Issues with hot water from cylinder.

ComicGeek

QrizB said:

ComicGeek said:

Your boiler temp of 60 degrees is probably too low for the size of cylinder. If you had a 300 litre cylinder then you could probably get away with a lower storage temperature, but 170 litres doesn't go far in the winter.

My 120-litre tank and 50C boiler temperature beg to differ. (OK we can't have three back-to-back showers.)

But I'm guessing that you don't have 4 bar mains pressure and 25 minutes of showering, so that's completely irrelevant...

QrizB

ComicGeek said:

QrizB said:

ComicGeek said:

Your boiler temp of 60 degrees is probably too low for the size of cylinder. If you had a 300 litre cylinder then you could probably get away with a lower storage temperature, but 170 litres doesn't go far in the winter.

My 120-litre tank and 50C boiler temperature beg to differ. (OK we can't have three back-to-back showers.)

But I'm guessing that you don't have 4 bar mains pressure and 25 minutes of showering, so that's completely irrelevant...

Mains pressure is irrelevant to the argument you're making.

25 minutes of showering is more relevant, but if 170 litres at 60C is good enough in the summer then 170 litres at 60C should be good enough in the winter too.

Edit to add:

Assuming 25 minutes at 40C (fairly hot for a shower) at 10 litres per minute, you'd need to blend 133 litres of 60C plus 66 litres of 0C. So 170 litres should be adequate for any supply temp where the water hasn't frozen.

ComicGeek

QrizB said:

ComicGeek said:

QrizB said:

ComicGeek said:

Your boiler temp of 60 degrees is probably too low for the size of cylinder. If you had a 300 litre cylinder then you could probably get away with a lower storage temperature, but 170 litres doesn't go far in the winter.

My 120-litre tank and 50C boiler temperature beg to differ. (OK we can't have three back-to-back showers.)

But I'm guessing that you don't have 4 bar mains pressure and 25 minutes of showering, so that's completely irrelevant...

Mains pressure is irrelevant to the argument you're making.

25 minutes of showering is more relevant, but if 170 litres at 60C is good enough in the summer then 170 litres at 60C should be good enough in the winter too.

Edit to add:

Assuming 25 minutes at 40C (fairly hot for a shower) at 10 litres per minute, you'd need to blend 133 litres of 60C plus 66 litres of 0C. So 170 litres should be adequate for any supply temp where the water hasn't frozen.

Mains pressure is highly relevant!

A shower uses more water at 4 bar mains pressure than it will at 2 bar (average UK domestic water pressure). The hot water flow rate from the Megaflo is increased as the mains pressure increases, subject to maximum flow rates through the hot water pipework and shower fittings. It might be 10 litres/min at 2 bar, and 18 litres/min at 4 bar - manufacturers have charts showing this relationship for each product. 

Taking your example: 25 minutes at 10 litres/min equals 250 litres, but you have only accounted for 199 (133+66) litres of mixed flow - where is the other 51 litres from?? If you do your calculation correctly, then it's 167 litres hot and 83 litres cold - not ideal for a 170 litre cylinder!

Also unlikely to be 10 litres/min shower at 4 bar, could easily be 18 litres/min - which would be 300 litres of hot drawn in 25 minutes. Cylinder storage temperature will be less than 60 degrees as well, as 60 degrees is the OP's boiler flow temp so always losses in primary pipework and losses in heat transfer across heating coils.

The OP is saying that they don't have a cylinder full of hot enough water when they want to draw from it - so whatever the storage temperature is at the moment isn't high enough. Either because they aren't heating the cylinder long enough for it to recover, or not heating it hot enough (boiler flow and/or cylinder stat).

ComicGeek

Above is a typical pressure/flow rate curve for a Grohe mixer shower, which is fairly typical.

Shower head flow rate of 13.5 litres/min at 2 bar, increasing to 19.1 litres/min at 4 bar.