Can I improve my COP?

QrizB

Thanks for the info. I know buffer tanks can reduce COP and have mostly gone out of fashion (replaced by volumisers) but beyond that, I'm getting out of my depth with guessing what's causing problems with your system.

Hopefully one of the refulars with more hands-on experience of a Daikin heat pump will be able to provide more help.

I think @matt_drummer is achieving COPs of greater than 4 with an Octopus-installed Daikin, although he had to do a lot of work to get it to its current state. He might have some pointers - or you could read though his previous posts on the subject?

Edit to add:

I think this is matt's system:

https://heatpumpmonitor.org/dashboard?id=72

Here's his current stats:

NedS

What time of day are you heating the hot water? Is it on a schedule or just on demand.

It's more efficient to reheat a cold cylinder rather than topping up a warm cylinder, so is generally more efficient to reheat from cold once per day if you are able. You can then schedule that reheat to occur during the warmest part of the day, generally around 3pm, when the heat pump will be most efficient. The exception would be where any ToU tariff savings would more than offset any gain or loss in efficiency, for example a cheap overnight tariff.

You can try adjusting the Weather Compensation (WC) curve down to the point where the heat pump is just able to maintain the desired temperature so you are running with flow temperatures as low as possible. How low you can go will depend on the size of your radiators, bigger radiators put out more heat so will require lower flow temperatures for the same heat loss.

As others have said, having a fully open system (TRVs all fully open and no rooms turned off) works best.

A buffer tank will hurt efficiency - you will see a temperature drop across the buffer. So, for example, a heat pump LWT of 50C may lose 5C across the buffer and you only end up with a LWT of 45C at the radiators. You can minimise the temperature loss by minimising any mixing effects in the buffer by ensuring the buffer is piped up correctly and that the flow rate across the buffer are the same by adjusting the primary and secondary pump flow rates. Generally though, it would be better in most cases to remove the buffer and ensure the pumps are of sufficient quality to move the water required to heat your home.

Nick_Dr1

Do you actually trust the data you are using to work out your COP? If its from the Dakin system, it has a reputation for not being reliable.
COP is a bit of a red herring anyway. Is your house warm enough and are you happy with the overall cost over the year?

matelodave

Nick_Dr1 said:

Do you actually trust the data you are using to work out your COP? If its from the Dakin system, it has a reputation for not being reliable.
COP is a bit of a red herring anyway. Is your house warm enough and are you happy with the overall cost over the year?

I agree, I was watching a video on internal COP rounding errors which when added really hammered the displayed COP so you really need to use a proper energy monitoring system (MID heatmeter) rather than relying on the heatpump display. 

TBH its a bit like the difference between the mpg display on the car dashboard and doing it properly by brim-filling your tank and doing your own sums. There's usually quite a big discrepancy and very seldom do they correlate with the manufacturer's quoted figures which are measured under laboratory conditions rather than out in the real world.

This is an interesting read - https://ukradiators.com/blogs/energy-saving-guides/is-the-cop-on-a-heat-pump-accurate.

In the end the only way to improve the COP is to reduce the flow temp to the lowest you can get away with (both for hot water and heating). Run the machine for longer without using external controls. The car analogy is appropriate again.
Cruising at 50mph is much more economical than thrashing along at 70-80 (reduce your flow temp). Use the accelerator to control your speed (like weather compensation controls your heating requirements) . Using an external thermostat (even something like Hive) is a bit like controlling your car speed by using the ignition switch.

In the end, don't get hung up on COP but tweak the system so you are comfortable and the running costs are manageable. My system runs so much better now I've taken all the controls off, adjusted the weather compensation to balance the heating load and then I let it get on with it. 

Netexporter

With outdoor temperatures being in the mid-teens, this week, your flow temperature should be at its lowest. If it isn't in the low to mid 30s then adjust the lower limit of the compensation curve.

Alnat1

"It's more efficient to reheat a cold cylinder rather than topping up a warm cylinder"

I heat the water once a day. Heating my tank from cold to hot would probably use around 3kWh but usually my tank is still warm, 30-35C when the daily schedule kicks in at 10pm. It generally uses 1kWh to get to 47C.

I could run off all the tepid water and heat the tank from cold, use more electricity but get a better COP. I don't consider that efficient though.

Strummer22

Alnat1 said:

"It's more efficient to reheat a cold cylinder rather than topping up a warm cylinder"

I heat the water once a day. Heating my tank from cold to hot would probably use around 3kWh but usually my tank is still warm, 30-35C when the daily schedule kicks in at 10pm. It generally uses 1kWh to get to 47C.

