Octopus Cosy - new tariff for ASHP owners

[George]

Bit of PSA, while it's probably cost neutral for me  at the moment, if combined with a battery it'd be a decent cost saving. Although I don't have a smart meter so can't get it

 

Two low/off-peak times and one peak time in the evening:

 

On the website said ~20p off, 50p peak and 33p in-between. 

 

 

https://octopus.energy/cosy/

 

EV Man's youtube video on it:

https://www.youtube.com/watch?v=2XkWWdJ2ga4

 

[ProDave]

It's that pesky 4pm to 8pm silly high peak that I don't want.

[Luke1]

In my area the peak is quite high. (Note this is a flexible tariff and so it will go up when the price cap changes!)

 

 

Unfortunately I can't see this working for most people that cook during the peak. Made worse by the fact that for approximately 6 months of the year you won't really be benefiting at all from the 'cosy rate' as you wont need heating. 

 

My fix ends at the end of January so I'm trying to find other deals and suppliers that may work better for me. 

EDF have a E7 flexible tariff which is currently 0.07p/kWh night and 0.055p/kWh day. Which actually works better for me than the Cosy deal.

I have also worked out that I could possibly 'save' ~£1000/year if I installed a 9kWh battery based on those rates. ROI approx 6 years at today's rates. However with the market being so unstable it's a big risk to know whether the E7 rates will always be this low in the cheap hours, especially with ever increasing EV charging on the grid. 

 

 

[ReedRichards]

Although this is being marketed to ASHP owners, typically you want to run your ASHP nice and steadily, not at a few hours of high power followed by a few hours of low/zero power.  My conclusion is either:

• Octopus don't really understand the needs of ASHP owners

or

• This is just a marketing gimmick to find an excuse for charging a high rate for peak-time electricity.

[Dave C]

3 minutes ago, ReedRichards said:

Although this is being marketed to ASHP owners, typically you want to run your ASHP nice and steadily, not at a few hours of high power followed by a few hours of low/zero power.  My conclusion is either:

• Octopus don't really understand the needs of ASHP owners

or

• This is just a marketing gimmick to find an excuse for charging a high rate for peak-time electricity.

 

That might be the ideal, but in practice neither I nor the other two heat pump owners I know have ever managed to get things arranged so the heat pump just runs constantly at a low power. Perhaps that's because it's annoyingly uncommon to get load compensation (rather than weather compensation), or because on a-bit-cold-but-not-freezing days the heat pump just can't modulate low enough to do anything else, but the end result is that except on the coldest days (and even after all the tweaking I've made to reduce short cycling), my heat pump comes on for 2-6 hours, then the house is up to temperature and it shuts off for a while.

 

It's not ideal but it's what's happening - so I guess you could regard this plan as a pragmatic acceptance of that pattern, saying "if you're going to have the heat pump running for 2-6 hours on 1-4 hours off anyway, why not play with your thermostat to get those hours to line up with not-peak-time".

 

Having to pay those peak time rates during the summer, though ... it's more like 7 months of the year for me, that I don't need any heating, so while in principle this plan isn't a terrible idea during winter, having to switch onto and off it every winter (and hoping you don't get stung by a more expensive rate every time you do) isn't appealing.

[JohnMo]

17 minutes ago, ReedRichards said:

Octopus don't really understand the needs of ASHP owners

But are pushing themselves as an installer - makes you wonder.

 

6 minutes ago, Dave C said:

because on a-bit-cold-but-not-freezing days the heat pump just can't modulate low enough to do anything else, but the end result is that except on the coldest days (and even after all the tweaking

Sounds like your heat pump is either too big, you need a buffer or bigger one, or you have the system flow constrained with too many zones closing down.

[PhilT]

It's got some attractions. The morning period is not far off our current switch on and get up times. The afternoon period is the time of highest daily temps = best heat pump efficiency,  and we still use gas for cooking.

[SteamyTea]

Makes my EDF E7 rate look very attractive, and I don't have to buy anything new.

 

[Dave C]

35 minutes ago, JohnMo said:

Sounds like your heat pump is either too big, you need a buffer or bigger one, or you have the system flow constrained with too many zones closing down.

 

Well, it's a 5kW heat pump (so probably not too big!), and I'm not shutting any radiators down so full system flow available all the time. That only leaves "I need a buffer" - but does that actually improve things that much? If the heat pump's coming on for 3 hours, then shutting off for 3 hours, am I actually losing that much efficiency? It's not short cycling, after all...

