Weather Compensation & ASHP Duty Cycle

I’ve been reducing my WL settings to increase ASHP duty cycle (and thus reduce the peak power consumption). Here’s a screenshot of a typical day, taken from my PV inverter monitor, showing household power consumption (15 minute rolling average). The spikes at 07.30 and 17.30 are kitchen loads (no other major consumers).

My observations: 1) ASHP is the major consumer (background consumers are only about 0.2kW - see before 06.00), 2) duty cycle looks to be 50-70% which I think is reasonable (but I’d like your opinion, 3) average net power consumption of ~1kW feels about right for a detached 3-bed house and an 8kW ASHP for mid-November (ambient 12degC). Comments?

I think I’d say timing maximum consumption to lowest unit cost is more important. Some off peak electricity units are a third the cost of peak and heat pump performance doesn’t vary as much.

Timing that so it produces comfortable temperatures all day is probably beyond the FTC5’s capabilities, sadly. It’s not easy with scripts and emoncms analysis.


Fair point, but I fear off-peak is of more interest to EV owners, as central heating is of limited usefulness at night unless your home insulation is near-perfect (in which case you don’t really need a heat pump).
I’d like to charge my PV battery on off-peak and run the ASHP off the battery during the day, but 1) I’d need an extra 10kWh battery, and 2) my power supplier won’t allow this (surprise…).

Hi Sarah. I’ve asked for this to be split into a new topic, so we can comment on your specific system.

(copied above from original topic for context)

You may be able to reduce the overall consumption by increasing the duty cycle of the heat pump, but this may come at the expense of performance, i.e. less heat being produced per kWh of electricity (known as COP).

Yes, this looks to be running reasonably well at around 3 cycles every 2 hours, which is okay. If it still cycles in colder weather (say 6C) then it might suggest the heat pump is too big for the property, or there aren’t enough radiators. You’ll want to have your TRVs on their highest settings to maximise radiator output, except for rooms that are getting too warm.

Net power of 1kW seems about right, looking at this other 8kW Samsung. In ideal conditions, it can run continuously on under 800 W of power with a flow of 35 C, producing 3 kW of heat to match the heat loss from the building. However, every house and every heating system is different, so we don’t expect them to perform identically.

I notice that the heatpump is only running between 8am and 9pm - there’s a school of thought that it’s better to keep them running for 24 hours with a couple degrees “setback” on the thermostat at night. By keeping the heat pump running “low and slow” throughout the day, it doesn’t have to work hard to get the house up to temperature.

It’s hard to infer much more about the running of a heat pump without sophisticated metering. Does the Samsung controller provide any additional insights?

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Have you looked at Octopus Cosy?
It gives you 2 off peaks (4h-7h and 13h-16h).

I charge our battery on both and run our hot water on both (tank isn’t big enough to get away with a single heat up). I also have a small setback on our ASHP (1 degree) that I raise at 6h so the start of this falls in the off-peak. The battery is then able to cover more of our heating load.

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I too have been using Cosy. I was originally skeptical that it would make sense for heating, but it’s been working out surprisingly well. See my analysis before I switched plus my experiences with it here:


I am using Cosy too.

I have a lot of batteries so more unusual but I have yet to buy any electricity outside of the six hours.

I heat the DHW at 13.00 and the heating runs all day. I turn it down 2c between 23.00 and 04.00 but start heating at 04.00 when the cheaper period starts.

I also charge my batteries in the cheaper six hours as necessary.

So far I always end up with the batteries fully charged at 16.00.

It works out OK, so ignoring charging losses all electricity import is at around 17p per kWh.


And another option is Octopus Agile which tells you the unit price a day ahead and often has “off peaks” at similar times to Cosy. It would be even more complex to program.

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Thanks Tim,

  1. I’m new to this community (and heat pumps) and the separate topic will help me tremendously - you and other responders are streets ahead of me. I’m a Chemical Engineer - comfortable with steady state/equilibrium, less so with cyclic systems/transients - so I’m learning fast from this community.
  2. I’ve been testing my new system (without the benefit of much monitoring hardware beyond the basic Samsung controller readings) because I suspect I’m not getting the CoP that Samsung advertise. I’ll report my findings to the community when complete (in case anybody else is interested).
  3. Hitherto, we’ve run the CH with a big setback (21.5degC daytime, 16degC night), mainly because 1) our current Octopus tariff has no off-peak component, and 2) running the ASHP at night appeared counterintuitive (who cares if the living room is cold when we aren’t in it?). My testwork is aimed at quantifying the financial benefit (if any) of starting the ASHP earlier each day, and whether to warm the living room quicker by increasing ASHP exit temp for a couple of hours after daily start-up (the Samsung controller offers an easy adjustment up to 5degC, which would re-warm the buffer tank quicker after its night-time shutdown).
  4. Tariffs. I spoke to Octopus several months ago about an off-peak tariff but was put off by 1) I couldn’t have one without an EV, and 2) the off-peak hours were offset by an increased tariff early evening (our second highest load after ASHP is cooking evening meal). Looks like I should ask again, as tariffs may have changed.
  5. I’m not clear how (if I did go for an off-peak tariff) how I tell my PV battery to charge itself from the grid during those hours. I guess I’ll just have to talk to my PV installer :frowning:
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What make is your battery and inverter, someone on here might be able to help.
I have a Fox ESS system and I can set up to 2 periods where the battery is charged up rather than discharging for house loads.

Hi Andrew,
I have a SolarEdge SE5000H inverter (max charge rate 5kW) and 10kWh SolarEdge battery. Currently no scheduling hardware/software as far as I know (demand priority: ex PV > ex battery till empty > import, production priority: to consumers > to battery charge till full > export).

Welcome! :slight_smile:

Yes, this is counter-intuative, and a departure to how houses have been heated in the past. The idea of a smaller setback is to avoid having the heat pump work harder in the morning when it could have been ticking over through the night keeping the chills at bay.

On the other hand, when using a TOU tariff, one wants to cram as much heat pumping into the cheap hours as possible, to optimise on cost rather than performance.


Hi all, I’m still struggling with this water law mode in warmer weather. It’s good until my heat loss is around 3KW and bigger and it follows what @SarahH said: “In ideal conditions, it can run continuously on under 800 W of power with a flow of 35 C, producing 3 kW of heat to match the heat loss from the building.”. But the hell begin when my heat demand is 2KW and lower in warmer weather conditions.

Seems no matter what, the stop cycle is always 3 minutes and 15 seconds. I would have thought the time until when the compressor is stopped vary but this is seems not the case. So it happened that when there is 10C outside and my heat loses are below 2KW the pump start to cycle very often because of these short intervals. My K1 radiators are not ideal for heat transfer at lower water temperatures but why the heat pump always stops only for 3 minutes. The internal logic should know maybe that this is too often and prolong the cycles so compressor is off for longer. Can I tell the pump the minimum time when the pump needs to be stopped.

Am I missing something here ?

Here’s the graph. Over night only few stop cycles but now as sun is shining and it’s warmer it started like crazy.