Home Energy Consultants, Inc.
PlugIn Stations Online
Enhanced Living, Inc.
Thermal Associates
Honest Weight Food Co-op
Real Goods Solar
Click ad for Sponsor details.

Consumers Guide to High Performance Boilers

Support our local Sponsors.

Click Ad for Sponsor details.

“The Consumers Guide”

To High Efficiency Gas Boilers

Choosing the right boiler for your home or business can be confusing.  Every heating contractor you talk to will tell you theirs is the best.

So, who do you believe?

To help you make an intelligent decision there are some THINGS you should know

We’ve compiled some FACTS for you to examine.

Keep in mind – these are facts based on science and physics – NOT our opinion.

The Myth of Efficiency Ratings

Heating boilers have a rating system called AFUE.

AFUE stands for Annual Fuel Utilization Efficiency.

The simple explanation for AFUE - the ratio of heat that goes into the living space vs. what is lost up the chimney.

Older heating boilers can have efficiency ratings as low as 50% while newer versions range from 83% to 95%.

In theory, if a boiler has a 90% rating, you should get 90 cents worth of heat for every dollar you pay.

But. . .

Higher ratings don’t always translate into lower fuel bills.

The rating system for boilers has many flaws because they are based on best-case, laboratory scenarios and assumptions that don’t really exist in real world applications.

In fact - many so-called high-efficiency boilers operate well below their stated efficiency because they use an inferior, out-dated control system.

Here’s why. . .

Boilers that have an AFUE rating over 90% need to operate with system water temperatures below 135 degrees in order to achieve the higher AFUE rating.

For every degree the water temperature drops below 135 degrees the efficiency INCREASES.

Conversely, for every degree the water temperature goes ABOVE 135 degrees the efficiency DECREASES.

Simply put –

The cooler the water - the more efficient the boiler

As you can see, the control system is a major factor to actual energy savings.

The typical heating cycle of a hot water system consists of 180-200 degree water circulated through a radiator or baseboard system.

Unless it is the coldest day of the year, these high water temperatures are unnecessary.

Believe it or not, at certain outdoor temperatures you can actually heat a home with 80 degree water temperatures.

Don’t Waste Your Money On A Boiler That Has An Old-Fashion Control System

Control systems are now available that adjust the water temperature according to the outdoor temperature.

As the outdoor temperature drops the boiler temperature increases.

This is called weather responsive control or outdoor reset.

Energy savings of up to 20% can be realized using this common-sense strategy if it is applied to the right boiler.

Low water temperatures and high efficiency gas condensing boilers with AFUE ratings over 90% are a perfect match.

High quality, high efficient boiler systems can cost up to twice as much as others, but you’ll recoup your investment through lower energy bills, less repairs and a longer lasting system.

Boiler Construction

There are many boilers still operating that are 80 -90 years old!

Though they may not be super-efficient, they have proven to be durable and reliable because of the amount of mass (material) in the construction of these vessels.

Compare these with today’s boilers that you can carry into a basement.


Some of these newer boilers are constructed of aluminum, however - the ones constructed of cast iron or high grade stainless steel are the best bet for longevity.

A well respected German manufacturer discontinued the use of aluminum because of the question of longevity.

What About Warranties?

Warranties on high-efficiency boilers are basically the same for all manufactures.

They offer a LIMITED Lifetime warranty on the heat exchanger (the most expensive part of the boiler).

The keyword is LIMITED.

Here are the LIMITS:

The 1st -10th  years you get a replacement heat exchanger at no charge (excludes labor).

After the 11th year and beyond the heat exchanger replacement is pro-rated.

I’m not a fan of these so-called LIFETIME Warranties but as I stated above, it’s typical of the industry. 

Some companies will replace the heat exchanger up to 7 years.  After that it’s pro-rated to the 12th year.

After the 12th year, you are out of luck.

When comparing warranties, don’t overlook the fine print - this is where some manufacturers try to weasel their way out.

Don’t Just Replace The Boiler

Some companies just come in, cut the supply and return pipes and attach the new boiler, give you a new circulator and if you’re lucky, you’ll get a new thermostat

Our belief is;

If we go as far as removing your boiler, it just makes sense and it’s cost effective to replace all the peripheral parts that are most likely as old as the boiler, instead of nickeling and diming you, one part at a time.

We both know that Murphy’s Law will always kick in – one of those old parts will break down on one of the coldest nights of the year.


  • Be sure the boiler will operate at water temperatures below 135 degrees for the majority of the heating season
  • Choose the highest rated AFUE rated boiler possible
  • Stainless steel is a superior material for boiler construction
  • Replace all the older parts of the heating system in the mechanical room
  • The lowest price boiler will always end up costing you more


Call us today for a FREE evaluation at 235-0311 or e-mail, [email protected]

or visit our website at: http://www.heating-and-air-conditioning-guide.com/

Comments on "Consumers Guide to High Performance Boilers"

  1. TheCenterforAppliedBuildingScience December 15, 2012 at 6:07 pm

    This is a really well explained article.
    I would add that the system design should start by doing two thermal models of the building (heat loss calculations): a low and a high, and then compare them to the energy bills (usage) the customer had for the last few years to get a third point of reference.
    Boiler oversizing has been/is a great problem in the US. Energy was cheap and your plumber didn’t care about your bills…he just wanted to be sure you don’t call him when it is -20F for a week and you complaint that you are freezing.

