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Is it KISS or constant – weather compensation vs. set flow temperature?

cliffhanger · 2025-03-10T22:15:32Z

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: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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cliffhanger

(@cliffhanger)

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Topic starter 16/03/2025 10:38 am

Away at present. Hence the delay. Will get back asap.

Cliff

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cliffhanger

(@cliffhanger)

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Topic starter 28/03/2025 9:53 am

Okay. I have setup the system using just weather compensation. There is something called 'thermal blockade' on my smart controller which turns off the stat rather than turn it to maximum.

I'm going to video the heat curve but at the moment it starts at 18(20c) and runs to 45(-30). -5c = 33c. The OAT is 10c and the system is stable at 19.5c. I tried a lower curve but the pump cycled as expected, 25c is the lowest it will go.

What's struck me is how cool the rads are. They don't feel they could warm anything! But they do of course. Now we are at the end of March the system's not going to be tested at lower temperatures which to me is the breaker.

To be continued

Cliff

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JamesPa

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28/03/2025 10:01 am

Posted by: @cliffhanger

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What's struck me is how cool the rads are. They don't feel they could warm anything! But they do of course. Now we are at the end of March the system's not going to be tested at lower temperatures which to me is the breaker.

Indeed. Its weird at first but you get used to it and actually it is more comfortable because the temperature gradients are less.

There is a school of thought pursued by some installers that having cool radiators leads to call outs (which is probably true). Thus some installers force a minimum FT of 37, which is roughly the point at which radiators feel warm. This of course increases the cost to the householder.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Old_Scientist

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28/03/2025 10:20 am

We ran our radiations at anywhere between 30-35C over winter. At 30C I struggle to tell they are on. If I place my face over the top of the radiator, I can just about feel warm air convecting on my face. At 32C I can feel they are warm to the touch, and at 35C they are (relatively) positively hot!

I still walk around touching the radiators to see if they are on - I just need to trust the room thermostat, which doesn't lie.

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.

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Grantmethestrength

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03/04/2025 4:09 pm

I had to laugh at this as I do exactly the same thing, I read on here someone explain it well, the body is running at 36c so anything less will feel cold. As I explain to friends it is actually magic that heats my house!

This post was modified 6 months ago by Mars

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Si
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cliffhanger

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Topic starter 14/04/2025 8:59 am

Just as an aside. My system is heating only.

What is the best way of turning a weather compensation setup off in the summer? I thought of turning the stat back on to its lowest temp, in this case 10c. Won't go any lower.

Or setting the heating curve to its lowest?

Cliff

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