Open Vented Heating System Problam

[gurus]

Open Vented Heating and Hot water system with 2 port valvs, 12 Radiators, are just warm and immersion tank takes ages to get hot. Boiler has just replaced last week but problam is still there. Pump has also been replaced.
New boiler is Vaillant 418. when the boiler runs in low flame then temprature rises very slow, but when goes in high flame then temprature shoots up to 80 degree and then comes back to low flame.

 

[doitall]

Was it installed by a gassafe engineer and what does he say about the problem.

 

[45yearsagasman]

Those symptoms you describe are circulation related.You should be getting approx 11C difference between flow and return pipes on boiler on high fire.Get you man back to check.