled pipe which keeps the DHW and the boiler water seperate but allows heat exchange).
There should be a thermostat on the cylinder between, vertically speaking, the boiler pipes. This will open the motorised valve and request heat from the boiler (if the water switch is ON) when the water temperature drops. There must be a heating/water control panel somewhere (behind a cover in the boiler or on a wall somewhere).
The big question is "why was the water circuit valved off?". I fear I can't imagine anyone doing this unless there was a problem! One likely problem (if there is one) is a leak between the heating coil in the cylinder and the DHW. If you run the system as it is intended, and there is a leak, eventually the DHW cold header tank will overflow (the pump pressure forces water from the boiler circuit into the DHW and "backfills" it). An unscrupulous seller might have just valved off the tank and switched to electric heating and not said anything. OTOH they may just have been stupid and not realised it was valved off! I hope it's the latter.
The kettling will just be an auditory annoyance but it may still be worth checking the immersion heater thermostat.
You are, at least, at the stage where you can experiment and found out how the system operates and if necessary repair it rather than be faced with a non-existent system.
There are some 'boxes' with electrical feeds strapped to the side of the cylinder with what looks like the wire cord used to hold up net curtains. One of these has a dial on it, and from your description I'm willing to bet (a small, nominal amount!) that that is the thermostat. There doesn't appear to be any 'kettling' whan the boiler (rather than the electrical system) is running the DHW.
Control panel - do you mean the dual hot water and central heating timer panel that's in the kitchen downstairs?
It wasn't actually turned off - it was open, but only just. All I've done is open it by a further half-turn or so. I take your point, however, about possible causes for that, and I'll chek the area for leaks as soon as I get home.
WRT to DHW header tank - presumably this would have to be physically sited higher than the top of the DHW cylinder? Is it possible the cylinder is fed at mains pressure? I ask because there is only one header tank in the loft area (that I have found) and that is level with the cylinder (the bottom of the tank is level with the bottom of the cylinder - I guess the top of the water level would be about level with the upper boiler pipe). I suppose I can check to see if this fills the tank by seeing if the ball-valve opens when a hot tap is turned on...
Aye, will do as soon as I get home. As I said before, at least I know what I'm loking for now...
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Heh, thanks - I sincerely hope I don't need it! Actually, I'm starting to enjoy this - it's amazing how quickly you start to understand things once you get over the fear...
Sorry ignore my other reply - hit SEND by mistake!...
Yes. There will never be kettling from the boiler fed system as the heat source is always well below boiling point - not the case with an electric element, where, if there's obstruction to free flow of water over it, localised temperatures can exceed 100C.
Yes
Yes you could do that. Try turning on the cold tap in the kitchen and trying to stop the flow by putting your thumb over the end - this will give a feel for the incoming mains hydrostatic pressure. Then do the same with the hot tap. If it requires much less effort to stop the flow, it must be run from a header tank, not the mains. If it's about the same, it will be a mains pressurised system. Some vented cylinders ("Fortic" et al) have the header tank integral as the top part of the cylinder. The other tank sounds like the header tank for the boiler circuit (which needs its own).
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