Some additional notes on potential automation of the Reclaim Heat Pump hot water system (and I suppose, others that use the same principles) are important.
TL;DR - please view this as a thermal battery not just a hot water system. The tank size is much larger than it is for a gas hot water tank. The temperature sensor is more like a binary “state of charge” reading (i.e., it’s fully charged, or not), and it is the “stratification layers” in the tank (and the temperature of each of these layers) that dictates the state of charge. Those are very hard to measure.
Another way to look at the state of charge for this system is to take your traditional battery charge indicator (like you see on a mobile phone) and turn it upside down. When the charge is low, there will only be one bar but it’s at the top of the indicator. For a full charge, all the bars will show in the battery indicator, but as you deplete the battery, it’s the bars at the bottom of the indicator which are consumed. As long as there is “charge” in the tank, you can consume it, and it will be hot.
It is very difficult (but I suppose not impossible) to try and use the temperature sensor as a battery state of charge indicator. If you were to log the temperature sensor, the curve is rather exponential. It sits as a very low temperature for hours, and then will rapidly jump up late in the charge cycle, so as I said earlier it’s almost a binary True/False. Without multiple temperature sensors mounted all the way up the tank, or using ultrasonics/thermal imaging, it’s going to be hard to get an accurate “state of charge” though. I would also say it’s not necessary, and we should just focus on boost control if and when you have a tank size which turns out to be too small for your household when using the built-in power saving modes of the unit.
What Reclaim uses the temperature sensor for is really only two things:
- To know when the system is not fully “charged” with hot water
- To avoid short-cycling the heat pump. They wait for the temperature at the sensor level to drop a fair bit before the heat pump kicks back in (aka “recharges”).
Some notes on its operation:
- Water is pulled from the bottom of the tank into the heat pump
- The water returned to the tank from the heat pump is always 63C as it exits the heat pump. The method by which it ensures that the water leaving the heat pump is 63C is how hard the compressor is working and how fast the water pump is operating. Water not 63C? Run the compressor harder or slow the pump down (or both).
- The water from the heat pump is returned to the top of the tank, so as to provide “fast hot water recovery” and “excellent hot water stratification”. Stratification being the layers of water in the tank and the different temperatures of those layers
- The water temperature sensor is close to the bottom of the unit
- The water temperature sensor can remain “stuck” at a fairly low temperature, even while the water at the top of the tank is 63C. The system monitors this temperature of course and controls the unit based on that temperature. When I had mine fitted yesterday, the temperature sensor in the tank read 17C for five hours, however after only two of those hours, three people had fairly long hot showers and did not run out of hot water.
If you’re curious why the heat pump heats the cold water in the bottom of the tank and returns the heated water to the top of the tank, it’s due to thermodynamics. Here’s what ChatGPT has to say about that:
- Faster hot water delivery: When the hottest water is at the top of the tank, it is readily available for delivery when needed. This reduces the time it takes to get hot water at the tap.
- Energy efficiency: By maintaining stratification in the storage tank, the energy used to heat the water can be reduced. This is because the colder water at the bottom of the tank requires less energy to heat than if the water were mixed.
- Reduced heat loss: Stratification minimizes heat loss through the walls of the tank, since the cold water at the bottom acts as a barrier, reducing the temperature difference between the stored hot water and the surrounding environment.
- Improved system performance: Systems designed to take advantage of stratification can deliver hot water more efficiently and effectively, increasing overall performance.