Free tool
Temperature rise (ΔT) calculator
Flow minus return across the appliance. Enter both and get the difference.
Let the system settle at the rate you are testing before you read it.
What the number is telling you
ΔT is the heat the water gave up on its trip round the system. Heating is commonly set to around 20 °C, and the manufacturer's instructions say what their appliance expects. On a condensing boiler the return temperature matters as much as the difference, because the flue gases only condense when the return is low enough.
| Reading | Usually means | Where to look |
|---|---|---|
| Wider than expected | Not enough flow | Pump speed, closed or seized valves, blocked filter, sludge, airlock |
| Narrower than expected | Too much flow | Pump running faster than needed, bypass passing too much |
| About right | Flow matches output | Check the return is still low enough to condense |
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Stop guessing what that model expects
Flint carries the manufacturer figures for 1,100+ gas boilers, so the temperatures, pressures and combustion targets you are checking against come from that appliance's own manual with the page cited, not from a rule of thumb.
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Common questions
What is a normal ΔT on a boiler?
Heating systems are commonly set to around 20 °C between flow and return. Manufacturers specify what their appliance expects, and a condensing boiler runs at its best efficiency when the return temperature stays low enough for the flue gases to condense.
How do I measure ΔT?
Take the flow and return temperatures across the appliance once the system has settled at the rate you are testing, either from the boiler's own display where it shows both, or with clamp probes on the flow and return pipework.
What does a ΔT that is too wide mean?
A wider than expected difference usually means not enough flow through the appliance. Look at the pump speed and settings, closed or partly closed valves, a blocked filter or strainer, sludge in the system, and an undersized or airlocked circuit.
What does a narrow ΔT mean?
A narrow difference means the water is moving through faster than the appliance is heating it, so the pump may be running faster than needed or a bypass may be passing more than it should.
Flint is a reference tool for competent, Gas Safe registered engineers. Everything here informs your judgement and never replaces it. Always work to the manufacturer's instructions and the current standards, and check every figure against the appliance data badge and manual.