- Boilers
- Gas boiler
A77
Beretta A77 boiler: low temperature thermostat alarm
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Code A77 with the bell icon on Beretta Ciao Green and Mynute Green E boilers signals a temporary stop from the low temperature system thermostat alarm (underfloor heating limit). It is self-resetting once the circuit cools; if the boiler does not restart, contact Technical Assistance.
Safety first
Stop and check this first
- Never bridge or bypass an underfloor heating safety limit thermostat to clear code A77, as that cut-out protects floor pipework and screed from temperatures above 45 °C, and if the alarm does not self-reset once cool you should call the Technical Assistance Centre.
- If you ever notice a gas smell near the boiler, open windows immediately, close the gas isolation valve if safe, do not touch electrical switches or create flames, leave the building, and call the gas emergency service from outside.
Safe checks you can perform
- Allow the heating circuit to cool and check whether A77 self-resets
- Try one single manual reset as your boiler manual describes
- Professional check of the T.B.T./A.G. loop (CN11 or M2) and PCB jumper JP7
Leave these tasks to a qualified professional
- Do not attempt this professional task: Test the external low-temperature safety thermostat and TbT (CN11) or M2 wiring
- Do not attempt this professional task: Configure PCB jumper JP7 for low-temperature installations (20-45 °C / T45)
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Test the external low-temperature safety thermostat and TbT (CN11) or M2 wiring
What they doVerify electrical continuity across the low-temperature safety thermostat and its connection to the white TbT jumper on 12-pole connector CN11 (Ciao Green) or terminal board M2 (Mynute Green E).
They must show youMultimeter continuity across the low-temperature thermostat loop and the terminal connections on CN11 (TbT) or M2.
In writingCut-out temperature of the external low-temperature thermostat and continuity test results at the boiler terminals.
Configure PCB jumper JP7 for low-temperature installations (20-45 °C / T45)
What they doCheck jumper JP7 on the control board and insert it on underfloor or low-temperature systems so heating flow temperature is restricted to 20-45 °C instead of the 40-80 °C standard range.
They must show youJumper JP7 fitted on the control PCB and the maximum heating setpoint capped at 45 °C on the display.
In writingConfigured jumper JP7 state (T45 20-45 °C range) and the heating flow temperature setpoint left on the boiler.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Allow the heating circuit to cool and check whether A77 self-resets
- Who:
- you, no tools.
- How:
- The official fault table states that A77 is self-resetting, so let the heating flow pipework cool down until the low-temperature thermostat closes, and check on the water gauge that cold pressure is between 1 and 1.5 bar (below 0.3 bar triggers A04).
- Result that matters:
- Once the low-temperature circuit cools, the safety thermostat closes and code A77 with the bell icon disappears automatically.
- If the boiler restarts automatically as the manifold cools, check that the heating temperature setting on the control panel is appropriate for a low-temperature system.
- If the pipes are already cold and water pressure is between 1 and 1.5 bar but A77 remains on screen, try one single manual reset via the mode selector.
- Evidence to keep:
- Photo of the display showing A77 with the bell symbol and the water gauge reading in bar.
Evidence: Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso · Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
Try one single manual reset as your boiler manual describes
- Who:
- you, at the panel.
- How:
- If the circuit is cold and A77 has not cleared on its own, make a single reset as your boiler manual describes (on the Mynute Green E the manual says mode selector to OFF, wait 5-6 seconds, then back to the winter heating position).
- Result that matters:
- Code A77 clears and the boiler resumes heating without tripping the low-temperature alarm again.
- If the boiler still does not restart after that single reset, do not repeat resets and contact the Technical Assistance Centre as instructed in the Beretta manual.
- If the boiler restarts but trips A77 again soon after firing, call an authorised engineer to inspect jumper JP7 (20-45 °C range) and the external limit thermostat.
- Evidence to keep:
- Note of the configured heating temperature on the display and the result of the single reset.
Evidence: Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso · Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
Professional check of the T.B.T./A.G. loop (CN11 or M2) and PCB jumper JP7
- Who:
- Beretta engineer.
