Heat Retention In Various Volumes

[antonbhoy]

Hi,
I've been toying with the idea of starting a heated tank, and I have a question about heating. for instance I assume that it would take less time to heat 20l of water compared to 35l of water, but my question is does the larger volume of water retain the heat longer, making it more economical to heat in the long run?
also does putting a larger capacity heater in a smaller tank have any effect on the length of time it takes to heat up the water?
Regards,
Anthony

 

[DrRob]

Yes, it takes less time to heat 20 litres of water than 35, assuming the same power of heater and the same flow.
 
Water has a very high specific heat capacity, which means that it'll actually take on a lot of energy to change temperature, and is equally slow at releasing it. The large volume will tend to be more stable, depending on it's shape to a degree as more will be lost through evaporation from surface area than through the sides of the tank as they'll be insulated by the glass. In the same way, larger tanks tend to have thicker glass and therefore more insulation.
 
So overall, larger tanks of the same shape tend to be more stable in their temperatures, but still cost more in energy to raise to that temperature and keep them there.
 
Oversizing heaters can cause problems with temperature fluctuations. We spend a lot of time trying to make sure that everything in our systems is nice and stable and a high power heater will do a lot of brief ons gettting the tank warm and then a lot of off time letting it cool. In an ideal flow scenario the heater thermostat will burn out earlier than expected as it'll be constantly flicking on and off, in a less ideal scenario the tank temperature will fluctuate more. Do remember though that heaters are rated for an average tank being kept in a warm room, so any tanks in colder areas will need bigger heaters anyway.

 

[antonbhoy]

Thank you for the reply, I never thought about surface area and evaporation.