Heat pumps

matt_drummer

Hi Oliver,

At the moment I am trying to get the heat pump running as efficiently as possible, that is get the most heat out of it as possible at the minimum electrical consumption.

I am making changes to the heating system to match the performance of the heat pump.

Once I have finished and I have the heat pump running as efficiently as I possibly can, that is in terms of COP and not cost I can then concentrate on getting the best balance for me between cost, COP and home comfort.

Cost isn't really an issue, as long as my solar exports during a year pay for what I import in the winter I don't care too much, I know it is still money but it's not money I actually have in my bank so I ignore it. I could, of course, get a refund of some of my credit balance.

The house runs from mains and batteries depending on load, the batteries are charged during the Cosy Octopus cheap periods..

My total electricity bill including the standing charge was £200 from 10 November to 09 December, not excessive considering how cold it was, how hot my house is and the amount of experimenting I have been doing. I would have spent more last year when I was on gas.

For the last week or so the heating has been running at a COP of about 4.25 so at my cheap Cosy rates and allowing for battery losses we could say my electricity cost might be 20p per kWh as I also generate some as well.

That makes my heating 4.71p per kWh

I don't know how much gas is but if gas is now 7p per kWh (?) and a boiler is 90% efficient then gas is 7.78p per kWh of heat.

My heat pump, with my batteries and solar is considerably cheaper at the point of use, I do know I had to buy the stuff to do it before anybody wants to remind me!

I have used more electricity in the last two months than I needed to just to heat the house, but I view this as part of the capital cost of setting the system up.

Efficiency is important to me, I did this as a project to see what I could do in terms of running a heat pump, indeed the whole house, entirely from solar power. It works but I would like to get the most efficiency I can out of the heat pump, just because I can and because getting the most heat I can from every kWh of electricity makes me happy.

I don't have a room thermostat as such, I have a Madoka controller in the hallway but it is more than a thermostat.

It can work like one but it does influence how the heat pump runs and I am not sure I like it how it has been working.

I will try again with it later.

Ideally I just want the heat pump to run continuously and the house temperature is controlled entirely by the temperature of the water flowing through the radiators.

It was another reason for the heat pump, I wanted continuous low heat , not huge blocks of heat for 15 minutes at a time, getting hotter and then waiting for the house to cool down before you get another 15 minutes of heat. I have never liked that about gas and oil and just want this to be different, It's another reason why I am trying so hard to get to a point where I can just leave it on a weather dependent curve and forget it.

That is how it is running at the moment, the efficiency is pretty good but it will get better after the next modification.

Even with water flowing at 32c through my radiators the heat pump produces far too much heat for the house so it gets hot, so I just run it for a few hours and then stop for a while.

I could just run it for 30 minutes in every hour but that means all the run time is in the worst part of my heat pumps heating cycle.

It takes 60 minutes to get into its stride and settle down.

I have one more modification to make and then I will concentrate fully on the settings, there is no point going too  far until I have finished my work.

If I had just turned my heat pump on ran it at the lowest flow temperature I could and then only ran it until the house got to a set room temperature then I think I would have used a bit less electricity than I have in the last two months, I'm not really sure as I haven't worked out what it might have been it total.

If I hadn't put the heat meter on it then I would have very little idea what was going on, my efficiency would have been poor and my heat pump gives basic data on that. But the house would have been warm enough and the electricity consumption wouldn't have been horrific most of the time.

I would never have known how badly it was actually working in terms of efficiency.

In the end I think the electricity cost would be 50% more over its lifetime than it will be now, quite a difference.

I would also have struggled to meet the original design temperatures in the weather we had a couple of weeks ago, I need to flow water at 50c at -3c outside because of radiator size, if the heat pump had ever got there, which I am not sure it would have done without the back up heater, then it would have cost a small fortune.

The back up heater is basically a kettle inline of the hot water outlet from the heat pump.

I had trouble with the back up heater for a couple of days, it comes on without you knowing even when it wasn't very cold outside, it consumed an awful lot of electricity as you might imagine!




The key to understanding heat pumps is that unlike a gas or oil boiler, 1 unit of fuel produces a huge variation in heat output, you know what you are getting from a cubic metre of gas or a litre of oil within a bit, what you get from 1kWh of electricity varies enormously depending on many factors.

matt_drummer

Hi Oliver,

To run a heat pump using the least electricity possible you need to heat the water to the lowest temperature you can.

