Importance of Derating a Boiler at Commissioning

Installing high efficiency innovative heating equipment requires more setup than hooking up the distribution system, the gas, vent, and controls. Gone are the days where you can hook up a piece of equipment, turn it on and it works.

Boilers need more setup than your typical heating equipment because they have been made with more versatility. In today’s market we have a piece of equipment that can be set to deliver water temperatures from 80 degrees to 180 degrees. The high efficiency boiler is also typically fully modulating. Some of the typical residential boilers can run from BTUs in the 10,000’s to the 100,000’s in the same boiler. That is both good and bad. It is great that 1 to 4 boiler models can cover a vast majority of residential needs. However, this versatility brings the need for more setup at commissioning. This setup is often thought of as unnecessary and that is “the bad.”

Modulating high efficiency boilers

One large misconception with modulating high efficiency boilers is that it will run up to what is needed and modulate at the BTUs the system needs for the day. Some of this misconception is driven by the marketing in our industry. It is true a modulating boiler will modulate to the amount of BTUs it needs within reason. Many modulating boilers will try to reach the desired set water temperature as fast as it can. Once it reaches that temperature, it will try to modulate down so the supply and return water temperature reach a manufacturer’s desired temperature difference. 

An example of this is if the heating load that is currently running is only 20,000 BTUs and you have 100,000 BTUs gas input burning, the water temperature will reach the desired temperature faster than the control system of the boiler can react. This will cause a seesaw of temperature which will inevitably result in a short cycle of the boiler because the max temp is reached, and true modulation won’t happen.

Why does it matter if the boiler short cycles? 

Short cycling will cause a high efficiency boiler’s parts to fail sooner and combustion impurities to clog the combustion chamber of the heat exchanger.  It will also cause the deterioration of the electrodes that spark for ignition and sense the flame.

How to avoid a boiler short cycle

Combat these problems at setup of the boiler during the commissioning process. Derating a boiler to lower than 100 percent rate is important in the reduction of service cleaning and part replacing. This is a parameter that often gets left alone because no one wants to take the risk of not providing enough heat to the house. This is why a Manual J report is important. One other way that isn’t quite as accurate but better than doing nothing is to determine how many BTUs the heat emitters can produce.

Reducing the maximum BTUs to the need of the space is typically pretty simple but the determination of how many BTUs are needed for the space can take some time. Reducing the maximum temperature to what you need for the type of emitter and load of the building will also be very beneficial to the efficiency of the unit and the longevity of the unit. Both the maximum temperature and the maximum BTUs should be set in the parameters when you get to that portion of a boilers commissioning process. Take the time at the beginning of an installation and do this. If you don’t take the time at installation to go through and set up a boiler to the space you are heating, you are definitely increasing the probability to have problems and increased service for that boiler. Most customers will understand the value in setup if it is explained that you are setting up for their situation and that the correct setup will contribute to less service frequency requirements. 

An example is comparing the boiler input BTUs to a car accelerator. If you push the accelerator to the floor every time you go somewhere your engine will function but not as long. A boiler can be the same way. If you don’t regulate your boiler’s BTUs, your boiler won’t have the longevity or reliability of operation that it could have.

Contact the B-Y hydronics specialist with any questions on high efficiency boilers. 

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