Q. How is heat transferred in a fuel fired boiler?
A. Heat flows from hot to cold (2nd law of thermodynamics) in three ways: RADIANT – electromagnetic waves (like sunlight) from the visible part of the flame strike metallic heating surfaces. CONDUCTION – heat passing through metal is absorbed by water on the other side. CONVECTION – heat from the non-visible part of the flame (the products of combustion or hot gasses) strike additional heating surfaces and through conduction heat water on the other side.
Q. What is boiler efficiency?
A. The amount of heat absorbed by the water in the boiler as a percentage of the heating value of the fuel being burned.
Q. Is the stated BTU input and output of a boiler related to efficiency?
A. Yes. For example, a 4 million BTU input boiler with a 3.2 million output would be an 80% efficient boiler. If the output were 3 million it would be a 75% efficient boiler.
Q. Where did the other 800,000 BTU’s go for the 80% efficient boiler?
A. Out the stack.
Q. Isn’t that wasted energy?
A. Not really. That extra heat creates the draft that’s necessary to pull the exhaust gasses out of the boiler.
Q. What if we could recapture all that heat going out the stack with something like an economizer? Wouldn’t that save money, fuel and reduce global warming?
A. In theory but here are the real world challenges: First you must have a hot water demand, or load, for the economizer to work. The cooler the water and the higher the stack temperature - the greater the temperature differential and the more heat you will extract. Conversely, the less delta T there is, the less heat you’ll extract. The more efficient a boiler is, the lower the stack temperature. A lower stack temperature means there is less delta T in the economizer. Swimming pool water and snow melting are examples of low temperature loads, but the problem is that unless you live in Siberia, these are probably not going to be continuous loads. A continuous low temperature load combined with a relatively inefficient boiler with high stack temperature is the best candidate for a stack economizer.
Q. What about pre-heating feed water for a steam boiler?
A. Most steam systems return as much condensate as possible and this water is very hot. As the temperature difference between the water flowing through the economizer and the temperature of the stack exhaust becomes closer, the delta T is lower and the savings go down. In cases where no steam is trapped and there is 100% fresh water make-up, a stack economizer would make sense.
Q. Wouldn’t even the slightest savings of an economizer be better than none?
A. That would be up to the owner but here are some other things to consider. At minimum, the economizer will require a pump to circulate water through it. Since the exhaust temperature out of the economizer will be lower, condensation is more likely to occur in the stack. Condensation of flue gasses can be mildly acidic to very acidic depending on the sulfur content in the fuel among other things. This usually means an upgrade to more expensive stack with traps to keep the condensation from migrating back to the economizer. Lower stack temperature after the economizer will create less draft. Less draft may lead to the requirement of a draft inducer. If this is the case, you have just added two electric motors to a system where you’re trying to save energy. After looking at the ROI, don’t forget to factor in maintenance of the additional equipment.
Q. Maybe I should get an electric heating boiler instead? Aren’t they 100% efficient?
A. Yes and there is no stack to worry about either.
Q. So it’s a no-brainer?
A. Not if you can afford the electricity to run it.
Q. So I’m better off buying the most efficient fuel-fired boiler I can and not worry about squeezing every last bit of heat energy out of it?
A. In general, yes.
Q. In that case I’ll buy a condensing boiler. They’re more expensive but with up to 98% efficiency I’ll get my money back in no time, right?
A. Not if you need steam and not if they don’t condense.
Q. They’re not available in low or high pressure steam?
Q. Well I have lots of buildings with hydronic heating systems too, big boy. I’ll use them there.
A. You can, but chances are they will rarely condense because most heating systems in the north America are not designed to operate at condensing temperatures. And when condensing boilers are not condensing, efficiencies can fall 10 to 15% from what they advertise.
Q. So I might only get 85-88% efficiency out of a condensing boiler if my heating system operates at 140 to 160 F and above?
A. Some condensing manufacturers claim higher than that. A lot depends on what the inlet water temperature is – the lower the temperature the higher the efficiency and vice versa. Latent (hidden) heat from combustion is released only when the hot gasses strike a cool enough surface. Picture your breath (hot air for some) on a cold window pane. Your breath condenses into small water droplets, and this change of state releases latent energy. When a surface is above a certain temperature for a given heat source – picture blowing on a warm pane of glass –condensation doesn’t occur. That’s because the glass isn’t cool enough to cause a gas-to-liquid change of state. When there’s no change, there’s no condensing – and therefore no release of latent energy.
Q. Even so, 85-88% efficiency still as good or better efficiency as most non-condensing boilers. And condensing boilers just look so sleek compared to a Rite. Why is that?
A. Rite Boilers were designed for boiler rooms with the boiler operator in mind. A boiler should be durable, easy to maintain and most of all, cleanable. Rite Boilers were designed to be functionally perfect, not runway fashionable.
