Yes, you’re right. You can set a bivalence point for both heating and DHW to decide when the backup heater should be enabled.
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So thats a caveat with my use of 70rps instead of eco. Might need to turn of NR (or switch to eco) when it’s <3C
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What secondary/backup heat sources does this support though?
- Second AHSP
- Boiler
- Backup heater in Hydraulic Unit
- Immersion in Vaillant UVC
- Immesion in some other UVC (e.g. Mixergy)
I always assumed it only supported hybrid configuration with a vaillant boiler. Not that I’ve looked into it in detail either…
@Ulric uses an additional gas boiler if I’m not mistaken and I have the heater in the hydraulic unit. Apart from that - no clue unfortunately 
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I also have 6kW heater in the hydraulic unit.
I know this is available and can be used if ASHP fails, but I’ve never understand if/when this is used in other circumstances. Do you? Do you have a bivalence point configured and is this backup heater used for this?
I had in the beginning, but as my HP is severely oversized I turned it off since there’s just no need. The time it was on it worked as it should and the backup heater was on in conjunction with the heat pump when the temperature dropped below the threshold (I had it at -10°C). It doesn’t seem smart as it will kick in even if the compressor isn’t at full capacity, so purely based on the configured temperature.
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I believe it anything that a relay can control. If not just turn on the immersion for all of cheap rate with a seperate outside thermostat indicates its very cold, or just enable by hand in this sort of weather.
Those relais (MA1/MA2 I believe) are separate, the bivalence control also modulates the backup heater so there’s some more control besides simple on/off.
Just turned my recent cold weather experience with Eco hot water mode into an Energy Stats blog post.
Hope it’s useful.
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Useful article, thanks!
Since switching to ‘Normal’ DHW mode a few days ago, the daily 1am timed DHW runs have all successfully reached the 50C target within the 2-hour schedule window, even during the last couple of nights when outdoor temps have been -3C.
Defrosting does take its toll on eficiency though; this morning’s DHW run at -3C consumed just under 4kWh to reheat a 250L tank from empty, presumably due to defrosts. Not a big deal in terms of cost, due to 5p/kWh during this time…
I am also having problems with defrost namely excessive noise with a fairly high pitched tonal quality during recovery. The 7kW unit is new and the noise is very much louder than the normal noise. I expected some increase but not the level I’m hearing. Do others experience this. It’s forced me to turn on nr mode at night, but that causes a slow fall in ft.
We have a problem with a noise coming through the primary pipework when on full power / defrosting on our 5kW. In fact we often can hear the pump inside the house as it changes power levels.
The problem is I’m not sure what is ‘normal’ but now winter is here I’ve decided to get Vaillant out to take a look.
Indeed, a gas boiler (because it was installed and cost me nothing if not running)
It’s connected on the ebus behind a VR32.
I’m still trying to understand how it behave under bivalence and trivai, it’s again the logic of vaillant engineers 
Gas boiler runs 1 min randomly when there is no demand. Jumping from S.31 ( no heat demand from ebus ) to S.0-1-2-3-4-5-6-7 .
While I am in inactive mode, when the temperature is well below the bivalence point or when in Trivai (via tariffs) below 1, it does not start until the ambient temperature falls below the desired temperature.
But then, it modulates well according to the defined heating curve. So it’s not following the inactive mode… I still have to dive in. Also I’m blind with the ebusd adapter not reading what’s behind the Vr32. ( don’t know what to do with this Bus coupler/bridge latency · Issue #109 · john30/ebusd · GitHub)
I posted a counter example here, where NR mode actually improved the defrost situation.
I did some testing with eco and NR on my 7kW here and this is what I saw:
eco:
- heating water:: 60rpm
- defrost: 80rpm max
normal + noise reduction 42% (70rpm):
- heating water:: 70rpm
- defrost: 80rpm max
normal:
- heating water: 110rpm
- defrost: 80rpm max
This shows that NR does not limit compressor speed when deforsting (at least with my unit). In fact the compressor speed during defrost on my system is always 80rpm regardless of DHW or NR modes. Given this I struggle to understand why using NR would be any worse than using eco for heating hot water.
Yes, given you had NR on, you obviously weren’t going to get the same output, but it looks like:
- the target flow temperature before midday was higher also (so maybe not directly comparable)
- you are getting more inefficient spikes with NR off.
In my case, this spikes cause flow temp (and in turn integral) to rise faster shortening cycle length. This is why I have NR turned on permanently. I was concerned based on your post that this was somehow creating an issue with defrost (e.g. by limiting compressor speed during defrost), but I can’t find any evidence for this.
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I’ve just spotted that the reported flow rate by the OEM kit has been steadily dropping from just below 20l/m (which is what it has been since balancing of radiators back in October) towards the end of December to around 16.3l/m today.
It’s still barely above freezing here so the system is working hard, with defrost cycles running, but it seems to be heating the house as well as can be expected at the moment - due to work in progress on ground floor.
I thought I’d read something on this forum about it being a heat meter issue, and the Vaillant reporting the correct flow rate value, but haven’t managed to find that thread again. I’ve had a lot of heat meter errors during DHW cycles, but thought I’d finally managed to get all the air out of the system. Until I bled a different top floor radiator on 2 Jan, and a heat meter error was reported on 3 Jan following a DHW cycle.
Question: Where would I check the Vaillant system flow rate? I have a SensoComfort but haven’t found anything that looks like current flow rate value yet.
Thanks for any pointers!
I think you can only get the flow rate on the interface using Vaillant kit.
Press both buttons on the interface > select Live Monitor then its the 2nd or 3rd bit of data shown by pressing Next.
First thing to check would be the filter on the heat pump is clear of gunk, whip it out and give it a clean. Short handled adjustable spanner is ideal for that.
Thanks @Willenic
Have found the live value. After converting l/h to l/m I get 17.1. This is what OEM is showing as well. Currently on DHW cycle, so will keep checking this afternoon when heating and during defrosts.
Sontex heat meters are very susceptible to gunk. Mine would massively under report flow values when that happened. I subsequently fitted a mesh filter which is easier to clean than the heat meter. I have to clean out the filter occasionally (but much less so now as most of the gunk appears to have gone) and the flow rate now only drops very slightly anyway. My current peak flow is around 29 l/m when the max should be about 32 so I’ll clean it again soonish / when it warms up a bit.
I have a Axioma Qalcosonic heat meter, which I know is very susceptible to air in the system. But given that the flow rate reported via OEM looks very close to that reported on the interface, it appears to not just be an Axioma heat meter issue.
The installer gave me a comprehensive verbal handover back in July, but I didn’t take good enough notes, and although I’m pretty sure I have a mesh filter - I can remember seeing it being cleaned, I have no idea where to find it now!
Time to get back to the installer I think…