Hydronic boilers are used in generating heat for residential and industrial purposes. The hydronic boiler operates by way of heating water/fluid to a preset temperature and circulating that fluid throughout the building. The fluid can be heated by any means...gas, wood, fuel oil, etc., but I will limit this discussion to natural gas.
TYPES OF BOILERS
There are many type of boilers, but for this article, we will look at the three most common - Atmospheric boilers, high efficiency boilers, and condensing boilers.
ATMOSPHERIC BOILER - a boiler in which combustion air/flue gases are drawn through the boiler without a fan. Modulating atmospheric boilers are designed to control the output (size of flame) to match the boilers variable load requirements. During this process the burner is designed to stay at the correct fuel / air ratios across the complete firing range ensuring maximum combustion and boiler efficiencies. Modulation refers to a boiler's ability to match firing rate - input - to meet the heating demand - output - of the system.
CONDENSING BOILER - Some new systems are fitted with condensing boilers for greater efficiency. These boilers are designed to extract the heat of vaporization of the flue gas water vapor. As a result of the lower flue gas temperatures, flue gas water vapor condenses to liquid and with dissolved carbon dioxide forms carbonic acid. The carbonic acid would damage a typical boiler by corroding the flue and fireside boiler heating surfaces. Condensing boilers solve this problem by routing the carbonic acid down a drain and by making the flue exposed to the corrosive flue gas of stainless steel or PVC. Although condensing boilers are becoming more popular, they are still less common than other types of hydronic boilers as they are more expensive.
HIGH EFFICIENCY BOILER - as a compromise for cost, high efficiency boilers are installed. The boiler control system allows for high temperature operation so that the flue gases do not condense. This prolongs the life of the boiler. while still achieve high operating efficiencies. High efficiency non-condensing boilers utilize a VFD for full air/fuel modulation with a smooth 4:1 turndown. The finned copper tube heat exchanger is wrapped around the burner and provides for enhanced heat transfer. Theseboilers are designed to discharge flue gases at the lowest possible temperature to prevent the formation of condensate. They can handle some condensate on start-up, and in some cases can handle a small amount of condensate in one part of the heat exchanger that evaporates elsewhere in the boiler. Because of this, the flue can typically run in PVC.
BOILER SUPPLY TEMPERATURES AND THE HEATING CURVE
So, you have designed a boiler / heating system for a building. Now you want to make sure it operates properly and at the highest effisceny possible. Based on your delivery system (perimeter radiation, fan coils, etc etc), the heating curve must be selected for the control system. Once a heating curve has been set, the control system will provide just enough heat to see that the required boiler temperature is reached for each outside temperature. The water temperature of the boiler will therefore change constantly depending on the variation of the outside temperature. One term for this is Outdoor Reset.
The boilers on outdoor reset for instance have lower losses than the boilers with a constant water temperature. For the constant water temperature control, the average annual water temperature may amount to 70C. For an outdoor reset temperature control it can be reduced to for instance 35C. Maintaining boilers at a higher temperature is therefore not advisable from an energetic point of view. Savings of 10% and more can be realized.
WHAT ABOUT THE FLOW?
Heating system flow rates are all based on delta T - the temperature difference between the supply and return water. the greater the delta T, the lower the required flow rate. Older style heating systems using atmospheric boilers used a delta T of 20F. As a result, large pumps were required to pump the fluid through the building. With the new condensing boilers, the delta T can be 40F or 50F. So, not only do you get effisciency from the boiler combustion, but you can also use smaller pumps therefore achieving reduced pumping costs.