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Air Source Heat Pumps (ASHPs)
Last Post by BobTSkutter 1 year ago
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 Richard24738
(@richard24738)
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Joined: 2 years ago
Posts: 99
Topic starter 20/02/2025 1:47 pm  

Quick Question:

Just had a look at the guage next to the red expansion tank for heating.

It is reading just off zero when the pump is idling and circulators off.

When the heat pump and circulators start up, the guage gradually increases to 1.9.

I have never noticed the gauge at zero before. Is this normal? 


Install 13 April 2024 - 4 Bedroom Brick Detached - Heat Loss 9,281w, Design 45c at -2 - Ecodan 11.2kw R32 - 25L Buffer - 250L Telford Tempest HP DHW - All 16 radiators replaced - Auto Adapt - Mel-Pump app - Octopus Cosy Fixed
19 Jan 2026 - 1 x Sigenstor 10 battery and 8kw Inverter


   
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bontwoody
 bontwoody
(@bontwoody)
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Posts: 1003
20/02/2025 2:02 pm  

I believe it should read just above one. it may indictate a slow leak or air in the system. I would top it up a bit and monitor it.


House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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 Richard24738
(@richard24738)
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Posts: 99
Topic starter 20/02/2025 2:13 pm  

@bontwoody Thanks. I did top it up a few days ago and that's what made me look at it today.

If the pressure is gradually rising to 1.9 when the pumps are on, would this indicate that it doesn't need topping up with water?


Install 13 April 2024 - 4 Bedroom Brick Detached - Heat Loss 9,281w, Design 45c at -2 - Ecodan 11.2kw R32 - 25L Buffer - 250L Telford Tempest HP DHW - All 16 radiators replaced - Auto Adapt - Mel-Pump app - Octopus Cosy Fixed
19 Jan 2026 - 1 x Sigenstor 10 battery and 8kw Inverter


   
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bontwoody
 bontwoody
(@bontwoody)
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Posts: 1003
20/02/2025 2:18 pm  

@richard24738 Thats what I wasnt too sure about but I would imagine that as the water heats up it expands and increases the pressure. If it gets too low your heat pump will kick up and error so its not too much to get worried about unless you have a major leak somewhere.


House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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 BobTSkutter
(@bobtskutter)
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Joined: 1 year ago
Posts: 94
20/02/2025 5:49 pm  

It sounds to me as though the bladder inside the expansion tank has failed.  I recommend you get this checked as soon as possible before excessive pressure causes damage elsewhere.

Regards

Bob



   
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 Richard24738
(@richard24738)
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Posts: 99
Topic starter 20/02/2025 6:19 pm  

@bobtskutter So, just to be sure I understand, it shouldn't drop to zero when the heat pump is idling?

In heating mode, it seems to be staying on or below 1.9.


Install 13 April 2024 - 4 Bedroom Brick Detached - Heat Loss 9,281w, Design 45c at -2 - Ecodan 11.2kw R32 - 25L Buffer - 250L Telford Tempest HP DHW - All 16 radiators replaced - Auto Adapt - Mel-Pump app - Octopus Cosy Fixed
19 Jan 2026 - 1 x Sigenstor 10 battery and 8kw Inverter


   
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 BobTSkutter
(@bobtskutter)
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Joined: 1 year ago
Posts: 94
20/02/2025 6:57 pm  

No it shouldn't, the pressure should stay fixed at what ever you set it to - which is normally about 1.5bar.  It should never go to zero and back up to 1.5bar when it gets warm.

Let me provide some back ground.

That red expansion tank contains a balloon filled with compressed air.  If the air leaks out of the balloon because there's an AIR leak in the schraider valve on top of the cylinder, the pressure will suddenly drop very low.  Which is what you described.

As the system warms up the water will expand and the pressure will rise, and go up higher than normal.  You said it gets to 1.9bar.  That means the pressure has increased by almost 2bar as it's warmed up - which is a lot.  And that makes me think the air balloon (proper name is bladder) has depressured.

You can test the expansion vessel by opening the schraider valve cap (looks like a car tyre air valve) and see if water comes out, but I recommend you get a plumber to check it.

Also, if your system was operating at 1.5bar and then it went to zero that's quite a lot of water to leak out.  Could be 1 or 2 liters depending on your system size.  I'm pretty sure you'd notice 2 liters of water on the floor.

