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Central Heating

Vaillant 637 not running on full flame

Neil Patel 24/11/2025 - 8.04 AM

I have a ecotec 637 system boiler, I have had 3 engineers out to diagnose an issue with boiler but either they don’t want to put labour to investigate or the boilers is is working as expected This boiler has been in commission since 2019 , I have seen it work since then, for the past month, when a demand for CH radiators is called the boiler doesn’t start on high flame to the max from cold. It is taking approx 20mins for the boiler to reach to a flow temperature of 65c that should really take no more than 3-5 mins . The reason is because it is not burning on full flame I am trying to identify cause but none of the engineers have been able to help The behaviour of flame not starting on full is same for HW and UFH demands Up until last month August , the boiler would reach the set flow temperature within minutes up to 5 and then the flame would modulate down to low for the rest of the time while it is servicing that demand What does this fault point to ?

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5 Answers

Art to Plumber

Rating: 5 out of 5
Sutton In Ashfield
It could be the is not enough gas going in so you need to to check the gas meter, or gas valve ,or the fan not moving fast enough
Answered4 November 2025
0

Blueflame plumbing and heating

Rating: 4.4 out of 5
Milford Haven
Hi, it doesn’t sound like the boiler is fully modulating, there must be flame rectification, as the boiler stays on.Personally I think it could be an intermittent problem with the pcb, as this Controls the modulation, Hope this helpd
Answered5 November 2025
0

Nic’s Plumbing and Heating Services

Rating: 5 out of 5
Gainsborough
Hi You need to check the status codes when the boiler is in operation as these tell you the “state” of the boiler. Yo can find information on how to access these in the boiler manual. Sometimes Vaillant boilers drop on low flame is they see a wide spread between the flow and return temperature - eg blockage or restricted flow. If this was the case the boiler would show S51 or something similar - status code S51. All the best
Answered6 November 2025
0

Infinity Gas, Heating & Plumbing Services

Rating: 5 out of 5
Bradford
The vaillant boilers always fire up on low load first, its to protect the heat exchanger thats perfectly normal. Things like insulation, pipe lagging and pipe sizing will make a difference also
Answered21 November 2025
0

AOS Heating and Gas Services

Rating: 5 out of 5
Leicester
Mass airflow needs to be replaced
Answered24 November 2025
0