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Conventional vs. Condensing Boilers

on December 2nd, 2019 in Boiler Install





Though on the market for over three decades, condensing boilers have only now began to gain the popularity they deserve, slowly replacing the “conventional” boiler. If you’ve been researching boilers, you’ve likely come across this term and wondered about the differences between conventional and condensing boilers. In short, the condensing boiler works much like a conventional boiler but comes at a premium price due to its increased efficiency and ability to save money on energy expenses.


If you’re considering buying a condensing boiler, this guide is up your alley. In this guide, we will look at the differences between the conventional boiler and the condensing boiler.


How Boilers Work

The boiler acts as the main source of a home’s central heating system. Within the boiler is a burner that requires a constant supply of natural gas, which is sent to your home from a pipe leading from your neighborhood’s gas main. A boiler begins working when you open the gas valve. The gasses flow into a combustion chamber, where the gasses are burned. These burned gasses are chemically converted into heat and byproducts, the former being transferred to cold water in pipes within the boiler via the heat exchanger, a tool within the boiler that transmits heat from one section of the boiler to another. The latter contains chemical elements such as CO2 and 2H20. It is how your boiler deals with the byproducts that determines whether it is classified as a conventional boiler or a condensing boiler.

How Conventional Boilers Work

The conventional boiler distributes those byproducts as fumes into the flue, which sends those fumes outside your home. The fumes themselves are superheated vapors, meaning they contain some of the energy that originally came from the gas pipes into your house. Thus, a conventional boiler allows some heat – energy – to escape. Allowing the fumes to escape safely from your home requires few parts – just a flue in many cases – ensuring the conventional boiler stays simple in design and low in cost.

How Condensing Boilers Work

A condensing boiler works in the same way a conventional boiler works but with extra functions added. Contrasted with the conventional boiler, which allows heat to escape your home, the condensing boiler recovers waste heat. A condensing boiler can recover heat because of its design, which either includes either a large heat exchanger surface or two heat exchangers.[1] The recovery of this energy starts when the vapor moves through the heat exchanger, at which time the vapor cools to an extent. If cooled to a low enough temperature – the dewpoint temperature – that vapor condenses, or turns into a liquid. This is problematic for conventional boilers because when the vapor condenses, it falls onto parts of your boiler – such as the heat exchanger – or house – such as a chimney. The vapor, now a liquid, contains corrosive chemicals that wear away at the objects on which it falls.

The condensing boiler condenses the vapors within the boiler so as to recapture the heat lost in flue gasses and has the beneficial side-effect of preventing corrosion resulting from condensed vapors. The larger or secondary heat exchangers within condensing boilers extract more heat from the exhaust gasses, cooling the exhaust stream to the dewpoint temperature, ensuring that condensation occurs within the boiler. In addition, the heat exchanger responsible for extracting heat from the vapors is built of high grade stainless steel to prevent against corrosion.

Regardless of which heat exchanger design the condensing boiler uses, it is capable of converting more of the superheated gasses into heat. In the end, more heat goes into the water for radiant heating and less out the flues, which is why the gasses leaving a condensing boiler are often only one-third to one-half as hot as the gasses leaving a conventional boiler.

This design gives a condensing boiler several advantages over a conventional boiler in exchange for a higher price tag.

Environmental Friendliness

Regardless of whether you’re running a gas or electric boiler, that boiler is contributing to global warming. The heat passing through your boiler’s flue contains carbon dioxide, which is a catalyst for global warming. A condensing boiler recaptures much of the heat released as carbon dioxide, reducing the harm done to the environment.


Because of how they work, condensing boilers conserve electricity and/or gas. This will be reflected on your monthly costs, as you receive the same amount of heat for less money. Though each boiler differs in its efficiency, you can expect to save, on average, 15% more when using a condensing boiler.[2]

The efficiency difference between a conventional and condensing boiler is to the tune of 75% efficiency vs. 90% efficiency. However, older conventional boilers can sometimes run at barely more than 50% efficiency, so a move from an older boiler to a condensing boiler can potentially save you 50% of your energy bill. Check your present boiler’s energy factor for a relevant comparison. [link]


Clearly, one of the reasons condensing boilers have not yet replaced conventional boilers is their premium price. As a condensing boiler necessitates more parts, it will always cost more than a conventional boiler. In addition, condensing boilers using single heat exchanger designs will require better materials to handle the increased heat – using stainless steel in order to resist the corrosive nature of the condensed gasses.

To recoup this higher initial price in the form of energy bill savings, you will need to ensure that you can benefit from the 90% and above efficiency that condensing boilers have to offer. That often means having to run your boiler during most of the day, which might make sense if you’re a landlord but probably doesn’t make sense if you’re at the office most of the day. However, owners of homes that must be constantly heated or warm-water swimming pools might find decent savings in a condensing boiler.

Install and Maintenance

With more parts and more complex in design, the condensing boiler seems like a nuisance to install. Indeed, many HVAC installation company avoid installing and servicing condensing boilers. When installing a condensing boiler, an HVAC company must make alterations and additions to your auxiliary system. For example, they must add a condensate drain, in which the corrosive condensate can be drained away from your home. They also typically make changes to your air separator, pipe system, circulation pump, switching relay, and expansion tank.

Similarly, the maintenance requires some extra actions: clearing the condensate drain, flushing the system, and maintaining all the other extra components added during your condensing boiler install. Also, in many cases, you will have to drain the condensate regularly as condensate can quickly build up in systems that make frequent use of heat. This is an easy task as a condensate pump can automatically drain the condensate for you.


As mentioned before, condensing boilers are becoming increasingly popular largely due to their energy savings. Yet because they are a new technology, your local HVAC company might not supply or install it; many HVAC companies are stuck in their ways and refuse to train staff for, store parts for, and inform clients of the newest HVAC technology. But as condensing boilers become cheaper, homeowners will start demanding them, meaning HVAC companies will follow suit.[3] Generally, if your HVAC company installs condensing boilers, you can trust that the company is up-to-date on the latest technologies.


In conclusion, the condensing boiler is quickly becoming the winner of the boiler wars at the current moment. The conventional boiler is losing its price advantage over the condensing boiler as HVAC technology continues to improve. There’s no better time than now to switch to a condensing boiler. That said, to know whether a condensing boiler is for you is the first step in making the change:

condensing boiler is better for you if:

  • You care about the environment.
  • You want to save on energy bills.
  • You use heat continually throughout the day.
  • You are a landlord.
  • You want to avoid corrosion.
  • You want a boiler that will last longer.

conventional boiler is better for you if:

  • Do not plan on residing in your current home for the long-term and do not care about resale value.
  • Want the cheapest option right away, without regard to long-term savings.


Ready to start? Call or click here to today to learn about how much a condensing boiler can save you!



[1] http://www.ahrinet.org/site/590/Homeowners/Save-Energy/Condensing-Boiler

[2] http://www.gsa.gov/portal/content/163495

[3] http://homeenergypros.lbl.gov/profiles/blogs/high-efficiency-should-be-a-drain-a-closer-look-at-condensing

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