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Can anyone explain the following behaviour with a Grant Aerona 3 R30 / Smart Controller?

Unsure · 2026-04-10T09:05:57Z

Occasionally, during a Set Back period the compressor either randomly comes on or cycles very rapidly (see graph). I’ve tried adjusting the ‘Set Back’ and ‘Decreasing Fixed Water Temp’ to stop the compressor activating in relation to flow temperature. Here is one example: During a night last summer, the outside, inside, and flow temperature were all between 20 & 24 degrees. The Set Back was at 5 degrees and Decreasing Fixed Water Temp set to 15 Degrees, yet the compressor came on and heated the rads up. The following night I changed the Set Back to 18 degrees, and the Decreasing Fixed Water Temp back to 4 degrees. Again, the compressor came on during various times in the night. If you look at the two graphs, one shows normal operation during ‘Set Back’ where the compressor remains off. The second graph shows the compressor coming on randomly. The internal and flow temp was around 22 degrees, and the outside temp is between 5 and 7 degrees, so the compressor should not have come on. Conversations with Grant, and reading the manual confirm with pump blockade off, the compressor is controlled by the flow temp and not the room temp. Yet despite the flow, inside, and outside temperatures being the same during different Set Back periods, the compressor remains off on some nights and randomly switches on during others. No one I've spoke with to date can offer any suggestion to what is going on.

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Air Source Heat Pumps (ASHPs)

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4834

12/04/2026 5:43 pm

Posted by: @unsure

↑

Mr Smith sets his WC and Stat on his Aerona 3 R32 / Smart Controller to achieve a house temperature of circa 20 Degrees. During the night, Mr Smith prefers it cooler, so the installer tells him to have a 3 degree Set Back (17 Degrees). Mr Smiths house maintains a minimum of 19 Degrees throughout the night. However, under the settings he was told to use by the installer, his HP comes on during the night (Set Back) heating the house up to 21 Degrees. How should Mr Smith set up his HP?

Unfortunately Mr Smith may need to adjust his expectations, more because of the house than because of the heating technology. He may be basing his expectations on what he knows from a relatively high loss house, which has a completely different dynamic, or on brainwashing by the aftermarket controls industry.

Having a low loss house, he nevertheless expects it to cool significantly at night to 17C from 20 even when it has been warm during the day and mild at night. That isn't going to happen whatever the heating technology because of the energy stored in the fabric of the house combined with the low loss. The only way Mr Smith could make this happen is active cooling.

He then expects it miraculously to rise back up to 20 in time for breakfast. That's a bit closer to possible but only if he starts heating it before he needs it to be warm, again because of the energy that needs to put into the fabric. His heating system does that but, because we don't yet have native controllers that use machine learning to deal with the vast variation in house response characteristics, it slightly overshoots, which the manufacturers probably consider is better on balance than undershooting.

The solution is probably for Mr Smith to abandon night time setbacks, which are unlikely to work in moderatey well or well insulated houses (whatever the heating technology), and instead do what MCS recommend, namely to set up rooms in which Mr Smith sleeps to be heated to a slightly lower temperature than the rooms in which he spends most of the day. This will probably result in lowest possible running costs and a high level of comfort.

If Mr Smith would somehow feel happier if the heat pump didn't come at all when the room temperature is above the target temperature he should put his heat pump into pump blockade mode. However he may experience both greater temperature swings and higher running costs if he does this, which he may or may not find more comfortable.

I hope that helps, happy to explain further or suggest solutions to problems experienced with the house comfort.

This post was modified 4 weeks ago 20 times by JamesPa

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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Unsure

(@unsure)

Active Member Member

Joined: 7 months ago

Posts: 13

Topic starter 14/04/2026 5:24 pm

Final Update.

Yesterday, I was going through the motions of looking at how each menu impacts the Set Back flow temp. I considered the combination of WC, Set Back, Decreased Water Temp - a parameter in the Circuit 1 installer menu ( although termed Decreased Fixed Water Temperature in the controller menu). I then compared the settings I have now with past settings and accounted for Flow, IAT, and OAT temps including WC. I concluded that on at least 90% of nights during the shoulder months, and almost throughout the winter, the flow temp needed to be 19 degrees or lower (inlet and outlet) before the HP came on during Set Back. It was at this point I walked away from the computer and used logic. Apart from an odd event, this has not been an issue for two years on the same settings I have now, so something as changed and it isn’t the weather.

Then I got woke up in the night with the Heat Pump going on and off AGAIN!

At around 2:45am the inlet and outlet temperatures were 28 and 29 degrees respectively (see image). The IAT was 22 degrees, and the OAT was 5.3 degrees. The curve was quite low at 6 with a -1 shift. For me, and other Aerona 3 owners I’ve spoken with this is not right. I’ve gone through months of graphs where during the night the IAT and OAT are similar and often much lower than last night (Inlet / Outlet 20 degrees, IAT 19 degree and OAT 3 degree) without the HP coming on.

This is the clearest example so far that something is not right. Ironically, since first posting this on the forum, this issue has gone from appearing once every four to six weeks to every night.

I found another Aerona 3 owner with a similar issue who found that a factory reset on the Smart Controller, and checking for loose or poor CAT 5/6 connections cured the problem. I’ve carried this out this afternoon.

If it happens again tonight, I’ll schedule the Pump off during the night and put up with the circulation pump pushing through the ABV during frost protection (for at least the rest of the cold spell). Something will give at some point. If not, then it’s time to ditch the Grant and replace it with another manufacturers HP. Hopefully, this post might aid other Aerona 3 owners if they experience similar issues, and like me, think they’re going insane!