Hi Jonathans,

I had an Aerotherm Plus installed a few weeks ago, the heat curve was also set at 1.2.  Check out the manual for your controller, I expect you will find 1.2 is a setting for a gas boiler.  It will also likely say that ASHP's should be set to 0.6.

I have been able to drop mine to 0.4 where it copes ok and runs at a low radiator temp even in the colder weather.  24 Deg at the 12.5 Deg outside at the moment, it's hardly ticking over.

Good luck

6.2.3 Heating curve In heating installations with a heat pump, the heat curve for all heating circuits is 0.6.

In all mixed heating circuits with boilers, the heat curve is 0.6.

In all direct heating circuits with boilers, the heat curve is 1.2.

Setting the heating curve (→Page 16)

I should mention also my set point on the sensocomfort is 21 Deg and it usually holds around 22/21.5.  A radiator valves fully open, running 24/7.  The property is 4 bed detached c1995, reasonable insulation, wall, double glazed windows, no floor, loft topped up.  Ok but not to a very high standard.

I feel it might be possible to drop the heat curve below 0.4 though I haven’t tried it and might not be sensible.  From what I can see the heat curve becomes a trial and error exercise to see what works over time.  1.2 was creating a large overshoot and with the above arrangement it seems to now work really well, its not long enough to get a feel for the cost though seems more efficient than oil on limited data.

Something called 'expanded' control setting was recommended to stop the overshoot which turns the heating off once the set point has been exceeded though the heat curve fixed the issue without having to apply that, lowering the flow temperature seemed to be the better solution.

Adding up the running totals, it looks like I have 2.8 COP though part of that is with the higher heat curve and through the colder weather and I'd expect that to improve with the lower setting.

According to Greenmatch: For comparison, in mild weather COP can be about 4.0, but when the temperature drops to 0°C, the COP can also decline to 2.5. On average, the COP of typical heat pumps has a seasonal variation of about 2.5-2.8.

So, for a retrofit likely about right and I anticipate will turn out better than average over a longer period.

Hi @jaspera -- thanks for that info. It looks like we have different controllers (I have a VRC720F) and in the manual Vaillant have removed the text you quote from 6.2.3 for some reason, so it's handy to have that here.

Over the past couple of days, with the heat curve at 0.8, I'm seeing a COP between 4 and 4.5. Of course the weather is very mild right now so it's hard to make a real judgement about if it's just down to the heat curve or not (though in a mild spell a couple of weeks back the best I saw was 3.5, at a heat curve of 1.2). I'll be interested to see whether this heat curve can still keep the house to the desired temperature when the weather cools off, and if so whether I can bring it down further.

Posted by: @jonathans

Hi @jaspera -- thanks for that info. It looks like we have different controllers (I have a VRC720F) and in the manual Vaillant have removed the text you quote from 6.2.3 for some reason, so it's handy to have that here.

Over the past couple of days, with the heat curve at 0.8, I'm seeing a COP between 4 and 4.5. Of course the weather is very mild right now so it's hard to make a real judgement about if it's just down to the heat curve or not (though in a mild spell a couple of weeks back the best I saw was 3.5, at a heat curve of 1.2). I'll be interested to see whether this heat curve can still keep the house to the desired temperature when the weather cools off, and if so whether I can bring it down further.

Hi Jonathans,

Further to my previous message. Expanded Adaptive Heat Curve control as shown in section 2.12.4 of the manual would probably be the most effective in your situation.

Posted by: @derek-m

Further to my previous message. Expanded Adaptive Heat Curve control as shown in section 2.12.4 of the manual would probably be the most effective in your situation.

I agree about the adaptive heat curve using the expanded room temp, and that's what I'm working toward. Since the manual asks that the suitable heat curve be set as part of implementing the adaptive one, I'm currently trying to determine what that is. It definitely wasn't 1.2. So far 0.8 is looking significantly better, but the warm weather obviously affects how much I can learn about suitable curves right now.

