Boiler and Megalow mistery

[Madrab]

do you think that if I hit the extra hr button before starting the first shower I’d produce enough additional hot water to have that additional shower that today I couldn’t have?

The short answer is .... Yes

My good lady does it, she pop's on a 1hr boost when she is off to do her weekly 'major' service. That normally includes her facials, long hair conditioning, shaving (it seems) her legs, arms, back and everything else she can possibly reach and all before a normal full body shower. She can use 150L of our 200Ltr cylinder and then everyone else is on a hide to nothing to have even a 5min shower @ 38deg, if it hadn't been overridden.

 

[RenzoP]

Thanks to everyone, you are all so helpful. I keep thinking that back in the flat with the simple boiler (no cylinder) it was all so easy… no need to plan anything. When the hot water was needed, the boiler would go on and heat it up..

 

[Johntheo5]

Thanks to everyone, you are all so helpful. I keep thinking that back in the flat with the simple boiler (no cylinder) it was all so easy… no need to plan anything. When the hot water was needed, the boiler would go on and heat it up..

A few things you can easily check out.
If you have a household thermometer measure the HW outlet temperature or/and ensure that the cylinder stat is set at 60C or a little higher at 65C which will give you a extra ~ 25L equivalent of 40C water.
Check the boiler flow target (setpoint) temperature. If this is set too low say at 55C then your cylinder cannot reach 60C/65C, it should set to a minimum of 65C to get a cylinder temperature of 60C.
Some boilers have or can be configured to give HW priority which means that when the cylinder stat calls for HW then the CH (if on) is switched off while the cylinder heating is on but more importantly it also changes the boiler target temperature to a max of say 80C which will certainly heat up the cylinder to 65C.
You can also check the shower flow rate by holding a container under the shower head for say exactly 30 secs, measure with a 1L container, X2 to give the flowrate in LPM, depending on what the result is you could consider installing simple flow restrictors in the shower head to still give a satisfactory flow rate and these steps might get you over the 4 showers in a row requirement.

 

[Rodders53]

Any HW cylinder with a air bubble will not have its full capacity of HW available.
Assuming the hot cylinder pressure is not allowed to reach more that say 5 bar then the HW dip tube in a 170L cylinder must extend "30L" downwards into the cylinder, there will be 165L of hot water at 5bar but only 140L of this is available as HW as the cold mains will start entering the dip tube end once the 140L is used up.

That depends on the manufacturer, surely?

For example: My Oso cylinder: the makers state that one can increase the tank capacity by circa 20% if an external pressure vessel is fitted. My nominal 210 litre tank without the dip pipe holds 250 litres according to the installation manual.

Heatrae-Sadia who make Megaflow may calculate their tank capacity differently of course?

 

[Johntheo5]

Of course they can calculate it differently, if I take it that your Oso cylinder has a air bubble of "40L" then a PRV setting of 3bar and heated to 60c will result in a final pressure of 5.54bar, the pressure will fall to 3.0 bar when 3.9L of HW is released.

The Megaflo has a baffle as well as the diptube so how are these removed to increase its usable capacity with a external EV I wonder.

 

[Johntheo5]

For anyone interested, you can change your own values here once you build your own spreadsheet. Can't post my Spreadsheet file.

 

[JohnD]

Thanks to everyone, you are all so helpful. I keep thinking that back in the flat with the simple boiler (no cylinder) it was all so easy… no need to plan anything. When the hot water was needed, the boiler would go on and heat it up..

A cylinder can easily do the same, but repeatedly running it for short periods to get the cold boiler hot, and then letting it stand to go cold again, is less economical on gas. Combi boilers do that every time, unavoidably, which reduces their efficiency and economy.

If you have a cylinder of reasonable size, timing the boiler to run from half an hour before, to half an hour after, regular bath and shower time will usually be ample, and more economical. The thermostat on the cylinder will switch off the boiler as soon as the cylinder is fully hot.

 

[RenzoP]

The thermostat on the cylinder will switch off the boiler as soon as the cylinder is fully hot.

I don’t see a thermostat on my cylinder.. I assume it’s only inside and cannot be set to a temperature (temp only set on the boiler) and that’s why I don’t see it ?

 

[Johntheo5]

It may be inside the cover in a plastic box about 1/3rd the way up from the bottom, it has to have a thermostat otherwise the boiler would only switch off when programmed off or turned off manually.

 

[Johntheo5]

The thermostat(s) is inside this box, factory setting is 60C and the setting appears to be pointing to ~ 1:30 o'clock, I wouldn't advise touching it, go away and measure a HW tap outlet once the cylinder has fully reheated.