Does the emoncms myheatpump model account for the energy dissipated during the off period of a heat pump cycle?
The myheatpump app calculates output energy based on the delta T between flow and return temperatures and the flow rate. This I understand, however, during the “on” part of the cycle when the compressor is running both flow and return temperatures increase in absolute terms as well. For example, with a set flow temp of 30deg over a period of time the flow temps will increase until it reaches the upper hysteresis limit set by the pump controller, in the example here it is +4deg (34deg).
When the limit is reached the heatpump compressor turns off and the flow and return temps quickly equalise (to 30deg) as there is no energy provided by the compressor.
As the room stat is still calling for heat the heatpump continues to run the circulation pump and this continues until the lower hysteresis limit is met for flow temp, in this case -4deg (26deg) whereon the compressor is turned back on.
I have calculated my heating system has a water volume of approximately 170L so in this example with the flow and return dropping by 4deg. there is an energy dissipation of 2.85MJ
This happens over a period of 140 minutes so I calculate the average energy dissipated is 338W yet the app shows heat output as zero.
This energy is clearly supplied by the heat pump in the compressor on period but can anyone clarify if this is included in the energy output displayed during the compressor on period.
Maybe there is a valid reason for not including this but I can’t think why off the top of my head.
Hello @janney666 the way to think about it is that the heat meter is measuring the heat output from the heat pump not the radiators. At the start of the cycle you can see a spike in heat as the heat pump runs a bit harder to heat the system water volume but the flow and return temps are relatively low and so actual output from the radiators at that point is lower than later on.
When the compressor turns off there is no longer heat output from the heat pump but there is heat output from the radiators as the system volume cools down. We are measuring that extra heat at the start of the cycle as the heat energy required to heat the system volume rather than at the end… hope that makes sense?
It’s really helpful to put that emphasis on how to think of this.
I had thought that the heat output shown in that app was exactly that, heat output but it’s clearly a bit more nuanced.
If flow rate is shown on the graph and a point in time is chosen anywhere in the compressor on cycle, the heat output is always the delta T x flow rate. At the start of the cycle the delta T is lower but the flow rate is higher and results in an increased heat pump output. This is fine because as you say it’s the heatpump output so nothing else needs to be considered.
However, whilst it is true that the output ends up as heat over a full cycle, the heat pump output, particularly at the start of the cycle is also storing additional energy in the heating circuit water volume.
I will now think of heat output shown in the app as energy output from the heat pump as I think that’s easier to understand for me.
What helped me understand this was that there are essentially three processes involving transfer of heat energy that are running in parallel. Your house loses heat to the environment, the radiators transfer the heat in the water into the air and the heat pump puts heat into the water in the first place. The heat meter only sees this last process. Your house will be at your chosen temperature if the first two perfectly match, i.e. environmental heat loss = fresh heat coming in from the radiators.
Ideally, these three are perfectly balanced - this is when the heat pump is running continuously and the house temperature stays constant. Cycling happens when the transfer of heat from heat pump to water is faster than the other two. Then the compressor needs to take a regular break so the time average of all three heat measures stays balanced and the house temperature approximately constant.
Hi André
Yes that’s another good way to consider the processes involved.
I do have some issues with how my own system does not balance those processes too well but I think my other concerns are better raised in a new thread as I think this question has been answered
