There are loads of Vaillant owners here that can guide you through the process of setting up your curves.

This short video we did covers and explains weather comp and curves:

That’s a great question, @davidb. I think your problem may be rooted in the fact the question is commonly a conflation of two distinct topics.

Firstly, if we imagine a world where the outside air temperature was always the same, we’d be setting a single target flow temperature. Even so, given the house is being used, the heat pump’s programming will tend to adjust various settings (e.g. modulating the compressor or varying the flow rate) to try to ensure a constant difference (delta T) between the flow temperature and the return temperature. Factor in the odd defrost cycle too and it becomes more obvious that variations do happen. In other words, the target flow temperature is not necessarily the actual flow temperature.

Weather compensation is just a layer on top of that. It’s a means of telling the heat pump what target flow temperature is appropriate to the system at any given outside air temperature. Your house is just a heated box and so will lose heat in a predictable way - more in colder weather, less when it’s warmer, and so using the outside air temperature as an indicator of how much energy the heat pump needs to pile into your home to balance the losses is a system that works remarkably accurately.

As for the practice of setting it up, @editor is bang on. Plenty of Vaillant users here who can help with the actual process.

Posted by: @davidb

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I am struggling to understand how weather compensation works.  I have done a search on this site, looking for a thread on the subject.  If it’s there, it didn’t come up using the search bar.

My 3.5 Aerotherm is currently set at .4.  (I know this 0.4 is low, I am doing an experiment). The ambient temperature is 7C so I expected the flow temperature to be around 30 to 31C - it’s actually 25.  The temp is set to 20 and the current is 19.5 so I expected the system to recognise the building needs heat and ramp up the flow to above 30.  I have struggled to understand how the control is designed to work.  Can anyone point me in the direction of a description of how it should work and the various factors which come into play?  

Basically, as @majordennisbloodnok says, WC adjusts the flow temperature (the temperature of the water leaving the heat pump) according to the outside temperature.  This changes the amount of heat emitted by the radiators (or UFH) so that it matches the loss from the house (which is determined by outside temperature).  This in turn ensures that the house stays warm. 

Because WC reacts to outside temperature ('OAT'), it responds much better to changes in conditions than control based on room temperature alone  This is somewhat counter intuitive (unless you study control theory, which few do) but nevertheless the case.  Its a concept initially developed for boilers, for which it works equally well.  Unfortunately we didn't adopt it for boilers in this country, unlike some of our more enlightened European neighbours.  The consequence is that we have been paying ~10% more for our gas heating than we needed to, and enjoying lower levels of comfort! 

WC leads (almost always) to better comfort and reduced bills, but the downside (with current technology) is that it has to be tuned to the house.  Once tuned to the house you just leave it alone, and you should end up with a much more stable and comfortable house than is possible with the controls we are used to in the UK.

Tuning it to the house on the Vaillant is quite easy once you get the hang of it.  Its described on page 4 of the sensocomfort manual ** (section 2.10 setting the heat curve), which you should have been given by your installer!.  Essentially the heat curve value (in your case currently 0.4) adjusts the slope of the curve (of flow temp vs OAT).  This tunes the weather compensation to fit your house.  

Once its right, which might take several days to a couple of weeks, you should be able to leave it, but you may find you want to make a minor tweak (perhaps 0.05 up or down) after experiencing it for a few weeks/a season.

In detail the full process is

• Set the target temperature to 20 or whatever other temperature you want the room where the sensocomfort* is to be
• Set room temperature mod to inactive.
• Set minimum flow temperature to something sensible, eg 22-25 for UFH, maybe 27 for rads.  This isnt particularly critical at this stage so long as its low enough.
• Operate your heat pump 24x7, with any thermostats/trvs set to max
• Adjust the heat curve value (currently set at 0.4) up or down, making no more than one adjustment per day, until the house just settles at the right temperature.  Adjusting it up raises the temperature, down reduces the temperature.
• You may need also to balance radiators if rooms settle at relative temperatures which differ from the relative temperatures you want.  There is a separate article on radiator balancing on this forum.

