Hard to say. If you're boiling the tank (from the noise) and the hot
water is not only slow but also luke-warm, then the only conclusion
would seem to be that a mixture of hot and cold is being drawn off
into the hot delivery pipe. Maybe the draw-off is mounted too low-down
on the tank?
The TRV could be there to control the primary circuit heating the
tank, but there are two problems there: TRVs are too small bore to use
on a delivery, or especially a primary circuit. More importantly, it's
sensitive to air temperature, not tank water temperature. Is there
any bypass circuit to the TRV? If it's _supposed_ to supply hot water
to the bath, but it's throttled by its diameter, could the bath hot
supply be pulling cold from somewhere as well?
I'm going to make a guess that your system is the same design as the
one we used to have: boiler heating water in the combined primary
circuit (with coil in the tank) and the CH side. DHW is obtained from
the heating effect of the coil. A red-handled gate valve allows
primary hot water through the CH system. DHW temp is controlled by a
capillary thermostat, that shuts off the primary flow to the coil when
the correct temperature is reached. There's an immersion heater for
backup. Somewhere there will be a pump bypass loop for when the
capillary valve shuts with the CH shut off.
It sounds to me like the capillary valve, either through the setting
or some fault, is turning off the primary flow to the coil at too low
a temperature, hence the lukewarm water. The boiling sound is probably
the primary water being heated too high, due either to too high a
thermostat setting on the boiler, or a fault with it.
Try turning down the boiler stat to about 2/3 of max, or try turning
it down to see if it clicks off when the bubbling sound starts, and
set it below that level. Turn up the capillary valve setting to 2/3
max (screwed out) and see if the water gets hotter. Failing that, open
it fully. Be careful when testing the DHW.
I regret to say i can't remember the maker of our capillary valve, but
it looked identical to yours.
I think serious scale build-up can cause boiling noises, too, with the
scale causing localised over-heating of the water. If there's a drain on
the HW tank it could be checked for sediment.
The electric heating element presumably consists of a coil of suitable
wire which gets hot when current passes through it. This wire will be
enclosed within a sealed metal tube.
If perchance this tube had corroded and sprung a leak, allowing water
to come in direct contact with the heating coil, I dare say this would
also result in boiling noises.
Dunno about that. Maybe AC and DC are different in this respect? When CH
systems start tripping RCDs, the advice given here is usually to check
the pump first - because that's the point in the system where water and
electricity are closest together.
It may be that the circulating water, with all its corrosion inhibitor
additives, is a good-ish conductor, but if we're talking about an
immersion heater, the water around it in the HW tank is just tap water,
which should be quite low on ions.
On seeing these, I think mine must have been an older version, but
obviously worked on the same principle.
With electronic controls being what they are today, I would have
thought that nice simple things like this would have died out by now.
Yes, maybe they're there as replacements for existing valves - I can't
see many people fitting these from choice on new systems. By sensing
tank temperature, they are at least an improvement over the earlier
Cyltrol valves. These just sensed return flow temperature on gravity
circuits, and if - as happened to me a few times - the primary circuit
decided to circulate the wrong way, you got no hot water!
You don't say whether you have just moved in or whether you have been
there a while. If you've lived there a while, have you always had poor
heating or has this happened recently? The reason I ask is that if
this has started to happen only recently, then something must have
gone wrong or broken recently. If it has always been like that, it
could just be a poorly designed system.
There may be a thermostat on the immersion heater but it will be
hidden under the lid (see below). Have you checked that there is not a
thermostat buried under the cylinder jacket? Many old systems did not
have cylinder thermostats. I know I had to add one to my gravity fed
HW system (now converted to fully pumped too).
This puzzled me too but other posters have said it could be a
capillary valve. My boiler is oil fired and they have to have a fire
valve. Whilst this is nothing like your set-up, my fire valve had a
fine capillary tube (a very thin metal tube only a couple of
millimetres wide) and I managed to break it. I've never understood
how, I was tightening the compression nut and was very careful to keep
hands and tools away from the valve at the time but I digress. I see
in your photo a Wickes jacket bag and I wonder whether you may have
snapped the capillary on your valve whilst fitting this? Don't be
embarrassed if you have, like I said we've all done it (well, me at
Before you fitted the jacket, what did the cylinder look like? Was it
bare metal or did it have a couple of inches of foam insulation on?
Do you hear the bubbling only when using the electric heater or when
using either electric or gas?
The water in the cylinder should get hot but it should never get hot
enough to boil. The recommended temperature is 60C: you need it this
hot to kill any germs living in the water.
Immersion heaters have a built in thermostat which can fail, causing
the heater to stay on. When this happens the water will boil. If the
boiling water gets into a plastic tank, it can make the plastic melt.
Then the boiling water pours everywhere. There have been some terrible
stories of people being killed when boiling water poured through their
ceiling onto them.
New immersion heaters now have two thermostats: one that you can
adjust and another set to, I think, 85C so that if the first one
fails, the second one will switch off the heater.
Often the thermostat is replaceable and you can replace it without
having to empty the cylinder first. You need to make sure the
electricity is switched off and remove the circular cap over the end
of the immersion heater and temporarily unscrew the wires (making a
note of which one went where - a digital photo may help you with
this). The thermostat should slide out.
You don't get second chances with electricity, so if you aren't sure
what to do, don't do it.
The thermostat should have a dial on it and what looks like a screw in
the middle. You put a screwdriver into the "screw" and turn it to
adjust the temperature. This will only adjust the temperature that the
electric heater will heat to; it will not make any difference to how
hot the gas boiler will heat the water. It may be that you need to
turn your setting up if the water is too cold or down if it is
The cylinder should vent into a header tank. Is this the small plastic
tank on the wall in your photo? Do any pipes run between the cylinder
and this? Does this plastic tank feel very hot, especially when you
hear the boiling?
It seems a very small tank. Usually such small tanks are used for the
CH not the HW. Is there a second, larger, tank somewhere?
Do you know which pipes you are feeling? Like you said in your
introduction, there is a HW circuit and a CH circuit. Are both hot or
just the CH ones?
Why would you need to top up with the electric? Use the gas to heat it
to the desired temperature. I think gas is the cheapest way to heat
your home. Electricity is nearly always the most expensive way, unless
you have economy seven and use the immersion heater overnight.
Have you tried the highest setting to see if that makes any
It depends on your system. Some systems might allow water through both
radiators and the cylinder at the same time and you are right, in some
systems this could cause the HW to not get enough heat. On the other
hand, if the system is designed to have both on at the same time, it
may be balanced to allow both circuits to get their fair share of the
heat. If you have a gravity fed hot water system, I wonder whether
that makes it more likely that the HW will be starved of heat when the
rads are on?
How many stories are in the house and where is the bathroom in
relation to the tank? Do all hot taps run cool or just the bath?
Other systems (particularly combi boilers, which you don't have) have
a valve that switches the heat to one circuit or the other. The combi
boiler in my last house would always give the water the priority, so
there was no CH whilst the bath was running.
Do your radiators have TRVs on them? The "rule" about having the
bathroom rad fully open is that if all your radiators had TRVS on and
the rooms got hot, they would stop the flow of water through the rads.
If the boiler came on, there would be nowhere for the hot water to go
and this could damage your boiler, so by having one radiator always
open, it means there is always somewhere for the water to flow, so you
never have the possibility of expensive damage to your boiler. It
doesn't have to be the bathroom rad; you should never have a TRV on a
radiator in the same room as the thermostat. This is so the thermostat
controls the radiator not the TRV.
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