HPAC Magazine

Replacing boilers in existing buildings

February 1, 2016 | By Mike Miller

Boiler replacement is sometimes viewed as an easy task, but it can be challenging if it is done right.

There is an opportunity to provide some additional value for the customer when it comes to boiler replacement. Doing it right will also set you apart from the competition. Think about the factors that will impact the overall investment for the replacement. Also consider the future operating efficiency and the extended life cycle of the equipment.

If the boiler seems to have failed prematurely, investigate the cause of the failure first. Was the failure related to flow, short cycling, piping or age? If the issue is not age related, spending the time at this point to identify and eliminate the cause of the failure will ensure that your customer is happy. What you should not do is replace a failed boiler with a like unit without considering the following points.


When a boiler needs replacing, do not assume that it was originally sized based on the appropriate building load. In fact, heating equipment used to be commonly oversized and grossly oversized in some cases. Apart from unnecessarily high operating costs, oversizing may have been a contributing factor to the premature boiler replacement.

The building may have undergone upgrades over its lifetime such as better windows, additional insulation, upgrades to the distribution piping and so on, that could impact the overall heat loss of the building. It pays to do a proper heat loss analysis based on the current building envelope. You may find that a smaller heating plant output capacity can get the job done now.

Perhaps the building was upgraded, but no changes were made to the heat emitters? Terminal units (fan coils, baseboard and radiators) in the building might be oversized for the upgraded building envelope. The fluid delivery temperature required to heat this building on the coldest day of the year may be lower than when the system was originally designed. That could impact what type of replacement boiler you choose.


Be aware of the application that the boiler needs to service. Does the system have many high temperature loads, which require setpoint operation for the majority of time? Those could include large domestic hot water heating needs, pools, spas, maybe even fan coils; anything that requires fluid to be delivered at 170F to 180F for a bigger part of the operating season. If that is the case, then a non-condensing boiler may be suitable in this application.

If there is no significant reason for a boiler to be operating at high temperatures for the majority of its operating life, a condensing boiler may be what you want to use. Condensing boilers operate much more efficiently in the long run if they are able to operate in condensing mode the majority of the time. Condensing occurs when the boiler’s return fluid temperature is at or below 130F.


Condensing occurs when the flue gas condenses. Different boiler and/or system designs and larger system fluid delta Ts may either promote condensing at higher return temperatures than that or prevent them at lower temperatures. Always consider the boiler’s design and noted operating benefits. The addition of a proper condensate drain is required.

Traditionally, smaller non-condensing boilers are on/off boilers, where larger non-condensing boilers can be staged or modulating with the use of power burners or modulating gas trains. Smaller condensing boilers typically come standard with modulating capabilities, which enables higher operating efficiencies.


Depending on the size of the boiler being replaced and available space in the mechanical room, it may make sense to replace a single boiler with two (or more) smaller boilers, which equal the same Btuh requirement. The benefit of multiple boilers includes some redundancy when not at design condition. The total boiler plant is designed to heat the building at the coldest day of the year, which represents maybe two or three days a year in all. Multiple boilers allow you to match the boiler plant output to the building’s needs year round.

For example, if you are replacing a boiler with two smaller boilers that are on/off firing, each of the boilers would provide 50 per cent of the load. Three boilers would provide 33 per cent of the load.

If the replacement boiler is a single modulating boiler, then depending on the boiler turndown ratio, the boiler can match its output anywhere between the minimum boiler modulation output to a  maximum of 100 per cent. Some newer boilers may provide up to a 10 to 1 turn down ratio, meaning the boiler could operate anywhere between 10 per cent to 100 per cent of its output. Many other modulating boilers have a turn down ratio of 5 to 1, allowing them to operate through a range of 20 per cent to 100 per cent of the needed output.


Is there a control system that is managing the boiler plant based on the building’s needs? If there is, the control system will have to be factored into the decision making process. Consider its ability to deal with the number and types of boilers you are opting for. The proposal you put together may include a controls upgrade as well. The cost of that upgrade could be minimal in comparison to the payback or life cycle costing of the new heating plant.

Controls are an invaluable part of any heating plant. If there is not a control system in place by now, there should be. The most basic addition a control should provide is the ability to reset the fluid water temperature delivered to the building based on outdoor temperature as it directly relates to the heat loss of a building.

If you opt for multiple boilers, pick a controller that can stage the types of boilers selected (modulating or on/off or maybe a combination thereof) and provides additional logic such as boiler rotation (on equal run time).

For non-condensing boilers the control would provide boiler return fluid temperature protection through the operation of a mixing device. If the building had multiple boilers to begin with and only one of them failed, then a staging controller needs to have the ability to exclude the older lower efficient boiler(s) from the rotation of the newer and higher efficient boiler(s). 


Depending on the boiler type, you may or may not be able to pump the full system flow through the boiler(s) directly. Many of the newer low mass boilers need to be hydraulically decoupled from the main building loop, especially when multiple boilers make up the total building capacity. In that case, the primary loop flow is equal to the system requirement where the flow through the boiler is a fraction, depending on how many boilers there are. For example, given a 20F deltaT between supply and return, three equal sized boilers in a 600000 Btuh system would flow 20 GPM each to a total primary loop system flow of 60 GPM (GPM = Btuh/(deltaT x 500)).

Hydraulic separation is accomplished through a low loss header or a set of two closely spaced tees (primary/ secondary), or other engineered fittings that serve the same purpose. Whatever the device, it physically decouples the generation equipment from the building system. Now each boiler can have the right size pump that it needs to ensure adequate flow through that boiler’s heat exchanger and the ability to control the pump’s output based on the boiler’s start-up sequence requirements.

The boiler’s venting and combustion air requirements may have changed with the introduction of a new device. Always check the boiler manufacturer’s instruction manual for proper implementation.


While you are replacing the boiler, also check on some of the following components to ensure the longevity for your newly installed device.

Verify the functionality of the existing expansion tank. You can verify its operation by watching the system’s pressure gauge when the boiler(s) fire. Its purpose is to capture a system’s pressure increase resulting from the expansion of the fluid when heated. If the expansion tank is operational, the pressure gauge on the system would maintain its fill pressure. If it failed, the pressure in the system will increase, or even pop the pressure relief valve.

Make sure that there is a functional device in the system that can capture the air and dirt in the system, particularly since the boiler plant was just worked on. Air and/or dirt introduced during that process can be detrimental to the new or existing components. It is always an advantage to have a device in place that can continuously remove dirt and air from a closed loop system with potentially ferrous components.

Some boiler manufacturers are very strict about the fluid quality and may require specific inhibitors and supplements to be used. That should be verified with the boiler manufacturer.

There is a lot to think about, but if you get the replacement right a trouble-free system won’t be eating up the time you could use to develop business and increase profits. Take the time to properly assess the boiler situation – the payback is worth it.

Mike Miller is director of sales, building services with Taco Canada Ltd. and is also chair of the Canadian Hydronics  Council. Contact him at hydronicsmike@taco-hvac.com.



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