You can adjust the heat curve value and minimum flow temperature either in the installer menu of the Sensocomfort or on the app.  You can adjust the target temperature in the user menu of the Sensocomfort or on the app.  Setting room temperature mod to inactive can only be done through the installer menu on the sensocomfort.

Once tuned for your house you can change (manually or through the schedule) the target temperature alone eg to achieve a setback or indeed to increase the target at certain periods to take advantage of ToU electricity tarifs.  This will actually change the flow temperature 'under the hood' which is the efficient and most comfortable way to do things, but you dont need to bother about that.

If some rooms get too hot occasionally due to solar gain or cooking, you can set the TRV to a couple of degrees above the desired value to act as a limiter for that room only.  There may be a case for setting room temperature mod: to active or expanded but only after you have done the above and experienced it for a few weeks.

Hope thats clear, as I say the concepts take a little getting used to but once you do get used to them it is easy to work and should result in greater comfort than with any system based on thermostats/TRVs alone..

* the sensocomfort should be on a representative room, not one affected by draughts, excessive solar gain or other disturbances that dont affect the rest of the house.

** like most electronics there are successive versions of the sensocomfort manual.  In mine the relevant instructions are on page 7, but its page 4 in the version to which I have linked.

Thanks for the posts and ref to the video - all of which largely reinforce what I believed I had understood and informed the way I have our system set up.  Valves all open, running continuously etc etc.

What I don’t comprehend is why the flow temperatures are nearly always below the temperature expected from the curve.  So in my initial post, flow was 25C when the outside temperature suggested it would be above 30C.  The room temperature hasn’t really changed (19.6 against a set temp of 20C) and the heating on.  The flow T is now 28 but still below the value I expected from the curve - roughly 2hrs later.  It’s this non correlation between the flow temp indicated from the curve (based on ambient temperature) and what the flow is actually indication under “status” which I am trying to comprehend.

A related factor is that I have been told by Vaillant technical that the system is designed to not overshoot the target temperature and backs off the flow temperature as the target is approached.   I was hoping there might have been a manual or technical paper of some sort which explained in some detail what is designed to happen.  With such a description it would then be possible to observe if what a system is doing is “correct” or something is amiss.

@jamespa comment re allowing the system to settle and only adjusting the curve every 24hrs is a very helpful reminder of the inertias involved.

All comments really appreciated on my road to actually understanding what variables Vaillant allow / have been designed to be in play.

PS.  For those who saw / responded to my post re a lack of flow through our UFH pipes, I now have a steady flow of 2.5l/m on each of my 2 circuits.  The flow regulators won’t allow anything lower and (at least last night) simply cut off the flow if I tried to set anything <2.  It was because the UF is now absorbing / emitting heat that I have dropped the curve from 0.6 to 0.4 to see how the building copes now all the emitters are working roughly as per design.

Thanks again for all contributions.

@davidb 

Hi, if you haven’t already done so then have a look at the link below and in particular read the bits about active, inactive and expanded modes for heating, the owner of that website has a Vaillant Aerotherm and is incredibly knowledgeably on them, a general browse of his site will teach you just about everything you might need to know.

https://energy-stats.uk/vaillant-arotherm-basic-settings/

Good luck.

Posted by: @davidb

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What I don’t comprehend is why the flow temperatures are nearly always below the temperature expected from the curve.  So in my initial post, flow was 25C when the outside temperature suggested it would be above 30C.  The room temperature hasn’t really changed (19.6 against a set temp of 20C) and the heating on.  The flow T is now 28 but still below the value I expected from the curve - roughly 2hrs later.  It’s this non correlation between the flow temp indicated from the curve (based on ambient temperature) and what the flow is actually indication under “status” which I am trying to comprehend.

