Octopus fitted us a Stelflow 210l slimline cylinder with the 6kW Daikin. Its just a tad under 2m tall but only 475mm diameter. We heat to 48C and have yet to run out with a house of 4. It uses around 0.8-1.4kWh per day to heat it.
Heat Pumps: anyone have one/thought about it?
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So I was inspired to find out how to set my Daikin 4KW Altherma 3M to "pure" weather compensation. Daikin refer to this mode as LWT or Leaving Water Temperature. I had to ask Grok a couple of times to help me find the options in the Daikin MMI menus. The instructions given don't quite match the menu structure in my MMI unit but I worked it out after 20mins of playing in the menus.
So in installer mode I enabled LWT mode (not fixed or WDC) and disabled the thermostat. This also disabled the schedule option so the HP would run purely off the WD curve. If you want to have setback periods then you have to re-enable the schedule mode but now it works on plus or minus temps rather than a target temperature. The Daikin Madoka thermostat is mostly for reference now.
So now I have a setback between 4-8pm to try to avoid running (4-7pm) when the unit price is +35%. Then after 10pm till 4am a night setback temp. Will adjust as required.
The Daikin Onecta app is more for reference now. You can't adjust the schedule directly like before but you can make temporary +/- adjustments until the next schedule window does a reset.
So in installer mode I enabled LWT mode (not fixed or WDC) and disabled the thermostat. This also disabled the schedule option so the HP would run purely off the WD curve. If you want to have setback periods then you have to re-enable the schedule mode but now it works on plus or minus temps rather than a target temperature. The Daikin Madoka thermostat is mostly for reference now.
So now I have a setback between 4-8pm to try to avoid running (4-7pm) when the unit price is +35%. Then after 10pm till 4am a night setback temp. Will adjust as required.
The Daikin Onecta app is more for reference now. You can't adjust the schedule directly like before but you can make temporary +/- adjustments until the next schedule window does a reset.
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I would turn the flow temperature up when it’s cheap, +5 or even +10 when it gets into the winter. Then do -3 or -5 at the regular rate and -10 when it’s expensive.
I’m using normal weather comp and use home assistant to do the automations on the leaving water offset when it’s cheap on IOG.
I’m using normal weather comp and use home assistant to do the automations on the leaving water offset when it’s cheap on IOG.
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LWT still uses the same WD curve as it turns out but it should match the output of the heat pump more closely to the heat loss of the house for any outside temperature. I think Daikin kept it buried as its easier for most users to think of their comfort via the thermostatically set temperate. Probably cuts down on support calls complaining about cold rads etc.
First morning with LWT enabled. HP started at 4am using ~330W after an initial start-up period to around 1KW. I didn't need it to run as the house was warm enough. Checked the Daikin MMI and the flow temp was 26C. Will just have to monitor this and make adjustments as the temps go down.
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Halifax and Lloyds are offering free heat pump to new mortgage customers:
To qualify, you’ll need to:
- be eligible for the government's Boiler Upgrade Scheme which will contribute £7,500 to the cost of the heat pump. Octopus Energy will apply for this and claim on your behalf
Lloyds will pay for a standard heat pump installation by Octopus Energy. Standard installation includes:
5-year labour and workmanship warranty.- apply for and accept your mortgage illustration for a new Lloyds mortgage. Or if you already have a mortgage with us, accept an illustration to switch to a new deal. You must do this between 15 October 2025 and 24 November 2025 to qualify
- open or already have a Club Lloyds bank account
- complete on your mortgage or deal switch
- install the heat pump within 12 months of completing on your mortgage or deal switch
- be located in England
- have a valid EPC rating of C or D registered on the government’s EPC Register
- pass a property suitability check for standard installation. Octopus Energy will do this and tell you if additional works are required. This could mean that you need to take action to qualify for standard installation or that you no longer qualify for this offer
- an air source heat pump (Octopus Cosy 6, Cosy 9, or Daikin).
- a compatible hot water cylinder.
- necessary radiators for your system and design.