Q. What do you mean by ’cleanable’? I’m picturing a soft rag with Turtle Wax.
A. Cleanable on the inside where the heat transfer surfaces are. Clean where it counts.
Q. Why is that important?
A. Because that’s where boiler efficiency can change. It’s not about the first day you start a boiler up and check it. That’s the catalog or brand new out-of-the-box efficiency. What owners and most specifying engineers forget to ask is this: What will the efficiency actually be over the next 1, 5, 10, 15, 20, 30, 40 years - or more – that the boiler is in service? The total fuel bill over the life of the boiler is infinitely more important than the first couple of months. In the real world, water gets lost in a system and water gets made up. Think of a car which has its own closed hydronic loop. As tiny as that system is, the radiator still has to be “flushed out” from time to time. Multiply that system by many times and you have a small hydronic heating system. Everything from a leaky pump seal to problems with the expansion tank can cause thousands of gallons to be made up. Instead of the radiator that needs flushing it’s the boiler needs flushing – or more accurately - de-scaling. If the scale can’t be removed, then the catalog efficiency will go down, fuel bills will go up and there’s not much anyone can do.
Q. You mean some of the high tech condensing boilers won’t let me inside to clean them?
A. That’s right.
Q. What about self cleaning boilers?
A. I would get it guaranteed in writing by the manufacturer.
Q. Wouldn’t it make environmental and economic sense to clean that scale out from time to time?
A. Yes. But the boiler must be designed around that feature.
Q. Are all Rite Boilers designed for full waterside access?
A. Rite Boilers are the most easily cleaned boilers in the world and that is why we emphasize that our boilers have efficiency you can maintain.
Q. So most other designs, even high efficiency and condensing boilers, can’t be easily cleaned?
A. Some claim they are “self cleaning” or can be de-scaled or de-limed with chemicals, but unless you can see all the waterside surfaces and have full access to them, it’s very difficult to get it all removed. Scale can be harder than cement.
Q. What about clean fireside heating surfaces? Aren’t they just as important?
A. From an efficiency standpoint yes. From causing damage to the boiler, no.
Q. Why is that?
A. Let’s look at soot, which is really carbon left over from unburned fuel. That is the most common fireside heat transfer problem. Soot will keep heat from getting to the heating surfaces. Scale on the other hand allows heat to the surfaces but won ‘t allow its conductance to the water. Metal can only absorb so much heat energy until it fails. So in both cases (soot and scale) heat energy bypasses the boiler, but only scale will cause additional damage to the heat exchanger.
Q. Even so, Rite Boilers aren’t as easy to clean soot out of as a fire-tube boiler, are they?
A. That’s true.
Q. Isn’t it just as important to remove soot from a boiler as it is to remove scale from an efficiency standpoint?
Q. Then why are Rite Boilers easier to clean on the waterside and harder to clean on the fireside?
A. Rite has made changes in that department. Access panels surrounding the tubes that were once tack welded are now fastened with screws and easily removable. Rite Boilers were developed when clean burning natural gas was on the rise and “dirtier” fuels, such as #2 - 6 oil, were becoming less popular. As a result, boilers that once required regular fireside cleaning no longer did. Fire-tube boilers are ideal for cleaning fireside surfaces which is understandable given that coal and wood were the primary fuels used in early boiler designs. In today’s world, something has to go seriously wrong for a natural gas or propane fired boiler to soot. Even #2 oil (diesel) rarely emits a visible smoke due to advancements in burner technology. And natural gas, low cost and abundant, is the cleanest burning fuel of all. Rite still offers a mechanical soot scraper as an option for our water boilers for #2 oil fired boilers. Today fireside soot is very rare and therefore fireside access is not nearly as important as waterside access when it comes to maintaining efficiency.
Q. A couple of miscellaneous questions to finish. In a water heating boiler, does the pressure of the water inside change the efficiency?
A. No. It could be 5 PSI or 125 PSI and if all else is equal, the heat transfer will be the same.
Q. What about steam pressure?
A. Yes. The higher the steam pressure, the higher the corresponding temperature – called saturation temperature – inside the heat exchanger. The higher the temperature inside the heat exchanger, the lower the delta T between the heat exchanger and the combustion process and therefore more heat will go out the stack, thus lowering efficiency.
Q. Does the temperature of combustion air change boiler efficiency?
A. For every 40 F rise in combustion air, the efficiency will go up about 1%. This is because the colder the excess air the more heat it will remove from the combustion process, leaving less heat for the heat exchanger.
Q. So sealed air combustion from outside on a cold day could lower boiler efficiency?
A. Yes. And not only that but as the air density changes with outdoor temperature change, the excess air measured in the stack will also change unless expensive combustion sensing equipment is used to and compensate for this. The more excess air, the lower the efficiency.
Q. So steady, warm combustion air temperature is one of the strategies to consider for maintaining good combustion efficiency all year around?
A. Yes. This can also put an end to seasonal power burner tune-ups based solely on changes to combustion air temperature.
Q. I know you get into low NOx burners in depth later on, but do low NOx burners improve boiler efficiency?
A. Many low NOx burners actually lower efficiency. That’s because most current low NOx power burners rely on greater amounts of excess air to lower the NOx than regular (standard emissions) burners. Remember, the more excess air, the lower the boiler efficiency.