Hot Water Solar & HVAC controls

[jdurand]

Our hot water solar collector shares water with our house heating boiler. The system has been rebuilt over the decades as parts wore out and to meet updated building codes.

We originally had off the shelf controls for the solar collector, tank backup heat, and house heating loops. That stuff was ...err... less than satisfactory and died after a short time in the 1980s. I replaced it with a PLC that also handled our house cooling system (swamp cooler that was switched into the heating ducts with automated 16 inch Y valves). That had separate 24VAC, 24VDC and 120VAC circuits to accommodate all the different things. That lasted until 2007 and then started acting funny.

By this time we were designing microcontroller systems so my wife and I designed this controller which has been happily running ever since. ;It's just a simple 16 bit microcontroller with 4 thermistor inputs, a 24VAC input from the heat demand thermostat, and four 120VAC relays for solar pump, heating pump, backup pump, and boiler. It has a supercapacitor which keeps the clock running for about 2 days without power.



I happened to mention it to the owner of the company* we were buying some new hot water panels from (old ones started leaking). He wound up ordering a bunch of a custom version for a power company trial.

Funny how things go.

* those replacement solar panels have been working great for many years, they were made IN CALIFORNIA by these guys
https://solarroofs.com/

[hemmjo]

I am looking for a new controller for my heat pumps. I have a dual heat pump, geothermal, hydronic radiant heat, chilled water A/C system. The display on one of the heat pump controllers has failed. I fear the other may soon fail. Replacements are no longer available.

Is there more information available for this controller?

John

[jdurand]

Well, the processor is an MSP430 series, I no longer have any of the development software for those. Do still have the programming pod, if that model is still supported.

But, any simple processor with analog inputs will work.

The thermistors are just standard 10K ones into a R-R voltage divider with a basic R-C filter. I sample those pretty rapidly and smooth the data with a delta-sigma filter (fancy sounding name for a simple filter, on my suggestion Marlin 3D printer software uses it...but only after making it more complicated than necessary).

A little bit of math, a lookup table and presto, you have 4 temperatures.

Then just a decision tree to decide when to start/stop which pumps.

The call for heat comes in through an AC opto isolator. Nothing fancy there. When the LED is on it latches in IRQ that's masked off. Poll that at a slow rate and clear the latch. This removes the AC part.

There's really not a lot more to it.

Oh, the LCD display is a standard 16x2, you drive that with a 74HCT125 and a single resistor. Make sure it's HCT, not HC.

I have 3 push buttons. When the device is idle pressing the UP button enables the hot water backup. Clicking the DOWN button disables backup. It can also be enabled/disabled by the internal clock.

Holding the middle button for 5 seconds enters menu mode. There you scroll through the settings, click the middle to change a setting, use UP/DOWN, click middle again and you're back to the menu. Wait 20 seconds and the system stores the settings and goes back to work with the LCD cycling through various things of interest.

We still have the design in the CAD archive if you're further interested.

[DIYer]

Thanks jdurand! We've added your Hot Water System Controller to our Electronics category,
as well as to your builder page: jdurand's Homemade Tools. Your receipt:

Hot Water System Controller by jdurand

tags: controller