I have recently noticed the new Hx column on Heat Pump Monitor.
My system is marked as H2.
I assume this is because I haven’t ticket the ‘Includes booster & immersion heater’ checkbox for my system.
My immersion is connected to an Eddi and is used for two things. Firstly if there is some excess solar, but not enough to use the heat pump it will divert power to the immersion.
Secondly the heat pump can trigger the Eddi to power the immersion for Legionella cycles (currently disabled as the diverted solar has been raising the temperature of the cylinder).
Power doesn’t come from the HP controller directly to the immersion.
I’m trying to work out if H2 is the correct designation for my system ad how I’d increase it to an H3 or H4.
How is diverted solar to immersion normally captured?
Hovering over the Hx value shows the following tooltips: (for air-source)
H1 = Heat pump compressor only
H2 = Outside unit only
H3 = Outside unit + Booster and immersion heater (if installed & used)
H4 = Outside unit + Booster and immersion heater + Central heating pumps & fans
If Legionella is disabled then it will assume immersion isn’t used and bump up to higher boundary.
If you do want to include immersion, then you probably want to include both the electrical and heat input. This requires knowing how many watts are going into the immersion and adding it to the power and energy feeds. Be aware that this will drop the COP, and will be at a disadvantage when compared to other H4 systems that never use the immersion.
Personally, I think it’s best to exclude the immersion entirely, as it’s not pertinent to the performance of the heat pump.
I have pretty much the same setup, with an immerSUN unit (the predecessor to the Eddi) driving an immersion heater in the DHW cylinder from any excess solar generation. This handles the majority of my DHW demand and I find it takes the cylinder to 60 degrees at least once a week - even in winter.
I do not include this immersion heater in the stats I report to HeatpumpMonitor (though I do have it metered so I could add it) - the rationale being as Tim has outlined, that it’s unrelated to the performance of the heat pump.
A slightly tricky one is how to deal with solar PV divert to an immersion heater? Should that be included in the system boundary of the heat pump? It seems like an unfair penalty for something that makes total sense economically especially if you dont have a battery to be used as a buffer for running the heat pump to do DHW in the summer…
There is an effect on the reported SPF from excluding the immersion, since the heat pump tends to run fewer DHW cycles (which are typically at a lower CoP than heating cycles) so the SPF will tend to be higher than for a system without solar PV diversion. However, including the immersion (at a CoP of 1.0) would be more misleading, in my opinion.
On my system’s summary page I have ticked the box to say it “Includes booster & immersion heater” - on the basis that the electricity consumption recorded for the heat pump does include the auxiliary heater within the main heat pump unit - which can be brought in to ‘help’ the compressor if required. @Timbones - is that the right thing to do in such a case, so should Andrew tick that box too (even though the immersion in the DHW cylinder is excluded)?
If one wanted to accurately capture this, it would be necessary to add the power from the immersion element to the readings from the heat meter, with additional process steps on the input.
So, a 3kW element would consume 3kW when it is running, and add 3kW to the measured heat. For Eddi-like integrations that can trickle in wattever, you’d need to measure the energy going in and append it onto the heat feeds.
If only the electrical input is considered, then the overall COP is unfairly penalized.
This reminds me of when I registered with the RHI program (the one before BUS), I was required to submit 3 meter readings every quarter: heat meter, electric meter and immersion. I complained to Ofgem that the heat meter didn’t include heat from the immersion, so that wouldn’t be fair. They actually agreed and changed my account so that I wouldn’t need to give meter readings from the immersion.
A backup heater, on the other hand, is inline with the rest of the heat circuit, so any electrical consumption there will be measured directly by the heat meter.
On the flip side not reporting and monitoring immersion usage allows someone to achieve a higher SPF if the heat pump doesn’t have to do any work heating hot water over summer?
I include it all, and when agile prices are negative with both the backup heater and immersion going it obviously hurts my efficiency stats. But this is why I’m glad the agile pricing is represented on the page.
I think it’s better to monitor it as an overall heating system rather than just how well the compressor/HP works, this takes into account the whole system and model design which are all based on a number of tradeoffs. There is the option/ability to track DHW separately anyway.
Not running the heatpump at all during the summer will actually reduce SPF due to the standby consumption being higher than zero. Better to run the heatpump daily to offset that standby with useful heat.
Im going to add the option select an immersion heater data feed as a separate data input on the heat pump dashboard. I might also add the option to select a separate booster heater feed and separate feed for secondary pumps… (this is going to get complicated!)
I think it would be good to change the options here, so that booster, immersion, primary and secondary circulation pumps are seperate items? and make them conditional on these components being installed and in-use?
This highlights an issue with the Hx model, which doesn’t distinguish between H4 systems that have (and use) immersion and those that don’t.
Yes, that’s a good idea, though doesn’t help with the boundaries. Even within the set of H4 systems with immersion, the difference between those that turn it on occasionally when Agile goes negative is quite different to those that put excess sunshine into the tank every day.
I think the original reason for measuring immersion energy was for when it was needed to make up for deficiencies in heatpump technology (same as backup heaters), which was important for evaluating them. With modern heatpumps that are more than capable of providing the demands of a household, immersion has a different role to play and is an optional bonus.
…and when the grid price is free or negative…
Might be worth taking a step back and asking what we’re trying to achieve here?
I have been thinking we need to start a thread on Total Cost of Ownership and see if we can find a sensible / realistic way of putting some numbers on that… Or at least to think through a few case studies…
That’s a good way of looking at it, and it is in those cases where recording booster and immersion heater use is most important.
Indeed. I literally turn mine off at the mains over the summer since otherwise the controller constantly drawing 45W has quite a big impact on SPF. I’m confident that the diversion of excess solar PV generation will satisfy all the DHW requirements April - September.
45 W is massive! Is that thing using 80s electronics? My Vaillant idles at 11 W, and that includes indoor unit, outdoor unit, control unit and Wi-Fi interface.
The economics of owning and running a heatpump has become incredibly varied and nuanced. Back when everyone was on a fixed tariff with no solar, then it was easy to calculate annual running cost from a couple numbers and compare to gas.
Now that every household can have some combination of “free” solar power, cheap time-of-use tariffs, paid exports and hot water diversion, the formula for “best” is different for every configuration.
With RHI, I get paid based on how much renewable energy I generate, so it’s most economic for me to run the heating as much as possible with the windows open
My NIBE F1145 GSHP has been installed for 8 years and the design is quite a bit older than that - it doesn’t even have a variable-speed inverter drive for the compressor - so I don’t think they were overly concerned about the standby power consumption back in the day.
That’s the main consideration, yes.
I’m on late-stage FITs with 50% deemed export so excess solar generation is literally free for me and having invested in the solar PV diverter I might as well use it.
I might be the only person making use of it in the short term, but if you are able to add a separate feed for secondary pumps that would be a much more ‘transparent’ way for me to report their consumption than lumping the data into the existing feeds as I was thinking I would have to do (as per my separate thread on the topic).
I could report booster heater consumption separately too, since the NIBE API tells me what that is (although I’ve configured the settings on my controller to only use the booster as a last resort). I’d have to subtract that from the electricity meter reading though, to avoid double-counting.
Then I do also have a sub-meter on the immersion heater in the DHW cylinder, mostly used for excess solar PV diversion, so I’d have the ability to exercise a separate feed for that too.
I’d be happy to be a beta tester when / if you are able to pull something together (though I don’t run a local emoncms so it would have to be on emoncms.org)
