Mitsubishi does not list a dedicated low water pressure error code in the Ecodan installation manuals we checked. Low pressure usually shows up as a low flow fault instead, most often L9, so check the pressure gauge first and then have an engineer look at the primary circuit if the code returns.
If it keeps coming back, book an engineer rather than topping up or restarting repeatedly. You can book a heat pump repair online or call 0333 041 5130, and our full Mitsubishi Ecodan heat pump error codes guide covers every other code.
Please read first: this is a guide only. We make every effort to keep it accurate, but manufacturers update manuals and codes differ between models and software versions, so always check the official Mitsubishi Ecodan manual for your unit. Do not attempt anything dangerous or anything that could void your warranty.
Is there an Ecodan error code for low water pressure?
No. In the Ecodan installation manuals we reviewed, the error tables contain no code that says “low water pressure”. The manual for the EHPT20Q-VM2EA cylinder, for example, sets a minimum primary working pressure and tells you to top up the water if the pressure falls below it, but it does not attach an error code to that situation. That is why this is a general low pressure guide rather than a single-code article.
What you are more likely to see is a flow-related code. L9 is defined as low primary circuit flow rate detected by the flow sensor (or flow switch on some models). The manual’s suggested checks are for water leakage, strainer blockage and clogged piping, plus the circulation pump and bleeding air from the primary circuit. A system that has lost water to a leak can easily end up with too little flow, so L9 is a possible knock-on effect of low pressure.
L3 is the circulation water temperature overheat protection. The manual notes that the flow rate may be reduced, and the same causes apply: leaks, a blocked strainer, clogged piping or a pump problem. It also says L3 can appear while the primary circuit is still being filled, in which case the filling should be completed and the error reset. LF is a flow sensor failure, which is a sensor fault rather than a pressure fault.
On figures, the EHPT20Q-VM2EA manual gives a minimum primary working pressure of 1.0 bar, a maximum of 2.5 bar, a pressure relief valve set at 3.0 bar and a commissioning pressure of 1 bar. Those numbers come from that particular cylinder, and other Ecodan units differ, so use your own manual’s figures and take the cautious reading if you are unsure.
Why does a Mitsubishi Ecodan heat pump lose water pressure?
The Ecodan primary circuit is sealed, so a falling gauge means water is escaping or has been absorbed into air pockets. Typical causes are:
- Leaks at pipe joints, radiator valves, underfloor manifolds or the connections on the cylinder or hydrobox.
- A failing expansion vessel whose air charge has dropped. Mitsubishi gives 1.0 bar as the charge for the vessel in the EHPT20Q manual, and a vessel that has lost it cannot cushion the pressure swings.
- Air in the primary circuit, which also raises the risk of an L9 flow fault.
- A pressure relief valve discharging after being over-pressurised or after a vessel failure. The outlet is often piped outside, so check there.
- Recent bleeding, radiator changes or maintenance that removed water without a proper top-up.
- A leaking automatic air vent on the hydrobox or pipework.
A modest rise in the gauge when the system is hot is normal because water expands, but a gauge that ends up lower after every cycle points to a loss of water or a tired expansion vessel.
What can you safely check yourself?
These checks need no tools and no panels removed. If you are not comfortable with any step, stop and call an engineer.
- Read the pressure gauge with the system cold and compare it with the minimum in your manual. For the EHPT20Q cylinder that minimum is 1.0 bar.
- Note any code on the controller, such as L9 or L3, and when it appeared.
- Look for visible drips at the cylinder or hydrobox, pipework, radiators and the relief valve discharge outlet.
- Check radiators and zone valves are open, including any thermostatic valves that may have been turned right down.
- Top up through the filling loop only if your manual allows it and you are competent, taking it up slowly to the correct figure and closing the loop fully afterwards.
Our guide to why a heat pump keeps losing pressure goes into the repressurising process and what a normal rate of loss looks like.
What not to do:
- Do not attempt any refrigerant work. Refrigerant handling is for F-Gas certified engineers only, and a low water pressure fault is not a refrigerant fault.
