Grant Aerona alarms: a heat pump in a house built for a boiler
Grant Aerona alarms frequently describe the water circuit rather than the heat pump. See why retrofit installations produce them and what to record.
What this means
Grant Aerona alarms divide between the sealed refrigerant side of the heat pump and the water circuit it serves, and the second group is where most retrofit installations end up. A heat pump moves far more water than the boiler it replaced, so a circuit that was adequate before can leave the appliance short of flow.
Equipment and model context
Grant Aerona air source heat pumps documented in the Grant fault and support material
- Aerona3
- Aerona 290
- Aerona R32 range
The Aerona name covers several generations with different refrigerants and different controllers, and the alarm set differs between them. Take the model and serial number from the outdoor unit and read the alarm against Grant documentation for that unit rather than a general heat pump list.
What you may notice
- An alarm appears on the controller and heating or hot water becomes unavailable.
- Flow temperatures fall short of what the controller is asking for, particularly in cold weather.
- The system had been performing acceptably and began alarming in its second or third winter.
Likely causes
- Water flow below what the heat exchanger requires can arise from a pump setting, a partially closed valve, or emitters that restrict the circuit.
- Air in the circuit displaces water and interrupts flow without any component having failed.
- Refrigerant-side conditions raise their own alarms and require certified attention rather than heating work.
Diagnostic paths
It alarms only during the coldest spells
The system runs through autumn without complaint and begins alarming when outdoor temperatures drop and the highest flow temperatures are demanded.
Log two numbers each time it happens: how cold it was outside, and what flow temperature was being asked for. Margin that vanishes only at the extremes is a design question rather than a broken part.
Some emitters never get properly warm
Certain radiators or underfloor circuits run cooler than others, and the pattern has been present since the heat pump was installed.
Map which emitters heat well and which do not. A heat pump needs flow through all of them, and a circuit balanced for a boiler rarely delivers that unchanged.
It appeared in the second winter, not the first
The system was acceptable in its first heating season and began alarming during a later one without anything obvious changing.
Report that timing explicitly. Debris accumulation, a strainer loading, and air gathering over a year are all gradual, and the first winter can mask them.
Heat delivered is flow multiplied by temperature difference. Halve the difference, as a heat pump does against a boiler, and the flow has to double for the same output, through pipework that was never asked for it.
- Same heatThe house needs what it always needed
- Lower differenceFlow and return sit closer together
- More flowVolume rises to compensate
- Same pipesWhich may not pass it
| Property | Boiler practice | Heat pump requirement |
|---|---|---|
| Flow temperature | High, around emitter design | Lower, so more flow is needed |
| Temperature difference | Wide across the circuit | Narrow, which demands volume |
| Emitter sizing | Sized for high flow temperature | Often needs to be larger |
| Pipework | Adequate at low flow | Can restrict at higher flow |
The retrofit problem nobody sees at handover
A heat pump fitted to existing radiators and existing pipework inherits a circuit designed around an appliance that worked completely differently. The boiler sent water out hot and got it back much cooler, so it needed only a modest flow. A heat pump works across a much narrower temperature difference, which means the same heat requires far more water passing through the same pipes.
In mild weather nobody notices, because the house needs a fraction of its design output and the circuit copes easily. The first hard freeze asks for everything, and a circuit that was always marginal reveals itself as an alarm rather than as a plumbing complaint.
What to hand over, and to whom
The first question about any Aerona alarm is which side of the machine it belongs to, because that decides whether a heating engineer may proceed or whether a certified refrigerant handler is required. Establishing that from the documentation before arranging a visit avoids an appointment that ends with somebody unable to open the unit.
For the water-side group, the record that matters is the emitter map, the outdoor temperature at each alarm, and the flow temperature being requested. Those three pieces together let an installer decide in advance whether they are attending a component fault or a sizing conversation. For background, the flow temperature and emitters page sets out the relationship between water temperature and emitter output, and the heat pump operating temperatures guide shows how capacity changes as the weather does.
What to give the technician
Give the installer the Aerona model and serial number, the alarm photographed from the controller, the outdoor and requested flow temperatures, the emitter map, and the age of the installation.
Safe homeowner checks
- Photograph the controller showing the alarm before anybody clears it, together with any text alongside the number.
- Write down how cold it was outdoors and what flow temperature the controller had been calling for when the alarm arrived.
- Feel every radiator and underfloor manifold during a heating period and map which heat well and which do not.
Is a reset appropriate?
Expect the system to run again once an alarm is cleared, and to keep running until the weather asks for the same output. What identifies a marginal circuit is the shape of a whole winter, so keep a note of every occurrence rather than the most recent one.
When to call a technician
- Where an alarm sits on the sealed side of the machine, only a certified person may open it: the F-Gas framework governs that in Britain and the European Union, and Section 608 does the equivalent job in the United States.
- Adjusting pump settings, balancing a circuit, or altering emitters on a heat pump installation should be done by somebody working to heat pump flow rates rather than boiler practice.
Questions people ask about this
Why does my Grant heat pump alarm only in cold weather?
Cold weather asks for the highest output and the widest temperature lift the system will ever deliver. A circuit with little margin holds through mild conditions and runs out at the extremes.
Can my old radiators work with a heat pump?
They can where they are large enough for the lower flow temperature and the pipework passes the greater volume required. Where either falls short, the result appears as underperformance and alarms rather than as an obvious plumbing fault.
Who should attend a Grant Aerona alarm?
It depends which side the alarm belongs to. Water-side alarms are heating work; refrigerant-side alarms require a certified handler, and identifying the side beforehand avoids a wasted appointment.
Evidence recordHow this page was checked
official manufacturer support article · checked 2026-09-15
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Grant UK technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article
- Scope of the definition
- Grant Aerona air source heat pumps documented in the Grant fault and support material
- Last checked
- 2026-09-15