Heat Pump System Pressure
Heat Pump System Pressure: What It Is, What’s Normal, and What to Do if It’s Wrong
Heat pump system pressure is one of the most common things homeowners see warnings about – and one of the most misunderstood. This guide explains what system pressure actually means, what’s normal, why it drops or rises, and when you can fix it yourself versus when you should call an engineer.
What does “system pressure” mean on a heat pump?
System pressure refers to the water pressure inside the sealed heating circuit of your heat pump system.
This is the same concept as a sealed gas boiler system – it is not refrigerant pressure.
The pressurised water circuit includes:
Radiators or underfloor heating
Pipework
Buffer or volumiser (if fitted)
Indoor heat pump components
👉 It does not include the outdoor refrigerant circuit, which runs at much higher pressures and is not user-adjustable.
Where do I see the system pressure?
You’ll usually find it in one of three places:
On the heat pump controller screen
On a round analogue pressure gauge (often near the cylinder or indoor unit)
Inside a hydraulic module
Pressure is measured in bar.
What is normal heat pump system pressure?
For most UK domestic heat pump systems:
Cold system (not running):
👉 1.0 – 1.5 bar
Warm / running system:
👉 1.5 – 2.0 bar
Anything outside this range can cause:
Error codes
Lockouts
Poor heating performance
Always check your manufacturer’s guidance, but these ranges apply to most Daikin, Mitsubishi, Vaillant, Samsung, Panasonic and similar systems.
What happens if the pressure is too low?
Low pressure is by far the most common issue.
Typical symptoms
Heat pump not heating
Error code relating to pressure or flow
System keeps stopping or locking out
Radiators warm at the bottom only
Common causes
Small water loss over time (normal)
Recent bleeding of radiators
Minor weep from a joint or valve
Expansion vessel pressure issue
👉 Most systems will not run below about 0.8 bar.
What happens if the pressure is too high?
High pressure is less common but more serious.
Typical symptoms
Pressure rises above 2.5–3.0 bar
Pressure relief valve (PRV) discharges water outside
Pressure drops again after cooling
Common causes
Expansion vessel has lost its air charge
Expansion vessel incorrectly sized
System overfilled
⚠️ Repeated high pressure usually means a failed or flat expansion vessel, which needs an engineer.
How to top up heat pump system pressure (DIY check)
Most homeowners can safely top up system pressure if it’s low.
You’ll need:
The filling loop (usually a braided silver hose)
Access to the pressure gauge or controller
Basic steps (general guidance)
Make sure the system is cool
Locate the filling loop valves
Slowly open the valves
Watch the pressure rise
Stop at 1.2–1.5 bar
Close both valves fully
⚠️ Do not leave the filling loop connected and open
⚠️ Do not exceed 2 bar when cold
👉 If you’re unsure, check your manufacturer-specific guide or installer instructions.
If pressure keeps dropping, is that normal?
A small top-up once or twice a year can be normal.
But pressure dropping repeatedly usually means:
A leak somewhere in the system
A faulty expansion vessel
A PRV that has discharged and not reseated
Signs to look for:
Stains on walls or ceilings
Damp patches near radiators or pipework
Dripping outside from a copper discharge pipe
👉 Repeated pressure loss = engineer required
Does underfloor heating affect system pressure?
Yes – but indirectly.
Underfloor heating systems:
Contain more water
Are slower to show pressure changes
Often have multiple zones and actuators
Low pressure can cause:
Poor flow to UFH loops
Cold floors even though the heat pump is running
The pressure rules are the same as radiators – the system still needs to sit around 1–1.5 bar cold.
Is system pressure the same as flow temperature?
No – these are completely different things.
System pressure = how full the system is with water
Flow temperature = how hot the water is
If your heat pump is running but your home is cold:
Pressure OK? 👉 Check thermostats
Pressure OK + thermostats OK? 👉 Check flow temperature
Radiators cold? 👉 Use the heat pump not heating helper
When should I NOT touch system pressure?
Do not top up the system if:
Pressure is already above 2 bar
Pressure rises rapidly when heating starts
Water is coming out of the discharge pipe
You’ve topped up multiple times recently
In these cases, topping up can make things worse.
Quick pressure sanity check
✅ Cold pressure 1.0–1.5 bar → Normal
❌ Below 0.8 bar → Top up required
❌ Above 2.5 bar → Likely expansion vessel issue
❌ Constant drops → Leak or component fault
Related guides
Heat pump power reset
Heat pump not heating (sanity checker)
Heat pump thermostat settings
Heat pump error codes (by manufacturer)
Key takeaway
Heat pump system pressure is simple but critical.
Most pressure issues are easy to identify, many are safe to fix, but persistent problems should never be ignored.
If in doubt:
👉 Check the pressure
👉 Top up once if needed
👉 If it happens again, call an engineer