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Mitsubishi 8kW - strange energy use readings

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(@gfletcher)
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243 kWhs
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Hi All,

I'm new to the forum having had a new Mitsubishi Ecodan 8kW installed 3 months ago. I already had PV's, and specified a Mixergy tank alongside the Ecodan. The PVs divert their excess into the tank. I was looking at the Mitsubishi display recently and found the bit where it says how much energy the heatpump has used and it shows that we used 90kWh in Aug, 91kWh in Sept and so far in Oct we have used 14kWh - with the majority showing as 'heating'. I am confused by this for a number of reasons -

1. we haven't had the heating on.

2. We have been self-sufficient for hot water through the PV's from about April to date (even today we have a full tank of hot water).

3. I have checked our half hourly data and that shows that in August we used a total of 122kWh of electricity, and in Sept 146kWh - so if the heat pump is right that means that we were using around 1kWh a day after the heat pump figures are taken into account - we aren't that frugal!

Any thoughts as to what is going on? I don't know where the numbers are coming from. I thought it might be measuring the amount of PV generation diverted but the October figure was 12kWh last night and it is showing as 14kWh now - we have probably dumped about 6kWh into the tank today.

Cheers

Gavin


   
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 HJD
(@hjd)
Estimable Member Member
240 kWhs
Joined: 3 years ago
Posts: 38
 

Hello and welcome to the forum.

We have the Ecodan too and I must say that I take the reported energy usage with a pinch of salt (that is unless you have one of the specialist energy monitoring packs added. We don't.)

Mine also recorded us having used energy on heating over the summer months when it was switched off - although only about 20kwh per month. I wondered if a) the Legionella cycle somehow gets recorded as heating rather than DHW or b) whether the heat pump does use some energy just spinning itself up from time to time and the energy monitor allocates this to heating.

Bizarrely, for me at least, since October started I have been getting COP readings for DHW that are close to 4, which we have previously never got anywhere near...so not really sure what is going on there either!

Anyhow, there is lots of Ecodan expertise on the forum so do reach out if you have more questions.

 

Howard


   
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(@gfletcher)
Trusted Member Member
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Joined: 2 years ago
Posts: 24
Topic starter  

Thanks Howard,

That's a lot of salt!

If it was the odd kWh here or there I could understand, but it is 3kWh a day on average through Aug and Sept - the legionella purge isn't happening, through that period the tank was regularly getting up to 70 degrees for the whole tank, so it wasn't needed. Just seems strange. I suppose it could just be inaccurate measuring of the PV input.

Cheers

Gavin


   
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(@derek-m)
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Posts: 4168
 

@gfletcher

3kWh/day equates to 125W/hour, which isn't a great deal in the scheme of things, but of course adds up over time. If possible check the reading on an hourly basis to see if it increases gradually throughout the day or is created by one or more jumps. 

If you have a Smart meter you could try switching everything off, except for your heat pump, and see if the meter registers any usage. If that is the case then go through all the controller settings, both User and Installer, to see if anything like base heater or pumps are set to operate. Also listen for any motors running. You could also feel around the outer casing for any warmer areas.

Read through the specification and see how much power the electronics use.


   
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(@gfletcher)
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Joined: 2 years ago
Posts: 24
Topic starter  

Thanks Derek. I suppose that from what I know about our household energy use I would notice if we were using an extra 3kWh each day, plus it isn't showing on the half-hourly data. So over August and Sept we used 3.9kWh a day and 4.9kWh a day respectively, that is the raw downloaded data from our smart meter. And this is in line with usage prior to the heat pump (3.8kWh a day in July) - 4 people with devices, cooking, TV etc. So 3kWh a day extra from the heatpump just doesn't show on the HH data - so I am wondering what the heat pump (or the controller/display unit) is actually measuring as 'use'?

Cheers

Gavin


   
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Mars
 Mars
(@editor)
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@gfletcher, welcome to the forums and your numbers definitely don’t stack up. We don’t have an Ecodan, but we often get weird readings when it comes to the iBoost doing its thing diverting energy from the PV to HW.

Because we have a hybrid system, we have two mandatory metres that were required for RHI purposes. These metres are supposedly very accurate and they often tell a different story to what the heat pump claims. 

Today was a prime example. Plenty of solar, heat pump in standby all day. After our morning showers, the PV reheated the hot water, heat pump never came on, yet the RHI metres registered that the heat pump used electricity and generated heat, which was simply not true. Bizarre.

So I typically go by what our a SolarEdge app is saying. Throughout the summer, we actually turned the heat pump off completely because it was drawing power. We started to discuss it here: https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/ashp-standby-electricity-consumption#post-8875

It’ll be interesting to see whether the other Ecodan owners here have had equally mixed data.

