Condensing Mode?

Viess

Will 140 degree return water be low enough for a condensing boiler to do it's thing? And will a concentric flue pipe setup help the condensing process at 140 return temp? Thanks.

Paul Pollets

Condensing mode

The dew point of natural gas is 128 degrees F. When the return water is less than the dewpoint of the type of fuel being burned, the boiler is considered in "condensing mode"  The concentric venting is preferred to CPVC venting, but has no effect on return water temperature. System design does.



I've been using the Vitodens on wall panel radiators for years and use the correction factors available for wall panel sizing to work at 140, rather than 180.  The panel rads are typically 30% larger, but work at lower temps and allow the boiler to modulate into condensing mode 90% of the time.

eluv8

Condensing Efficiencies

Just to add to what Paul has mentioned, Dew point within a boiler as explained to me is relative to several factors. Humidity, Return Water Temperature, and Excess Air. There are charts that show the relationships and how they effect condensation and efficiency, as excess air increases dew point decreases as well. I have been to several jobs where the boiler was supposed to be condensing with 115 return water but due to the levels of excess air and probably other factors I don't understand the boiler was only operating at about 89%, yet combustion numbers where all within factory spec. I would really like to see a real world test replace the current testing procedures, however I am not holding my breath. Meanwhile I will continue conducting my own study.

Viess

Condensing?

Thanks guys.