Cost of running a Nibe ASHP , controlled by SMO20 for DHW only

Good afternoon all
I am new to this forum so please bear with me. I wonder if anybody has advice to offer in terms of the high running costs of my ASHP. I do not mean due to the large rise in unit costs of electricity. This issue has dogged me since the outset, but I am away from home a lot, leaving just my partner there and we kept putting it off, partly because the cost per kWh previously was less.
We lived in a caravan for two years to build:
A 220m2, 4 bedroom PassivHaus self build (not taken to certification), airtightness test of under 0.4, MVHR, ASHP, triple glazing, Passive raft type slab (400mm EPS, 100mm reinforced concrete slab above to FFL), Loxone automation
No formal heating save:
1000w of resistive UFH in main bathroom, on for 1.75 hours a day in winter. Not relevant to ASHP of course.
There is a single zone wet UFH in the downstairs slab fed from a buffer tank off the ASHP, but this has a very low flow rate of approx 28d and is meant to help the thermal mass of the slab and take a “chill” out of the porcelain slabs downstairs. This is is brought on by a mean average of thermistors in light switches downstairs and one remote. I must stress that is rarely on and when it is, not for long, say one hour, usually in early morning.
So, the ASHP. Nibe 8kw (5kw model was not approved when we built), two x 200L mains pressure cylinders (not thermal stores) with equal drawdown (obviously engineered for larger family occupancy), SMO20 with one external sensor and three if I recall (not there atm) sensors. Engineering and sale was by NuHeat in Devon (who I have contacted but don’t really progress in the conversation much!)
I appreciate cylinders have a daily loss of heat. I also now appreciate that in cold snaps, particularly like recent ones overnight, the ASHP has to do some work in defrosting to protect itself, but we have high running costs for just DHW. I know we should have taken readings way back on the heat pump meter but my partner has tended to look at overall house consumption on the smart meter. We are now logging just the ASHP. Early days of logging, about a fortnight but seeing some 17Kwh on some days (yes cold period as discussed earlier) down to 10/11, but even this is an awful lot for one shower max and a possible slight bit of hand washing up. I see some peoples figures that are ridiculously low even when combined with space heating. Not sure I could “shut down” one cylinder easily so am with this Y cord draw down of both together, but the daily thermal loss can surely not require the ASHP to be on this much? I have turned off the immersion purge cycle as everywhere I read it is unnecessary for such sealed systems.
I understand countries like Sweden pay much less for their electricity. I understand the principle of COP, but this is usually more relevant for space heating. Purely for DHW 99% of the time is an expensive way unless any of you agree that there must be a fault somewhere. I had engineers out last year from a NIBE approved company who lowered the curve slightly in an attempt to get it cycling less, but still I believe there is an issue. Or are you all going to say 10Kwh a day fro an ASHP to deliver DHW is ok?
Any thoughts ? I would really appreciate if any of you have some good advice. Thank you so much for reading this.

Comparing to my own system (Ecodan 11 kW), 17 kWh per day just for hot water seems way more than it should be. I have a single 250L cylinder than is heated up to 43 degrees once per day, and typically uses between 1-2 kWh per day, never more than 4. Average COP heating water during this period was 2.9.

Space heating is always the more significant power draw (during the colder months).

Some questions that might help us understand what’s happening with your system:

  • How hot are you heating the cylinders up to?
  • How often are you reheating the cylinders?
  • How are cylinders connected to ASHP? (not via buffer I hope)
  • How is the ASHP being controlled for space heating?
  • How hot is the ASHP heating flow temperature to for heating?
  • How is the UFH being turned on/off?

I suspect that the ASHP may be running for much more of the day than you think, and is trying to do space heating even when the UFH is turned off. This means it will spend a lot of energy heating up the water in the buffer tank when it isn’t being used.

17 kWh means it’s averaging 700 watts per hour, or 1 kW for 17 hours. I’d expect a Passivhaus to have much lower consumption, say 10 kWh per day, possibly even less for UFH.

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Thank you Tim for such a prompt response. My answers to your questions are below in red. Some are from memory as we installed this a while ago, but I work in a technical world and am fairly confident they are all accurate.

