Solar Hydronic Floor Heating System

Solar Hydronic Project Overview


The system consists of:

- Four 4x10’ flat panel solar collectors
- A repurposed 80g water heater used as a drainback tank
- Two circulator pumps: one for the solar panels, and one for the floor hydronics
- An ESP32 controller connected to various temp sensors, light sensor and relays
- An LCD remote display inside the house


System Description and Site

The system heats the 5” concrete slab on the first floor of our small house.  Combined with the passive solar design of the house, the thermal mass of concrete walls, plus R-21 to R-52 insulation in the walls and ceiling, this system heats the house without any other heat source required.  We’re on a south-facing slope at 4000’ in the Sierra Nevada foothills in California, so we have snow and actual seasons, but the winters are on the mild-to-medium side.

The panels we used are two different kinds (different manufacturers, slightly different sizes), and 3 of them had the glass either completely or partially broken out.  I cobbled together glass from old sliding doors and combined that with some left over double-walled acrylic greenhouse wall material to re-seal the panels.  They’re placed at 45° tilt matching the orientation of our house, about 172° AZ.  We live in a conifer forest, so trees to block the panels early and late in the day, but overall the site was chosen for excellent exposure.  I built a custom rack out of scrap steel to hold the panels.


Drainback Design & Plumbing


The ~80g water tank is from an old Heathkit system—ironically designed for a solar system, but I’m not using it as intended; an old electric water heater tank would work just as well.  There are two independent circulation loops in/out of the tank, each with it’s own pump: one loop to and from the solar panels to collect heat, and one to and from the PEX tubing in the concrete floor to heat the house. There were two heater element ports in the side of the tank that I removed to use as the outputs for each of the loops: colder water off the bottom of the tank to go up to the panels, and hotter water off the top of the tank to go to the floor.  Both loops return to the tank through the ports in the top.  There’s also a hot water manifold in the house to distribute and balance the flows through the various zones in the floor.


Controller


Here’s a list of the sensors:

At the solar panel array:
    • Panels interior temperature (inside insulated panel enclosure, not touching copper)
    • Ambient exterior temperature (behind panel, in shadow, sheltered from wind)
    • Diffused direct sunlight value (inside sealed enclosure with a frosted window)

Inside the mechanical room:
    • Water tank interior temperature (hanging inside tank, immersed near top of water)
    • Panels loop send temperature (epoxied to brass fitting near exit from tank)
    • Paenls loop return temperature (epoxied to copper return line near tank)
    • Floor loop send temperature (epoxied to brass fitting near exit from tank)
    • Floor loop return temperature (epoxied to brass fitting near tank)

Inside the house:


Software & Logic Design

In theory, this setup is pretty simple: read temperatures from various sensors and decide when to turn on either of the two independent pumps to collect, store or distribute heat.  But since I’m not a programmer, and reality is always messier than theory, it took some tweaking and adding some extra temp sensors to arrive at the best working version of this system.

The floor loop was definitely the simpler of the two, so I’ll start with that.  Basically the logic looks to see if a) the house needs heat, and b) there’s heat available in the tank; it then turns on the pump (regardless of the state of sunshine or heat in the panels).  It uses software-assigned temperature differentials to trigger the pump, and keep it from triggering too often.  It uses just two sensors: tank temperature and house temperature.  There are copious notes in the sketch explaining this.

The panels loop is more nuanced, with more sophisticated logic to determine when to turn the pump on, when to leave it on, and when to turn it off.  It looks for a minimum sunlight value and higher temperatures in the panels than in the tank; once the pump turns on, it will stay on if those conditions persist, but it will stay on as long as the water returning from the panels is hotter than the water going in.  Again extensive comments in the software on this.

There are some hard limits on the system running; for example, there’s a master ON/OFF switch at the remote which will shut the whole thing down.  There’s also a maximum tank temperature setting, currently 150°F at which point the panels pump will not run under any circumstances.

Conclusions & Thoughts

Basically this system works great.  When the sun shines, it works, the house gets warm, and we don’t have to light a fire or think further about it.  It’s a magical kind of heat, and lasts for a long time.  

Initially I didn’t have temperature sensors on the panels loop; adding those, with the condition that ‘once the pump is on, keep it on as long as the temperature returning from the panels is warmer than the water going in’, really smoothed out frequent pump cycling due to clouds passing, a bit of shadow, etc.  

In our case, our house has huge thermal mass with concrete floors and walls; as such, it’s unlikely that we’ll overheat the concrete.  Also, I did not install a temperature sensor in the slab itself, which I had intended to do.  I can see there would have been issues with it; like how close to set it to existing pipes (a bit of a hazard when installing it to be sure), and how the slow change in heat in that mass would affect pump cycling, etc.

Potential problems: if the sun is shining, but the panels pump isn’t running, the panels can get incredibly hot; if the pump suddenly kicks in, the water will boil in the pipes and there’s a tremendous hammering and pressure buildup as the water returns to the tank.  It might be necessary to install some kind of heat diverter at some point, or allow the tank temperature to rise higher—say 180°F; or at least have ThingSpeak send an email when temps rise too high.  By this point, if we have these conditions frequently, it’s probably time to just turn the system off for the season: basically covering up the panels with a big piece of shade cloth to avoid cooking everything.

Hope it’s helpful, send me any questions!

Mark