Basic Terminology?

Mark Mathys

Can someone explain the difference between a condensing and non-condensing boiler? Also, what is "blowdown"?

I see these terms all the time but don't know their meaning.

Thanks

Mark

Tom Blackwell

The term "condensing" means that there is enough heat transfer to lower the temperature of the products of combustion (flue gas) to a point below the dew point. The water vapor and associated compounds will condense into liquid and must be drained out. Current non-condensing designs go to great lengths to make sure that this point is not reached to prevent corrosion of the heat exchanger. Condensing designs must be manufactured using non-corrosive materials, hence the high cost. It should be noted that to realize savings from a condensing design the water temperature must be low enough to make this happen. The payback is not there if you must have 180 degree water. "blowdown" is a term used with steam boilers and is the draining of boiler water to reduce the amount of dissolved solids in the boiler water. Hope this helps- I tend to explain too much.

Dale

Terms of confusement

The term blow down is most often used (with residential steam boilers) when the float type low water cut off is "blown down" periodically to flush out the solids that accumulate under the float that would eventually keep the float from dropping if a low water condition existed. The steam pressure above the water arround the float is used to force water and solids DOWN through the valve at the bottom of the low water cut off. Other things can be blown down like gauge glasses.On commercial steam boilers lots of things are blown down to keep junk out of the boiler. It's easier to use the boiler pressure than it is to take the covers off and dig out the muck.
Condensing refers to gas boilers where the hot products of combustion are mainly CO2 and water vapor, think steam, as the other post said, if the boiler heat exchanger is effecient enough ( and the return water temp cool enough) the exhaust can be cooled down and the "steam" in the exhaust condensed to give off it's latent heat of vaporization. The exhaust temp drops from about 300 degrees to about 95 degrees. This increases the effeciency about 10%. The water that's left from the exhaust must go to a drain. As the other post said there's no free lunch. The effeciency gain requires more sophisticated components.

Kal Row

not just the solids in the water...

but especially those sitting on the float of the low water cut-off or coating/cloging the sensor on the electronic ones, you can see, the blowdown valve, is sitting right under the cut-off/feeder control - and - you should blow it down with a good head of steam maximize the cleaning effect - do it into a pail, an use gloves - da vet shteem kud shprraay's baahk!!!