Peter Vasey
mobile: +44 (0)7774 692847
email: [email protected]

If I am understanding you correctly, you think the perhaps not brilliantly insulated 100L buffer tank may think it has to keep reheating to a temp, via the SMO20 controller, even though the UFH has no interest in harvesting that heat. Obviously I am not using any of their “heat controls” in the menu. I am merely energising the pump circulator on the few times it is required. This was my perhaps crude way of combining the two systems. But if I am not using their heating system, other than the heat curve for the overall, how and why would it keep coming on for the buffer if that indeed is what is happening?


Hi Peter. I have an 8kW NIBE GSHP in a nearly-Passivhaus (kudos for the 0.4 ACH; I’m at 0.9 hence the ‘nearly’) with a single mains-pressure 200 litre DHW cylinder. My real-time data is here on HeatpumpMonitor (I’m the only Ground Source system on the list). You can spot the DWH cycles since they happen at 00:30 (start of the Octopus Go cheap rate period) and go to 50C - though not every night since I use Solar PV diversion to drive the immersion heater when it’s sunny and the PV would otherwise be exporting to the grid.

I typically see 1kWh electrical input (and 4kW heat output) to reheat a 200 litre DHW cylinder to 50 C so 17 kWh just for DHW is massive. Like Tim says, maybe you’re dumping more heat into the UFH than you think.

Do you have NIBE Uplink monitoring available? That’s really very good for showing exactly what the heat pump is doing.


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Hi David
Yes I saw you were on the monitor list a few days ago.
I currently do not use any timing method for DHW. As I read views and opinions, they appear split on this issue when using an ASHP. What made you decide to use timed and I assume this is just for DHW, not you space heating, for which I see you have radiators. I imagine they are doing very little work in your passivhaus?
I am on the Tesla energy plan at he moment, battery not car as source It manages my gateway and thus solar is exported. I remember toying with idea of diverting solar (I have 5.4k), but did not. So many choices to make in a self build.

Timing of DHW with a GSHP is uncontentious; the ground loop stays warm overnight so no CoP penalty from running at 1am (when I pay 7.5p on Octopus Go) - though there would have to be a heck of a hit to CoP to make that a bad choice with an ASHP too.

I have some radiators but also a fair bit of wet UFH (and a duct heater on the MVHR). Typically using around 35 kWh of heat per day just now to keep 315 m^2 at 21C. The heating runs 24x7 October - March and my only timing is to ‘block’ the heating 16:30 - 19:15 (since I’m on the Octopus Energy EQUINOX trial, checking that it’s OK to have heat pumps dodge the evening usage peak).

I’m a fan of solar PV diversion. I have an immerSUN unit and it does 75% of the DHW annually for free.


Hi Peter - your answers didn’t come through to the forum - can you post them again?

That doesn’t work. You are better to go to the website than try to reply using the email interface.

  • How hot are you heating the cylinders up to?
  • How often are you reheating the cylinders?
    Nothing timed at this point. Have always left the whole system to run itself, which may well be an issue
  • How are cylinders connected to ASHP? (not via buffer I hope)
    Absolutely not. Buffer is simply for “UFH”. Happy to share diagrams if it helps
  • How is the ASHP being controlled for space heating?
    My automation energises the UFH pump and the loop flows through the buffer tank and harvests the heat. If this makes the buffer tank go below temp, the thermistor tells th ASHP, via the control unit to start warming the buffer again.
  • How hot is the ASHP heating flow temperature to for heating?
    28d. As I explained in my introductory essay, it is not “heating” per se, but a nudging of the thermal mass of the Passiv slab. No other heating is in the house.
  • How is the UFH being turned on/off?
    Automation de energises the UFH pump, so the reverse of above.

I suspect that the ASHP may be running for much more of the day than you think, and is trying to do space heating even when the UFH is turned off. This means it will spend a lot of energy heating up the water in the buffer tank when it isn’t being used.
It certainly runs much more than I ever assumed it would before I owned one. It is distanced, for planning reasons from the plant room by approx 10m of highly insulated pipe; 2 x 32mm pipes within 150mm insulated tube, buried ostensibly at 750mm although it does get more shallow as it reaches house wall I discovered fairly recently, maybe 450mm, but that will have approx another 100mm of soil when the final section of front is finished, if it ever stops freezing or soaking the pile of topsoil!

