The world of boilers has changed dramatically the last few years in terms of efficiency and controls. This has been driven in part by energy costs, but also by the ability to install programmable real time controls. Today’s boilers are termed Mod-Con Boilers. Mod-Con stands for Modulating and Condensing.
Modulating (control): Today’s boilers turndown at least 5:1, which means that a 100,000 BTU nameplate boiler can operate at 20,000 BTU/Hr and do so efficiently. This is important, in order to keep the boiler from short-cycling or on-off firing when the boiler demand is low. Heating systems are sized to maintain home comfort in worst case conditions, the coldest time of the year. So a system, sized at 100,000 BTU/Hr, will seldom run at that level. Modulation is a very nice feature for the residential boiler that was only available on large commercial and industrial boilers in prior years.
In a bit more detail a boiler might be programmed to increase boiler water temperature 1F for every 1F outside temperature drop; a typical pre-programmed value used in the initial boiler setup. The actual ratio is very building dependent. A house that is well insulated and air-sealed might be as low as 0.5F rise for every degree temperature drop outside. The Mechanical Contractor based on the input of the designer will program this value.
A typical boiler control package with Indirect domestic hot water heater is shown below: Residential: Condensing Boiler (outside reset) with Indirect dhw Tank
The Indirect Water Heater is a nice feature with one heating device maintaining all of the heating needs and at high efficiency. Also the Indirect Water Heaters are made of much better materials for long life. Usually a stainless steel tank and exchanger and very well insulated to keep standby losses low.
Most boilers have “Priority Firing” to support the Indirect Water Heaters. In other words, the Indirect Water Heater is first priority when the boiler controls see tank temperature dropping. A 3-way valve (or a circulating pump) will direct all of the boiler energy to the Indirect Water Heater until the temperature setting is maintained. In the well insulated home, domestic hot water use can often set the boiler size.
Commercial: Two Boilers (non-condensing, existing) at 80% Capacity and Hydraulic Separator.
Condensing (energy efficiency): Today’s boilers can be in excess of 95% efficiency vs. the non-condensing boilers which are in the low 80%. “Condensing” the water generated from the combustion process lowers the stack gas temperatures and allows for maximum heat to be extracted from the fuel and turned into useable heat.
The non-condensing boiler can have a stack gas temperature in excess of 400 F, necessary in order to prevent the water of combustion from condensing. The condensing boiler stack gas temperature might be as low as 100 F depending on return water temperature. If fact in most cases the material for venting the stack gas is plastic pipe for the condensing boiler.
Condensing boilers are possible because of the use of stainless steel heat exchangers, which can withstand the condensed acidic water vapors that are the result of the combustion process. The older boiler construction was generally cast iron or copper, which would corrode and fail within a short time if stack temperature were not maintained high enough to prevent condensation. One does need to collect and treat the condensate but that is part of the boiler package. A drain is needed in the Mechanical Room for disposal of the treated condensate but that’s always a good idea where water is present. It also a good idea to have a water source in the Mechanical Room for packing (filling) the system.
A graph of the efficiency curve for a condensing boiler is shown below. The graph is for a commercial boiler but a residential boiler will have a similar curve. Note how the lower Return Water Temperature increases Thermal Efficiency. Today’s boilers, and a hydronic system that is designed for low temperature, are a good fit.
Another aspect of control, for today’s boilers, which improves efficiency is – inverse temperature control based on outside temperature. Just enough temperature to maintain comfort. From the graph above it can be seen the lower temperatures increase efficiency.