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ASHP fault for DHW only with high set point – P1 error on 6kW Grant Aerona 3

 
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Air Source Heat Pumps (ASHPs)
Last Post by 2019ashp 2 days ago
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 2019ashp
(@2019ashp)
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Topic starter 16/02/2026 10:58 pm  

Hi, has anyone seen an issue where an air source heat pump warms radiators fine, and warms Domestic Hot Water fine when the setpoint is set lower to 40c, but gives an error when the DHW set point is set higher to 45C?

For context it's a 6 year old 6kw Grant Aerona 3, and it's been working fine until recently where we started seeing a P1 error. 

Resetting it gets it running again but we've noticed that it's always heating DHW when it trips, and that lowering the DHW set point seems to fix the issue permanently. 

Still, doesn't seem right that it can't deal with a 45C DHW setpoint.

System pressure is 1.5 bar, all valves are fully open, the mag filter is clean and the 2 port motorised valve for the DHW tank appears to be working fine so I don't believe there is any water flow restriction into the tank that would account for the P1 high pressure error. 


This topic was modified 3 days ago by Mars

   
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JamesPa
 JamesPa
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17/02/2026 8:51 am  

Hi

 

Im not a Grant expert but the Grant R32 manual says the following - it this the error?

 

image

I wonder how it detects this error.  One would think it was the flow meter but it could be temperature sensors.  You seem to have checked the obvious, the other things I can think of are: Is there a fine mesh filter somewhere (I have one on my Vaillant system that wasnt obvious until the installer pointed it out), is there a real time readout of flow rate which could confirm.  Are there any other real time diagnostics that might tell you whats going on?

I suppose its possible that its a compressor fault - has it been serviced recently?


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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 2019ashp
(@2019ashp)
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Topic starter 17/02/2026 12:32 pm  

Thanks for the reply. Yeah there isn't a mesh filter or strainer anywhere in the system, just the mag filter which is clean. 

It probably is due a service alright. Just curious that it only happens when DHW setpoint is increased. I'll try find the flow diagnostics too, to confirm it's a flow rate issue. 



   
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Mars
 Mars
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17/02/2026 3:27 pm  

@2019ashp I’ll ask Grant for their thoughts tomorrow… strange that it only happens in DHW.


This post was modified 3 days ago by Mars

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 ASHP-BOBBA
(@ashp-bobba)
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17/02/2026 10:25 pm  

Posted by: @2019ashp
↑

Thanks for the reply. Yeah there isn't a mesh filter or strainer anywhere in the system, just the mag filter which is clean. 

It probably is due a service alright. Just curious that it only happens when DHW setpoint is increased. I'll try find the flow diagnostics too, to confirm it's a flow rate issue. 

I don't know the Grant system but this is common when there is air trapped in the tank circuit which causes the water to have reduced flow and reduced heat exchange causing this fault. If there are thumb valves or AAV's on that section of line you should check they are open or open and bleed them, its effectively the same result as having air in the top of a rad, seems like it gets hot on the pipes but not doing much on the emitter.

 I would find the code that give you the flow rate on the water circuits and check it in heating and hot water and compare them.

 


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 2019ashp
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Topic starter 18/02/2026 12:15 am  

Cheers, yeah there's no AAVs or thumbvalves approaching the outdoor unit.

One of the first things I done was check the AAV on the outdoor but itself and purge some water through.  

The p1 error is often caused by an airlock but system is well bled in this case. 

 

 



   
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JamesPa
 JamesPa
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18/02/2026 8:02 am  

Posted by: @2019ashp
↑

Cheers, yeah there's no AAVs or thumbvalves approaching the outdoor unit.

One of the first things I done was check the AAV on the outdoor but itself and purge some water through.  

The p1 error is often caused by an airlock but system is well bled in this case. 

 

 

I think @ashp-bobba was referring to a possible airlock in the DHW circuit, near the DHW tank itself.  As this is usually a high point in the DHW circuit there should be a bleed valve (automatic or manual) near the tank. 

Given your symptoms that does seem very plausible to me; @ashp-bobba does this for a living so should know!

 


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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 ASHP-BOBBA
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18/02/2026 8:45 am  

Posted by: @jamespa
↑

Posted by: @2019ashp
↑

Cheers, yeah there's no AAVs or thumbvalves approaching the outdoor unit.

One of the first things I done was check the AAV on the outdoor but itself and purge some water through.  

The p1 error is often caused by an airlock but system is well bled in this case. 

 

 

I think @ashp-bobba was referring to a possible airlock in the DHW circuit, near the DHW tank itself.  As this is usually a high point in the DHW circuit there should be a bleed valve (automatic or manual) near the tank. 

Given your symptoms that does seem very plausible to me; @ashp-bobba does this for a living so should know!

 

Yes, I have seen "many times" where the indirect coil connections raise up but no air bleed, it only takes a small amount of air in the line to effect the flow rate.

Perhaps we can get a picture of the set up to see how it is? 

 

 

 


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 2019ashp
(@2019ashp)
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Topic starter 18/02/2026 8:46 am  

@jamespa Of course that's what he meant, I was half asleep 🙂  

I had look for a way of bleeding this last week but there doesn't seem to be a bleed valve on the grant monowave tank, nor an aav\thumbvalve on the plumbing circuit near the tank. Poorly designed IMO. 

But yes it does seem very, very plausible indeed with the symptoms. 



   
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 ASHP-BOBBA
(@ashp-bobba)
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18/02/2026 8:48 am  

@2019ashp can you post a pic?

 


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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JamesPa
 JamesPa
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18/02/2026 8:54 am  

Posted by: @ashp-bobba
↑

Yes, I have seen "many times" where the indirect coil connections raise up but no air bleed, it only takes a small amount of air in the line to effect the flow rate.

Thats worrying, its like an obvious place for air to collect; why would you not put a bleed there (no need to answer that!)


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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 2019ashp
(@2019ashp)
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Topic starter 18/02/2026 8:56 am  

@ashp-bobba thanks - I'm away for the day now but will do later, cheers



   
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