  2. TheCenterforAppliedBuildingScience December 15, 2012 at 6:08 pm

    This ties up to Ed’s recommendations in the article about using low temp water to get max. efficiency because if you oversize the boiler it won’t be running at its best efficiency.
    Another point to further expand into controls and outdoor resets is to understand that they will increase comfort and decrease heat losses (=more efficiency as an installed system). And to some extend will help reduce the size of the boiler (in certain circumstances): if it gets down to -20F in a brick building (high thermal mas) the boiler will start firing a few hours ahead of the game at a higher rate…so in reality when you multiply the kbtus/h by the number of hours the boiler is actually firing at a higher rate compare to an old system it will allow you to “under-size” the system in some cases (A high thermal mass building without controls may take a long time to cool down for the thermostat to even know it is real cold outside).

  3. TheCenterforAppliedBuildingScience December 15, 2012 at 6:09 pm

    I think there are several other issues that are culturally “built” into the plumbing/hvac trade in the US since high efficiency boiler and tankless water heaters are so new over here.
    Even if you install an outdoor reset/control any plumber who comes to service your boiler will turn the boiler temp up to 180F because of acidic condensation and their belief that a house can only be heated at 180F. Then he will leave your basement window open or cut a large whole somewhere so your boiler can have enough air… now you have your basement full of air at -10F stealing the heat from your pipes before they go into the house.

  4. TheCenterforAppliedBuildingScience December 15, 2012 at 6:10 pm

    Another important point is that “combi boilers” (they provide domestic hot water and heating)(as you call them here) have been used oversees for decades. They are compact, fit into a kitchen cabinet, and have everything built in (a circulator, an expansion tank, and pressure relief valve). There are few being sold here, and some brands even force you to plumb them primary-secondary (because of several not ideal installations).
    The point is that one thing is the efficiency of the boiler as rated and another thing “as installed” overall when installed in an unheated basement with many feet of copper pipe going back and forth to add all the extras.
    I do understand that the size and configuration of most US homes is not ideal to a “combi boiler” a lot of times, and that so many installations have been screwed up by just cutting the pipes and replacing the boiler as Ed points out.

  5. TheCenterforAppliedBuildingScience December 15, 2012 at 6:12 pm

    Basically I am trying to make a call to architects and designers to understand that if a house is designed with the kitchen and bath not too faraway, a combi boiler in a kitchen cabinet may be the most efficient solution (eg. ranch houses). Even a larger two floor house with further bathrooms could get a second tankless install behind the bath wall if it is too far away.
    Another cultural myth in NY is that copper cannot be used for gas pipe…while it is used all around the world…then I walk into the law library I picked some NY building code and stated that it is legal as long as the gas has less than xxx-parts of sulfur (which NG does). Then I found a US-copper website association promoting its use.
    the point is that if you route gas with copper pipe through the outside of the building adding a second tankless heater on the second floor may not be as difficult as running black pipe inside the building.
    Things have been working in a different way in other countries for decades.

  6. TheCenterforAppliedBuildingScience December 15, 2012 at 6:13 pm

    9 years ago everyone was fighting PEX tubing in NY with all kind of myths(although used abroad for 40y)
    A house is a system and we need a new field=Buiding Science; its biggest enemies are cultural misconception and conservatism in the industry and trades. Visit us:
    happy greening!

  7. TheCenterforAppliedBuildingScience December 15, 2012 at 6:23 pm

    Hi Ed and all,
    there is a topic that it is not clearly or mistakenly explained in your article: cast iron versus aluminum and stainless boilers etc…
    the problem with cast iron boilers is not their longevity but the fact that they have to be fired at 180F and that you need the basement windows basically open plus the fact that most of them are oversized and short cycling leaving all that thermal mass to cool and go away thorough your basement window loosing tons of btus and creating great discomfort.
    Adding an outdoor reset without adding an external condensing recovery unit (which are not available for house boiler sizes) to lower the boiler temp to 135F even if technically possible would probably never pass any inspection.
    This is one of the “old good” myths that the cast iron boiler is better because it is so heavy (with a few exceptions).
    Aluminum heat exchangers are/could be good as anything else stainless, cast iron or stainless…it is a matter of looking at the whole system.

Leave a Comment

You must be a Member to participate - please Login or Join!

Let us know how helpful or informative this article was.

You must be a Member to participate - please Login or Join!

Ratings for this Entry:

  1. Arlene Nock October 01, 2011 at 12:42 pm

    This explains very well the differences in boilers and helps to understand the logic behind spending more at the outset of boiler replacement in order to save on gas bills and future repairs. Thanks!

  1. Howard Stoner September 29, 2011 at 3:46 pm

    This is a informative article and very well written by someone who covers the basics and is well informed. hs