Poor differential across flow and return.

[dawoodseed]

Hello,

I have a Gas condensing combination boiler.

Make: Vaillant

Model:ecoTEC+ 937

Recently out of interest I decided to see what is the return temperature when the boiler is doing CH, I found that with a flow temperature of 60 degrees the return temperature was 54 degrees. Every time I check there appears to be only a 6 degree differential across the flow and return, I assume this is not so good and the differential should be around 11 degrees or 20 degrees.

Is there way to make the differential greater?

Is it do with the radiator sizes?

Thank You.

[Andrew Gabriel]

It doesn't have to be, and at 60C flow, you are most unlikely to achieve a 20 degree differential.

Does your boiler control the pump speed? In this case it's doing the right thing, maintaining the return temp below 55C, which is actually what matters for condensing operation.

Yes, and flow rate. Were your radiators sized for a condensing boiler (70/50) or a conventional boiler (more like 81/70)? If they were designed for 81/70, then at 60 flow I would expect exactly the drop you're seeing.

So it could be that boiler is modulating burner and pump speed exactly correctly, and/or that your radiators were designed for

81/70 -- they both give the answer you're seeing.

[dawoodseed]

Thank you,

I don't know how they were sized, but I forget to add that if the boiler is running both heating zones the differential is about 12- 15 degrees. One zone has 10 radiators and the other has only one. The 6 degree differential was with the 10 radiator zone.