Condensing Application

Now that many condensing boilers are in service for hot water heat and domestic water application, it is good to review what is being learned.

First, condensing boilers don’t condense unless operating water temperatures are in the 100 °F range or less. Non condensing boilers will often provide 85% fuel to water efficiency which is very good for large systems that require water at 179o F –200 °F and may be higher than a condensing design that is not condensing. As we know the extra 10-13% efficiency gains come from capturing the hydrogen losses (water) that normally discharge up the stack. The byproducts of combustion are mostly CO2 and water (with other elements). In condensing boilers that condense, this moisture in the boiler transfers heat into the water in the heating system. Believe it or not, all boilers are condensing during cold start-up. The big difference is that “condensing” units are designed to drain the condensed liquid, and they are constructed with material that won’t corrode. The reason for corrosion is the condensed water that causes oxidation (rust) and it is mildly acidic on natural gas, and very corrosive with oil (sulfur acid).

When picking a condensing capable product, the choice by most engineers and buyers is to use stainless steel tubes although there are designs that offer coated copper tubes and even cast iron. Coatings can fail, and iron will rust.

To determine if a condensing boiler makes sense you need to

1. Calculate run hours where the water temp in the boiler will allow it to be full condensing
2. Calculate the load during the low temperature condensing hours. (Often when the systems runs at low temp the load is very light thereby lowering actual savings).
3. Look at the hours and load when it is running non condensing mode.
4. Add those costs and compare them to a comparably sized non condensing boiler.

Then make your decision based on R.O.I. Don’t forget possible utility rebate money if it will run in condensing mode. Life cycle cost and initial cost should also be considered. The initial cost of a 6 million Btu per hour hot water plant can be much more for condensing boilers than non condensing and if they do not run in condensing mode, there will be no fuel savings. Further, life cycles of non condensing boilers are well established (30-40 years on a firetube for example) but condensing is not expected to be anywhere near that, especially if actually condensing, due to the moisture and acidic water.

As always, when you have a design worked out, it is always a good idea to speak to a knowledgeable boiler representative, and we are pleased to discuss applications anytime.

If you have questions, contact your Rounds Company sales engineer.