Having scale on heating surfaces is like having all the traffic lights in your town turn red at the same time, and stay red. Most movement stops. In the case of a boiler, that movement is heat. It's trying to get out of the fire, through the metal, and into the water, but the scale is blocking its way. So the boiler just keeps getting hotter as your building gets colder. That's not good for either one.
Pipe scale is rust. It usually shows up on the inside of the pipe and when the moving water causes it to flake off, we call it scale. Get enough of it going around and it will gum up the works.
Mineral scale is something else. In a boiler, it's mainly stubborn calcium carbonate, which never wants to dissolve in hot water. Behind this leading pest, other minerals will line up, and in this order of appearance: calcium sulfate, magnesium carbonate, iron, silica and manganese. And you can add to that menu, phosphates and hydroxides of any of those minerals. It pays to know your enemy, and the only real way to do that is to get the water analyzed if there's a problem.
It also pays to understand the simple magic that creates scale. First, think of a hot cup of coffee. You add a spoonful of sugar and give it a stir. The sugar instantly dissolves, and the hotter the coffee is, the quicker that will happen.
But the minerals that cause scale in a heating system do just the opposite. They are inversely soluble. That means that the hotter the water gets, the less those minerals will want to dissolve. Cold feed water arrives with the minerals in solution. The burner runs and the minerals come out of solution and cling to the hot surfaces. That's when the traffic lights start going red.
Most hydronic heating systems are closed to the atmosphere, so if you're getting a lot of scale, suspect that too much water is entering the system. Close the automatic feed valve and watch the gauge. If you're losing pressure, the system probably has a leak. Go find it and fix it.
And watch your circulator location, especially on commercial jobs. If you're pumping toward a compression tank. the circulator will show its differential pressure as a drop in suction pressure. If the circulator's ability to produces pressure exceeds the system's static fill pressure, the pressure at the pump suction will be sub-atmospheric. If there are automatic air vents nearby, they'll suck in air, and that will cause the ferrous parts of the system to rust. Say hello to Mr. Scale.
If you suspect the system is getting too much air, take a water sample and dip a magnet into it. Iron particles will stick to the magnet and you'll be on your way to the solution. I'm a fan of the new hydronic dirt separators that include magnets. It's amazing what those neat devices will pull out of the flowing water.
Scaly steam boilers
Scale can cause lots of problems in steam boilers, especially if you're using the boiler for process and you're dumping all or some of the condensate. Lost condensate means lots of fresh, cold feed water, and that brings in the calcium and magnesium, which will cling to the hottest surfaces of the boiler, slow heat transfer, raise fuel bills, and cut years from the life of the boiler. Most owners think that's unavoidable, but it's not.
Here's how to shine: You're replacing a boiler in a bakery, where they use steam to put the crust on bread, or in a seafood restaurant, where they use steam to cook shellfish. None of that condensate comes back to those boilers so the cold feed water will kill them.
Instead of feeding the boiler with cold water, feed it through a small water heater. The calcium and magnesium will settle out inside the water heater instead of the boiler. Sure, they'll lose the water heater at some point, but water heaters are cheaper than boilers.
One more trick: On the hot-water feed line from the water heater, install a microbubble air separator, the same sort you'd use on a hydronic heating system. Run your hot feed water through that and it will act like an inexpensive deaerator, removing the air before it hits the boiler. Less air means less corrosion, and that means less scale.
Now go amaze your customer with your street smarts.
Sludge is the softer version of scale and it will often wind up in your radiators if the hardness in the water gets a chance to react with any phosphate or alkaline treatments someone may have added. Sludge looks and acts like paste, and it will murder heat transfer. If the hot water can't pass its heat to the air, the rooms go cold and the heating bills go up. So take care in adding chemicals to any hydronic system, unless you know what you're doing. I've often watched contractors pour chemicals into boilers because they were frustrated with a problem, and they thought the product had a cool name. And they usually over-apply whatever treatment they use, believing that if one pound of sugar is sweet, well, then 10 pounds of sugar is 10 times sweeter. It's not. Know what you're doing before you do it.
The boiler was cast iron; the system had been leaking. Oh, and the fill valve added a lot of fresh, hard water. The minerals did their inversely soluble thing and clogged part of the boiler's passages so the water couldn't flow through there. The burner kept firing. Steam formed, expanded like crazy and then collapsed within the clogged section. It did this again and again, hammering away at the boiler's gut. When I got there, that beast was bouncing on the floor. Had I not seen this I wouldn't believe it possible. I hit the off switch and got the heck out of there. You would have done the same.
Time for a new boiler.