Boiler Cycling / HR92 hysteresis

[Peahead]

Hi All,

Firstly thanks Evohomeshop for providing this forum, seems like an excellent resource!

My Setup -
Open Vented System with a Worcester 24Ri Boiler. Y Plan
Evohome HR92 x 5 (Lounge/Bed1/2/3/4) + DHW

My Issue -
Through the night, the boiler will continuously cycle on and off, I assume trying to maintain the temperature in the bedrooms. However, it seems to do it for such a short cycle, I can't understand why as the rads don't really have time to properly heat up. I have a couple of temperature probes on my boiler flow and return and a raspberry pi reporting the temps over Wifi. See the graph below:



As you can see, between Midnight and about 05:00 it cycles 14 times, with the boiler flow never getting above 30C (Note the rate of data capture is fairly slow so it may peak a little higher) The bedrooms are set to 18C.

My question, is this normal behaviour? It almost seems like there is little hysteresis with the HR92s (I.e. it drops to 17.5C on the heating comes, then off at 18C, rinse and repeat) I'm thinking the only way I can solve this is to programme more set points over night, i.e.18C at midnight, then 16C for 2 hours, then 18C again etc etc. I cant set the cycle times as I don't have a boiler relay, just 2 x BDR91 driving the 2 way valve. I do have an additional BDR91 spare I could use if it helped.

Any thoughts welcome

[Peahead]

Hi,

Its a raspberry pi Zero running this :

https://temperature-machine.com/

with some ds18b20 temp sensors

[welcometothenorth]

Ours does the same, I assumed it was normal behaviour. 30 degrees flow is enough to maintain your bedrooms at 17.5 - 18, and should be more efficient for the boiler running at a lower temperature. The 0.5 degree range on the target temp is normal behaviour for this system.

[Peahead]

Thanks for the feedback, I was assuming it wasn't good for the boiler to cycle for such a short time each time and would cause premature wear. I think I need to do more research on it!

[welcometothenorth]

You can change the cycle rate in the parameter settings. Default is 6 per hour.

[Peahead]

I don't think I can do this, as I don't have a "Boiler" relay, just two BDR91's feeding the Y-Plan valve, I seem to remember this setup prevented access to the cycle menu (I think! its been a while since I set it up)

[Richard]

TPI cycling is very normal. When you set a room temperature, evohome is trying to maintain this.

If the room only needs 500W and you have a 24000W boiler, it is going to have to cycle it (turn it on and off lots) to provide the correct amount of energy into the system.

[Peahead]

Thanks all for the support. I've managed to capture a lot more data now, as you can see it appears to be the demand from one TRV that is causing the constant cycling (Yellow line on bottom graph, although the red line demand also increases) The boiler fires 14 times for no longer than ~4 minutes a time, and if you look at the top graph (room temps) they barely move at all.

For reference, the top graph shows the TRV reported room temperatures, the 2nd graph shows the BDR91 relay switching on and off (Orange line) the boiler flow temp (yellow line)

Bottom graph shows TRV demands overlayed on the BDR91 relay switchpoint. Colours on the bottom graph match the colours on the top graph, so the thin yellow line on the bottom is the Daisy TRV

I'll put it on economy mode tonight and see how it looks and see how much the temperatures in the rooms drop by. I suspect I will need to adjust the schedules to change the overnight set points (currently 18C) to something much lower to stop this. A quick look at my energy meter and this cycling through the night is using about 20kWh which whilst not massive, does have an impact on my gas bill