I've a pentair solar control panel for my solar system for my pool. On one sunny and very warm day in California, the water in the pool remained cold after a few hours of circulation. Therefore, I was suspecting either the temperature sensor is not working or the control panel is malfunctioning. I eliminated the fact that some of the tubes in the solar panels on the roof might be blocked because the water temperature did not change at all... Meaning it didn't even warm up a few degrees after a 4 hours of operation... Anyway, I had a technician came out a few days ago and indicated both the sensor and the control panel are working properly. However, the same problem still existed. Can someone please share some insights here? Thanks!!

Do you mean your heater is saying water sensor open? Or your control?
Whichever one it is , simply replace the sensor, if it's the system giving you this msg the. Find a probe with a bandclamp on your plumbing and replace it .

If it's the heater giving the code then depending on what type heater you have, the temp sensor is located around the header area

Usually toilets do not require hot water.
Is the plumbing divided into hot and cold water pipes?
If toilets are hooked to hot water, how many flushes and how many gallons per flush per day.
What is the expected consumption per each worker?
What is the latitude from equator, and expected solar input. Further north, and hazy atmosphere reduce input. Beijing has smog I believe.
How often is the solar panel going to cleaned of smog and dirt for maximum collection?
If the area has hard water, then the entire system is prone to sediment build-up and failure unless water is artificially softened.
Solar water heater temperatures can exceed 180 degrees at times. This temperature will damage storage tank that is not made for that temperature.
Most solar water heater systems have pressure regulating valve and expansion tank to accommodate occasional high temperature and pressure.

A good fellow to contact might be at following website:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

Solar collectors are usually connected to a circulating pump, and then the heated water is stored in a tank.
The pump circulates water continually from solar collectors through a storage tank.
This type of system would use ordinary tank-type water heater for storage.
Most of these systems also utilize a back-up heating element inside the tank.

More complicated systems circulate special liquid through spiral tubes inside the water heater.
This type of heater would be an indirect water heater.
Indirect heaters are used for solar collectors, and used with boilers, or any outside heat source.
http://www.bockwaterheaters.com/Products/products_coil_tank_water_heaters.html
http://www.hotwater.com/products/residential/cirrex_solar.html
http://waterheatertimer.org/images/solar-water-heaters-on-roof_lbiRv_5784111-500x282.jpg

Temp sensor is bad.

You may actually use any pool heater with a solar water heating array. I do not however know about any that come as a package deal. You will need a solar array, a solar controller, a actuating valve to hook to the solar controller, and a new bypass assembley for the plumbing system. All of this is made by manufacturers such as : Jandy, Pentair, Raypak, Lasco, Buderus, Hayward, etc....

Both of those heaters are fine I prefer the Pentair for it's smaller footprint and ease of installation as you can turn it for access to plumbing. The Hayward is tougher to turn the exchanger on and you only have right or left as options for the plumbing to come out. Sta-Rite is still my favorite for installation being easy to plumb, wire and run gas to. It's very easy to change from natural to LP also.
Most heaters today are the same in reliablity and that is mostly up to you to keep the chemicals correct. Being in the business I always look to ease of install and access for repair.

DELAY IS ON FOR THE SOLAR PANEL OR TH E SPA IS ON

if you pump is dual speed,try turning to lower speed and water will stay in solar system longer,thus having a higher heat gain

Depending on the manufacturer, your solar panels may be replaced under warranty. Our installer says that once the panels begin leaking, they tend to deteriorate fairly rapidly. For example, three leaks the first year, 7 leaks the second year, 30 leaks the third year.

Many solar panels are warrantied against freezing, but that is not likely to be the future of the warranties, because they inevitably DO freeze and leak.

If there are just a few leaks, it is easier to fix them - a Fafco Solar panel repair kit can usually be bought from your installer for about $25.00.

If you turned the pump on to manual to circulate the water through the panels, that should prevent a freeze - but will cost you the electrical cost to run the pump.

Our experience has been that it is better NOT to drain the panels - we had many fewer leaks the years we have not drained the panels, despite occasional hard freezes in our part of California.

In addition, your rooftop installation should help radiate some heat up to the panels to keep them from freezing.

Check your warranty and get the panels replaced if there are more than a few leaks.

the lower the flow rate through the panels, the more heat rise you will get. The most efficient rise is about 10 degrees. Try and adjust the valve so you will get at least a ten degree rise consistently with the solar panels. Make sure you are turning the correct valve and not shutting off the return line completely.