E135 on a CTC heat pump means "risk of freezing": the water leaving the heat pump got too cold during a defrost, so the controller raised an alarm to protect the heat exchanger. You will see it on the controller the heat pump is paired with, such as a CTC EcoZenith or EcoLogic display, and it is most common in a cold snap.
One alarm after a hard frost is not always serious, but a repeat is. If E135 comes back after a reset, book a heat pump repair so an engineer can find the cause before the system locks itself out. For every other CTC alarm, see our full guide to CTC heat pump error codes.
Please read first: this guide is here to help you understand what your CTC heat pump is telling you. Every effort has been made to keep the information accurate, but manufacturers update their manuals and codes can differ between models and software versions, so always check the official CTC manual for changes before acting on anything here. Do not attempt anything that could be dangerous, and avoid any work that could void your warranty. If in doubt, call a qualified heat pump engineer.
What does E135 mean on a CTC heat pump?
E135 means the controller has spotted a risk of the heat pump's water side freezing during defrost. CTC's manual says it fires when water leaving the heat pump falls below 15 °C during defrost, or when the inlet and outlet temperatures differ by more than 15 °C for 20 seconds.
CTC links it to low circuit temperature, low flow, or faulty HP in and HP out sensors. Its guidance is to keep the return temperature at 25 °C or more during defrost and to check the pump, filter, pipework and sensors.
E135 is a critical alarm. It needs acknowledging in the installer's coded settings, and after the fourth critical alarm the heat pump locks and shows a repeat code from the E218 to E221 range until an installer attends. Two related codes often appear alongside it:
- E211: water flow stayed under 10 litres a minute during defrost (15 on the 622 model), on EcoAir 600M units with a flow sensor and the 700M.
- E163: defrost ran past its maximum time without clearing ice from the evaporator, the finned coil on the outdoor unit.
Is ice on a heat pump normal?
Yes, some frost on the outdoor coil is normal in cold, damp weather, and the heat pump clears it by running a defrost. CTC's manuals note that a cloud of vapour after defrost is normal and should stop within seconds. Our guide to the heat pump defrost cycle explains how that works.
What is not normal is heavy ice that stays after a defrost, ice that keeps building through the day, or a defrost that triggers E135, E211 or E163. CTC lists poor heating or heavy ice after defrost as a sign of a faulty defrost control, a refrigerant leak or extreme weather. Our article on a heat pump frozen outside unit shows the difference.
Why does a CTC heat pump freeze up in cold weather?
It usually freezes up because not enough warm water reaches the heat pump during defrost. CTC's manual names low circuit temperature, low flow and faulty HP in or HP out sensors for E135, and for E163 it says the evaporator has not cleared of ice.
The following causes are general engineering knowledge rather than CTC's own wording:
- Sludge in radiators or pipework, which slows the flow the defrost relies on.
- A blocked strainer or dirt filter, which CTC also lists as a check for flow alarms.
- Air locks that stop water circulating properly.
- Low system pressure, often from a small leak; see heat pump losing pressure.
- A small system with no buffer tank, holding too little water to defrost safely.
- A failed defrost sensor or valve, so the unit cannot judge or complete a defrost.
- A frozen or blocked condensate drain, so melt water refreezes at the base.
- Snow, leaves or debris blocking airflow, or a unit sitting too low to the ground.
- Low refrigerant, which is an F-Gas engineer's job to diagnose.
What can you safely check yourself?
You can safely make a few outside checks, but leave anything inside the unit to an engineer.
- Keep the power on. Frost protection needs power, so do not switch the heat pump off at the isolator.
- Clear around the outdoor unit. From outside, brush away loose snow, leaves and debris around and under the unit by hand.
- Check the condensate drain path. Make sure melt water can drain away. If the drain outlet is frozen, thaw it only with lukewarm water, never boiling.
- Check pressure and valves. Read the pressure gauge against the range your installer set, and check radiator and isolation valves are open.
- Use holiday or frost settings, not the off switch. If you are going away, use the controller's holiday or away mode; our guide to heat pump holiday settings explains why.
- Reset once. Press Reset Alarm on the EcoZenith or EcoLogic display. Critical alarms need installer acknowledgement, so if E135 returns, stop and call us.
