Solar plumbing

[Telemachus]

Tl;dr: the greater the difference between the surface temperature of the solar panels and the water flowing through, the more heat energy transferred to the water.

I read what you are saying... I'm just trying to find a law in thermodynamics that would support that.

If I add a kilojoule of energy to 52' water, or to 62' water, it is still a kilojoule of energy added.

As long as the temperature sensor in the system is measuring water coming from the large body of water (pool water before either heater) then both heaters will keep adding their respective energy until the solar finally gets shut off.

I can think of a scenario where this can be a problem (which you already pointed out). If the ambient temperature is 70, pool water is 64, and gas heater is heating the water to 74 prior to going out to the solar panels, then the ambient temperature could actually radiate away the heat gains from the heater... I'm just not sure how much this would matter in real use... the solar panels could be programmed to shut off if the differential is 6‐8' less, making this a minimal case.

Certainly not enough in my mind to justify the difficulty in plumbing it this way.

 

[jimmythegreek]

Usually on a pad like that it's best to replumb a good chunk instead of trying to sneak Ts in. I agree with solar then gas heater, you want max temp water from solar gain entering your heater not after it. Most solar controllers have a 8 to 10 degree temp ambient over water temp requirement to activate the solar valve. With a SWG you don't need a heater bypass unless it's a heat pump

 

[Telemachus]

Thanks for the opinions and advice. I really appreciate it.

 

[MITBeta]

I read what you are saying... I'm just trying to find a law in thermodynamics that would support that.

If I add a kilojoule of energy to 52' water, or to 62' water, it is still a kilojoule of energy added.

Heat flow is driven by temperature differential. If you have a 3500*F gas flame you will transfer slightly less heat to 62* water than to 52* water.

Same goes for solar panels: the colder the water input, the more heat will be transferred for a given panel surface temperature. (Of course, there is a relationship between the surface temp and the water temp, but this is generally correct.)

 

[Telemachus]

Heat flow is driven by temperature differential. If you have a 3500*F gas flame you will transfer slightly less heat to 62* water than to 52* water.

Same goes for solar panels: the colder the water input, the more heat will be transferred for a given panel surface temperature. (Of course, there is a relationship between the surface temp and the water temp, but this is generally correct.)

Then couldn't it also be argued that the heat transferred from the gas heater to the already heated solar water will also be decreased? The gas heater heats up coils which then use the same Fourier's law of heat conduction to transfer heat to the water...

I'm misunderstanding how the order of the two heaters matter.

 

[MITBeta]

You want the heater where the fuel is free to see the greater potential and do most of the heating, and then for the heater where you the fuel costs money to make up whatever difference.