Hi

We're building a new house and I'm reviewing the geysers.

Is a 150L or a 100L more energy efficient?

1) The larger uses more energy to warm up, and needs 'less re-heating'. 

2) The smaller is faster to heat.

Notes:

1) Not sure if a larger or smaller geyser looses more heat when not in use.

2) How much hot 65 Deg C 'geyser' water do you need on average for a shower? Does anyone know the ratio of hot water these use as I assume this will help work it out.

I'm not so worried about the price difference as it's not very big.

I can't answer around the heat loss but I can provide my own experience, I have a 200l geyser and replaced the element with a 3kw element, in summer my geyser runs from 10:00 to 14:00 and set to heat to 65 degrees, in winter there is a extra cycle in the mornings from 5:00 to 6:00 set to 50 degrees, this works for us 3 grown ups and 2 children. The kids bath at night and we take 3 hot showers in the morning. The geyser heats from Solar during the day and I have enough battery to heat the morning cycle so no eskom is used to heat water.

My 150l geyser normally ends up at around 30-35C in the morning.
It takes between an hour and 90 minutes to get to max 65C thermostat setting.
My timer kicks in at 10 but when I see enough solar energy then I manually switch it on earlier.
This is the typical winter time graph and you can see the 3kW element running plus the base load and there were two spikes where the kettle was used too.
I switched it on just after 09:45 and it was at max just after 11:00

3 hours ago, AlexanderR said:

Hi

We're building a new house and I'm reviewing the geysers.

Is a 150L or a 100L more energy efficient?

1) The larger uses more energy to warm up, and needs 'less re-heating'. 

2) The smaller is faster to heat.

Notes:

1) Not sure if a larger or smaller geyser looses more heat when not in use.

2) How much hot 65 Deg C 'geyser' water do you need on average for a shower? Does anyone know the ratio of hot water these use as I assume this will help work it out.

I'm not so worried about the price difference as it's not very big.

The larger geyser has more hot water in it, so more heat to lose, and a greater surface area over which to lose it. However, the ratio of surface area to volume becomes lower when you go for a bigger geyser, so a bigger geyser will retain heat more easily relatively speaking. One reason why you won't find really big animals on land, they'd struggle to lose heat.

A lot depends on your water usage patterns. If you heat up a 150l of hot water to 65 deg, but you only use/need 100l, then you've wasted energy. You could of course lower the temperature on a bigger geyser to maybe 55 deg. Just for re-sale value to the house, better to just go with a 150l as most families will need or expect that size.

If your geyser is at 65 deg, the cold water is at 25 deg, and you want to shower at a reasonably warm 45 deg, then you need to mix in equal parts of hot and cold water. You could play around with a water heating calculator here: https://www.spiraxsarco.com/resources-and-design-tools/calculators/water-mixing/water-mixing?sc_lang=en-GB

It doesn't suit everyone's installation,  but a low pressure solar geyser works much better than a high pressure system for the simple reason that as soon as you open a tap on a high pressure system, the cold water starts mixing with the hot water, and cools the water down quite quickly. Also the high pressure geysers don't heat the water directly, but use a heat exchanger, which is less effective than low pressure geysers where the water is inside the heating tube.

With a low pressure geyser, if you set the water inlet to fill quite slowly, then you can use just about the entire volume of hot water before the cold water has any effect.

We have a 300litre high pressure and a number of 150litre low pressure geysers, and the low pressure work much better.