- How:
- With the 230 V supply isolated, the technician tests continuity across the 24 Vdc low-temperature thermostat / generic alarm input (2-pole clamp on the cut white TbT jumper of 12-pole connector CN11 on Ciao Green, or Low temperature thermostat / general alarm terminals on board M2 of Mynute Green E) and checks PCB jumper JP7 (inserted for 20-45 °C T45 floor heating; removed for 40-80 °C T80 standard heating).
- Result that matters:
- The external limit thermostat loop is closed at normal operating temperatures and jumper JP7 matches the installed heating emitters.
- If an underfloor heating system is installed without jumper JP7 inserted (allowing 40-80 °C flow water), the engineer fits JP7 to enforce the 20-45 °C range and resets the manifold safety stat.
- If the external limit thermostat has failed open or the TbT/M2 jumper wire is broken, the technician repairs the termination or replaces the faulty thermostat.
- Evidence to keep:
- Service sheet confirming T.B.T./A.G. loop continuity on CN11 or M2 and the position of jumper JP7 (20-45 °C T45 vs 40-80 °C T80).
Accessing the 12-pole CN11 harness, terminal block M2, and PCB jumper JP7 requires opening the electrical enclosure where 230 V mains voltage is present, which must be left to an authorised technician.
Evidence: Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso · Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Beretta
- Product type
- Gas boiler
Known code variants
- A 77
- Beretta A77
- Beretta boiler A77
- Beretta error A77
- caldera Beretta A77
- error A77 Beretta
What it means
In the Beretta fault tables for Mynute Green E (English and Spanish) and Ciao Green (Spanish), code A77 with the bell icon (figure 34) means 'Low temperature system thermostat alarm' ('Alarma termostato instalaciones baja temperatura') and triggers a temporary stop. It activates when the external low-temperature safety thermostat or general alarm contact opens (wired via the cut white TbT jumper on 12-pole connector CN11 on Ciao Green, or across board M2 after removing the U-bolt on Mynute Green E). The alarm is self-resetting once the contact closes, and an A77 readout alone never proves any part is faulty, as missing jumper JP7 (40-80 °C vs 20-45 °C) or a loose TbT/M2 bridge can cause it.
Warnings and stop conditions
- Mediumsource verified
Never bridge or bypass an underfloor heating safety limit thermostat to clear code A77, as that cut-out protects floor pipework and screed from temperatures above 45 °C, and if the alarm does not self-reset once cool you should call the Technical Assistance Centre.
Evidence: Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso · Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
- Highsource verified
If you ever notice a gas smell near the boiler, open windows immediately, close the gas isolation valve if safe, do not touch electrical switches or create flames, leave the building, and call the gas emergency service from outside.
Evidence: Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso · Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
Symptoms and causes
Probable causes · in order
- External low-temperature or underfloor heating limit thermostat trippedCommon
When flow water temperature into a low-temperature manifold exceeds the safety limit thermostat setting, its contact opens the T.B.T. / A.G. circuit and places the boiler in temporary stop A77 until the pipework cools.
Evidence: Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso · Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
- Jumper JP7 not fitted or heating flow temperature set above the 20-45 °C rangePossible
On both Beretta series, PCB jumper JP7 sets the heating temperature range, so without JP7 the boiler runs in standard mode at 40-80 °C (T80), whereas inserting JP7 restricts flow temperature to 20-45 °C (T45) for floor installations.
Evidence: Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso · Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
- Open circuit or loose jumper on the TbT loop (CN11) or terminal board M2Possible
If the white jumper marked TbT on the 12-pole CN11 connector (Ciao Green) or the U-bolt bridge on terminal block M2 (Mynute Green E) is loose, cut without a closed thermostat connected, or has damaged wiring, A77 stays active.
Evidence: Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso · Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
Virtual technician
Does A77 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Beretta Ciao Green 25 y 29 C.S.I., manual de instalación y uso
Beretta · 2015 · manual
- 2
Beretta Mynute Green E 25 C.S.I. y 30 C.S.I., manual para el instalador y usuario (EN/ES)
Beretta · 2015 · manual
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