To comment on this I would need to know a lot more about your system/house.  Even then 'spot' observations of FT are always near impossible to interpret, plots vs time are needed.  Perhaps your installer enabled 'active' room temp mod mode which does an element of auto-adaption of the WC curve, however thats just one of many possible explanations.

Posted by: @davidb

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A related factor is that I have been told by Vaillant technical that the system is designed to not overshoot the target temperature and backs off the flow temperature as the target is approached.  

I believe thats the case (assuming that the 'temperature' in question is the flow temperature).  There are however some circumstances when it will overshoot, deliberately so far as I can tell. 

In room temp mod = expanded it shuts off altogether when the measured room temp exceeds set temp plus a small amount (I cant remember how much).  I use expanded mode at the end of the season when solar gain tends to be a dominant factor, and inactive mode for the rest (ie the majority) of the season. 

The exact algorithm of room temp mode = active is not disclosed, which is why its difficult to recommend it.  Basically it does some autoadaption of the WC parameters but within undisclosed limits.  I presume, but dont know, that it works well in at least some houses provided the WC is set manually in the right ballpark and the homeowner doesn't fiddle too much

The openenergymonitor forum has some technical descriptions of the Vaillant modes, created I believe by 'reverse engineering'

@davidb

When my 7kWh Vaillant heat pump was installed the target temperature was set to 19C. I checked the temperature in various rooms with the sensoCOMFORT controller. Just handling this unit altered the display temperature. I ended up fixing it to the wall in the hall.

I had a digital thermometer and used it on all the radiators aiming the dot at top centre of the radiators. This showed that all the radiators were at roughly the same temperature. While the target temperature in the sensoCOMFORT was 19C, it often did not get to that, or so it seemed.

I purchased a cheap digital thermometer and checked the termperatures in all the rooms. This showed a uniform temperature of around 19C, I then sat it on the coffee table in the lounge where it displayed 19C even when the sensoCOMFORT in the hall showed 18.5C or lower. After that I largely ignore the sensoCOMFORT and keep an eye on the thermometer. The heat curve when the outside temperature drops to about 10C is set at 0.6 and this seems to work well. The flow temperatures always seem a bit lower than the curve suggests but the room temperature is stable at 19C. When the outside temperature drops further and the lounge thermometer will not rise above 18.5C, I adjust the heat curve to 0.65. When it is 0C or below and there are strong winds I raise it to 0.7. My heat curve has been dropped back to 0.6 since mid-January.

@robert-m Hi Robert, your observations are similar to mine in terms that the temperature from the sensocomfort is regularly below the target temp.  I don’t know if you have been following my thread over the past 2 weeks but I suspect your 7kW system might be better matched to your property than mine.  My heating need, even at peak, is 25% less than the capability of 3.5kW pump I have.  At -5C the sales blurb said COP of about 4.4 with a flow temp of 35c.  Accordingly I am dependent on the system cutting back output and probably reducing COP accordingly.  What sort of COP have you been getting over the past week?  Do you know the heat requirement of your property?

Hi David. The calculated heat loss of my house was 7kWh hence the recommendation for a 7kWh heat pump. In practice, I have not seen the heat pump use more than 3.5kWh even during the coldest weather. According to the Vaillant App, the COP for heating so far this week is 5.37 and last week it was 5.21.

When the outside temperature rises, the heat pump is very effective in adjusting down its effort, using less energy. At tickover, it will drop to 22W. My old gas boiler only ran at one temperature when it was on.

Posted by: @robert-m

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In practice, I have not seen the heat pump use more than 3.5kWh even during the coldest weather.

That would be 3.5kW of electrical power in. The power output into the house will at least double that - often more. A 7kW Arotherm+ will never consume more than 3.6kW of electrical power.  The 7kW refers to the rated output power, not input power.

Posted by: @robert-m

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At tickover, it will drop to 22W.

If its only consuming 22W of electricity, then the compressor is not running at all, just the water pump. This is not 'idling', its basically off.