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Disclaimer: Not an engineer

I need some help determining the required performance from a heat exchanger for a pseudo-instant hot water system on a boat. Using a heat exchanger, and a reserve of hot glycol, I want to heat domestic water. I have tried to find the answers and figure it out myself, but am not getting anywhere.

I have a diesel furnace that maintains a reserve of glycol at $80^\circ\ C$. I am happy to leave the furnace and hot glycol reserve out of the equation by stating that I simply have an unlimited supply of glycol at $80^\circ\ C$. I am also happy to state that the heat exchanger suffers no heat losses other than the desired heating of the water.

So here is the calculation I need help with:

If I want to raise the temperature of domestic water by $20^\circ\ C$ at a rate of $8$ litres per minute, how do I determine the required wattage of the heat exchanger in relation to the available flow of glycol?

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Under these conditions, you can just use the definition of specific heat:

$$Q=cm\Delta T$$

for heat energy $Q$, specific heat $c$, mass $m$, and temperature difference $\Delta T$. Differentiating with respect to time, we get the expression for power $dQ/dt$ as a function of mass flow rate $dm/dt$:

$$\frac{dQ}{dt}=c\frac{dm}{dt}\Delta T$$

From here, we need the density of water $\rho$. Substituting $m=\rho V$, we get the power as a function of the volume flow rate $dV/dt$:

$$\frac{dQ}{dt}=c\rho\frac{dV}{dt}\Delta T$$

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