What is a Condensing Boiler?

If you use a boiler for heat, it’s possible to save up to 15% or more on energy by switching to a condensing boiler.

A condensing boiler, also known as a high-efficiency gas boiler, recycles extra heat from the exhaust gases, thereby wasting less energy. It’s a great solution for both new and existing homeowners looking to run their homes more efficiently.

How Does a Condensing Boiler Work?

If you’re new to boilers, read our boiler buying guide. As fuel (natural gas, propane, or oil) burns inside your boiler system, it produces exhaust in the form of carbon dioxide and water vapor. In a traditional boiler, this exhaust escapes out the flue, taking 20% of the heat energy with it.

A condensing boiler works as follows:

1. As fuel burns, it heats the water flowing through the boiler and produces exhaust
2. The hot exhaust flows through the boiler to a secondary heat exchanger before exiting out the flue
3. The secondary heat exchanger collects some of the heat from the exhaust gases to preheat the incoming water, saving energy
4. As the exhaust gases cool, they produce water/condensate, which is drained from the boiler

By using a secondary heat exchanger to collect heat from the exhaust, a condensing boiler can preheat the incoming water and use less energy during the main heating process. While a standard boiler typically has an efficiency rating of around 80%-86%, a condensing boiler has a rating of 90% or higher.

Condensing Boilers Pros and Cons

The biggest pro of switching to a condensing boiler is the energy savings. When it comes to efficiency, condensing boilers simply cannot be beaten. You can likely save at least 10% depending on your previous boiler.

Another pro involves piping material. In standard boilers, exhaust gases are much hotter, so they need to flow through heavier, more expensive piping that is galvanized or stainless steel. Since condensing boilers cool the exhaust gases, they can use lighter, less expensive exhaust piping like PVC.

Finally, many states and utilities offer energy rebates as incentives for homeowners to switch to efficient technology like condensing boilers, so you can get money back on your purchase.

It used to be the case that condensing boilers were more expensive than standard boilers—that is no longer true. In fact, with the annual energy savings, potential rebates, and piping material savings, condensing boilers could be less expensive in many cases, especially new construction.

With a condensing boiler, however, you will need a way to drain the condensate water. On the same token, condensation will prevent the use of chimney venting because the acidity can eat away at the metal. Instead, you will want to use direct venting through the wall or roof. In general, condensing boilers are also higher maintenance.

Finally, swapping out a standard boiler with a non-condensing boiler may require reconfiguring your plumbing system, which could be expensive. Check with your plumbing professional.

Should You Get a Condensing Boiler?

If you are building a new home, there’s no reason you wouldn’t want a condensing boiler. Everything, from the venting to the condensate drain, can be designed to handle it. Plus, you won’t really notice the cost in the context of the whole-home.

For existing homes, you can substantially save on your annual heating energy usage. Of course, some older homes will require more adjustments to handle condensing boilers. If you are replacing an old cast-iron boiler, for example, you will have to change out your venting and add extra piping. Installation will be more difficult, but you will still save energy long-term.

Overall, any downsides to a condensing boiler are typically overcome by the benefits. If you still have questions, please contact us to speak with a boiler expert.