Short cycling, third party thermostat help

Hardware Heatpump
1

Could anyone help with how I can identify if my vaillant arotherm plus 7 is short cycling. I rely on my vaillant app for data, daily energy usage. I’m new to heat pump and technology associated with it and any help and advice anyone can offer would be very appreciated.

Our property has a 3 zone under floor heating upstairs operating from 3 individual thermostats, downstairs is all radiators. I appreciate it is more beneficial to have the heating system on an open loop but this was troublesome for our system. The installer has connected a third party thermostat, hive, so that downstairs can be operated from it and separately from our upstairs UFH thermostats.

I believe our ASHP has been aggressively short cycling as a result and because we are not operating sensoCOMFORT controller and thermostat. I appreciate the upstairs under floor heating, with pump, actuators and the downstairs hive will be effecting the efficiency and COP.
Our installer, who has been amazing throughout, has agreed to change our system to an open loop and this should hopefully happen soon, they will remove the UFH pump, actuators and other hardware to achieve this.

I have only recently became aware that vaillant do not recommend using third party thermostats, I’m hoping when the change is made, that it will resolve the issues of what I believe are significantly higher daily running costs, significant short cycling and low CoP.
Our system was commissioned on 1 December last year, so not very long ago, but the app shows 919 on / off cycles in 40 days and 615 kWh electric energy consumption, heat and water during December with a COP of 3.27, 1860kWh heat and 1530 kWh hot water generated. During January, 9 days so far, 285kwh electric energy consumed with 2.66 CoP for heating and domestic hot water. The app shows 700.2kwh heat generated, 58.2kwh hot water generated. We have a 300L cylinder.

The daily power usage seems high, e.g December 8th consumption for both heating and hot water was 35.9, generated heat 85kwh, hot water 8.8kwh with indoor temperature of 19.4C and outdoor temperature of 1C. The temperature in downstairs rooms always overshoots and is on average between 20.5 and 23.5 during the day when hive schedule is set at 19.5C, the setback is 18C. I’m optimistic that the change to single loop system and removal of hive and use of sensoCOMFORT controller and stat will resolve the constant on/ off cycling, low CoP and reduce the overall performance and efficiency of our system.

Our property is a 1960’s 3 bed bungalow with 3 room loft conversion and is pretty well insulated. Any thoughts, help and advice would be very welcome. I would also like to learn if our ASHP, as I expect is significantly short cycling, I just need to understand how we can check this from my vaillant app as we do not have a heat meter installed to collect data.

Thank you in advance

2

Hello, and welcome to the forum!

This averages out to 23 cycles per day, which is less than once per hour - not too excessive.

Having multiple thermostats and valves is a common cause for cycling. Converting to an open loop will probably help a lot.

This seems about normal for that size of heat pump, given how cold it’s been recently.

  • Do you know the projected heat loss for the property?
  • Can you provide stats for the coldest day this month?
  • What flow temperature does the system run at?

There are ways to collect data directly from Vaillant (or indirectly from their cloud service) if you’re technically inclined… Being able to view the behavour of the heat pump in charts makes it much easier to tell if it’s working properly.

3

I greatly appreciate the reply Tim.

The proposed flow temperature 45°C

Seasonal CoP 3.4

Down stairs outside temp - 2.7°C

The DT 5°C

Area of building 161.58M2

Average heat loss 51 W/m2

Total heat loss 8221W

Main water temp 45.5°C

My vaillant app shows 35.9 kWh for January, 6 with outdoor temperature of -1.3°C (outdoor temp would have been lower that day) and 17.7°C indoor temp which always overshoots by 2-3°C because of third party, hive thermostat, 2.54 CoP

35.1 kWh, January, 5, outside temp -2.5°C , indoor 18°C (20-21°C), 2.4 CoP

December, 13, 19.5kWh, indoor temp 20°C, outdoor 5.9°C, CoP 3.07

Thank you

Dan

4

Data collected by HeatpumpMonitor.org shows that that properties of a similar age and size have consumed between 400 and 800 kWh of electricity in December, so the figures you’ve shown do not look particularly high.

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The COP figure is on the low side – we’d expect to see it closer to 4.0 in that month – but the temperature measurement can be off sometimes. You can check if the sensors are matched by finding a moment when the compressor is off but the pumps are running, as André describes here: Why doesn't a low flow temp translate into a decent COP - #27 by Andre_K. There follows a calculation to work out the true COP.

Heat pump efficiency could be improved if it ran at a lower flow temperature, say 40°. Converting to open loop should help with that.

There’s a chance that the heat pump is slightly oversized for the property (surprisingly common), with actual heat loss closer to 4 or 5 kW. This can cause more cycling when the weather is mild, particularly if not all zones are open. This is quite normal for heat pumps though, and up to 3 times per hour is deemed acceptable by manufacturers.

Anyway, it sounds like you’re already making steps to improve the system.

1 Like
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That is reassuring. Thanks a million Tim for your time, help and advice. I’m very grateful.