Heating Help

condensing boilers

Are condensing boilers strictly hot water boilers? Can a condensing boiler be a steam boiler? if so, please explain. Thanks!

Sort of

So far as I've heard, all the condensing "boilers" for residential or smaller commercial use are hot water, not steam.  Furthermore, they will only operate in the condensing mode -- where their efficiencies are quoted -- at relatively low (like 140 or so) output temperatures.  If the water temperature is cranked up -- maybe through an outdoor reset on a cold day, or just to compensate for undersized radiation, they won't condense any more.

The reason is not hard to figure: if you want to condense water at atmospheric pressure, whatever gas the water vapour is in (that is, the combustion exhaust) must be cooled below 212 or so -- actually significantly below.

Which is not to say that a steam generating system can't be condensing: it can.  But the condensing part must be a separate feed water heater, not part of the boiler -- and this jolly complicaton just isn't worth the effort for a smaller boiler.

Not possible.

In order to the boiler was to be used strictly for steam production, it would be impossible to condense the flue gas.

Flue gas must be below 144 or so degrees in order to condense.

Theoretically speaking the only way to make steam at that temp is in a deep vacuum.

Now as far as industrial steam boilers like in a power plant, or some such application, they often use a separate heat extraction device that the exhaust from the steam generation boiler passes through. That heat extraction can be used in many different ways, but the most common in combustion air pre-heat.

This would not be found on residential or even light commercial application, at least nothing modern. There used to be residential sized heat extractors back in the 70's and maybe the 80's I believe, but most of those were more trouble then they were worth, and they were not really meant to condense the flue gases, just recoup some of the waste heat.

As far as a packaged steam boiler designed to condense in any application, there are none.

Nothing on the market

As said it would require another heat exchanger to reach condensation and this would  need placed in the return water to be of any use. The combustion gases would also need to follow a top down path with the hottest gases at the top of the boiler and them exiting low in the colder water of the return.

Combustion air preheat has been done...

on residential size boilers....the Dunkirk Quantaum Leap boiler (not the later, simplier Quantum series).  Flue gas temperatures would typically be about 20 to 30F colder than return water temps with this set up. On mine, on the typical winter day, I would see exhaust temps of about 90F.  Combining this concept along with an efficient primary heat exchanger, would easily move steam boilers into the condensing mode.  All it would take is a little (probably very little) work on some manufacturers part to do this with a smaller boiler.