Any tradeoffs for high gain cylinders (besides cost?)

[choff]

Hi there. Converting from a vented to unvented HW system, off a condensing system boiler (already fitted). S plan layout.
We've got through the winter with flow temps of around 55, to keep the boiler in condensing range. One guy quoting suggests a high gain cylinder. That seems to make sense, esp. if it made us ready for a heat pump in the future.

Besides cost, is there any tradeoff? Will they fur up with timescale or fail faster for some other reason?

 

[old duffer]

so a high gain cylinder means a longer coil....which has a greater surface area...which means deposits have a larger area on which to adhere.
So rate of deposit will be the same but the area of deposit and therefore mass will be greater inside the tank.
Will the rate of deposit be greater? No
Will having a greater surface area mean higher rates of transfer? Yes
Will it fur up at the same rate as any coil? Yes

Is this a trick question?

 

[JonathanM]

There is a video here from a very respected poster. At the 9 minute mark he seems to be saying that in some situations a high gain cylinder can lead to excessive stratification, so that the hot water in the tank is a lot hotter than the water at the bottom. I think in this video he sets the cylinder to 60C (the cylinder thermostat is a the the bottom) but the water from the taps is 72C. I've no idea how big an issue this is, but he doesn't seem happy with that outcome when he measures the tap temperature at the 16 minute mark.

 

[Exedon]

Besides cost, is there any tradeoff? Will they fur up with timescale or fail faster for some other reason?

if your in a hard water area whatever cylinder you chose be better to take steps to stop scale building up.
High gain cylinders work well with hot water priority systems..

 

[Johntheo5]

There is a video here from a very respected poster. At the 9 minute mark he seems to be saying that in some situations a high gain cylinder can lead to excessive stratification, so that the hot water in the tank is a lot hotter than the water at the bottom. I think in this video he sets the cylinder to 60C (the cylinder thermostat is a the the bottom) but the water from the taps is 72C. I've no idea how big an issue this is, but he doesn't seem happy with that outcome when he measures the tap temperature at the 16 minute mark.

I'd say a 10C dT between cylinder top and bottom is what would be expected, a 20kw coil circulating at 25LPM will have a dT of 11.5C so this will be reflected in the cylinder, some rapid recovery cylinders quote, I think, ~ 35LPM coil circ rate so a 20kw coil will then still have a dT of 8C,. I Ihave a twin coil 150L cylinder with a very short stack of solar coils right in the bottom of the cylinder, the (solar) coil top inlet is only 400mm from the bottom of the 1500mm high cylinder so all this heating is done in less that a 1/4 way up the cylinder, there are three PT1000 cylinder probes, one just above the solar coil, one at ~ 650mm opposite the centre of the oil fired coil and one at ~ 1120mm in the top of the cylinder. With a cold cylinder and using the solar coil only, the cylinder heats up completely uniformly there is occasionally 1C difference between any of the probes and normally none as its heating up, if you draw off say 35L of HW the whole solar coil is then immersed in cold water and the coil will heat this up first, and will keep heating the lower part of the cy;linder until the temperature is the same as the the mid cylinder probe, at which time both will start rising uniformly together, same with the top probe.
I would imagine a electric heating element in the bottom the cylinder would give the same readings. Why not circulate any single coil cylinder from the bottom up?.

 

[Notch7]

Converting from a vented to unvented HW system, off a condensing system boiler (already fitted). S plan layout.
We've got through the winter with flow temps of around 55, to keep the boiler in condensing range.

Wouldnt you be better converting from S plan to hot water priority, if its possible with your system

I have a hot water priority with a ultra fast recovery vented hot water cylinder, the boiler flow temperature is set using weather compensation and over the past couple of weeks when outside temps have been down to 3 deg, the flow temp has been a maximum of 48 deg and a return of typically 38-40 deg. When the boiler is in hot water mode it fires at 80 degs.

I was put off an unvented system because its more complicated and has more servicing costs

 

[choff]

Only 2 outlets in the house are drawing cold water from the loft tank; everything else is mains. The pressure imbalance is dramatic, baths take forever to run, and the shower pump is super noisy. This vented system is doing us no favours, and has reached end of life.