We’ve received numerous questions recently about the hourly consumption of our air source heat pump (ASHP) from people that are considering switching to energy providers that offer flexible, smart metre tariffs.
To help you get a better understanding of our consumption, we will be posting our data on a monthly basis over the colder autumn and winter months to see where our electricity peaks and troughs occur.
We will preface our first post by saying that we are not on a smart metre. We have an old school electricity metre, and we have a competitive fixed tariff with our provider.
To get the ball rolling, let’s deconstruct October’s numbers. The data provided below shows how many kWh of electricity were used to drive the pump on each day in October.
To give you a more detailed insight, we’ll take a closer look at October 26, 2020, which was the day that our pump worked the hardest. In order to do that, we’ve provided the data below from our SolarEdge app, which shows our total electricity consumption as a household on an hourly basis.
On October 26, our ASHP used 49.9 kWh of electricity over a 24 hour period. According to SolarEdge our overall consumption was 59.7 kWh (11.6 of which was offset by our solar panels).
If you haven’t watched our videos, what you need to know is that our heating runs 24/7 to maintain a base temperature of 21C across most of the house. This also helps maintain steady temperatures for the underfloor heating.
The first spike at around 9:00 is the increase in temperature to 22C in our main living area downstairs, and it also reflects the water reheating after our morning showers.
The increase in electricity consumption from 16:00-21:00 involves heating the TV room to 23C (which is in the coldest part of the house) followed by the master bedroom, and also takes cooking into account using the oven for dinner (which resulted in the significant spike at around 18:00).
From the graph above you can ascertain that when the ASHP runs 24/7 the consumption pattern is constant. The peaks in this pattern rise and fall from day to day, and are based on the ambient outdoor temperature and how much electricity the ASHP needs to deliver the required heating for our property.
You can draw further insights from our ambient outdoor temperature for October 26. There wasn’t a huge variation in day and nighttime temperatures, which is why our ASHP never really exceeded 2 kWh per hour.
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Another interesting post thanks.
So my two cents about your heat pump energy use, your house seems to be very ‘heavy’ thermally and so sometimes a change in outdoor temp doesn’t cause a reduction in compressor energy use until the next day. But if we focus on a few days where the outdoor temperature was roughly stable say around Oct 11, then, 32KWH on the compressor assuming a COP (efficiency multiplier) of 2.5 then you produced 80KWH of heat. Let’s deduct 6KWH for hot water use, so 74KWH of heat for heating. Assuming your target house temp was 21C by underfloor heating and the outside temp was 7C then that’s a temperature difference of 14C on average. To maintain that temperature difference you needed 74KWh of heat, so doing the maths you need 74/14 = 5.3KWH of heat for each degree of difference indoor-outdoor temperature. Going back to the COP of 2.5 that means your heat pump should be drawing/consuming 5.3/2.5 = 2.1KWh (or 2.1KW continuously for an hour) all day long and this seems to be right from your second chart.
If we extrapolate what this means for freezing temperatures outside, if the outdoor temp is 0 and your indoor is 21C (average for the day), then you need 21*5.3=111KWH of heat for that day. Assuming a COP of 2.5 then 111/2.5=44.5KWH of compressor energy consumption per day. Assuming 30 days a month then 44.5*30= 1,335KWH of compressor energy use per month. This is not far off what I think you reported for January this year. To put this in context my 1980’s surburban brick home would be using 75KWh of gas heating a day (or 2,250KWh in a month like January) so your heat pump is doing really well in reducing your energy use and saving you money (even though you need more heat than this house, you still use less energy overall because of the high COP in the heat pump).
On energy conservation and emissions: oil boilers are really dirty so moving to an ASHP was a much cleaner option (and it looks like your heat pump actually has more than enough spare power and is using less energy than a gas boiler in a ‘modern’ home like mine). Just an idea but it could be worth asking the heat pump to do more on the underfloor circuit (raise the temp of that) that feeds your living room to get it up to 23C given that it has quite some capacity spare. This might save a little on any direct electric heating your are using (underfloor heating is only slowly responsive so I assume you need a hot air fan to create that boost?) . Thanks for reading this.
Thanks for the incredible analysis once again Steffan. I will read through this thoroughly later today, but at first glance, there’s a lot here that’s made me think again regarding our central heating. Great analysis.
So I spent a long time reading your detailed comment last night, and the formulas you’ve presented are really great. I’ve not come across an interpretation along these lines before, and it’s a hugely beneficial way of looking at heat pumps and comparing their performance against other forms of heating. I particularly find the formula how you get to compressor energy usage per month interesting and helpful. It’s just such a great way to understand ASHPs.
In response to your question at the end, we don’t have a hot air fan. Do you mean an electric plugin fan?
Given that underfloor heating only slowly allows you to change temperature, I was wondering how you heat-up rooms specially in your house at certain times, etc in the mornings and evenings etc. It could be you don’t use an electric heating fan, I just has an idea you could use that spare capacity in the heat pump to heat up your lounge to 23C. In an ideal world you could run the heat pump harder during the day to store heat in the building as you have higher air temperatures and solar power working in your favour, rather than at night when you have neither.
I hope your blog inspires others to take the plunge to an air source heat pump. I will definitely point you out to a friend of mine out in the sticks. Roll on the Winter.