Instead of trying to test different curves, do you think it would be better to just implement the adaptive heat curve using 0.8 as the suitable building curve & see what happens as the weather cools off? My plan had been to keep watching this curve for a while as outdoor temperatures are supposed to drop some later in the week, and then perhaps try 0.6 if that seemed possible. However, I'm also eager to try the adaptive curve setting so I welcome any shortcuts to getting there.

Hi @jonathans 

My understanding of what Vaillant describe as 'Expanded' control, is that the controller initially sets the required water flow temperature with reference to the outside air temperature, but also measures the indoor air temperature and then adjusts the water flow temperature to achieve the desired indoor air temperature. If the indoor air temperature is below the setpoint, then the controller would increase the required water flow temperature slightly, so as to raise the indoor air temperature.

I would estimate that with your system it should be possible to set your controller for Expanded control with the present weather compensation curve without causing any problems.

Hi jonathans

It would be interesting to see how expanded works out for you if you try that.

For me it did work well to stop a temperature overshoot up to 24 Deg with 21 Deg setpoint with 1.2 curve. Although it seemed to fix a problem introduced by the curve being set too high.

Just my experience but I found it introduced a sawtooth type control where it would heat to exceed the set point, switch off, the temperature would drop, switch on and repeat, which is what it seems designed to do.

– The zone is deactivated: Current room temperature + 2/16 K > set room temperature
– Zone is activated: Current room temperature < set room temperature - 3/16 K

I didn’t find the hysteresis of this pleasant as it dropped the temperature in our lounge excessively before it would restart once the sensor (positioned elsewhere) had detected that the temperature was too low.

Overall the heat pump is much, much better than our oil boiler in that the temperature is now a very pleasant c21/22 Deg all the time, no freezing cold mornings or other off periods while the heating catches up. Very stable and consistent.

Having spent a small fortune on this advanced system I don’t feel like going back to an on/off arrangement and moving away from the gentle top up of heat and consistent temperature. I might achieve a more efficient configuration possibly but the lower curve setting seems to have eradicated the overshoot and led to a better control for me. I'd be interested to try the adaptive setting and if you also do, how you find it works out.

Thanks both, I appreciate your input. I think I'm going to leave things as I have them til the end of the week, for data collection, and then try switching to expanded mode. I'll let y'all know how that works out.

I didn't wait til the end of the week and switched over to expanded mode + adaptive heat curve last Tuesday. I'm not wholly sure what to make of the data I'm seeing.

When we had a heat curve of 1.2 the graphs of temperature changes in the house were fairly spiky. Changing the heat curve to 0.8 smoothed them out quite a lot, but with theadaptive heat curve on they've gone back to being spiky, like @jaspera describes above. I'm also overshooting the requested temperature in places. The flow temperature has gone up a little bit, from around 35C now up to 40C.

Presumably as a result of the increased flow temperature the radiators are noticeably hotter, and hotter than they were when I had the heat curve at 0.8, which was heating the house just fine -- so clearly hotter than they need to be. In addition, the power use seems to be about the same now as it was before I made any changes to the heat curve. So something is going on in the background here because there is a lot more heat and no correlated increase in cost.

It's hard to see what difference the expanded mode change has made to the COP on its own, because of the 1.2-to-0.8 heat curve change coinciding with the warm spell at the end of December, but I can at least compare where I am now against similar outdoor conditions from before that heat curve change, and it looks like there's an approx 30% improvement in the COP. So again, something is clearly going on in the background because I have an increase in flow temperature, hotter radiators, and a higher COP. This is the opposite of what I would expect to have happened to the COP.

I do not need the radiators to be this hot. The drop to a 0.8 heat curve showed me that cooler radiators could warm the house as much as I needed. Since I do not have direct control over the flow temperature I'm going to try to lower the flow temperature.

The next experiments I want to try are:

• First, keep the adaptive heat curve on and lower the "desirable heat curve" to 0.6
• After evaluating that, keep the adaptive heat curve on and change from having some periods of higher heat request (morning and evening) to having just a single constant temperature through the day
• Finally, if none of those lower the flow temperature then try disabling the adaptive heat curve and just letting the system run on a 0.6 curve

I would be interested in any thoughts anyone has on what I'm seeing here and my plans to change things.