[ProDave]

1 hour ago, ReedRichards said:

or

• This is just a marketing gimmick to find an excuse for charging a high rate for peak-time electricity.

It is a marketing tool, to try and convince the customer they will save money and encourage them to get a smart meter to enable it.

 

I would love to see the statistics for domestic usage in the pesky early evening peak.  We are constantly being told that is when demand is higher. So a lot of households must use a substantial part of their daily usage in that peak time?

 

For us it is really just the oven that bothers me and not something I want to have to time shift.

 

But if we all scrap gas, there are going to be a LOT more electric ovens..........

[JohnMo]

53 minutes ago, Dave C said:

If the heat pump's coming on for 3 hours, then shutting off for 3 hours

If it's running for 3 hours that's not short cycling. If it runs for less than 10 mins then possibly yes.

[Dreadnaught]

It struck me when I saw this tariff that the second afternoon cheap window could be a good time to heat the DHW cylinder when the heat pump needs to work at its hardest. Overnight on the other hand would be good for space heating.  (I have no PV, nor a battery).

 

2 hours ago, Dave C said:

heat pump just can't modulate low enough

 

I am presently negotiating with the wholesaler supplying my ASHP to swap the 5kW model that I originally ordered for the smaller 3.5 kW model exactly for this reason.

 

2 hours ago, Dave C said:

it's more like 7 months of the year for me, that I don't need any heating

 

This is a good point.

[Luke1]

9 minutes ago, Dreadnaught said:

 

I am presently negotiating with the wholesaler supplying my ASHP to swap the 5kW model that I originally ordered for the smaller 3.5 kW model exactly for this reason.

 

 

Do you know what the 5kW model can modulate down to? I can't see the 2.5kW model modulating down much more. And for piece of mind, especially with recent posts regarding cold snaps and constant defrost cycles then I would something slightly larger that necessarily required.

 

For info my Nibe 8kW heat pump has a max draw of 4.0kW, and at its lowest setting it draws a little less than 500W. 

 

All you need to do is match your output of UFH and radiators to maxmise what heat can be extracted from the system and ensure this matches or exceeds the lowest output from the heat pump. (It would only then 'cycle' when the house has reached temperature)

I'm sure someone more technical could explain that better. 

 

[JohnMo]

34 minutes ago, Dreadnaught said:

negotiating with the wholesaler supplying my ASHP to swap the 5kW model that I originally ordered for the smaller 3.5 kW model exactly for this reason

If your heat pump modulates down to 2.5kW and your system volume that remains always open to the heat pump is above 45L then you will have a run time over 10 mins with a load of only 0.5kW.  so you are not short cycling.

[Nick Thomas]

Hmm. I'm trying to like this tariff, but I'm not sure I can.

 

My PV generation overlaps the 1-4pm period; that's already sufficient to run the heat pump and charge the battery, so it'd only really be useful on cloudy days. Even today, winter solstice, I got a cool 7kWh out of the panels.

 

The 4am-7am period ends before my setback does, and I'd be reluctant to move it earlier. If the house is too warm, I struggle to sleep. I could charge the battery in this one, but I don't see it compensating for the 4-8pm period.

 

Judging from the messaging octopus has done around heat pumps on their other schemes, they imagine that you're happy to heat the house to be hotter than usual before an interdicted period, and leave the pump off during it as the house cools back down to what would usually be your thermostat temperature. I'm sure that works for some, but I really don't like an overly warm house, day or night. During the day, the thermostat is 20°C and during a saving session I let it cool down below that. I can always pop on another layer, after all.

 

[ProDave]

47 minutes ago, Dreadnaught said:

I am presently negotiating with the wholesaler supplying my ASHP to swap the 5kW model that I originally ordered for the smaller 3.5 kW model exactly for this reason.

I would not go lower than 5kW otherwise I think the DHW reheat times would be too slow.  They are already much slower than a gas or oil boiler.

[Dreadnaught]

53 minutes ago, Luke1 said:

Do you know what the 5kW model can modulate down to?

 

The heat-pump model I am buying is the Vaillant Arotherm Plus. At 2º air temperature and output water at 35º the 5kW heat pump can modulate from 3.00 to 7.40 kW. Note that higher top figure is a lot higher than the nominal "5kW". Conversely, the 3.5 kW model can modulate from 2.10 to 6.90 kW. The house I am building is close to passive levels of performance and I predict that the space heating demand at 0º ambient temperature is around 2kW or so.

