Daikin A5 fault codewhat it means and how to fix it
High pressure peak cut in heating, freeze protection in cooling. On an Altherma, Daikin’s first four checks are all water-side: pressure, flow, stop valves and air. Not the indoor filter of an air conditioner.
Take care
Which Daikin appliance is showing A5?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
An Altherma heat pump with radiators or a hot water cylinder
This page is yours. Daikin’s Altherma checklist starts with water pressure, flow, stop valves and air.
Read the pressure gauge, check zones are open, then book your installer.
A wall-mounted air conditioner with a remote control
A different fault under the same letters. On a Daikin split, A5 is an indoor unit freeze-up or high pressure code and a dirty filter is a published cause.
Use the Daikin A5 page in the air-conditioning section of this site, where cleaning the filter is the first step.
What A5 means
A5 is the clearest example of why an Altherma owner must not read a Daikin air-conditioning table. On a wall split, A5 is an indoor unit code about a frozen coil, and a dirty filter is a legitimate published cause you can fix yourself. On an Altherma, Daikin’s table gives "OU: High pressure peak cut / freeze protection problem", and the service manual explains the trigger: pressure too high in heating or domestic hot water, too low in cooling. The unit stops and the reset is manual. The checklist is what makes this page different from its air-conditioning namesake.
Daikin’s first four steps are the water circuit — check the water pressure, check the water flow, check that all stop valves of the water circuit are open, purge the water circuit — and only then do they move to the outdoor air thermistor, the expansion valve, the refrigerant stop valves, a clogged circuit, overcharge or shortage, and non-condensables. The logic is that in heating mode the high side of the refrigerant circuit is the plate heat exchanger where heat passes into your water.
If the water is not carrying that heat away — because pressure is low, flow is restricted, a valve is shut or air is sitting in the exchanger — refrigerant pressure spikes and the unit cuts out to protect itself. That puts A5 in the same family as the 7H flow codes from the owner’s point of view: the causes you can see are a low pressure gauge, a filter nobody has cleaned, a zone that has shut, and air after recent work. The refrigerant items further down the list are certified work, but they are reached only after the water side has been proven.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Water pressure too low | Daikin’s first check. Read the gauge before anything else; a system that has quietly lost pressure will not move enough water. | Common |
| Water flow too low | Daikin’s second check. A blocked magnetic filter, a closed zone or every radiator valve shut leaves the heat with nowhere to go. | Common |
| Closed stop valve or air in the water circuit | Daikin’s third and fourth checks. A valve closed for other work and not reopened, or air introduced by that work, both produce a pressure spike in heating. | Common |
| Refrigerant-side fault | The rest of Daikin’s list: outdoor air thermistor, expansion valve, refrigerant stop valves, blockage, charge, non-condensables. Reached only after the water side is proven, and certified work. | Occasional |
What to try yourself
Read the system pressure gauge. Low water pressure is the first item on Daikin’s own list for this code on an Altherma.
Check that heating zones are open and no isolating valve near the indoor unit has been left closed after other work.
Think about recent work on radiators, valves or the cylinder. Air in the water circuit is Daikin’s fourth check and follows any drain-down.
Book your installer and ask when the filter was last cleaned. If the water side checks out, the refrigerant items on Daikin’s list need an F-Gas certified engineer.
Do not clean the filter or purge the circuit yourself unless your installer has shown you how on your system. The primary circuit may contain glycol and the indoor unit carries mains.
Get an F-Gas certified heat pump engineer to look at your Daikin A5
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What it costs to fix
Typical repair cost
£90–£800
Time on site
20 minutes
An installer visit is typically £90 to £180 and covers pressure, purging and a filter clean. If the water side is proven and the fault is on the refrigerant side, an F-Gas certified engineer’s visit and any recovery or recharge takes it higher.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Daikin fault codes
- 7HGet it checkedA water flow problem. The sub-code matters more than the code: 7H-01 clears itself, 7H-04 to 7H-06 need a manual reset, 7H-08 is the pump, and 7H-07 is the unit freeing a stuck pump.Related: water flow
- C0Get it checkedFlow sensor malfunction: the sensor sees water moving 45 seconds after Daikin’s pump has stopped. Low pressure, air and an external pump running are on the list before the sensor itself.Related: water flow
- Bill has jumpedGet it checkedThe most important heat pump fault has no code. If your electricity cost has stepped up with no change in habits, the backup heater is probably doing the work.Related: same brand
- Ice on the outdoor unitGet it checkedNormal in cold damp weather, and the unit clears it itself. Daikin specifically forbid helping — no hot water, no de-icer, no scraping.Related: same brand
- Water at the tap is too coldGet it checkedDaikin give two causes and a remedy that costs money. Before using it, check what your schedule is actually asking the cylinder to do.Related: same brand
- U0Get it checkedShortage of refrigerant in the Altherma outdoor unit. Daikin’s own checklist ends with a leak test, the circuit is sealed, and on an R32 unit this is F-Gas certified work by law.Related: same brand
Own a Daikin heat pump? Save it and its codes come first next time.
Related guides
Common questions about Daikin A5
Should I clean the filter like the air-conditioning page says?
Not that filter. The air-conditioning A5 is about the indoor unit’s air filter, which an Altherma does not have. The filter that matters on a heat pump is the magnetic filter in the water circuit, and Daikin’s checklist reaches it through water flow rather than airflow. It is an installer job on most systems.
Why does a high pressure code have water-side causes?
Because in heating the refrigerant gives up its heat to your water in the plate heat exchanger. If the water is not moving enough to take that heat away, refrigerant pressure climbs until the unit cuts out. Daikin put pressure, flow, valves and air at the top of the list for that reason.
Is this the same as 7H?
Related. 7H is Daikin detecting low water flow directly; A5 is the consequence on the refrigerant side when flow or pressure is inadequate in heating. If you have seen both, the water side is almost certainly the place to start.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.