IBC Technologies · Error codes
IBC VX “Max. delta T exceeded” alert
Error – Max. delta T Exceeded
The manual lists this alert when the outlet water temperature is more than 45°F above the inlet (return) temperature, or when the outlet temperature is rising faster than 9°F per minute. Both readings point to too little water flowing through the boiler for the heat being produced.
Applies to
This alert is shown by the V-10 touchscreen controller on VX Series residential boilers (VX 110, VX 150 and VX 199), as documented in IBC Installation & Operating Manual — VX 110 / VX 150 / VX 199, doc 120-347E6, sections 9.2–9.4.
Controller: V-10 touchscreen controller. Source manual: 120-347E6 (2026-08-07).
Why a code is not a diagnosis
A large temperature difference across the boiler usually means low flow: a failed or air-bound pump, a closed or partially closed isolation valve, a stuck zone valve, or a system design problem. It can also be a faulty or mis-wired temperature sensor. The alert does not distinguish these.
Safe things to observe
- Note the alert text and whether it appears only on some zones or all the time.
- Check that any isolation valves around the boiler are in their normal operating position (but do not force valves).
- If radiators or in-floor loops have manual balancing valves, note whether some rooms stopped heating at the same time.
Do not
- Do not open or adjust valves beyond their normal position without knowing what they do.
- Do not keep running the boiler repeatedly while a no-flow condition may exist.
- Do not attempt to bleed the boiler yourself unless your installer has shown you the correct procedure.
How professional service helps
- A technician verifies pump operation and current draw, checks for a stuck or air-bound pump, and confirms flow direction.
- Isolation and zone valves are checked, and the system is balanced where needed.
- Supply and return temperature sensors are compared and replaced if out of tolerance.
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Frequently asked questions
What does delta T mean?
Delta T is the temperature difference between the water leaving the boiler and the water returning to it. A healthy condensing system runs a modest, steady difference; a large or climbing difference means the boiler is making more heat than the water flow is carrying away.
Boiler locked out? Call with the alert text ready.
The brand, model and this alert are passed along when you request service online, so the technician arrives prepared.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
- IBC Installation & Operating Manual — VX 110 / VX 150 / VX 199, doc 120-347E6, sections 9.2–9.4 · 120-347E6 · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.