Custom water mains temperature workaround?

I am trying to model a solar water heating system that can boost the temperature of water in a tank from a heat recovery system, as well as pre-heat water from mains. It appears that there is no way to set a custom hourly mains temperature. However, I am wondering about a possible workaround.

Suppose I have a tank of water containing sitting at 500 gallons at 140 degF, which I must raise to 170 degF. (dT=30)
Suppose that I need an additional 500 gallons of water raised from mains temperature (50 degF) to 170 degF. (dT=120)

It is straightforward in SAM to simulate raising 500 gallons of mains water by 120 degF.

To simulate that I am ALSO heating the 500 gallons of 140 degF water in the tank, will I get a reasonably close answer if I simply regard that volume as though it were a much smaller quantity being raised by 120 degF, rather than the full volume being raised by 30 degF?
i.e.
[(170-140)/(170-50)]*500 = ...
(30/120)*500 = ...
0.25*500=125 gallons

Thanks for your help.

Dear Jarrett,

As you know, SAM's solar water heating model calculates the water mains temperature from the ambient air temperature data in the weather file. Unfortunately, there is not a way to provide water mains temperature data as an input to the model.

Best regards,
Paul.

Thank you for your response. Since there is no way to manually input water mains temperatures, my question is really about the possible work-around.

What I'm trying to get at is this:
Since I cannot change the water mains temperatures, will SAM give me a *reasonable approximation* if I simulate my process by simply reducing the volume of the water draw by the appropriate amount?

So, if the volume of water X that I actually need to heat is already half-way to my target temperature set by the auxiliary set temperature, I would input that I draw a volume of water X/2.

Would that work?