I could run off all the tepid water and heat the tank from cold, use more electricity but get a better COP. I don't consider that efficient though.

I do think some of the advice here is along similar lines - it may improve your COP, but make little difference to overall energy consumption and cost, which is presumably what you want to minimise. 

For example, opening the radiators in rooms that you don't need to heat will just waste heat, no matter how efficiently that heat was generated. You've indicated that the volume of the system excluding these radiators is enough to prevent short-cycling, so I would advise that you don't waste the heat! 

Using the lowest flow temp that can keep you warm is good advice though. Also, if your heat pump has an inverter and a limiter (ours has a limiter under something called 'sleep mode' - to make the compressor quieter). I've found that the compressor is more efficient at 50% power than being allowed to run at full whack (it tended to do this for DHW, and when starting up for CH). It takes longer to reach target temp but that's not noticeable in day-to-day use. If your heat pump has this option and is still effective with the limiter in place, try this to see if the COP improves.

NedS

Alnat1 said:

"It's more efficient to reheat a cold cylinder rather than topping up a warm cylinder"

I heat the water once a day. Heating my tank from cold to hot would probably use around 3kWh but usually my tank is still warm, 30-35C when the daily schedule kicks in at 10pm. It generally uses 1kWh to get to 47C.

I could run off all the tepid water and heat the tank from cold, use more electricity but get a better COP. I don't consider that efficient though.

That's not what I was suggesting.

It will be more efficient (and cost less) for you to reheat your tank from 30C to 47C once per day as you are doing than it would to reheat your tank to 47C every time it drops to, say, 40C. Some installers set the tank to reheat automatically based on a preset hysteresis (7C in this example). This will be less efficient than reheating from a cooler temperature once per day, but runs the risk of running out of hot water, hence why installers often leave the tank set to reheat automatically.

Exiled_Tyke

Thanks for all the helpful advice and sorry for the delay in responding. It's a busy week for me.   A couple of thoughts from all this. 

1. Yes, water is heated once a day, overnight on TOU tariff.  I've experimented with how low I can set it and I think have found the optimum.  Interestingly the installer did indeed have it set to reheat during the day, which I changed almost immediately after he left. 
2. The big question for me remaining, I think, is do I run for longer (e.g. all day) at lower flow temperatures and possibly setting the ASHP to an even quieter setting (I note that the quietest one should be avoided as it won't run a defrost cycle on sub-zero days)?  Currently I have the system switching on (or to be more precise,  lifting the temperature from its set back) in the middle of the afternoon (when there is the greatest chance of a bit of spare, PV available in winter).  But would I be better of simply running all day and throttling the system back further?  I guess there are no easy answer to this one.  

NedS

Exiled_Tyke said:

2. The big question for me remaining, I think, is do I run for longer (e.g. all day) at lower flow temperatures and possibly setting the ASHP to an even quieter setting (I note that the quietest one should be avoided as it won't run a defrost cycle on sub-zero days)?  Currently I have the system switching on (or to be more precise,  lifting the temperature from its set back) in the middle of the afternoon (when there is the greatest chance of a bit of spare, PV available in winter).  But would I be better of simply running all day and throttling the system back further?  I guess there are no easy answer to this one.  

Set backs are complicated, as there are many factors that are unique to your property and heating system. There was an excellent discussion here by retired physicist Michael de Podesta:

https://protonsforbreakfast.wordpress.com/2022/12/19/setback-should-you-lower-heating-overnight/

which concluded that there's not a lot in it, and whether or not it will save money probably depends on how efficient your heating system is when output is higher to recover the lost heat compared to continuous running.

To summarise, all else being equal, running a set back and then later returning to normal levels requires less heat overall (and therefore may cost less). However, if you are having to thrash your heat pump to recover from the set back, that may more than offset any savings you just made, so it depends.

A good analogy here is a car on a motorway trip. Driving at a constant 50mph is more efficient than driving half the journey at 40mph (setback) and the other half at 60mph to make up the lost time.

Conversely, if you have an oversized heat pump, you may find it easy to increase heat output to recover the setback whilst still operating in an efficient window. To use our car analogy again, imaging that car journey in a car with a top speed of 60mph vs a car that can easily do 120mph, where doing 40-60mph is all in the same efficiency range so makes little difference.

Of course the study above only really considers energy use and efficiency. It does not take into account ToU tariffs where applying a set back during an expensive period, and then recovery during a cheap rate period would almost certainly provide monetary savings even if the recovery were less efficient, as this would be more than offset in monetary terms by the ToU savings.