We all know this, heating up the water less costs less, simple physics.

So I can only tell you about my heat pump in my house, the principle is the same but the numbers will be different in other houses.

What we know about my heat pump is that its minimum electrical input is 900w, it doesn't not matter what I do, how low I turn the water temperature down, this is the least it will ever consume whilst running.

In tests this model of heat pump gets a COP of 5.0 when it is 7c outside and the water flow temperature is 35c

Great, that's what we want then isn't it, a COP of 5.0, it's available and I would like that sort of efficiency.

The heat produced from 900w at a COP of 5.0 is 4,500w, awesome.

Except when it is 7c outside my house only loses around 2,500w of heat.

Anyway, we give it a go, but when I flow the water at 35c in October I don't get a COP of 5.0. I don't get 4,500w of heat and I am using more than 900w of electricity, something has gone wrong!

It's my radiators, whilst they can keep my house warm, even too warm with 35c water flowing through them they cannot deliver 4,500w as they are not big enough.

After some calculations (which I had actually done before the heat pump was installed) it turns out my radiators can only deliver about 3,000w of heat at 35c flow and that drops as the house warms up (the heat output of radiators depends on the difference between the flow temperature and the room temperature)

The situation gets worse as the house gets hotter, the heat production falls and the electricity consumption increases and eventually the water going back to the heat pump is too hot and the heat pump has to stop.

Then it tries again, uses a lot of extra electricity in the first 60 minutes or so and then settles down to an efficiency between 3.0 and 3.5, it's OK but it is not the claimed efficiency of 5.0

Then I forgot some of the things I knew, panicked a bit and thought if I can only get a COP like this when it is quite warm how bad will it be when it is really cold?

But with some help, and when I had calmed down I explored changing the heat pump for a smaller one, I still might but probably not.

I decided that my radiators needed to be much bigger so they are now about 200% of the capacity they were and I can now run a low flow temperature and get mot of the available heat production out of the heat pump.

I don't know if it is Daikin only behaviour, I suspect not, but once you are at the minimum electricity input then the heat output is determined by the ability of your home to take it, in my case the radiators could not take the full heat produced so the heat pump produces less, the problem is that as the electricity input is at it's lowest there are no savings to be made.

From what I can see, the heat pump adjust the flow rate and the dT (difference between the flow and return temperatures) to try and keep producing as much heat as it can for as long as it can but as the radiators are the limiting factor you don't get all the heat output even though you you are consuming enough electricity to produce more heat, the heat pump just can't do it and that is the problem with oversized heat pumps, undersized radiators and too low flow temperatures.

For a time I had to run at a flow temperature of about 43c to get a respectable performance a COP of about 3.5, but this uses 50% more electricity than running at 35c flow, I didn't want to carry on doing that.

The best way to deal with it is find somewhere for the extra heat to go, fortunately I have a plan for that, otherwise it is just wasted.

The other smaller issue which is more a Daikin thing than some other heat pumps is that so far it takes a long time to settle down, you can see from the graphs the electricity performance in the fist part of a heating cycle.

The COP isn't always that bad in this initial period but when you run off batteries will a limited power output I don't want 3+kW going out of the batteries when I could limit it to 900w if the heat pump runs for longer.

I hope it makes sense.

I'm not an expert in heat pump design or heating system design. These are just my experiences and findings from quite intense following of the data, talking to people who know stuff and experimenting.

Oversized heat pumps are pretty common and they all suffer from the same issues.

It is just this particular model of heat pump has quite a limited range of heat output and a long warm up period making it a little tricky to deal with.

Had I known what I know now I would never had let Octopus install this heat pump here.

I made a mistake, I foolishly believed that although I knew the heat pump was too big I could just turn it down and run it really efficiently and it would all be great.

You wouldn't really expect a 9kW heat pump to have an effective minimum output of 4.5kWh (if you want it to run at the quoted efficiency) and a maximum output when you need it of only 6.3kWh.

This is the other thing, when it's a bit cold, it doesn't always have to be that cold, it spends 30% of it's time defrosting, so not producing any heat.

