Also: my technical knowledge is very limited so please explain any technical terms you use…thanks!
In brief: I got a NIBE F2040/SMO S40 system installed for my mother in law a couple of years ago. She lives in a 2-bedroom semi-detached house in southern Scotland. Some insulation in the roof and possibly cavity wall but not sure.
Is there any info out there on the amount of kWh such a system would typically use? I’m concerned that her electricity consumption seems excessive for someone who lives modestly by themselves with very little technology in the house. Her smart meter isn’t working (I’m trying to get it fixed) so I have limited accurate info, but from mid-Sept to the start of Dec she used an average of 25 kWh daily. That’s about the same as my house but mine is at least twice as large as hers and we have considerably more stuff. She has very limited income and I just want to make sure everything is as it should be for her. The installer is helpful and is looking at the system later this week.
I think the person best placed to advise you if this specific system is working well, is the HP installer during their visit this week.
For rough comparison, the 8kW heat pump in my 180 sq m house is using around 10-15 kWh per day in this mild December in southern England. This has kept the house at a steady 21 degrees and hot water for 2 people at a CoP of 4.6 (which is pretty good efficiency).
It will be very helpful if you can get to the bottom of the issues with your mother’s smart meter because then you can begin to get a much better picture of the electricity usage pattern during each day. Speak to the energy supplier: I’m sure they have teams who can help you set things up correctly.
That’s really helpful David thank you. I know the installer will want to help his client, I’m just trying to get some independent benchmarking data, of which yours is the start and is very useful!
I’m trying to get the smart meter fixed through the energy company but it’s really hard going! Hopefully it’ll be fixed in the new year.
There are plenty of other systems on heatpumpmonitor.org and each system inputs the building’s configuration so you should find a few that more-or-less match your mother’s property.
The standard response when this question comes up on Reddit is “it’s always the immersion” and after checking, it usually is. This would be my starting point.
The F2040 uses R32 refrigerant which can typically only get hot water in the cylinder up to around 55-60 degrees. If the target temperature for the cylinder is higher than this, it will have to use the immersion heater to heat up the extra few degrees, which will use a lot more electricity.
It could also be down to anti-legionella cycles being done too regularly. There’s a lot of mixed advice out there on how often you should carry out an anti-legionella cycle but the advice generally varies from once per week, to never. Once per week would be the safer option, especially for the elderly. If it’s doing it more regularly than this, you could consider reducing it. You could also check the target temperature for anti-legionella cycles. I think 60-65C if guaranteed to kill everything, so if it’s going a lot higher than this, the immersion is needlessly using more energy.
Personally, I do not carry out anti-legionella cycles and only heat the water up to 45C, but my household is only made up of 2 young, healthy (mostly), adults and we cycle through almost all of the cylinder volume every day. I have an R290 heat pump, which can heat the water up to 70-75C degrees if I wanted it to though.
Thanks for the reply Jake, appreciated. I discussed with the installer and I decided to keep the legionella settings as is - he said it’d only save approx 2 kWh a week and I didn’t want to risk my elderly mother in law’s health. Thanks for the tip though.
Thanks again David. The installer says the consumption is as predicted pre-installation so in that respect I have no grounds for complaint, but the consumption is very high for a property of its size. The installer says the size isn’t the deciding factor but the quality/type of build, which I understand, but there is some installation in there so it’s surprising and disappointing that the consumption is so high as it’s not saving my mother in law any money (though the house is warmer).
Yes there’s insulation in the attic and cavity walls. I’ve now got a bit more information. Looking at the graphs I can see that the hot water is taking up the vast majority of the consumption since April (which is the furthest back I can see on the “total energy” graph) and in November it makes up half the total kWh which the pump is producing, around 1000kWh. The installer has lowered the temperature range to 38 to 44 degrees and the water demand has been set to “small”. My mother in law lives by herself and doesn’t use any water other than for washing up and one shower a day - and that runs off an electric shower, so isn’t part of the problem anyway.
Any other ideas on how to reduce the amount of energy going on water heating please?
