Boiler Replacements

Nicole Imeson, P.L.(Eng.)

A condensing, high efficiency boiler captures heat energy from the flue exhaust to preheat the return water, achieving higher efficiencies with the same amount of fuel as non-condensing mid-efficient boilers. The return water temperature must be approximately 80-130°F to achieve condensation of the flue gases, which is an indication that the boiler is operating at higher efficiencies. The return water temperature is lower than traditional non-condensing boiler systems that maintain a minimum return water temperature of 140-150°F. Traditional, non-condensing boilers must maintain higher return water temperatures to ensure that the condensation does not form and cause premature corrosion and risk thermal shock.

 Figure 1. Non-Condensing Vs Condensing Boilers

In an existing building with traditional non-condensing boilers and higher return water temperatures, the existing heaters—baseboard heaters, heating coils, unit heaters, etc.—are sized to meet peak design heating loads with higher supply and return water temperatures. When replacing an existing boiler with a high efficiency model, lowering the supply and return water temperatures to achieve condensing of the flue gases will reduce capacity of existing heaters. This can cause heating concerns during severe winter conditions, depending on your location and climate.

The supply and return water temperatures directly impact the boiler's efficiency, which can range from the mid-eighties to high nineties in high efficiency, condensing boilers. The efficiency curve varies by manufacturer and model, so consult the boiler specific chart, like the one from Lochinvar Crest below, which illustrates boiler efficiency based on firing rate and return water temperature.

   Figure 2. Efficiency Curve from Lochinvar Crest Boiler

Outdoor Air Reset Schedule

Most boiler plants implement an outdoor air reset schedule to adjust the heating water supply temperature and the boiler's output capacity based on outdoor air temperature. As outdoor temperatures rise, the supply water temperature decreases within a specified range. This control strategy ensures that the heating system's output closely matches the building envelope heat losses. The boiler incorporates an onboard control panel to manage the reset schedule using an outdoor air sensor.

When installing a new high efficiency boiler in an existing building that requires higher supply water temperatures during peak winter conditions, the outdoor air reset schedule range can be adjusted to facilitate high-efficient condensing mode during milder winter conditions and shoulder seasons. Performing a heat loss load calculation will help to determine the building's heating requirements across various outdoor air temperatures. Comparing this with the output of the heating equipment at different supply water temperatures ensures the system maintains adequate heat distribution from peak heating to warm weather shutdown.

Pumps

In addition to assessing how frequently the new high-efficient boiler can operate in condensing mode, other factors play a role in a boiler replacement project. Piping arrangements vary between buildings, but typically involve dedicated circulation pumps for each boiler. A primary/secondary piping arrangement is a commonly used configuration, which hydraulically separates the boiler circulation loop (primary loop), from the building heating system (secondary loop). This configuration requires a constant flow pump for each boiler on the primary loop, while allowing the secondary loop pumping system to fluctuate based on the building load demand. The two loops are connected through a common low head loss piping path, which prevents the primary and secondary loop pumps from interfering with one another. Replacing existing boiler pumps may be necessary to match the flow rate requirements of the new boilers, if the existing pumps cannot be rebalanced to achieve the boiler specific requirements.

Venting

Existing boiler flue exhausts typically uses single or double wall B-vent construction. High-efficiency condensing boilers require stainless steel venting to support condensation within the flue exhaust stack without compromising the material. Although PVC/CPVC venting listed for this application can also be used where the flue temperature is low enough to not exceed the rating of the plastic material. In most cases, the existing venting cannot be reused for the new condensing boilers.

Upgrading the boiler flue exhaust can affect other appliances in the mechanical room. Locating a high-efficiency appliance in the same room with an atmospheric mid-efficient one can create issues. The high-efficiency appliance can cause the atmospheric mid-efficient appliance's chimney to downdraft, which is a violation of the Natural Gas and Propane Installation Code CSA B149.1. To address this, the high-efficiency appliance may require a separate combustion air duct from outside directly connected to the appliance, instead of drawing combustion air from the room. Alternatively, replacing the other appliances in the room with high-efficiency, non-atmospheric models alongside the boilers can mitigate this issue.

Gas

The input capacity of new boilers is often comparable to or lower than that of existing boilers due to increased efficiency. If the heating plant is expanded or model availability changes over time, the existing gas piping may need review to ensure it can accommodate the new gas load. Additionally, high-efficient condensing boilers may require direct acting gas regulators to respond quickly to changes in output. The gas pressure regulators at the appliance should always be reviewed to ensure that it is calibrated to operate within the acceptable range and style for the appliance.

Turndown

Boiler sizing for peak heating loads results in rapid cycling at part loads, which can stress the boiler and its components. Modern high-efficient boilers offer turndown options to reduce output and operate at part loads, aligning with expected heating demands. This reduces cycling frequency, enhances equipment longevity, improves efficiency, and enhances occupant comfort by maintaining consistent heating water temperatures which reduces radiant temperature disparities.

Water Treatment

It’s always advisable to flush and clean the entire system when upgrading a hydronic plant to prevent debris from entering and fouling the new boilers. However, in older buildings, calcified leak points that are currently sealed may open after flushing; provisions should be included for repairing potential leaks during the installation of the new boiler plant. Reviewing the water pH also ensures the piping material is compatible with the boiler system. It is recommended that a chemical treatment and water maintenance plan be developed to ensure longevity of the system. 

Replacing an existing boiler with a high-efficiency model improves energy efficiency by capturing heat from flue exhaust. Condensing boilers require lower return water temperatures than traditional non-condensing boilers, to achieve the higher efficiencies. Extending the outdoor air reset schedule range optimizes performance in varying conditions, which allows for higher efficiencies in retrofit applications. Upgrades to piping configurations, venting, and gas supply may be necessary, and should be reviewed for boiler replacement projects. These steps ensure a smooth transition to high-efficiency boilers, maximizing sustainability and operational effectiveness in heating systems across buildings.