So, to sum up, I have a heat pump that is too big for my house when it is over 5c outside and a only really just big enough when it is -3c outside. It probably does better when it is even colder as the defrosts stop.

It's really limited in what it can do and making it fit your home and lifestyle whilst trying to get the best efficiency you can is pretty difficult.

I would never buy another Daikin heat pump, I didn't want one in the first place, I wanted a 7kW Vaillant but couldn't find an installer. I have the MCS and Ipswich Borough Council to thank in part for this.

It has been a stressful journey and not something I would like to do again. Thankfully I know so much more now than I did a year ago.

oliver1951

Matt, that’s all really informative. I’ve now read it about 3 times and I’m going to have to read it a few times more to fully comprehend it. It’s British Gas (BG) who’ll be doing my install, if the council ever give PP. They will install a daikon altherma 3, but an 8kw. I don’t have anything line an EPC a, it’s an EPC E rating. Although some of that rating is based on there being a very old as in 1970 gas boiler and a hw cylinder that has no thermostat! 

I’m really interested in your observations about the radiators and their capacity to push out the heat. Did octopus install the radiators or were they existing? I’m having everything replaced, a total re-pipe and new radiators, because the existing system is a one pipe. I’m not sure what my minimum power consumption is, probably just a little less than yours. But the thought of that being consumed 24/7, would be quite terrifying. 

The rads they will be fitting are HUGE , and I would be aiming to have a low water temp. Also my aim for house temp would be around 16oc , which will sound low to a lot of people, but currently it’s around 12, but only in the warmest room. So based on your experience I’m guessing it’s probably right sized, it’s the running costs I’m uncertain about. Looking at that link you gave me, it did seem that the daikins didn’t seem to be achieving the best cops. But also, and again bearing in mind your point about the rad sizes and ability to use the heat generated, how is the cop actually measured, how did you figure out you were running at a lower cop rating etc.

That is all great info and I’m going back to re read it again. Thanks for the time taken to respond. I really appreciate it. I did ask BG what info they had on their installations and customer experience info and I was told they had none and if they did they wouldn’t be able to share it. Not exactly comforting.

matt_drummer

Hi Oliver,

I can change my heat pump for the 8kW, it's a much smaller heat pump, it's different class altogether and probably more appropriate to my heat loss most of the time.

It's minimum power consumption will be lower than mine.

The 9kW is actually a de-rated 16kW heat pump and that is where the issue lies, the four heat pumps in my range (9, 11, 14 and 16) all have the same minimum electrical input, the same compressor and the same circulation pump.

There are three in the range you are being offered, a 4, 6 and 8kW, they are all the same so there is no point in having anything other than the 8kW.

Running at a house temperature of 16c means you won't need to run for long periods I would imagine.

They will not design it to run at that temperature though, the MCS require the room temperatures to be higher than that. As you will be getting a grant the system has to be designed to a specification that will suit any owner of your home in the future who may not be happy with 16c.

How big is your house and what is the heat loss?

It is the heat loss that really matters, they tend to overstate them leading to oversized heat pumps.

I suspect that won't be an issue for you, the 8kW should be fine although make sure it is actually big enough.

Octopus only replaced two of my radiators.

I have now replaced all of them including the ones replaced by Octopus, they just weren't big enough.



Looking at my heat pump output of 4,500w I need radiators to put out that much heat when my house is 22c and the water flowing through the radiators is an average of 32c.

That means the difference between my room temperature and radiator temperature is 10c, called a delta t of 10c.

Radiators are sold in this country with a heat rating based on a delta t of 50c, that would be mean flow temperature of 70c with a room temperature of 20c.

I currently have radiators with an output of 40kW at a delta t of 50c which means that at a delta t of 10c they are capable of delivering 4,800w of heat. Enough to run my heat pump at 35c flow temperature.

At installation I only had 18kW of radiators, they looked massive but they were less than half of what was required to run efficiently at a low flow temperature.

It is also important to match the size of each radiator in a room to the heat loss of the room at the required room temperature.

Ideally you don't want zones or trvs reducing the available heat output but you can only do that if the radiators give out heat in proportion to each other relative to the rooms they are heating.

If some radiators are too big for the rooms then those rooms will get too hot or the radiators have to shut, which reduces potential heat output and damages efficiency.