- Do not remove the outdoor unit or indoor unit panels. Live electrics, moving parts and pressurised components sit behind them.
- Do not keep over-pressurising to be safe. Pushing the gauge towards the maximum can lift the pressure relief valve and stress joints.
- Do not ignore repeated top-ups. If you need to refill the system more than once or twice a year, water is escaping somewhere and it needs finding.
- Do not open or clean the built-in strainer yourself unless you know how to isolate the circuit. Opening it on a pressurised system will release water quickly.
When should you call an engineer?
- L9 or L3 appears more than once, or the heat pump stops heating altogether.
- The gauge falls within days of a top-up.
- The pressure relief valve is dripping or has blown.
- You can see water around the cylinder, hydrobox or pipework.
- The pressure jumps up sharply when heating starts, which suggests the expansion vessel has failed.
Booking early matters here. A small leak caught in week one is usually a cheap fix, while months of topping up can let air and fresh oxygen into the circuit and cause sludge and corrosion.
How can you stop it happening again?
- Book a yearly service so the strainer, expansion vessel charge, air vent and gauge pressure are checked. Our annual service checklist lists the items an engineer should cover.
- Fix small leaks early. Our guide to a heat pump leaking water helps you tell condensate from a system leak.
- If the circuit is dirty or the strainer keeps clogging, consider a heat pump powerflush and a magnetic filter.
- Have the system commissioned properly. Our commissioning checklist covers filling, flushing and noting the cold pressure.
- If the unit stops heating during cold spells, read our winter tips so freeze protection does not catch you out.
Other Mitsubishi Ecodan codes you might see with low pressure
Mitsubishi lists no pressure code, so these are the flow-related Ecodan codes most likely to appear when pressure or circulation is poor.
| Code | Meaning | First step |
|---|---|---|
| L9 | Low primary circuit flow rate detected by the flow sensor or flow switch. | Check the gauge and valves, then book an engineer to check for leaks, strainer blockage, air and the pump. |
| L3 | Circulation water temperature overheat protection. Flow may be reduced by leaks, a blocked strainer, clogged piping or a pump fault. | If the system was just refilled, let the filling finish and reset. Otherwise book an engineer. |
| LF | Flow sensor failure. | Leave this one to an engineer, who will test the sensor and cable. |
Code letters and numbers can vary between Ecodan generations and controllers, so match yours against the manual for your exact cylinder or hydrobox.
Need help with low pressure on a Mitsubishi Ecodan heat pump?
UK Heat Pump Servicing engineers find the leak, check the expansion vessel and air vent, and carry out heat pump repairs with a 12-month guarantee. We repair Mitsubishi Ecodan systems, clearing strainers, recharging expansion vessels and putting the primary circuit back to the correct pressure. Our annual heat pump service catches pressure problems before they stop your heating, and we also offer cover plans if you would rather spread the cost of looking after the system. If your gauge is low or a code is showing, contact us or call 0333 041 5130.
Frequently asked questions
What pressure should an Ecodan be at?
It depends on the model. The EHPT20Q-VM2EA manual gives a 1.0 bar minimum and 2.5 bar maximum working pressure, but always use the figures in your own installation manual or the label your installer left.
Why does my Ecodan pressure rise when the heating is on?
Water expands as it warms, and the expansion vessel absorbs the extra volume. A gentle rise is normal, but a big jump or a pressure that never recovers when cold means the vessel needs checking.
Is L9 always a leak?
No. L9 is a flow fault, and a blocked strainer, trapped air, a seized pump or a faulty flow sensor can all cause it. A drop in pressure is one possible cause among several.
Can I reset an Ecodan error myself?
You can acknowledge a code through the controller if your manual allows it, but resetting does not fix the cause. If the code comes back, stop resetting and book an engineer.
Does a low pressure fault stop hot water too?
It can. If the primary circuit flow drops, the unit may lock out and stop heating the cylinder as well as the radiators, so an engineer should look at it promptly.
