Thermal heating efficiency at various water flow levels

[cujowoof]

Hey everyone...I am certain this question goes beyond a lot of the forum content, but being the fact that I now have a variable speed pump installed, I started to wonder the heating efficiency of water at various flow rates, and whether a heater can be more effective by heating water more effectively when the rate is slowed. I have to think that the heater companies normally do all of this type of calculation, but if someone knew that by running a lower rate through the heating elements would improve how well the water is heated, I'd be all for it.

Has anyone gotten some good quantifialble data on this, or might be able to shed some light on this topic?

 

[JasonLion]

The heating efficiency is highest when the water flow is fastest. Heat transfers more quickly when the temperature difference is maximized. Low flow rates allow the water to heat up more, reducing the temperature differential and reducing efficiency. However, very high flow rates will damage the heater, so there are some significant practical limits.

Most modern heaters include internal equipment to regulate the flow rate through the heat exchanger to try and optimize this process to some extent, so in practice it makes very little difference (unless the heater goes into low flow rate alarm mode).