I have heard say, before heatpumps became popular, that in your mother in law’s situation, a more cost-effective solution might be to use instant water heaters instead. I have no idea whether this is generally accurate, or whether it is accurate for a heat pump. An instant water heater has a COP of 1.0, but you have losses only while it is actually heating the water, not 24 hours per day from a tank. It will be a matter of getting the data and doing the sums. And yes, I accept this is a lot easier said than done.
Can you turn off DHW from the heat pump and can she use an electric kettle to heat enough water for washing-up, and compare electricity consumption before and after?
Thanks for the reply Robert. What you’re suggesting makes sense but given all the cost associated with the installation of the ASHP I want to explore why it’s using so much energy to heat water unnecessarily first.
Heat Pumps are most efficient when they are pumping up a small temperature differential between source and target. They become less and less efficient as that dT increases.
That’s why all the advice is to keep the flow temperature to the radiators / UFH as low as possible.
DHW gives lower CoP than CH because of the necessity to run at a larger dT.
Heating cold water (15C) gives better CoP than heating hot water (40C).
Frequently topping-up the temperature of a DHW tank (say from 40 to 50 degrees) gives worse HP efficiency than waiting and using the whole tank of hot and only reheating when the water in the taps is not hot enough to do the job.
This isn’t as bad as it sounds because there is fairly strong stratification inside the tank and the hot water at the top of the tank (where the take-off pipe is) tends not to mix too much with the cold from the inlet at the bottom.
In a small household like your mum’s you may get away with a few days between DHW heating cycles.
Would you be able to share a picture of the hot water tank? I wonder where the tank temperature sensor is located? is the tank temperature sensor securely fastened far inside the temperature sensor pocket? How large is the tank in Litres?
Thanks for the reply and sorry for the delay in replying.
So I think what you’re saying is that she might only need to have the water heated (say) every three days or so rather that it constantly being heated? (Unlike with air temperature, where I think you’re saying it’s better to maintain the temp constantly, which certainly was my understanding of heat pumps).
Looking at the Uplink app, the hot water temperature for her system is currently at 53 degrees, whereas for my house it’s currently 48 degrees. I don’t know how significant that difference of 5 degrees would be? Neither of us have “smart control” engaged. Hot water demand is “small” and hot water boost is off.
My take on HeatPumps is that every single degree of flow temperature lifting has a real and measurable cost. Reducing flow temps is the name of the game.
The 5 degrees extra DHW target temp at your mum’s installation does (at least) 3 things
It means the HP has to flow 5 degrees hotter at the end of the cycle. These 5 degrees will consume much more electricity than the 5 degrees at the start of the cycle. (Maybe twice as much)
The DHW cycle will take longer, and this consumes max power for longer.
53 degrees water in the DHW tank is close to scalding temperature. The water will probably need to be mixed with cold at the point of use. So it’s possibly counter-productive to store it hotter than you will use it.
Like you, we have our DHW target set at 48 degrees. There are folk on here with targets in the mid-40’s.
Try turning the target down and scheduling reheats less frequently - and see whether that is acceptable/comfortable for your mum.
Hi again David and anyone else interested in this thread.
I contacted NIBE who were very helpful. They connected to the heat pump via the app and sent this reply:
“I can see the problem, the hot water sensor (BT6) is incorrectly positioned or out of position.
Your hot water cylinder is, in reality, very hot yet the sensor thinks it’s cool (or below its target) and is throwing all the heat pumps energy at it to raise the temperature of BT6.
This is usually a very simple fix, the sensor is in a pocket halfway up the cylinder, it needs a little contact paste and to be pushed further into the socket.”
This makes complete sense; given that I could feel the water was very hot despite the amount of energy being used on it, I thought either the controls and/or the sensor couldn’t be working.
I’ll send this info to the installer tomorrow and hopefully he’ll be able to fix it soon.
So sorry for the late reply. Looks like you correctly called it as I got this reply from NIBE:
“I can see the problem, the hot water sensor (BT6) is incorrectly positioned or out of position.
Your hot water cylinder is, in reality, very hot yet the sensor thinks it’s cool (or below its target) and is throwing all the heat pumps energy at it to raise the temperature of BT6.
This is usually a very simple fix, the sensor is in a pocket halfway up the cylinder, it needs a little contact paste and to be pushed further into the socket”
I hope this means the installer can fix the problem very soon. Very much appreciate your time.