A water tank is used to store heat at $61C$ for domestic hot water and heating demand.

The volume of the tank is $259L$ and the diameter is $0\mathrm{.}97m.$ Assume water Cp $=4,184$ J$/($Kg K $)$ and density $1,000k\frac{g}{m^3}.$

If the overall loss heat transfer coefficient is $4\frac{W}{m^2(K)}$ and ambient temperature is $21C,$ estimate the time constant $[$hours$].$

Use $2$ significant figures