- Plumbing, pipework and electrical work to Octopus Energy design standards.
- asbestos testing, not including removal if required.
- 8-year product warranty (Cosy) or 5-year warranty (other brands).
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@SkyTerran you really need to include links to the relevant information.
For Halifax they offer £2000 cashback for a hear pump, using any accredited installer, plus £100 bill credit if using Octopus.
Halifax will also give £1000 cashback for solar panels, they have other offers as well.
Lloyd's do appear to offer a free heat pump, but also have £2000 cashback offer.
For Halifax they offer £2000 cashback for a hear pump, using any accredited installer, plus £100 bill credit if using Octopus.
Halifax will also give £1000 cashback for solar panels, they have other offers as well.
Lloyd's do appear to offer a free heat pump, but also have £2000 cashback offer.
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So last night was the first cold night since we got the heat pump. Outside temp was about 6C, which equated to about 39C leaving water temp. The power use was pulsing quite a bit compared to the day before which was pretty steady (apart from the DHW cycle at half 4). What would be causing this then? COP was above 4 on the first day on the chart and barely 2.9 on the second despite only 3C difference in leaving water temp, so it was much less efficient doing whatever it's doing.
Mine pulses quite a lot, I just figured it’s a Daikin thing. I’ve seen others report it as well.
Edit: unless you have an open energy monitor system on it, I would take whatever the Daikin MMI is telling you with a large gain of salt.
Edit: unless you have an open energy monitor system on it, I would take whatever the Daikin MMI is telling you with a large gain of salt.
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It's wired into the Smart port on my Sigenergy gateway now, so I get actual power usage figures.
Unsurprisingly, the Daikin MMI power usage figures are nonsense. They read high almost without fail, by up to 20% some days.
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I've been looking into a hybrid set up to compliment a gas boiler my 1930's house needs too much work at the moment for a heat pump to be feasible as it stands now.
I think Im definitely going to have to reduce the weather curve. It was 2C this morning outside, so the heat pump was going up to 43-44C leaving water temp and making a racket as it kept fluctuating. It was LOUD. I set an offset of -4C on the leaving water and it then sat stable at around 900w usage and under 40C flow temp rather than up to 2kW before and was pretty much inaudible.
The house was very warm after running only 4 hours, so I think Octopus' default curve (50C at -3C) which I've already dropped by 4C is still too high. I think the 5.39kW heat loss they calculated is too much. That sort of noise was too high. I could accept it in the depths of January, but not October!
The house was very warm after running only 4 hours, so I think Octopus' default curve (50C at -3C) which I've already dropped by 4C is still too high. I think the 5.39kW heat loss they calculated is too much. That sort of noise was too high. I could accept it in the depths of January, but not October!
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My Arotherm was set to 0.8 on the heat curve, I've reduced that to 0.55. Last week I had a COP of 4.99, this week currently at 5.26, this includes hot water, as reported by the myVailliant app.
0.8 gives a flow temperature of 44°c at zero outside, 0.55 gives 38°c flow.
Apparently a good way to home in on the correct whether curve is when outside temperature is at or close to design temperature, set the system to work on pure weather compensation (it will ignore the internal temperature sensor), then adjust the curve, temperature should stabilise at your design temperature, if the house is too hot lower the curve, too cold raise it a bit. Allow 24 hours between each adjustment.
0.8 gives a flow temperature of 44°c at zero outside, 0.55 gives 38°c flow.
Apparently a good way to home in on the correct whether curve is when outside temperature is at or close to design temperature, set the system to work on pure weather compensation (it will ignore the internal temperature sensor), then adjust the curve, temperature should stabilise at your design temperature, if the house is too hot lower the curve, too cold raise it a bit. Allow 24 hours between each adjustment.
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I’m at 45C at -8 ant the top and 30c and 11 at the bottom of the curve and it’s still too high so the house overshoots.
The original curve was 50 at -2 and 25 at 20c with a 5.5kw heat loss at -2.
In this weather you also need to adjust the bottom of the curve, changing the top will not help that much.