17 kWh means it’s averaging 700 watts per hour, or 1 kW for 17 hours. I’d expect a Passivhaus to have much lower consumption, say 10 kWh per day, possibly even less for UFH.
Me too, even less as under floor space heating is rarely engaged and at 28d

Thank you David for all this info. I do remember looking at the immerSUN, but because I was going down the battery route (pre TEP of course), I decided to divert PV to this instead.
I only have the basic NIBE uplink available, not the paid version so my data is very limited; basically just DHW temp, outside temp, and " climate system"which is the supply and return temp, along with charge pump supply and return temps

Yes, that’s my theory (caveat: I’m no plumber). Can you restrict the heat pump so that it only operates when it’s needed, i.e. when UFH loop is running, or water is being heated? Maybe let it only be active during certain times of the day?

Plotting this data over a few days could be enlightening…

As a test Peter you could swap the heat pump from Auto to Manual mode (in menu 4.2; OP MODE) so it will continue to cover the DHW demand but won’t try to do any space heating. (I do this with mine over the summer.) See what happens to the electricity consumption over a day or two.

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The paid version of NIBE Uplink only gives you longer retention of the history and the ability to remotely manage the settings via their app; the same values get recorded, paid or free. I don’t pay for this and instead grab the data from the API every couple of minutes with a script that runs on a timer and store it in my own database - from which I then upload to the HeapumpMonitor website via another script.

It’s non-trivial to call the NIBE Uplink API but I have some notes on how to navigate the OAuth2 authentication here: NIBE Heat Pump Monitoring via NIBE Uplink API (Python Version) | Marsh Flatts Farm Self Build Diary

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Thank you David. Although I am abroad, I think I can persuade my partner, over a video call, to change this menu and we can see what happens. I like the idea as it would narrow things down to get a clearer idea.

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Thank you for the link David. What an amazing looking building!
Other than some manual plotting, which I can do whilst I am out here abroad, anything further will have to wait until I return home.

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So this is the thorny area of whether one runs the ASHP on a timer or let it plot its own course. A very split jury from what I have read although interesting to note that the controllers themselves include timer functions. I begin to think a scheduling is called for, but that of course is global so I would have to accept my floor warming loop would be constrained to the same timed functions wouldn’t I ?

For the avoidance of doubt, you don’t need to be at home to do some more real-time monitoring:

  • The heat pump is sending data to NIBE Uplink in the Cloud (on NIBE’s servers) anyway.
    • You don’t get to control what it sends, so it’s sending all the data all the time.
  • There’s no easy way to tap into the data feed locally, so you just have to pull the data back from NIBE’s servers using the NIBE Uplink API.
    • This is what the SmartPhone App does (which is why they wrote the API in the first place).
    • You can do that from literally anywhere on the Internet - you just need somewhere to run a Python script - and then somewhere to store / send the data (like

I’ll send you a Private Message with further info.

[And thanks for the comment about the house - it’s been a long journey and the artist’s impression on the website still looks slightly better than the real thing (and it didn’t quite make it to Passivhaus certification) but really it’s a privilege to have had the opportunity to build it.]

In a typical building, with relatively low levels of insulation, there’s certainly an argument for some scheduling so that the heat doesn’t all leak out before anyone gets to benefit from it - although there are many, many variables related to thermal mass of the building, electricity pricing at different times, sizing of the heat emitters, outside temperature at different times of day, daily occupancy profile etc. etc.

However, in a Passivhaus, the insulation is so good that it really doesn’t matter when the heat is delivered - since it leaks out so slowly - so you might as well keep the building warm 24x7 and trickle-feed the heat at low temperature to maximise the CoP. IMHO the main reason for any scheduling of heating would be to do a bit more heating when electricity is cheaper and a bit less when it’s expensive (or high-carbon) - or with an ASHP, do a bit more when it’s warmer outside so the CoP is better.

You’ve got a slightly different problem though Peter, since something seems to be ‘wasting’ heat along the way, and scheduling the timing (NIBE call it Schedule Blocking) could be a way to minimise that.

A simple sketch of your heating circuit would be useful. My UFH pump runs constantly, circulating via a Low Loss Header and Buffer Tank, with the Heat Pump’s Weather Compensation controlling the temperature, so I know the heat is all coming out of the Buffer and going into the floor (and the rads, which have their own loop and pump, also running constantly). You seem to have the opposite situation, where your UFH pump runs intermittently and maybe the heat pump keeps topping up the buffer ‘just in case’ - like Tim was suggesting.

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Does their smartphone app show graphs or anything that would help us understand what the heatpump is doing all day? Like hourly consumption or flow temperature?