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Is it KISS or constant?

 
Air Source Heat Pumps (ASHPs)
Last Post by Mars 3 hours ago
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 cliffhanger
(@cliffhanger)
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Topic starter 10/03/2025 10:15 pm  

Hi

I'm new to the forum.

I've just had the latest Grant Aerora 290 fitted.  The 9kw version.

I'm experimenting with the best way to run this ASHP and it gets confusing. 

It's either the KISS method(1) see:

https://renewableheatinghub.co.uk/the-abcs-of-ashps-a-jargon-free-introduction-to-heat-pump-basics

Or

The 'constant' method according to Grant (2)

I've tried KISS using the OAT and heating curve, without any thermostat or other stats, and it was economical but I couldn't get the room temperature wanted it kept over shooting by up the 3 degrees.  I reduced the heating curve but was in danger of a ridiculously low curve(0.5) nowhere near my design temperature.

So I rang Grant. I was advised that KISS wouldn't work as I'm relying on water temperature only without a stat to achieve the desired room temperature.   They say I've got to run it constantly!   But that constant is a misnomer.   The pump must be left on but under the control of the stat on the smart controller.   One can schedule 'setbacks', especially at night, and according to lifestyle.   The LWT mustn't be allowed to cool too much.    This method also proved economical.

So which is it 1 or 2?   KISS is favored not I can't control it at present.

Cliff

 

 


   
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Mars
 Mars
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10/03/2025 10:28 pm  

Cliff, welcome to the forum.

You’re running into one of the classic debates in heat pump operation. Here’s my take. Others might disagree.

The fact that you’re seeing temperature overshoot with KISS isn’t surprising, and it highlights a key limitation of that approach, especially with some heat pump brands.

Grant is right in saying that if you rely solely on flow temperature without a stat, you don’t have a direct feedback loop for room temperature control. Weather compensation works on the assumption that the external temperature is the only variable affecting heat loss, but in reality, things like internal gains (appliances, people, solar gain) can throw that off, leading to overshooting. Your experience of a 3-degree overshoot suggests that either your system is responding too aggressively or your house has more thermal lag than expected, meaning heat keeps building up even after the system has backed off.

The issue with dialling down the heating curve too much is that, as you’ve found, you risk setting it so low that it won’t deliver enough heat when you actually need it. If you went down to 0.5 and still had overshoot, that suggests the system is struggling to modulate correctly at lower loads.

Grant’s recommendation of running “constantly” with a thermostat is a bit misleading, because as you noted, it’s not truly constant. It’s just ensuring that the system remains active, with the smart controller managing small adjustments. This approach can work if the thermostat has a wide enough hysteresis to prevent short cycling. If it’s too tight (e.g., calling for heat every half a degree), you’ll end up with inefficient on/off cycling, which defeats the purpose.

Since you’ve tested both approaches and found that Grant’s method also proved economical, it sounds like that’s the better fit for your setup. You might still be able to refine it, though. If your controller allows for it, setting a lower overnight setback and using a slightly less aggressive heating curve might help balance things out. It’s also worth checking if Grant’s controller has any form of adaptive learning, where it adjusts over time to reduce unnecessary swings.

At the end of the day, the best method is the one that keeps your home comfortable and efficient without excessive cycling. If KISS isn’t giving you the control you need and Grant’s method is working well, then that’s probably the way to go. That said, if you’re still fine-tuning, it would be interesting to see what happens if you use weather compensation but introduce a room stat with a very broad range; something that only kicks in if the room temperature strays too far from target rather than micromanaging every degree.

 

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