I have no trvs or zones, the whole lot is permanently open and the radiators are balanced with Danfoss RLV-D lockshields.

Radiator balancing is important, each radiator needs to work fully as designed.

Maybe tell us what size radiators you are planning on fitting?


The COP is the relationship between the heat produced and the energy consumed in producing it.

1kW of electricity giving heat output of 4kW is a COP of 4.0

The Daikin controller does give basic information on electricity consumption and heat produced but not in enough detail to really work out what is going on. It is probably enough for most people.

You can extract the information from the heat pump using something like ESpAltherma and then analyse the data in more detail.

If have electricity meters and a heat meter installed.

To know what the COP is you only need to know the electricity consumption and the heat produced.

The heat produced is calculated by the heat meter in my case.

The heat pump also has this information and can work out the heat produced.

Water has an energy value like everything else and if you know how much of it flowed through the house and what temperature it was when it arrived at the house and left the house then you know how much energy it lost in the process. That is the heat delivered to the house, the heat produced by the heat pump.

It takes about 1.16Wh to heat 1kg of water by 1c.

If 1,000 litres of water flow through my house in one hour and it comes in at 35c and leaves at 29c we can work out the energy lost by the water cooling down.

1,000 litres is 1,000kg of water so it is 1,0000 x (35 -29) x 1.16 = 6,960Wh

That is 6,960 kWh of heat 

If that used 1.50 kWh of electricity then the COP is 4.64 (6,960/1,500)

MultiFuelBurner

Hey Matt one comment I wanted to make was we looked at Cosy Octopus for our heat pump but don't have the batteries and Solar that you do to make it work for peak times.

I was wondering if you have compared what the cost would be on Agile? It may be a switch to Cosy when Agile prices are high then back to Agile when low. I don't think either have restrictions on swapping.

matt_drummer

Hi MFB, I hope you are well.

I used Flux during the summer as it gave me the best export prices and I import nothing for about 8 months of the year.

I would have just gone back to E7 with Octopus if I could but there seems to be an issue with that switch so I just chose Cosy at it gave six lower priced hours and apart from 16.00 to 19.00 the rest of the day is SVT rate.

I don't have to think about it too much but I could probably do better. I have an EV but no charger at home so some other tariffs are not available.

Right now I could do without the distraction of a variable electricity tariff.

I used to build plastic models and play the drums until I got a heat pump!

I will look at it at some point but my biggest gain is getting the most out of the heat pump at the moment and it is what I have been focusing on.

It would have been much easier if my heat pump wasn't the one I have.

matt_drummer

I haven't written down anything like this before but I will try to sum up and illustrate where I have go to with my heat pump. I need to do something like this to work out the most efficient overall way to run it now I know how it behaves and what it is capable of.

I apologise in advance, it may not be that easy to follow!

If I pick a day where it is reasonably cold outside, say an average of 2c and I want my house at 21c  on average I need something like 108kWh of heat. I will ignore DHW production as you can't heat the house and DHW at the same time and it will jusy over complicate it.

On my old radiators at 35c flow temperature my heat pump would produce around 3.5kW of heat, not enough to keep the house at 21c all day, the heat loss exceeds the heat production.

I would need to run at 45c flow temperature to produce the 4.5kW of heat I needed I would need to run continuously to do this and That would require about 1.8kW of electricity so my daily electricity consumption would be 43.20kWh and the COP would be 2.50, not great.

On my new radiators I can produce 4.5kW of heat at 35c flow and I only use 1kW of electricity to do this so my electricity consumption for the day is now only 24kWh and my COP is 4.50. 

The radiators make a huge difference.

A cold day is easy to deal with where the heat loss of my house matches or exceeds the minimum output of the heat pump.

If we now look at a day where I only need half the heat to keep my house at 21c on average I only need 54kWh of heat. Now my heat loss is only 2.25kW, half the minimum heat output of my heat pump.

If I run the heat pump all the time the house will get too hot if the heat pump is producing heat at its full minimum output of 4.5kW.

On my old radiators I got about 3.5kW of heat at 35c consuming 1kW of electricity. So if I ran it for about 16 hours I would produce 54kWh of heat and consume 16kWh of electricity and the COP would be 3.38. 

My old radiators would give me a COP of between 2.50 and 3.38 in typical winter conditions, an average say of 2.94, not very good compared to others.

