Low flow temperatures and cycling

7 Aug 2008
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United Kingdom

I have been trying different flow temperatures and have found the following:

With flow temperatures of about 45 degrees to 60 degrees, the boiler switches on at the right time in the evening and starts to warm up, the flow temperature rises to what I have set (55 degrees) and I can hear the boiler lowering the output (modulation?) then the boiler switches off. The house has still not reached the desired temperature. After around 5 minutes the boiler comes back on and cycles until the house has reached the temperature.

With a flow temperature of 75 degrees, the boiler comes on and stays on until the house has reached the desired temperature the house warms up quicker which is what I would expect.

Do you think it is better to leave the flow temperature at 75 degrees?

Thanks in advance!
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Can't remember if you had just a new combi or a new system now and too lazy to read other posts again :rolleyes: :LOL:

I always leave my combis set for a ch flow of 60 on a new system I have installed or 65 on an existing rad system.

From your post I would go with the higher flow temp, as getting the rooms hot quickly so the external controls then shut the boiler off will be more economical.

I suggest you try 65-70C. The lower the return temp, the more the boiler will condense. It should always have a return of 56C (or is it 57) or lower, otherwise above this it will not condense.
I always set to ecotecs to a 70 deg c flow temp unless a 360 prog is used. if you want to economise try 65 in moderate weather and 75 in colder weather. You should consider weather comp and if you have a 360 prog the boilerstat should be on full as the 360 modulates the temp as per the load. ;)
if you have a 360 prog the boilerstat should be on full as the 360 modulates the temp as per the load. ;)

Well you learn something new everyday :rolleyes:

I only thought that you left the temp on full when using the 400 or the 430 :rolleyes:

Ta ;)
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I have a Vaillant ecoTEC+ 937 connected to a VRT360F soon to be replaced with a Honeywell CMT927. TBH I haven't tried 65 degrees, with 75 degrees radiators were burning hot and house warmed up very quickly which felt brilliant. Our old boiler (Baxi WM) never got the house to the right temperature.

Will try 60 degrees in mild weather, 65 in slightly cold weather and 70- 75 in very cold weather.

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