By Francesco Lo Presti
Information taken from Grant Erickson’s, agency sales manager at AquaTech, presentation on “Condensing Boiler Design Information and Hydronic Application” on Feb. 7.
Gas-fired hydronic boilers are a revolutionary technology transforming how homes heat their water systems. These boilers, powered by natural gas, are designed to efficiently heat water for household use and play a crucial role in maintaining comfort and convenience within a home.
Hydronic boilers use water as the heat transfer medium, circulating it through pipes to radiators or underfloor heating systems. Gas-fired hydronic boilers are particularly popular because they can modulate heat output based on demand. These boilers ensure optimal energy usage by only heating water when needed, reducing energy consumption.
Gas-fired hydronic boilers can also provide consistent and reliable hot water throughout a home. They are especially effective in achieving an even and comfortable heat distribution, eliminating hot and cold spots that may occur in traditional heating systems.
Condensing vs non-condensing
Now that we understand how gas-fired hydronic boilers operate and what their role is, let’s discuss the differences between a condensing and a non-condensing boiler. Essentially, all boilers are condensing units, but some are better equipped at handling combustion than others. What typically makes one better equipped than the other is the materials or the types of heat exchangers used.
Condensing products are made with materials that can handle condensate, like stainless steel, aluminum or a dual heat exchanger. Non-condensing products are made with materials that are a little less expensive, such as copper, steel, or cast iron.
Next is understanding what happens with combustion. With stoichiometric combustion, you have a fuel source; for this conversation, it will be natural gas. So, you take the natural gas, which is the methane, and you’re going to combine it with oxygen from the air. Then, you’ll get a source of ignition, a source of heat, which starts that chemical reaction. Ideally, what you will produce is water vapour and CO2.
Now, since there is no such thing as true stoichiometric combustion, there will be other things happening, but the main thing you have is CO2 and H2O. The water will then take the form of gas.
From there, the focus turns to flue gas dewpoints. With condensing boilers, you’re getting below the dewpoint where that water vapour will condense down from a gas into a liquid, which releases energy. That energy gets put back into your thermal balance, which allows you to get higher efficiency with condensing boilers.
Reaching the dewpoint
Notably, the dewpoint of natural gas will vary depending on the fuel used and the type of combustion you have. For example, a fan-assisted product with natural gas will have a dewpoint of 130 F, which represents your return water temperature. Anytime you operate above that number (most manuals will say 140 F, whether a condensing or non-condensing boiler), you will be in the non-condensing mode, and you will have an efficiency of about 86 to 87 per cent. Anytime you operate below that number, you will be in condensing mode, and that water vapour will condense and turn to liquid. Again, this varies depending on the fuel source. Additionally, the combustion type will depend on the amount of CO2 in the stack.
The key focus is on changing state and getting latent heat, as that’s where the extra efficiency comes from. When we look at water vapour, roughly one lbs. will give you about 1,000 BTUs.
In terms of gallons of condensate, one gallon of condensate can produce approximately 9,000 BTUs. When we look at how this latent heat is applied, for every 100,000 BTUs of input, approximately one gallon of condensate is produced per hour. A 600,000 BTU unit will produce approximately six gallons per hour, and a six million BTU unit will produce approximately 60 gallons per hour.
The stack is another key thing to remember with condensing and non-condensing. If you are operating in that condensing mode in a condensing boiler, you will need a stack that can handle condensate, usually stainless steel, CPVC, or polypropylene. Many non-condensing boilers will not have those types of stacks; they typically have B-vents. That is fine in non-condensing mode, but the stack can rot when condensing.
Heat exchangers
Typically, the three main heat exchangers you see in condensing boilers are recuperative (dual), aluminum, and stainless steel, which feature water and fire tubes.
A dual heat exchanger is essentially a two heat exchanger design. You will have a primary heat exchanger that doesn’t condense and a second one that can condense. In this type of product, the return water will go into that second heat exchanger and be exposed to those flue products, which will have cold temperatures. The flue products will then condense, and that water vapour will go from a gas to a liquid. The latent heat will go into the primary non-condensing heat exchanger.
Cast aluminum heat exchangers also work well in condensing situations. They can handle condensate, and these products typically mimic cast iron. Cast aluminum heat exchangers have various heat transfer pits. The flue gas products are found at the top and are forced downwards. As they get closer to the bottom, a counterflow design will start to condense and pool up at the bottom.
Low-mass boilers have a low water volume in the heat exchanger and generally tend to be water tube appliances. Water will be found in the tube, with flue product surrounding it. These appliances tend to have very specific flow requirements and a narrow range of minimum and maximum flow rates. For that reason, primary and secondary piping is recommended. In this setup, the boiler loop is typically referred to as the primary loop, while the system loop is commonly used as the secondary loop. The key to making this system work is having closely spaced T’s.
Naturally, high-mass boilers are the opposite. They have more water volume and tend to be fire tubes in condensing applications. In this design, the flue products are in the tube and are surrounded by water. This allows for more flexibility with flow rates, as they generally have a much larger flow range. As a result, you tend to have more flexibility with piping schematics.
Water quality
It is important to remember that if there are issues with water quality, there will usually be issues with the boiler. Especially in retrofit applications with things like magnetite, as soon as you start to replace the boiler or rattle the pipes, all that magnetite and other things will come out of the pipe.
The fresher water being taken on with untreated water results in more scale forming. When heating water, scale will precipitate out. When there is a heat source, scale will adhere to it and form a hard substance. This will result in a loss of efficiency and uneven heat transfer.
The first steps in water protection can be done prior to the initial fill and start-up by isolating the heating appliance and flushing the entire system. You can also clean the system with an approved pre-commissioned cleaner and clean all the water filtering devices.
Another step can be monitoring your makeup water. Typically, there should be less than five per cent of your total system volume per year in your makeup water. If there is more than that, you will know there is a leak and scale accumulation in the hydronic system. :