For our 14kw Ecodan ASHP for October we used 1164kw/h of electricity. 493kw/h for hot water and 671kw/h for heating.
On the 26th October the ASHP used a total of 47.2kw/h of electricity.
We keep our house fairly warm downstairs as it takes a long time for the underfloor heating to react and heat up the concrete slab. We use the Heatmiser Smart system downstairs and most rooms are set to 24 degrees all day and the hall and the lounge in the original house are set to 25 as these areas struggle to maintain 24 so setting them a degree warmer helps maintain the temperature close to 24. I think the neo thermostats run warm as upstairs we have set to 21 on the honeywell evo system and it feels similar temp wise.
I think the ASHP is great for central heating but not great for hot water. Each month the ASHP consumes between 400-500kw/h to heat a 500l unvented cylinder to 52 degrees with a maximum drop of 5 degrees allowed (otherwise people complain of cold baths). We do have between 6 and 11 people living in the house so consumption is high. £600 a year seems a little steep for hot water but my judgement might be wrong with that.
I did have a quote for a 17kw solar system with solar iBoost that would have worked well with our system but I couldn’t commit to being in this house in 12 years time when the payback would be complete.
Here is a basic spreadsheet for the current running costs/consumption. *Its based off what the ecodan software claims consumption figures are so I can’t verify they are 100% correct as I have no way of monitoring it.
https://drive.google.com/file/d/1uZLsVpahsJQ_7mMUHp9aFHkYciFQcuNe/view?usp=sharing
Thanks for the great feedback. I’ve just realized that with all the numbers and charts, I’ve not shown our overall ASHP electricity consumption for October, which was 1031.50 kWh.
You run your house warmer than we do, but I’m tempted to raise the temperatures because I don’t think we’ll use that much more power. I assume you’re running your pump 24/7.
I don’t actually think your hot water is that bad given the amount of people you have in your home. That’s a lot of showers, baths and washing up. For us, it’s just me and my wife, so our hot water requirements are limited, hence the bulk of the ASHP activity is driven at taking care of central heating.
Do you know what your COP is?
I think our November numbers will not be great because it’s been cold, damp and dark (little solar).
You said you’re not verifying your data. Are you on the RHI scheme?
I’ve not calculated the COP as I’m not quite sure how to? I’ve got the energy usage data from the ecodan app/control panel but the reason why I say its not verified is that in the app it says *estimated so I don’t have a way of confirming the energy usage. Any advice on how to calculate the COP?
I’ve found running the underfloor heating thermostats a degree higher works much better. The installer mentioned that sometimes it can take up to two weeks to fully heat up the concrete slab and you want to maintain it to reduce a peaks and troughs of the temperature throughout the day. When its really cold and I haven’t got certain rooms warmed up enough before the cold weather you end up playing catch up trying to heat that room and then the upstairs radiators will struggle in the evenings if the whole house is calling for heat.
[…] If you’d like to see what our numbers were like last month, please refer to our October figures. […]
Hi, many thanks for your blog, it’s been very encouraging to read about other peoples systems and their energy consumption.
We run 14kw Mitusibishi Ecodan in a 5 bed, detached, old house with as much insulation as we can install but it has solid brick walls and a cellar so is not ideal for a heat pump. It was installed in September and so far we are very pleased with it. We used to burn between 3,500 – 4,000 litres of oil a year depending on how cold it was in the dark months.
However I have a query which I can’t explain and was hoping that those with more experience might be able to help. The system is set to run off a single stat which is set to 19 degrees and has a variable flow and return operating temperature set to 30-55 degrees which the algorithms in the ecodan control depending on external/internal temperature and historical activity.
However every night usually between 2.00am and 3.00am according to the hourly temperature log, the system goes in to overdrive and spikes the flow temperatures to about 60 degrees for about 20 minutes. It then falls back to the required level. I would upload a picture of the spike from the app but I can’t on this website. It have no idea why it’s doing this or if it’s a bug in the system giving spurious temperature readings. It’s nothing to do with the legionella protection as I’ve checked.
Any ideas?
Nick
Thanks for sharing your feedback and comments about your system Nick.
Pertaining to your 2-3am spikes, do they always happen at exactly the same time? We had a spike like this when we installed our iBoost and it was the immersion heater that was set to come on every evening at midnight. Took me ages to figure this out. Do you have an immersion heater with separate settings? That’s one thought. The confusing thing is that you say the flow rate is bumped up to 60C (not the hot water tank). This is an odd one.
The other spike could be a system defrost, but that’s highly unlikely.
Over and above that, the only thing I can think of is that there’s a setting instructing this weird spike. You could drop Mitsubishi a support email and ask.
This is an interesting one, and would be great to hear what the issue is once it’s been solved.
Excellent website!
Thank you Karl. Your feedback is greatly appreciated.
I have the same ‘issue’ as Nick (with the Ecodan) only the spikes are not systematic as in the same time every day, my flow temperature is set at 27 degs, however it more than doubles (reaches 60) for a about a 20 min spell. Ours is a new build which we’ve only been in for a few months and I’ve only just got round to using the app etc. to see performance. Is this something I should be concerned about (looking at the charts alarms me!)