Where the heat loss calculation is out is probably on the air changes, that’s not abnormal and it’s a complete assumption. When it’s very windy here we don’t overshoot anywhere near as quickly.
In regards to noise, mines inaudible inside when going full tilt on hot water. What’s brand have you got?
If it’s Daikin, they may not have removed the transport bolts on the compressor. If they are left in it rattles a good one.
The original curve was 50 at -2 and 25 at 20c with a 5.5kw heat loss at -2.
In this weather you also need to adjust the bottom of the curve, changing the top will not help that much.
Where the heat loss calculation is out is probably on the air changes, that’s not abnormal and it’s a complete assumption. When it’s very windy here we don’t overshoot anywhere near as quickly.
In regards to noise, mines inaudible inside when going full tilt on hot water. What’s brand have you got?
If it’s Daikin, they may not have removed the transport bolts on the compressor. If they are left in it rattles a good one.
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Yes, its a 6kW Daikin. I don't think its the transport bolts as its silent during the DHW cycle and at lower LWTs. The design temp is 50C at -3C which I've lowered to 46C, but it was still pushing 43C LWT this morning at 2C outside so I think it probably needs several more degrees off. The noise is definitely a pump noise as you can 'hear' it through the radiators. Outside its just the noise of air moving but nothing too offensive. Its probably cycling due to being too high as power usage is approx 1.7kW and COP drops off a cliff when its doing it.
I found the mechanical pump noise is barely audible over the fan at high demand such as in DHW heating. Some noise does travel to my bedroom through the external pipes but I have to stick my ear against the wall to hear it. I was having more issues with a high 25 l/h flow rate in my bedroom. I did find a lock shield valve still had a couple of turns on it and that helped a lot with water noise through the rads so worth checking. Everything is fully open now.
As you can better graph your pump power you could try altering the Delta T between 5C (Fan Coil) and 10C (Radiator). I'd be interested in seeing if that changes the amplitude of the pulse rather than completely eliminating it. I can only see a potential pulse in LWT mode via the Octopus app "live" power reading. Maybe it's just hunting either side of the WDC calculated target.
If you believe you have plenty of heating capacity then you could try the Quiet mode. I tried this as part of my initial high flow rate issue. I currently have it set to Automatic - Mild - 10:00PM to 07:00AM (Daikin don't recommend the other two modes!)
That will reduce your overall heat output by 20% by limiting compressor duty and fan speed. In my case it also lowered the flow rate from 25 to 21 l/h. There may be a slight increase in the overall heat up time from cold so it's a trade off. My DHW heating is not during the quiet period.
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I changed the weather curve overnight to 42C at -6 and 28C at 15. It always runs a couple of degrees warmer than the curve, so this gave a leaving water temperature of 31-33C when it was 11-13C outside through the night. It ran very nicely indeed, doing a 12 hour run where it used a steady 350-400w of power, consuming about 4.8kWh in total. COP was about 4.8.
The house is a nice 21C throughout, despite a very strong wind all night. I need another cold night like the night before to test it properly though, as last night was mild. It should tick along at 37-38C LWT, instead of the 40-42C of yesterday morning, which it didn't like. I suspect with the Daikin, it's happy until the LWT is too high for the heat loss of the house, so it can't shed the heat quick enough, return water temp rises too high and it keeps cutting in and out. As soon as I ran an offset yesterday of -4C, leaving water dropped to 38C and power use dropped from an average of 1.8kW to 900w, for a mere 3C drop in LWT.
The house is a nice 21C throughout, despite a very strong wind all night. I need another cold night like the night before to test it properly though, as last night was mild. It should tick along at 37-38C LWT, instead of the 40-42C of yesterday morning, which it didn't like. I suspect with the Daikin, it's happy until the LWT is too high for the heat loss of the house, so it can't shed the heat quick enough, return water temp rises too high and it keeps cutting in and out. As soon as I ran an offset yesterday of -4C, leaving water dropped to 38C and power use dropped from an average of 1.8kW to 900w, for a mere 3C drop in LWT.