Green Part 8: The Smartest HVAC on Earth

My home-grown combination hydronic HVAC and domestic hot water system worked remarkably well, given its overall simplicity. However, there was considerable untapped potential to:

• provide more heat at any given time,

• be more rigorous about when and for how long to run the SanCO2 heat pump,

• be much more intelligent about how much water to heat every day to match the actual demand and minimize thermal losses from the tank,

• enable the exterior vent to be used in an automated manner for cooling at night,

• and much more...
  

To accomplish this, we upgraded the system with the Harvest Pod, a product from Harvest. This Pod is a very smart controller that enables the hot water tank to become a smart thermal battery. The upgrade was a small plumbing job to remove a pump and insert the Pod, and then to do a little electrical rewiring to remove my original relay-based controller.

Here is a link to Harvest's description of their system: https://www.harvest-thermal.com/product.

The smart thermal battery stores heat in a water tank, which makes the heat pump way cheaper and cleaner to run. The Harvest Pod checks the forecast, learns our routine, and optimizes the SanCO2 heat pump runtime to utilize the energy generated by our solar system (or the grid electricity when it’s cheapest and cleanest). So, we get a combination of house heating and hot water. load shifting of all our thermal loads, and night cooling, all in one well-thought-out and programmed system. So, by adding some smarts to when and how to heat water, the Harvest Pod has transformed our system into a smart thermal battery that stores the energy generated by our solar array during the day and then supplies hot water and house heat when the sun goes down.

We set out thermostat to 70F during the day, and set it back to 66F at night, and we use hot water when we need it for showers and baths, as well as for the kitchen.

Here is some data for the system running during the month of January, 2025. Below, I will explain what everything means.

The first two pie charts, along with the numbers below them, illustrate the average electricity we used per day and the average thermal energy we consumed daily, during the month of January 2025. If you divide the thermal energy used by the electricity consumed, you obtain a coefficient of performance. Our system is 319% efficient in its use of electrical energy, meaning we generated 3.19 kWh of useful thermal energy (the energy we actually used in the house) for every 1 kWh of electrical energy we consumed. This is the magic of heat pumps! Although the SanCO2 heat pump was even more efficient (339%), the system efficiency accounts for the energy required to run the Pod and pump the thermal energy into the house, as well as the thermal losses from the system. In our case, it is 319%.

For geeks out there, these SanCO2 and system efficiency calculations includes all start-up, defrost, and shut-down cycles on the heat pump, which are a key factor in the overall efficiency of a heat pump. This means these real-life efficiency numbers will never be as good as the manufacturer's numbers on the nameplate. Most high-efficiency heat pumps will get somewhere between 200 and 300% when you account for startup, defrost, and shutdown cycles.

The lower graph displays the hourly electricity price in blue bars. We are on TOU-C (PG&E), so our price jumps up from 3pm to midnight. The orange line shows the average hourly thermal heat used by the house (and us, because it includes domestic hot water use). The green line shows the electrical energy used by the heat pump. Notice that the heat pump was able to generate and store enough heat during the day, such that we almost never ran the heat pump between 3 pm and midnight. This is the magic of the smart thermal battery, as it allows us to load shift our evening thermal loads into electrical loads during the day (with an efficiency of 319%).

We generated, on average, 24 kWh of energy per day in January of 2025 from our solar system. The heating and hot water system used only an average of 12kWh per day. On only 4 days of the month did the solar system generate just less than 10kWh. So, even in the winter, we were generating enough solar power to heat our home on ~90% of the days (and on those days, we had plenty of extra power to run the fridge, cook, and operate our lights and computer equipment, and cover a decent amount of charging for our EV)!

See also Green Part 9: Growing some food, and the start of rainwater management and Green Part 10: A deck, free from pressure-treated wood.