Buy Bodge Buster – Homeowner Air Source Heat Pump Installation Guide: https://amzn.to/3NVndlU

Follow our sustainability journey at My Home Farm: https://myhomefarm.co.uk


   
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(@gfletcher)
Trusted Member Member
243 kWhs
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Topic starter  

Thanks @Mars - interesting to hear your experience, it at least helps me feel like I'm not going mad or stupid! Interesting the link between the RHI meter and the 'reality' of what is going on - seems like there could be something for RHI recipients to exploit should they be minded to! So what are your RHI meters reading? Do they take information from the heat pump and is the heat pump reading PV diverted energy as heat pump generated energy?

But the real question assuming that the information is wrong, is what is going on and how can manufacturers try to get to the point where the information displayed is giving useful information. So if I didn't have a handle on my energy usage I would look at that and wonder why the heatpump was costing me excess money seeing as I had invested in solar PV and a diverter. 

Cheers

Gavin 


   
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(@derek-m)
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@gfletcher

It will probably be measuring the electrical current flowing into the unit along with the supply voltage, and thereby calculating the power consumption. It will integrate these power readings over a period of time and store the result. It operates in the same way that your mains meter works.

With your 8kW ASHP the full operating range of the measurement system may be 4kW or even 5kW, so the 125W it is allegedly measuring would account for 3.125% or 2.5% of the systems full range. All measurement devices are built and calibrated to a tolerance, which tends to vary with how much they cost, the more accurate, the more expensive.The accuracy of a mains electricity meter used for billing, I believe is +/- 1%.

Unfortunately I don't know if it is possible to see the current and voltage readings, let alone correct any errors. Because it accumulates over time, any slight error can produce the results that you are experiencing.

I would suggest that you contact Mitsubishi technical help and ask them what may be causing the erroneous readings and how the problem may be resolved. I would be interested in what they have to say.


   
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(@gfletcher)
Trusted Member Member
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Posts: 24
Topic starter  

Thanks @Derek M - I'll get in touch with Mitsubishi and see what comes back. I'll let you know what response I get. 

Cheers

Gavin


   
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(@derek-m)
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@gfletcher

I forgot that you mentioned that you have solar PV.

Applying Kirchhoff's First Law, which states that the sum of the currents flowing into an electrical junction, will be equal to the sum of the currents flowing out of the junction.

So during the day when your solar PV is generating lots of power, this power will be supplying all the electrical devices that are connected, including your ASHP. Any excess will then be powering your diverter to the point where there may be a small amount of power being exported to the grid.

Your ASHP controller could therefore be measuring and registering energy usage, but this will not be measured by your mains electricity meter.

Let me give you a simple example. If your PV is producing 1kW of power, of which 50W is powering your fridge, 50W is powering your freezer, 50W is powering your laptop, 125W is powering your ASHP, and 700W is being diverted to your immersion heater, with the remaining 25W being exported back to the grid. Kirchhoff is happy.

Whilst your ASHP is measuring power consumption, your mains electricity meter is not.

When your solar PV can no longer supply all the demand, then power will once more start to be imported and recorded by your mains electricity meter. If your heat pump is drawing a consistent 125W each hour of the day, then only a proportion of this will be recorded by your mains electricity meter.

I hope that this makes sense.


   
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(@gfletcher)
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243 kWhs
Joined: 2 years ago
Posts: 24
Topic starter  

Thanks Derek - I just don't see what it would be drawing. So I can see that there is a need to circulate sometimes to make sure that parts don't seize etc and that the fluid in the system is ok. But 125W each hour every hour - what is it doing? There is no hot water demand, there is no heating demand, and surely there is only so much circulating that it can do. So for example I am sat here watching TV with the internet on and a light on, plus there is the hall light on (both lights LED's) and the smart meter says I am using 152W. I know there are +/- aspects to this - but it just seems out of kilter completely.

So I suppose that the conclusion is that the heat pump is recording some energy that is diverted from the PV's to the tank. But it isn't an exact science. 

As you suggest, I'll ask Mitsubishi about it, the accuracy is misleading from a consumption perspective. 

Cheers

Gavin


   
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(@kev-m)
Famed Member Moderator
5561 kWhs
Joined: 3 years ago
Posts: 1299
 

@gfletcher 

there have been many discussions on other forums about Ecodan standby power consumption. Some users report up to 200W on standby.  Mine uses 24W (14kW Ecodan R32) on standby. It seems to depend on the model and whether it's a R410A or R32 based ASHP.  The power is supposed to provide some sort of heating to enable the compressor to start quickly.  I know mine is accurate because it uses the RHI metering solution.  The only sure way to measure if you don't have proper metering is to switch everything else off.     

As others say the internal Ecodan internal measurements may not be that accurate. It also depends how your ASHP is wired up and what is actually being measured. There are several different options.   


   
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