Hot Tip #22 – Boiler Blowdown Exchanger

Boiler blow-down occurs from two different boiler locations. The first location is from the bottom of the boiler water reservoir.

This is known as “bottom blow-down” or “rapid blow-down”. It is usually less than 10 seconds in duration and depending on water quality is performed once or twice per shift.

This blow-down is intended to remove “mud” from the bottom of the boiler. The high velocity and short duration of this blow-down do not make it a good candidate for heat recovery.

The second location is from the top of the boiler water reservoir. This is known as “top blow-down”, “surface blow-down” or “continuous blow-down”.

This method of blow-own is continuous in duration and the volume of blow-down depends on water quality; as determined by boiler water chemical analysis.

This method of blow-down is intended to remove contaminates that are floating on the boiler water surface. The majority of all boiler blow-down water is from this “Surface blow down” method.

I measured a heat exchanger on a 400-horse power boiler that was heating water, which had a flow rate of 2.4 GPM, 90 degrees.

This boiler had fairly clean make-up water and blow-down seemed to be in good control.

At $8.00/MM Btu and 80% thermodynamic efficiency, this exchanger was recovering $1.04 per hour.             

At 4,000 run hours per year, the total savings would be = $4,323 per year.


2.4 GPM * 60 Minutes/Hour * 8.34 pounds/Gallon * 90 degree rise * 4,000 Hours/Year * 4,000 Hours/Year * $8.00/MM Btu / 1,000,000 / 80% efficiency = $4,323/Year.

Note: If an exchanger is to be installed, comply with all safety codes! You do not want to install this wrong!