Dreadnaught

Good advice.

Ah, I see. Well done. I have emailed them. Let's see how they reply.

Ah I see, that's unfortunate news. I will have to see how much Vaillant will charge to swap mine out in the heat pump itself. Did you ask Vaillant to change yours?

@Dan F, turning to these settings … "Room temp. mod." "Set-back mode" "Set-back temperature" I am in two minds about these. The best way to run a heat pump is "low and slow", which would suggest running it 24/7 at the lowest temperature commensurate with a comfortable internal temperature. But the CoP at night will be worse. So turning the compressor off at night could lead to better CoP overall. These settings above could allow a half-way house. Leave the heating on at night but lower the "target flow temperature" overnight to minimise the impact of the lower temperatures during that period. Any thoughts, @Dan F?

Good question. Short answer "not any more, I think". Long answer: when I bought the UniTower it had the bug in the controller yes. Then my installer got in to a bit of a pickle and called in the Vaillant flying squad to help (the problem turned out to be installer-error 😄 but that's another story). In problem solving, the Vaillant engineer (with a little bit of encouragement from me) swapped out the controller board inside the controller, which, as you would imagine, I was very pleased to see him do! I now have "351.09.01" on the controller in my UniTower but the version in the heat pump itself is still with the bugged "351.06.07", hence "I think" above.

Reflecting on what you've told me, for my system … "Compressor hyster. Heat." could easily be set at 15º (its highest possible value). Nothing bad will happen to the UFH pipes or my 55-tonne concrete-raft foundation-slab if the water circulating is 15º hotter than the "target flow temperature" and it will mean that, in the circumstances where "Energy Integral" is not shutting the compressor down, that the compressor will run for longer – good for CoP. "Compr.start heat. from": similarly I can set this at -120 ºmin (the lowest possible value, a reduction from the default at -60 ºmin) for similar reasons, so that the "Energy Integral" takes longer to shut the compressor down, again helping CoP.

This all now presents another question: what are the best settings for a well-insulated house with a simple heating system* for … "Compressor hyster. Heat." "Compr.start heat. from" "Room temp. mod." "Set-back mode" "Set-back temperature" Are there any other settings that are relevant for heating that I have missed? * Mine, unlike yours, is, hydraulically, a very simple system: flow is to the UFH manifold without a secondary pump and without buffer or low-loss header. Single zone, no controllable zones; no wiring centre; all manifold valves are fully open. Low system resistance (big fat pipes throughout). Basic system diagram code "8". As simple as can be. I calculated that my total system volume is about 100 litres.

@Dan F, thank you. That makes full sense now. It's effectively a double limit for when the compressor is turned off: (i) the energy integral; (ii) and a ΔT between the "target flow temperature" and the "actual flow temperature" (not the return temperature) . Local and system conditions will define which limit is hit first, as you say. An example: if the "target flow temperature" is set at 25º, the compressor will operate until the flow temperature reaches (25+7) = 32º (the return temperature is irrelevant), OR if the "Energy Integral" has reached "0" (from its user-selectable starting value, with default -60 ºmin). Is that correct? Regarding when the compressor restarts after being shut down, do you happen to know whether it is only the "Energy Integral" that does so, or whether "Compressor hyster. Heat." also does so at the other extreme (in my example above 25-7 = 18º)?

Thanks @Dan F. Very interesting. With the Arotherm Plus, do you happen to know how the user-selectable setting of "Compressor hyster. Heat.", which seems to define a ΔT, is relevant? Its default is 7℃ and the range of possible settings is 0 to 15 ℃. (The setting is not accessible via the VRC 720 but only via the heat-pump controller - which in my case is part of the UniTower). In operation, my guess has been that the controller targets a ΔT according to the value set for this setting and uses the "target flow temperature" only for the "Energy Integral" calculation, to define when the heat pump turns off (or on), but I might be over-simplifying. With a constant flow rate (which we know is an idiosyncrasy of the Arotherm Plus), one might think that targeting both "ΔT" and "Target flow temperature" at the same time would more often than not be contradictory when the only parameter subject to control is the compressor speed, hence my enquiry.

