- Thread starter thinfourth
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Kayos
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The 14kw is used to calculate the size of the boiler and nothing else, give it 14kw and it will recover in 20 minutes, and heat from cold in 30-40, doesn’t mean it’s undersized
A boiler stat can fail and the overheat will be 105 deg c, anything over 90 is classed as uncontrolled. It also isn’t guaranteed to have a manual reset overheat, if it’s auto that’s not allowed either
Kayos
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Give it 14kw and it will recover in 20 minutes,
Those stats are not energy cut outs
@thinfourth since you both like long hot showers the solution might be to fit a heat exchanger to your shower waste, you plumb them in so the waste water going down the drain pre-heats the water feed to your hot water cylinder so the incoming water doesn't cool the tank as quick so it also takes less time to heat the tank back up to temperature. Me and my missus have the same problem but we just stagger our showers to solve the problem.
I've had electric showers in the past but nothing beats the soaking you get from a pumped shower fed from a hot water tank, until the tank runs cold...
I've had electric showers in the past but nothing beats the soaking you get from a pumped shower fed from a hot water tank, until the tank runs cold...
I Have a 200lt unvented tank run off a 24kw boiler and if its calling for heat it will recover quicker than the water is used with 1 shower running. It will struggle to keep up with 2 at the same time for long periods but never really a problem.
Like you say the Coil may be capable of heating at a rate of 14KW/hr but that's dependant on heat input to the coil which could be effected by pipe sizing, incorrect balancing of system, boiler set temperature or even just the central heating robbing heat when its on too. Most systems are never balanced properly which can cause all sorts of issues on larger systems.
When installing unvented cylinders i used to always fit solar cylinders as are almost the same price new. I would double up the coils to give a much faster recovery rate and it also gave the option of upgrading to solar later on without having to change the tank again.
Like you say the Coil may be capable of heating at a rate of 14KW/hr but that's dependant on heat input to the coil which could be effected by pipe sizing, incorrect balancing of system, boiler set temperature or even just the central heating robbing heat when its on too. Most systems are never balanced properly which can cause all sorts of issues on larger systems.
When installing unvented cylinders i used to always fit solar cylinders as are almost the same price new. I would double up the coils to give a much faster recovery rate and it also gave the option of upgrading to solar later on without having to change the tank again.
eh? the coils in my tanks are rated for 14kw of heat transfer with a decent temperature difference between tank/coil - those numbers are from the technical spec. sheet for my tanks, the coils have a known surface area so you can calculate the heat transfer through them. number isn't just there for a boiler sizing guide. (really it would be for a tank sizing guide anyway, because you size the boiler for the house/load and then find a tank to suit, not the other way around)
200 litres of water, 50'C rise (10'C to 60'C) - that comes out as 11.72kwh by my maths
which is 50min re-heat time
ok... those numbers don't look so bad but that's with no heat loss and 100% efficiency... as the tank warms up the temp difference between the tank/coil will drop and the heat transfer between them will drop pretty quick too
maybe 50min is do-able with a 14kw immersion heater, but not from a tank coil
(not mine anyway, obv. there's a lot of different tanks and coils out there)
if the boiler stat and the tank stat both fail, it's going to boil the water in the tank anyway, with or without the phe pre-heater?
thinfourth, I was thinking about this last night... next time the tank is cold maybe you should check the feed and return pipes to the coil, see if it's doing much? laser thermometer thingie would be great, but just grabbing them should give you an idea... maybe your tank/coil is covered in limescale so the coil is insulated from the tank water ?
would also give you an idea if there's a problem with the flow rate etc. etc.
thinfourth
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I have checked the inlet and outlet to the heating coil while it is running
The inlet = OUCH that is hot
The outlet = That is quite warm
So i think that it is getting succulent flow
It might be some build up on the coil but without anyway of getting the coil out there isn’t much i can do about it. It is unlikely to be limescale as we are very soft water here.
I shall talk to a few professional plumbers locally but i suspect the answer will be to rip out the tank and boiler and replace them with brand new ones.
The inlet = OUCH that is hot
The outlet = That is quite warm
So i think that it is getting succulent flow
It might be some build up on the coil but without anyway of getting the coil out there isn’t much i can do about it. It is unlikely to be limescale as we are very soft water here.
I shall talk to a few professional plumbers locally but i suspect the answer will be to rip out the tank and boiler and replace them with brand new ones.
normspanners
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Now just a thought, on my tank coil, std ish vented system, I have a gate valve on the return to act as a restriction to the flow to stop the boiler circulating the water too quickly and stop hysteresis, might he have a similar valve that might restrict the flow ?
another thought would be a separate pump in the line to the coil but dedicated to the shower use and switches on when the shower is running giving max flow through the coil when the shower is used, but the normal pump would still circulate through the heating circuit, when showering has finished the normal flow would resume through the stationary extra pump, heating the water as normal but for the whole time the shower is used you would have a boost to the coil circuit.
gaz1
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your forgetting a few things here you might not be aware of
14kw it may quote dosnt mean its going to produce that heated pipes are not cleaned like they are with stoves therfore when new will produce that amount of heat transfer
your also forgetting flow through the pipes faster the flow the quicker that heat gets back to the boiler shutting it down dosnt mean the systems is being effective of heat transfer
too slow of flow and you cause thermal shock at your boiler most new boilers
14kw it may quote dosnt mean its going to produce that heated pipes are not cleaned like they are with stoves therfore when new will produce that amount of heat transfer
your also forgetting flow through the pipes faster the flow the quicker that heat gets back to the boiler shutting it down dosnt mean the systems is being effective of heat transfer
too slow of flow and you cause thermal shock at your boiler most new boilers
normspanners
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Ok another thought how about the fphe just in the the line to the shower, again with its own separate (shower switched) pumped supply from boiler so when showering the circuit to the shower gets a 5-10 KW to boost the shower as you said the shower can handle it coz it is thermo controlled ?
normspanners
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Yes as said, hysteresis, that is why I have a control valve on mine to stop that happening.
But mine has been adapted from the old idea of the DHW being thermosyphon off the seperate 28mm sercs off boiler.
If its a conventional boiler you should have around a 11 deg differential between flow and return and if a condensing around 2O deg. Obviously will vary a bit depending on tank temperature but is what to aim for when tank is cold. If set up properly modern condensing boilers also require around 20% bigger radiators and cylinder coil over conventional boilers to get the same performance as before due to the bigger temperature drop. This is often over looked when people just upgrade there boilers on older systems.
In older domestic system design only around 6kw was used for HW when calculating the boiler size for a house and imagine a lot of cylinder coils were built round those figures too. Things have moved on a lot now with design and we also have much higher demands. Probably no fault with yours, just system design.
Only way to guarantee not running out of hot water is an electric shower(poor flow) or large combi boiler (no back up) which also have there downfalls. lol.
In older domestic system design only around 6kw was used for HW when calculating the boiler size for a house and imagine a lot of cylinder coils were built round those figures too. Things have moved on a lot now with design and we also have much higher demands. Probably no fault with yours, just system design.
Only way to guarantee not running out of hot water is an electric shower(poor flow) or large combi boiler (no back up) which also have there downfalls. lol.
