Is there a non intrusive way to measure if water is flowing through a copper pipe or just standing still in it? The copper pipe belongs to my heating system and is always under pressure with water, flowing or not.
Non Invasive flow meters typically use ultrasonic sensors clamped to the pipe. The keyword is simply non-invasive flow meter.
But regular flow meters are much cheaper. All you need is the appropriate size, cut the pipe and thread it, attach t he meter and done. It's done all the time.
You can use two piezo ceramic transducers in combination with a TDC1000 AFE from TI to measure the flow.
So what you're hoping is to keep circulating after the thermostat has shut down the furnace, in the hope of extracting as much heat as possible from the water? Your question only asked about flow, not temperature.
The problem, I fear, is that you're not looking at the entire system's gains/losses. The best thing you can do, by far, to minimize waste heat, is to insulate the pipes everywhere except the radiators themselves. The next largest waste heat source is the furnace & exhaust system, and unless you want to put in some passive air feed to send furnace-room air into the house proper, there's not much you can do about that.
Compared with those two items, the proposed heat recovery from warm water in the pipes is miniscule at best. Yes, it is sensible to set the circulator to continue for a few minutes after the thermostat triggers, but you get diminishing returns in a hurry. For one thing, the rate of heat transfer in the radiators is a strong function of the temperature difference. You're going to end up burning more electric power to run the pump than you'll ever get in room warmth.
Mount two surface temperature sensors (or the hot and cold junctions of a thermocouple arrangement) to the pipe say 100mm (4") apart. Place a resistive heater between the two temperature sensors. Add 5-20W of heat to the pipe with the heater. Compare the temperatures.
- When water is stationary you will measure the same temperature at both locations.
- When water at above or below ambient flows (and stops) you will initially see large change in both sensor values and small differential due to additional heat indicating flow direction.
- Water flowing at ambient temperature will only result in small change in one sensor indicating flow direction.