Finding small changes have a decent impact on run time and overall efficiency of the system as a whole. Just a matter of finding something close to the sweet spot - putting a decent amount of energy into the floor, but not too much or too little.

Have found at low loads a reasonable run time is better than a short run, overly long runs end up running hotter than need and hit CoP.

Last night and the night before were very similar temperature outside (around 9 Deg).  Changed around half a degree in compressor start stop hysterisis, this extended run time and as well as off time.  Resulted in 3 off longer runs (20 to 40 mins on and 1 to1.5 hrs off), compared to 5 shorter runs the night before over a 7hr period. Overall CoP has remained high, running CoP is the same. House was slightly warmer at just over 21 compared to just under.

Tonight and the next night are down to 5 then the night after is down to 3, so will be a better test. Going to leave set as currently and see what happens, suspect as heat loss increases run time will increase to naturally compensate for additional heat losses.

Today's running so far.

Posted by: @declan90

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@johnmo I did consider asking if you had found overall heat demand / electric usage had increased when you had reduced the cycle time but increased overall "on" time (despite overall COP improvements) when you first posted it - but couldn't work out how to phrase it without coming across negative/unhelpful. Everything I've been looking at for my own unit seems to have people chasing the longest possible cycle times (albeit in peak heating season).

For me I'm only really currently interested in the "while running" figures on my own installation at the moment as the unit is predominantly in standby, and that's a reported standby electricity usage of ~10W. 

At 35 W standby I can see how the "off" periods will impact the daily figure. 

As this will be our first winter (unit installed in May) its still hard to let our old habits die, as such only DHW runs at the moment. Finding the internal temperature is only really dipping to between 18C and 19C when we have a day or two of all day drizzle and gloom. Will be interested to see how the forecasted dips to single digits overnight later this week will impact things!

Recently switched onto Octopus Cosy as it does feel like the summer has ended! I'm holding off for now switching on the weather compensation (which I will want to fine tune when I have some proper heating demand) and have set ours to run in Vaillant's "expanded mode" where it only comes on as called for by the controller acting as a thermostat and it then modifies the weather compensation curve depending on how far away from the target temperature it is reading.

It'll be interesting to see how long I can get away with just running some heat in during the 3x cheap periods this tariff provides!  Although the ability for truly scientific experiments might be limited by how the willing test subjects family react...

Hi @declan90

We are in an almost identical situation to you (except I'm staying on Agile for now). ASHP installed in June, so this will be our first winter too. I'm also just running occasionally ATM to put a bit of heat into the house as required. Agile prices are near zero today, the room stat had dropped to 18.5C and running with a LWT of 35C warmed the house to 20.5C in just under an hour, and which point I switched off demand for heat. We will see how quickly the house now cools.

There has been much discussion of the Cosy tariff (see @toodles thread) for heat pump users. We don't have batteries, so I'm undecided if it would work for us in the depths of winter, but would certainly allow us to put sufficient heat into the house during the cheap slots in spring and autumn. For those with large enough batteries, the chance to lock in 11p electricity is very appealing. The Octopus Compare app has previously had Agile as cheaper for us, but recently it is a lot closer with Cosy (I will continue to monitor).

I think (S)COP is a distraction - the main consideration for me is the running cost, and that must often be traded with comfort. For example, on Cosy the cheapest running cost to deliver a given amount of heat is to run the heat pump only during the 3 Cosy cheap slots. You may get a better (S)COP by running it continuously (although the 3h cheap slots are not exactly short cycling), but if it costs more to deliver the same amount of required heat then it's false economy. But as you identify, comfort is also a factor and large swings in house temp between cheap Cosy slots may not be to the taste of all family members. Hence the attraction of Cosy to those with large batteries who can smooth the heat delivery over longer periods as battery capacity allows. @toodles will tell you they are able to run continuously, only charging batteries during the cheap 11p slots.

@old_scientist Yes indeed, with 27kWh of storage* capacity, the 3+3+2 hours at the lowest rate each day ‘carries through’ with a good margin for me. During the 8 hours of lowest rate, house consumption is straight from the grid so the fully charged battery only ever has to ‘take the strain’ for a maximum of 6 hours once a day and shorter periods during the rest of the day; my probable maximum consumption during those 6 expensive hours will be less than 4.5 kWh so the grid need not be used at all. In addition, there might also be some contribution from solar PV to assist. Come next March, I will know how well my theory works out in practice!😉

*The NetZero app informs me that the actual battery capacity at present is just short of 30 kWh.

Regards, Toodles.

To correct myself, I should say 4.5 kW and that level of draw could be supplied by the battery for the full 6 hours.

OK, back to sleep now…Toodles

@old_scientist I made the jump to cosy the other weekend after a few days of lousy solar and no sign of cheap (windy) rates on agile for the weekend in question. Having batteries certainly made it an easier choice! 

Agile and no batteries feels brave (but I'm a natural worrier/planner)

Planning on IOG longer term but that'll be at least a few months away. 

In two minds at the moment whether to try my best to fully top up the batteries at each available opportunity. If so it would all be 11p but could be putting a lot of cycles through the battery. 

As you say it's cost not cop that's key (cop will help of course!). I'm hoping that if I can get my average unit rate for import down to 15p that my total solar production/export will net it all out. Will be interesting to see if it pans out as expected...

Posted by: @toodles

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To correct myself, I should say 4.5 kW and that level of draw could be supplied by the battery for the full 6 hours.

OK, back to sleep now…Toodles

I'm currently on Agile but getting 8kw Daikin fitted next week, and looking to go on Cosy for ease of use.

It would be good if I could get by on my 6.4kwh batteries through the 6 hours dearer Cosy rates. 

I was initially thinking I may need more batteries.

@toodles 

Thank you for reply. 

I suppose I've got to weigh up the cost of additional batteries v cost of hitting the higher Cosy tariff occasionally. 

Although missing the much higher 4pm to 7pm tariff should be achievable at even the coldest days if my usage isn't too far from yours. 

Cheers

@njt It’s certainly worth doing the sums as even if you use peak rate occasionally it still might not be economically sound to increase your battery size. I have only a 5kWh battery but I can’t make the case to buy another module given the limited months when it isn’t enough.

Posted by: @njt

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suppose I've got to weigh up the cost of additional batteries v cost of hitting the higher Cosy tariff occasionally. 

Or just accept sometimes you will use higher rate electric. You should still come out way cheaper than standard rate electric.

@johnmo I sometimes have to pause and prod myself with a reality check; I inwardly complain that ‘I have to pay 11 pence per kWh for all the grid power I consume!’ Then I think…. but some people have to pay two to three times as much for every kWh they use and possibly have much higher demand due to poorly insulated housing and less efficient heating systems. Reflectively, Toodles.

@njt As the Cosy peak rate (4-7pm) immediately follows a cheap slot, your battery should be fully charged and at least should avoid importing during the peak.

Further, in cold weather, you may be able to set back your temps slightly or adjust your ASHP settings (maybe lower LWT - I'm not familiar with Daikin settings) to just trickle heat into the house at 1kWh thus staying within the capacity of the battery and not allowing the internal temperature to drop significantly, getting you through to the next cheap slot where you can raise the temps again and recharge your batteries. I do this on my Samsung in Weather Compensation mode which allows the target LWT to be easily adjusted up/down by +/- 5C on the controller, setting back the LWT overnight or bumping it up in very cheap Agile slots to deliver more heat at cheaper prices.