Condensing boilers are the newest great thing. But what's so special about them? Do they actually work? And what about the old fact that condensation kills boilers?
A condensing boiler can be more efficient than a conventional boiler. In a correctly designed system, it can be 90% to 98% efficient. The catch is that the efficiency does not happen in just any application. The boiler must be able to condense flue gases inside the heat exchanger, and that takes a special circumstance.
What is condensing? To go back to science, condensing happens when a gas is cooled enough to become a liquid. You see it all the time. The humidity (a gas) in the air becomes water when it is cooled by the outside surface of a cold drink container. The same thing happens at night when dew appears outside on the grass and windshields. What we don't see is that heat is given off in the process. Applied to the heating business, this is an exciting event — heat that costs nothing!
In a boiler or furnace, combustion causes flue gases. With a conventional boiler, we want those gases to go right on up the chimney. Allowed to condense in the boiler or flue, they would result in considerable damage. That's why it is important to keep return water temperatures high in a conventional boiler.
But if you had a boiler that could withstand the corrosive liquid — called condensate, you could condense the gases and keep the resultant free heat. That is exactly what a condensing boiler does. But it does not do it without help from the return water temperature. Low temperature return water is required as a coolant to condense the flue gases inside the heat exchanger.
Condensation inside the boiler begins when flue gases drop to about 130°F. For full efficiency, the temperature in the heat exchanger needs to be 60°F or less. Flue gases tend to be about 7°F warmer than the return water. This is going to require some cool return water.
Here is where old industry standards go head-to-head with the new technical requirements and create a problem. From many decades-old heating practices we have inherited supply water temperatures of 160°F for cast iron radiators, 180°F for copper fin tube and 190°F for fan coils.
We also have an industry standard that says we should aim for a 20°F temperature drop (ΔT) from the supply water to the return. So if we use 180°F supply water and we have a 20°F ΔT, that gives us 160°F water returning to the heat exchanger. The flue gases leaving the heat exchanger will be about 167°F. The flue gases will not condense in the heat exchanger, and the expected efficiency will not occur.
Clearly the only way to get expected efficiencies from a condensing boiler is to deliver lower return water temperatures than are appropriate for conventional boilers.
Here are some ways to have lower water temperatures:
Do a load calculation (also known as a heat loss calculation)
Of course it would not be wise to simply lower water temperatures. But a load calculation may show that a lower water temperature than expected can meet the heating needs of the building.
After all, buildings are much better insulated and more tightly constructed than they were in the days when those temperature guidelines were developed. Load calculation software is easy to use and is available — often at no charge — from equipment manufacturers.
Use an outdoor reset control
A way to get low return water temperatures much of the time is to use an outdoor reset control. Because outdoor temperatures are usually above design temperatures (there are more warmer days than colder ones), outdoor reset can enable condensing 60-70% of the time.
For example, if the outdoor temperature is 40°F and the boiler provides 125°F water with a 20°F ΔT, the return water would be 105°F. Add 7°F (see above) and the flue gas would be 112°F. It's condensing! That would take the boiler from 89% to 92-94% efficiency because of the outdoor reset.
Look for ideal applications
Ideal applications for condensing boilers are whenever you can provide low return water temperatures. With the lower design temperatures of in-floor radiant and snowmelt, these are perfect targets.
The bottom line is that regardless of the high efficiency rating of a condensing boiler, it cannot deliver that energy savings unless flue gases condense in its heat exchanges. That's where the free heat comes from. Condensing happens only if a coolant is provided, and that coolant is low temperature return water. It all depends upon appropriate system design and application.
Former Honeywell rep Carol Fey is a technical trainer who has spent more 25 years in the heating and controls industry. She has published four books and a DVD in the “Quick & Basic” Series, including Quick & Basic Hydronic Controls. She can be reached at [email protected] or on the Web at www.carolfey.com.