E78 error code
ACV HeatMaster TC E78 error code – Mixing sensor short circuit
What does ACV HeatMaster TC condensing boiler and hot-water generator E78 mean?
ACV documents E78 for the HeatMaster 25, 35, 45, 70, 85 and 120 TC range as: Mixing sensor short circuit.
The code identifies the condition detected by the product; it does not always identify the failed component on its own. Confirm the full code and exact model before ordering parts or attempting a reset.
The ACV HeatMaster TC condensing boiler and hot-water generator E78 error code indicates aCV documents E78 for the HeatMaster 25, 35, 45, 70, 85 and 120 TC range as: Mixing sensor short circuit. This guide explains what the code means, the symptoms and likely causes, the checks that are safe to make, and when to stop and arrange an authorised repair.
Symptoms you may notice
The ACVMax display shows E78. The HeatMaster may stop firing, lock out, restrict heating or hot-water operation, or display a warning while protection remains active.
Common causes
The official HeatMaster TC fault table associates E78 with mixing sensor short circuit. The exact sensor, wiring, circulation, pump, combustion, gas-supply, configuration or controller condition must be confirmed on the appliance.
Before you start troubleshooting
Use the display, rating label or product menu to confirm the exact model. Similar-looking codes can have different meanings across product generations and regions.
- Record the complete code before switching the equipment off.
- Check only user-accessible controls, filters and external connections described in the manual.
- Do not open gas, refrigerant, combustion or electrical compartments.
Troubleshooting steps
Record E78 and the exact HeatMaster TC model. Check only the visible display, pressure, power and external conditions permitted by the user instructions. Internal sensor, wiring, gas, combustion, pump and controller tests require an approved engineer.
Can you fix this error code yourself?
The safest next step depends on the exact model and the cause behind the code. External checks may help, but a returning fault needs professional diagnosis.
A successful reset is not proof that the underlying issue is fixed. If E78 reappears, note when it happens and give that information to the service engineer.
Reset instructions
Use reset only after the condition has been corrected and only as described in the official ACV instructions. If E78 returns, stop resetting and contact an approved installer.
Applicable products and models
HeatMaster TC condensing boiler and hot-water generatorHeatMaster 25 TC; HeatMaster 35 TC; HeatMaster 45 TC; HeatMaster 70 TC; HeatMaster 85 TC; HeatMaster 120 TCManual A1005625-664Y6900 revision E, January 2021When to contact a professional
Contact ACV support or an approved heating engineer when E78 persists, returns after a permitted reset, or requires internal testing, adjustment or component replacement.
Frequently asked questions
What does ACV HeatMaster TC condensing boiler and hot-water generator E78 mean?
ACV documents E78 for the HeatMaster 25, 35, 45, 70, 85 and 120 TC range as: Mixing sensor short circuit.
Can I fix ACV HeatMaster TC condensing boiler and hot-water generator E78 myself?
Some external checks may be suitable for the user, but the result depends on the exact model and cause. Use the official manual and arrange service if the code returns.
Should I reset the product after code E78?
Use reset only after the condition has been corrected and only as described in the official ACV instructions. If E78 returns, stop resetting and contact an approved installer.
When should I arrange a repair?
Contact ACV support or an approved heating engineer when E78 persists, returns after a permitted reset, or requires internal testing, adjustment or component replacement.