Optimal system design to supply solar energy to a spa?

[davidesstein]

I'm a new board member at a condominium association in the Los Angeles area. The complex has an outdoor spa with two heating systems: natural gas and solar. That's where I can use some help figuring things out.

First issue: the control system design puzzles me. The two systems' temp. sensors and controllers are independent of each other. The main spa pump works on a timer, running steadily from 6 am to 10 pm. It circulates water through the gas heater but not (or only a little) through the solar collectors. Each morning, by the time that the sun is high enough to heat the collectors, the gas heater has long since heated up the water to its service temperature.

So here's the control system design issue: Logically, because the gas heater's thermostat will maintain the proper water temperature regardless of what the solar system does, this suggests that the solar system's controller will hardly ever find that the water temp. is cool enough to warrant turning on the solar pump in order to contribute hot water from the collectors, even during midday sunny conditions. (Occasional observation indeed suggests that the solar pump is hardly ever running during midday.)

Am I missing something? Is this really the optimal way to supply solar energy to a spa?

 

[Bama Rambler]

Sounds like one of two things need to happen. Either the gas heater needs to have it's setpoint backed off when the sun is providing enough heat to maintain the spa or it needs to be shutdown completely when there's enough solar energy.

What controllers are being used for each system?

 

[JasonLion]

Welcome to TFP!

A system like that can work if it is setup correctly and users can tolerate some temperature variation in the spa. As long as the target temperature for the solar controller is a couple of degrees warmer than the target temperature for the gas heater and both of those temperatures are acceptable in the spa it can work. If the temperature in the spa needs to be more tightly controlled, there needs to be an integrated controller.

My guess is that it was setup correctly originally, and one way or another it is no longer setup correctly. The key issue is that simply a lack of maintenance, or did someone mis-adjust it to some purpose, and if there was a purpose, can the system be setup correctly and still satisfy that purpose.

 

[davidesstein]

To Bama Rambler, who asked: "What controllers are being used for each system?" Answer: For the solar system, a Heliotrope General's Helio-Matic HM5000A (which appears to have two setpoints: "For differential 6°/3° jumper in; 9°/6° jumper out"). The gas heater is a Jandy LRZ with an internal electronic dual-thermostat control. (What does that tell you?)

To JasonLion, thanks for the welcome. The temperature variation tolerance is probably about 4°, given that under about 101° the users are likely to complain that it's too cold, while above 105° is prohibited by the county health department.

As for system setup, it may well be that when the gas heater was replaced a few years ago, no thought was given to integrating its controls with the solar system. Also the solar system installer has a better reputation for his salesmanship than for his technique. But yes, let's see if we can the system adjusted to work correctly.