The other place water can leak without you knowing is via the pressure relief valve.  It could be anywhere on your system, if you know where it is have a look at the outlet pipe from the valve, if it's wet then the pressure relief valve as failed.

Regards

Bob



   
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 Richard24738
(@richard24738)
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Topic starter 20/02/2025 7:06 pm  

@bobtskutter Thanks. Chasing up my installer.


Install 13 April 2024 - 4 Bedroom Brick Detached - Heat Loss 9,281w, Design 45c at -2 - Ecodan 11.2kw R32 - 25L Buffer - 250L Telford Tempest HP DHW - All 16 radiators replaced - Auto Adapt - Mel-Pump app - Octopus Cosy Fixed
19 Jan 2026 - 1 x Sigenstor 10 battery and 8kw Inverter


   
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1
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 Richard24738
(@richard24738)
Estimable Member Member
Joined: 2 years ago
Posts: 99
Topic starter 21/02/2025 11:33 am  

@bobtskutter Hi Bob. Installer asked me to check the pressure relief valve on the top of the Red Expansion tank. Only air came out; pressure was 1.5 and the pump was running. So, unfortunately maybe not the expansion tank.

I have topped up the water so the pressure is back up to 1.5.

Not sure what a PVR looks like. Does it vent to the outside? There is a pipe on the outside - no noticable water but maybe that only occurs when the pressure drops to zero?

Pump is idling in this mild weather and is still at 1.5 but I will keep an eye on it.

Installer will come early next week.


Install 13 April 2024 - 4 Bedroom Brick Detached - Heat Loss 9,281w, Design 45c at -2 - Ecodan 11.2kw R32 - 25L Buffer - 250L Telford Tempest HP DHW - All 16 radiators replaced - Auto Adapt - Mel-Pump app - Octopus Cosy Fixed
19 Jan 2026 - 1 x Sigenstor 10 battery and 8kw Inverter


   
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 BobTSkutter
(@bobtskutter)
Estimable Member Member
Joined: 1 year ago
Posts: 94
21/02/2025 5:42 pm  

The valve on top of the red expansion tank is what's used to fill the air bladder inside the tank.  It's good that water didn't come out.  The installer should check the pressure inside the bladder to make sure it's set properly.  The proper value depends on how much water is inside the system, but it's typically about 1.5bar.

I've attached a pressure picture of a pressure relief valve, but the one you have might not look like this.  It sounds as though you've found the outlet line and it's not wet, so that means the valve isn't leaking.

Please keep an eye on the system pressure, if it starts to get to 2.5 or 3 bar then I recommend you turn off the heat pump and wait until the installer has checked things.

Regards

Bob

PRV


   
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 Richard24738
(@richard24738)
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Joined: 2 years ago
Posts: 99
Topic starter 21/02/2025 6:14 pm  

@bobtskutter Thanks. Pressure remains steady at 1.2 with the system idling after House Heating.

No idea what's trigering the drop in pressure.

I have attached a photo of a valve on the side of the Hot water tank. Is this what you are refering to?

PSV

 


Install 13 April 2024 - 4 Bedroom Brick Detached - Heat Loss 9,281w, Design 45c at -2 - Ecodan 11.2kw R32 - 25L Buffer - 250L Telford Tempest HP DHW - All 16 radiators replaced - Auto Adapt - Mel-Pump app - Octopus Cosy Fixed
19 Jan 2026 - 1 x Sigenstor 10 battery and 8kw Inverter


   
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 BobTSkutter
(@bobtskutter)
Estimable Member Member
Joined: 1 year ago
Posts: 94
21/02/2025 6:27 pm  

OK, that is a pressure relief valve for your hot water cylinder, not your heating system.  That valve is to protect the cylinder from bursting.  There is another one somewhere for the central heating system.

Is the red expansion tank you checked attached to the hot water cylinder or to your central heating system.  If you're unsure then getting the installer back to check things is a good idea.

You can loose pressure for several reasons.

The expansion vessel could have failed.

There could be air in the system that's being vented through an auto air vent, but loosing 1.5bar is a lot of air to "suddenly" vent from the system.

There's a water leak.

How long did it take to pressure the system from 0 to 1.5bar?  Did you have the fill valve fully open or only slightly?

You've got the installer coming to look, that's good.

Regards

Bob



   
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