Optimal placement of solar temperature sensor

[Rocket J Squirrel]

The prevailing wisdom seems to be that a pool's solar temperature sensor should be near but not touching the thermal solar panels. This way, the sensor is getting the same sun as the panels without being affected by the panels.

I now wonder whether this is non-optimal. It seems to me that the sensor should be in contact with the panels, so it can read the temperature of the water inside the panels. Then, as the water in the panels cools due to circulation through the pool, the sensor would correctly read lower and might even disable solar if the water temperature in the panels got low enough.

Comments?

 

[ajw22]

Searching around the net I find both recommendations.

The Hayward Aqua Solar manual ( https://hayward-pool-assets.com/assets/documents/pools/pdf/manuals/aquasolar-owners-manual.pdf ) says:
Pool Sensor
The pool sensor should be mounted in the filtration plumbing after the filter, but before any conventionally fueled heaters.

While Pentair (https://www.pentair.com/content/dam...r-installation-guide.pdf/subassets/page35.pdf ) says:
CAUTION! DO NOT DRILL HOLE AND CLAMP SENSOR INTO SOLAR PIPE.
1 Mount the sensor on a flat surface, with the same exposure to sun as the solar collectors (next to
the collectors is recommended) or any sunny location. Do not let the sensor touch the panels. For
glazed panels, install the sensor between collector and glazing.

 

[Rocket J Squirrel]

You were looking at the pool sensor, not the solar sensor, in the Hayward manual. Hayward actually says

Use a screw or silicone adhesive to attach the solar sensor near the solar collector array. The sensor does not have to be attached to the collectors. It is only important that the sensor be exposed to the same sunlight as the collectors. Additionally, the underside of the sensing element may be covered with silicon to minimize wind cooling.

What I am questioning is the assertion by both manufacturers that the sensor should not touch the panels. I find that my solar sensor reads too high relative to the water in the collectors when both are in full sun, resulting in the solar being engaged when it is not significantly heating the pool.

 

[magiteck]

The solar controller that I bought, won't engage the panels unless the "surface temperature" is at least 4 degrees above the water temperature. If 80 degrees is hitting the panel area, and the water is 76, the panels would then turn on. But if the sensor was touching the panels, the heat gain within the panels would be (most likely) less than 4 in this scenario, so the panels wouldn't engage -- even though (albeit minimal) heat gain could have been obtained.

At least in my case, I'd always want my panels on if there's an opportunity for heat gain, even if it's minimal. Basically I'd prefer the panels be on anytime they wouldn't be effectively cooling the pool.

If you for some reason didn't want your panels engaged unless they would be substantially heating the water, then having the sensor touch the panel might help with that.

 

[MSchutzer]

What is significantly heating the pool? The better the solar panels conduct heat the closer the panels will be to the water temperature. My panels are copper and when water is running through them they are only a couple of degrees above the pool temperature regardless of the the outside temp even when in full sunlight.

I know the Aqualink RS system needs to see the solar temperature sensor at least 5 degrees above the water temp to activate the solar actuator valve and as it drops below that level it will cycle the valve on and off accordingly.

I guess I don't see the problem if the solar temp sensor reads the temperature in full sunlight (even when not connected to the panels) as it means there is heat gain to be had even if that temp is only 5 degrees above the pool water's temperature. Another thing to think about is the thermal mass of the panels. If you have on and off windy conditions that cools the sensor below the threshold I think you would rather turn off the solar valve when the sensor cools rather than waiting for the whole thermal mass of the panels to cool down. I think it would work either way, but I would think the sensor on its own would react faster to wind gusts or passing clouds.

As long as the panels are not actually cooling the water I don't think it matters if you don't get a significant heat gain in marginal conditions?

 

[Pool_Medic]

Optimal IMO is pool temp sensor is mounted inside the plumbing after the filter. The solar temp sensor should be mounted near the solar panels, ambient air temp should be mounted under an eave.

 

[ajw22]

I think the solar temp sensor needs to be anticipatory.

The solar temp sensor will heat up before the water in the panels do. That gives the solar system time to react and get the pump running and water flowing in anticipation of the water in the panels heating.

And when the sun goes behind a cloud the solar temp sensor sees the loss in solar heat before the water in the panel cools and shuts down the system.

So the solar sensor being placed in the air give a better system response to what will be versus lagging the waters response.

 

[1poolman1]

Properly working solar panels will be cold to the touch when in operation. The "solar" sensor should be able to sense what heat is available in similar conditions to the panels themselves. If the sensor is attached to the panels, they will cool it to the point that it will give bad info to the control, at best, and may not "sense" enough heat to activate the control at all or over heat beyond what you would like. All controls operate at a 3-4 degree differential; setting is 75 solar will activate when water temp hits 72 AND solar temp is above 75 by about 3 degrees. Otherwise the system would be constantly turning on and off.

Your home HVAC works very similarly.

 

[MSchutzer]

Poolman,

A three to four degree variation in water temperature is a LOT in a water temperature control system. In the case of the Aqualink RS system if the water temp set point is 75 degrees the heating (solar or otherwise) will run until the water temp sensor reads 75 degrees. When the water temperature sensor drops by 1 degree (to 74 degrees) the heat will activate again to return the water temp to 75 degrees. The solar actuator valve control works the same way as the call for heat for a gas heater as long as the solar temperature sensor is at least 5 degrees above the actual water temperature.

 

[1poolman1]

The vast majority of consumer temperature controls, thermostats, have a hysteresis of, on average, 3 degrees. That includes heater temperature controls, solar temperature controls, automation temperature controls. Some more sophisticated t'stats will allow you to adjust the offset. Jandy controls have (or had) the ability to calibrate temperature for this purpose.
Talk to the engineers/designers of these items and you might be surprised what actually is happening in the electronics before you see a change in the display.
While there are areas on the human body that are sensitive to minute temperature differences, on the whole most people, on average, won't notice an increase or decrease of less than 3-5 degrees, the design parameter for pool equipment. Pools work by being accurate, not precise. Perhaps your pool equipment schools have taught something different. Engineers that I know cringe at the "inaccuracy" of pool electronics. Yet, most people were happy in the era of "bulb and bellows" mechanical temperature control with much wider temperature swings especially as they aged.