@PhilT, thanks. That's interesting. What's the reference source for that graph? Is it as google found: "PV Optimized Control of Modulating Heat Pumps regarding PV Self-Consumption" (January 2019) by Christina Betzold and Arno Dentel? Link. I had a quick scan read. When I encountered: "The bi-quadratic-polynomial equation for describing the heat pump characteristics" I thought I might leave it to @SteamyTea and went to pour myself a drink instead.

Ah, @mk1_man, yours shows a ΔT of about 3.2º. Interesting again. Are you able to look at the compressor modulation % during that period?

Ah, @JohnMo, Google tells me you have a Viesman heat pump, possibly a "Vitocal 100-A" or similar? Is that so? Mine is a Vaillant Arotherm Plus 5kW. For the Arotherm, the circulation pump speed is effectively fixed (860 l/hr for the 5kW model) and there is a setting for the ΔT (albeit a setting that can only be accessed by an "expert").

Oh that's interesting, @JohnMo. Which manual is that? Its not in my manual.

Thanks @JohnMo I see the ΔT shown in your chart is about 3.5º. - Is your's set at the factory-default of 7º ΔT? - Would you be able to overlay on that chart a line for compressor modulation (%)? My theory is that, with the well-insulated houses that are common to those on this site, such heat pumps will hit their lowest-level limit of modulation for much of the time (which is 25% max compressor speed) because heat demand from the house is just so low. That includes yours during the period shown in your chart and would explain the ΔT.

I wonder how then is compressor modulation controlled by the system? A direct response to ΔT (within the operating limits of 25% to 100%)?

Thanks @IGP. Yes, I had read that post too. All very interesting. Another thing I have noticed is regarding ΔT. My understanding is that heat pumps generally target a ΔT of about 7º (or sometimes I have seen 5º referenced), which is unlike gas boilers that target something around 20º. However in @Dan F's chart above, the ΔT is just 2º after about 20 minutes of heating (24.5º supply, 26.5º return), which surprised me. Its just an observation and I am sure there is a good explanation for it. I wonder if this is just a consequence of the modulation flexibility of the compressor in the Arotherm Plus. The compressor can only modulate down to a minimum 25% of full speed, not lower. (I do note that the ΔT is user definable (setting "Compressor hyster. Heat." in the installer-level heat-pump controller settings.)

@Dan F, as an amateur non-engineer, I have been thinking deeply about the control system for the Arotherm Plus heat pumps. This thread, and your graph above in particular, has been helpful for me. I wonder do you happen to know whether "Hc2ActualFlowTempDesired" is targeting the temperature of the flow to or return from the home? I could imagine it being either.

@Post and beam, I have used that manifold on the supply side of my system (on the return-side, I use a smaller Ubbink manifold). These manifolds come with sound attenuation sponge pads that are inserted inside. Before the manifolds, on the larger diameter ducting (160mm Aerfoam) connection to the appliance, I have in-line flexible sound attenuator on both supply and extract. My system is effectively silent from the bedrooms. It is performing very well.

3.1m x 1.3m. Feels generous. One end has MVHR. The other end has cylinder and heat-pump internal bits. Will use some of the space for storage with wheeled shelving units which I will push around as needed and even remove if I need to access something big. A flexible space. (Personally I wouldn't put any type of house battery inside the house. Precautionary principle.)

In case anyone else is not familiar with "VDI 2035", as I wasn't, this is a good quick intro… https://www.heatgeek.com/heating-water-treatment-explained-vdi-2035/

Very interesting. How is the filter fitted? Do you just remove the valve and the filter wedges between the valve and the valve adapter? With my Ubbink/Brink system, the valves are quite difficult to remove once fitted. Is that a problem for you when changing the cone filter? Are these the cones… https://www.aliexpress.com/item/1005006704266648.html

Hi @Barnboy, I am afraid I don't know, sorry. This is my one… https://underfloorparts.co.uk/product/ivar-uni-mix-underfloor-heating-ufh-pump-mixing-kit-underfloor-heating-manifold-control-unimix/

Of any interest? Still available.

I wonder, are there prescribed widths or depths for a channel so as not to need to de-rate an electrical cable for being inside insulation? Nice video.

The owner of the team who poured my concrete raft was well known to be wealthy from his trade with properties owned abroad. One of his team was originally a chippie. He eventually left to join a team of small-scale house builders. Within a a few months he had convinced his new colleagues to down tools and take up concrete pouring as a means to make much more money.