What not to do
- Never chip, scrape or pick at ice on the coil or fins.
- Never use boiling water, a heat gun or a hairdryer on the coil.
- Never remove panels or touch wiring inside the unit.
- Never touch the refrigerant circuit, which is legally restricted to F-Gas certified engineers.
When should you call an engineer?
Call an engineer as soon as E135 returns after one reset, because each repeat brings the heat pump closer to locking out. Book one straight away if:
- E211, E163 or a code from E218 to E221 appears.
- Heavy ice stays on the coil after defrost, or keeps building through the day.
- Pressure keeps dropping after a top-up.
- The house is losing heat and the controller shows the heat pump in alarm.
CTC links heavy ice after defrost to a possible refrigerant leak or defrost control fault. Refrigerant work can only be carried out by an F-Gas certified engineer, so do not let anyone without that certification open the refrigerant circuit. If the whole system has stopped, our guide to an air source heat pump not working covers the wider checks.
How can you stop it happening again?
The best prevention is keeping water flowing freely through the system, which an annual service does. These steps are general engineering knowledge rather than CTC instructions:
- Book an annual service. An engineer checks flow, pressure, sensors and the defrost, as set out in our heat pump annual service checklist.
- Keep the dirt filter or magnetic filter clean. A blocked filter is one of the quickest routes to low flow.
- Deal with sludge. If the system water is dirty, a heat pump powerflush can restore flow.
- Protect the water circuit. Where fitted, the correct antifreeze or inhibitor, or freeze-protection valves, help guard outdoor pipework.
- Insulate exposed pipes between the outdoor unit and the house.
- Raise the unit and keep drainage clear. Feet or a stand lift it above snow, and a clear path lets melt water away.
- Keep the area around it open. Avoid enclosing the unit or letting plants grow close.
For more seasonal advice, see our heat pump winter tips and guide to heat pumps in cold weather.
Other CTC codes you might see in a cold snap
Cold weather can trigger several other CTC alarms; each code links to more detail in our CTC error codes guide.
| Code | What it means |
|---|---|
| E211 | Risk of freezing, low flow during defrost |
| E163 | Defrost maximum time exceeded |
| E218 to E221 | Repeat alarms after four critical alarms; heat pump locked |
| E061 | Max thermostat (F10) tripped; CTC lists storage in extreme cold as one cause |
| E01, E02 (older EcoAir 105 to 111) | High or low pressure switch; an iced evaporator is one listed cause |
| E10, E11 (older EcoAir 105 to 111) | Defrost sensor 1 or 2 fault |
Need help with a frozen CTC heat pump?
Yes, we can help: our F-Gas certified engineers repair CTC heat pumps across the UK, including E135, E211 and E163 faults. We find out why the defrost is failing, whether that is flow, sludge, a sensor or the refrigerant circuit, and fix it with a 12-month guarantee.
If you have no heating in a cold snap, our emergency heat pump repair team aims for same-day visits where possible. Otherwise, book a heat pump repair. Regular heat pump servicing and our cover plans help stop freeze alarms coming back. See the heat pumps we work on, call 0333 041 5130 or contact us online.
Frequently asked questions
Is E135 the same as a frozen outdoor unit?
Not quite. E135 is about the water side getting too cold during defrost, while a frozen outdoor unit is ice on the air side coil. The two are linked, because a weak defrost leaves ice behind, but you can see E135 with very little visible ice.
Why does E135 only appear in winter?
The heat pump only needs to defrost when the outdoor coil frosts over, which happens in cold, damp weather. That is when a weak flow or a small system is put under the most strain.
Does E135 mean my refrigerant is low?
Not usually. CTC's listed causes for E135 are flow, circuit temperature and sensors. Low refrigerant is a possibility when heavy ice stays after defrost, and only an F-Gas engineer can test for it.
Could E135 damage my heat pump?
The alarm exists to prevent damage, so it is doing its job when it appears. The risk comes from repeated resets without fixing the cause, which can lead to freezing in the heat exchanger and a locked-out system.
How long does it take to fix a CTC freeze alarm?
Many causes, such as a blocked filter, low pressure or an air lock, can be fixed on the first visit. Sludge may need a powerflush, and a sensor or refrigerant fault may need parts ordering. We will explain the timescale once the fault is diagnosed.