Edited December 21, 2022 by Dreadnaught

[Dreadnaught]

18 minutes ago, ProDave said:

I would not go lower than 5kW otherwise I think the DHW reheat times would be too slow. 

 

My DHW cylinder is a matching Vaillant UniTower (190 litres). I calculate to heat that tank from stone cold (8º) to 55º would take 1h 39min at 5kW, which is well within the power output of both the "3.5 kW" and "5.0 kW" models (see above).

 

Obviously you'd rarely need to heat the whole tank from cold. More realistically, two showers (say 80 litres in total) would take 42 minutes at 5kW to heat to 45º, which is the likely tank temperature I would choose (unless I had visitors staying).

 

(This assume 100% heat transfer from the heat pump to the hot water tank. No losses. So the figures are under estimates). 

Edited December 21, 2022 by Dreadnaught

[JohnMo]

17 minutes ago, Dreadnaught said:

I calculate to heat that tank from stone cold (8º) to 55º would take 1h 39min at 5kW

Think you may be disappointed looked at the user manual and it says with 5kW the reheat time is over 3hrs.  Which may be down to the small surface area of the coil.

 

 

[ProDave]

20 minutes ago, Dreadnaught said:

My DHW cylinder is a matching Vaillant UniTower (190 litres). I calculate to heat that tank from stone cold (8º) to 55º would take 1h 39min at 5kW, which is well within the power output of both the "3.5 kW" and "5.0 kW" models (see above).

 

Obviously you'd rarely need to heat the whole tank from cold. More realistically, two showers (say 80 litres in total) would take 42 minutes at 5kW to heat to 45º, which is the likely tank temperature I would choose (unless I had visitors staying).

You have a very different household to me.  At times our 300L tank is barely enough.  Our showers can run in excess of 10L per minute so you are expecting a woman with long hair to shower, wash, rinse and condition their hair in 8 minutes.  You have not met my wife and daughter.  Double that time and water consumption and my 300L tank barely does 2 lady showers, certainly not if it has done 1 much shorter man shower first.  Tell them to turn it down to a trickle and they don't see why they have to put up with sub standard.

 

And nearly 2 hours to re heat it "sorry dear you have to wait 2 hours if you want a shower as well" lands you in the dog house.

 

Oh and most ASHP's (probably all) stop heating the house when they heat the hot water.  Ours likes to only do DHW in half hour chunks.  You can fiddle with parameters to make it do DHW for longer before reverting to space heating.

 

 

[JohnMo]

Plus you have an anode to replace 😥

[Dreadnaught]

30 minutes ago, JohnMo said:

Think you may be disappointed looked at the user manual and it says with 5kW the reheat time is over 3hrs.  Which may be down to the small surface area of the coil.

 

Thanks @JohnMo. I am reading that now. DIN EN 16147 seems to be a set of standard testing parameters but unfortunately it seems one has to pay £££ to read the details online. The target temperature does not seem to be specified in the manual you pasted but I suspect it's quite high, it could be 70º given how long it takes. Of course, charging to lower temperatures would take dramatically less time. (I am not worried about Legionella at all).

 

Edited to add a correction: the temperature is indeed specified in the manual: 49.9º. 

 

29 minutes ago, ProDave said:

At times our 300L tank is barely enough. 

 

My life is unimaginably simpler than yours sounds 😄. Out of interest, what temperature do you keep your tank at?

Edited December 21, 2022 by Dreadnaught

[ProDave]

8 minutes ago, Dreadnaught said:

Out of interest, what temperature do you keep your tank at?

48 degrees, found by experiment as the hottest task you normally have to perform is kitchen washing up and with no cold added, I can just, and only just put my hands in the water without it being painful.  I see no need for any hotter than that, but if such a need arrises, there is the boiling water tap.

[Dreadnaught]

46 minutes ago, ProDave said:

48 degrees

 

Off topic. I have been reading online that shower mixers need a minimum hot-water-feed temperature of 55º to 60º to function correctly. Does keeping your tank at 48º cause your shower mixers to go haywire 😄?

[SteamyTea]

2 hours ago, Nick Thomas said:

My PV generation overlaps the 1-4pm period; that's already sufficient to run the heat pump and charge the battery, so it'd only really be useful on cloudy days. Even today, winter solstice, I got a cool 7kWh out of the panels.

That is why they are offering cheap power I suspect.  A lot of the people that take it up will already have PV.