Someone needs to make a relay module

I confess that I cheated here, and limited the flow temp so that it wouldn’t get too excited [like MyForest does]. However, I don’t really like doing this as it adds more complication to the system, and is another thing to go wrong if I mess up. Sometimes the house struggles to get warm as a result. I’d prefer to let the HP do its own thing, overriding as little as possible. Interesting to experiment with though.

A few further thoughts…

  1. Today was much colder and I saw my first defrosts of the season. I was worried that this “relay” was interfering with that process, though looking back at the data maybe not as the compressor was still being used during the defrost.
  2. I also worry that turning the HP off (via MelCloud) is perhaps too abrupt for the system; I’d hope that the controller would stop gracefully, but I don’t know if it does.
    An alternative strategy I’m trying is to drop the target flow temp right down for a short while after each cycle, letting the HP come to a stop when it’s ready.
  3. After the DWH cycle finishes, I actually want the SH cycle to start immediately, without any forced pause [another thread]. This means making the “relay” more sophisticated so it can tell the difference between cycling and a mode change.

I did give that sort of thing a try, but the Ecodan controller decides it’s no longer worth running and shut itself down quickly anyway. Maybe give it a try with your system and see how it behaves.

Hmm, I think I must be missing a reference in an earlier post where you are chatting about the pause.

If we gloss-over my stupid two-minute polling problem, you can see mine was:

  • doing space heating
  • decided it had done enough of that
  • switched to DHW
  • came out of that and decided it was worth doing some space heating
  • eventually decided there was nothing worth doing

2022-11-19T20:04:05.992 Setting initial target flow temperature to 35.5 °C. The flow is currently 31.5 °C
2022-11-19T20:04:06.316 Turning on so we can do some heating. Has been cold
2022-11-19T20:18:04.651 Setting target flow temperature to 37.5 °C so it’s gently increasing. The maximum is 38 °C.
2022-11-19T20:24:04.637 Setting target flow temperature to 38 °C so it’s gently increasing. The maximum is 38 °C.
2022-11-19T20:34:04.944 We’re going to heat the tank to 39 °C because it’s at 16.5 °C which is lower than the trigger temperature of 18.0 °C and the flow is at 38.0 °C
2022-11-19T20:48:04.725 The tank temperature is 40.0 °C which is at least as hot as the target of 39.0 °C so stopping forced hot water mode
2022-11-19T20:50:13.373 Setting target flow temperature to 39 °C so it’s gently increasing. The maximum is 39 °C.
2022-11-19T20:56:04.946 Turning off because the average recent target temperature 38.3 °C is higher than the maximum of 38 °C.

Are you seeing some other behaviour as you switch modes?

Bear in mind that mine is set to block hot water on the FTC5. It only ever goes over to DHW because my script turns on “forced hot water” mode.

John proposed a simple relay that detects when the compressor turns off, and keeps it off for a certain period (the pause). The idea is to prevent it cycling so much, and let the flow temps drop a bit before starting a new cycle. This could improve COP overall.

I’ve been experimenting with a couple software implementations of a relay. One of the considerations I pointed out above is that we don’t want to put in a pause between the heating and hot water cycles, as per your chart.

If I were to implement this in hardware, I think I’d put the relay on the “call for heat” line from the thermostat, reading state of the compressor and 3-way valve. When compressor stops, turn off the call for heat, unless we were previously heating water.

I have just joined the two-minute polling club too :frowning:

Apologies, I was being dim.

I know my complicated, expensive and hard-to-reproduce solution isn’t the answer, but this chunk of code is what’s been working well for me. I hate that it has so many magic numbers which are local to my setup. As Brian keeps reminding us, this will not get fine-tuned by the installer.

Anyway, feel free to use as much of that logic as you wish, I’ve put it online so you can see it.


@johncantor John - is that not what degree minute (DM) control would give you? Vaillant, NIBE and I think a few others manufacturers have it built into to their controllers. Example data from my NiBE F2040 8kW showing the heat/electric data for 3 cycles at low demand and also the DM data from the controller over the same period.

Yes indeed, and Nibe have been using degree mins for very many years. I guess many of the the air con derived systems have controls that are somewhat crude. Its a complex topic to grapple with.

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