With my new radiators I have choices.

I can always run at 35c now but my heat pump consumes more electricity in the first hour of running than it does in the subsequent hours. To produce 4.5kW of heat I use 1.5kW of electricity during the first hour and only 1kW after that.

My most efficient way to get 54kWh of heat is to run my heat pump for one heating cycle of 12 hours (54/4.5)

That would use 12.5kWh of electricity and the COP would be 4.32 (54/12.5) but the house comfort wouldn't be ideal

I could heat in two blocks of six hours. That would use 13kWh of electricity ((2 x 1.5) + (10 x 1)) at a COP of 4.15 (54/13)

I could heat in four blocks of three hours and that would use 14kWh of electricity ((4 x 1.5) + (8 x 1)) at a COP of 3.86 (54/14)

I could heat in six blocks of two hours and that would use 15kWh of electricity ((6x1.5) + (6 x 1)) at a COP of 3.6

Or I could heat on for one hour and off for one hour that would use 18kWh of electricity (12 x 1.5) at a COP of 3.0 (54/18)


It is clear that the first hour of running is the most expensive, minimising the first hours has a massive effect on overall electricity consumption and COP. 


This is the problem with an oversized heat pump when it is not that cold outside, when the minimum output of the heat pump exceeds the heat loss of the house.

The most efficient way to run the heat pump is for as long as possible, at the moment that means the house gets warmer than we might otherwise choose.

I have over simplified it but I think you should get the idea.


I have choices, get a smaller heat pump, accept that the house will be warm, run at a lower house temperature and accept the reduced efficiency or find somewhere for the excess heat to go.

I have chosen to keep the heat pump, accept a slightly higher house temperature at times and I have found somewhere for the excess heat to go.

If it works out I should be heating a place that I currently heat by other means for no additional electricity use from my heat pump and a house that is easier to keep at a sensible temperature.

oliver1951

Matt, a lot of information again. I’ll dig out my figures and reply on that later. Where are you going to put the excess heat. I’m still not entirely clear about that aspect of excess heat. 

matt_drummer

Hi Oliver,

I have the heat pump running at the moment at 35c flow using 9020w of electricity and delivering 4,500w to the house.

The water flowing through my radiators at the rate it is and the difference between the incoming and outgoing temperatures is 4,500w of heat.

I have a 20m2 timber building on the back of my garage that I currently heat in another way.

If I put enough radiators in there to deliver 1,000w of heat to that building then only 3,500w will be going to the house instead of the 4,500w currently.

That may be hard to understand but all I am doing is increasing the size of my house (and therefore the heat loss) and adding extra radiators to deliver the heat produced by the heat pump.

The water capacity of the whole system increases so as long as the flow rate and the difference in temperature between the incoming and outgoing water remains the same then the heat produced will remain the same, it is  just that the heat will be distributed over a bigger area, less of it will be in the house as some of it will now be in my timber office building.

My heat pump won't use anymore electricity, I may have to run it for longer to keep my house and the office warm but that is exactly what I want as my heat pump is more efficient the longer it runs.

Overall I will be better off as I won't need to spend electricity running the current heat source of the office.

The plumbing is a little tricky though.

Although the circulation pump in my heat pump is massive and designed for houses two to three times the size of mine I am not convinced that the water flow will be good enough just using this.

So I plan to add a circulation pump to feed the office radiators but that needs hydraulic separation so I am going to do that using a low loss header. Rather than just feed the radiators in the office directly from the heating circuit as you might when just adding an extra radiator I am going to feed a low loss header that can then be pumped to the new radiators.

That new pump should match the flow of the one in the heat pump but to avoid any conflict between the two pumps I have decided that hydraulic separation is the way to go.

It's an interesting project for me, it may not work as I am no heating engineer, but I think it will. We'll see in due course.

Hopefully, what I will be able to do is heat the office when it is mild to take heat from the house that I don't need and then if it gets really cold and I want more heat in the house I can just shut down the radiator supply to the office and heat that by other means as I do now.

oliver1951

I seem to have misplaced my paperwork. I have asked BG to forward me a copy. To my recollection the heat loss calc gave just under 8kwh, based on -2oC etc and with a house temp of around 21oC . The house is 130sqm habitable per the EPC.