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Is there anyone left who still pipes a modulating-condensing boiler with the full flow of the system? In other words, no closely spaced tees, no hydraulic separator, no low-loss header, no buffer tank, not any primary-secondary piping whatsoever? No mechanism to decouple the boiler piping and pump from the system piping and pump(s)?
If your answer is yes and you’re using Viessmann’s Vitocrossal CU3A, I’m in complete agreement with you. That heat exchanger has as much or more water capacity as most cast-iron boilers. The pressure drop across it is negligible, and it rarely makes sense to go that route with this beast of a boiler.
If your answer is yes and you’re installing a boiler with a water-tube heat exchanger, my response will almost always be, “Don’t do it.” I can see some rare instances where a heat exchanger manufacturer using larger water tubes with less pressure drop could work. Viessmann’s Vitodens Inox Radial heat exchanger comes to mind. Off the top of my head, I can’t think of another one I’d even consider. I’m not much of a gambler when it comes to taking hydronic risks.
Fire-tube heat exchangers fall somewhere in between. The pressure drop through these boilers is considerably lower than that of the water-tube heat exchangers. However, if you choose to pipe them without some primary-secondary piping arrangement, you should do your homework before committing to it.
You should be aware of the boiler’s minimum and maximum flow rate limitations. Every manufacturer I know lists these in the installation manual; yet another great reason to crack those pulp nonfiction documents open. They’ll typically look similar to the Lochinvar Knight fire-tube boiler (Figure 1).
Use a hydraulic separator if the system flow rates are greater than the boiler’s maximum flow rate or less than the boiler’s minimum flow rate.
I have a different approach based on my personal preference and what I know will work. On every modulating-condensing boiler, other than the Viessmann CU3A, I install a 4-in-1 or 5-in-1 hydraulic separator. This gives me or my customer air separation, hydraulic separation, magnetic separation, dirt separation, and a port that can be used for a system sensor or an analog temperature gauge.
In my opinion, I consider all these essential when installing a modulating-condensing boiler. The facts are simple. This single component cleans up the piping and makes things easier on the installer — a no-brainer choice for me.
Below is a quick summary of some heat exchangers of different types, manufacturers and characteristics.
1. Fire-Tube Heat Exchanger
Low pressure drop.
Concentration of heat at the top; may cause issues with tube sheets and gaskets.
Lochinvar Knight uses a larger heat exchanger than most.
2. Water-Tube Heat Exchanger (Figure 3)
High pressure drop in most of them.
Fewer welds that can fail.
Better flame spread.
NTI, Utica, Viessmann and others have a larger diameter water-tube style.
Easier to service as everything is front-load.
1. Kiturami Heat Exchanger
Korean-made fire-tube heat exchanger used in many boilers.
Smaller heat exchanger used in Combis and other boilers.
2. Sermeta Heat Exchanger
Manufactured in France and China.
Water-tube heat exchanger that typically has high pressure drops.
Common in dozens of boilers.
Sermeta Mono heat exchanger most common in 153-246 MBH boilers.
316L stainless steel.
3. Viessmann Heat Exchanger
Made in Germany.
Inox Radial — large waterways, less pressure drop.
316Ti stainless steel.
4. Giannoni Heat Exchanger
Made in Poland.
Pumping away and
Let’s look at an expansion tank, a circulator and a boiler heat exchanger with a somewhat significant pressure loss. And by significant, I mean 24 feet of head on a residential boiler with a water-tube heat exchanger.
If you look at some manufacturer-supplied installation manuals, you’ll see it in black and white. Manufacturers recommend circulators such as a Grundfos 26-99 to get the proper amount of water flowing through them. On the flip side, you’ll see some residential fire-tube heat exchangers with 0.3 feet of head. That’s an eye-opener if ever there was one.
Looking at the piping schematic in Figure 4, I’ve had people point out that you’re supposed to be pumping away. My response is that I am. There’s still some confusion as to what we’re pumping away from.
The key to this setup is that I am pumping away, which always means pumping away from the point of no pressure change. That point is always going to be the expansion tank; it doesn’t matter where it’s located in the system. I’m clearly doing that here.
I’m also pumping directly toward the boiler’s heat exchanger. Some get the two confused. You want to pump away from the expansion tank and pump toward the heat exchanger of a modulating-condensing boiler.
The reason for this is simple. In a closed hydronic system, when the circulator bangs on, it creates a pressure difference between its inlet and outlet. Knowing that and the placement of the boiler circulator in Figure 4, we’re ensuring the heat exchanger will see a slight increase in pressure. Flow will improve, preventing overheating.
The added pressure will help force microbubbles into the solution until they hit the air separator. It also will aid in keeping the inner surfaces of the heat exchangers clean and all but guarantee against any water flashing to steam.
If we were to put the circulator on the outlet of the boiler, the heat exchanger would see a slight decrease in pressure. The boiler is more likely to overheat. Air can become a problem and the chances of premature heat exchanger failure increase.
1. Pipe them right. Use hydraulic separation.
2. Pump them right. Use the right-sized boiler pump to overcome the heat exchanger pressure loss. Most manufacturers will give you options on which circulator to use; others will include the right pump with the boiler. At the very least, they give you the necessary heat exchanger data to run the numbers yourself. They teed it up for us; all we have to do is hit it.
3. Keep them clean. Annual service and proper water treatment. Imagine an 80,000 BTU/hr. modulating-condensing boiler with a heat exchanger that has a pressure loss of 24 feet of head. Now, gunk up the waterways and see what happens next. An early death for the heat exchanger is almost certain.
Keep it clean
In every modulating-condensing boiler installation instruction manual, you will be given parameters for the proper water chemistry of the fill/makeup water. And if you’re not testing, you’re guessing. Test kits are readily available, as well as the ability to send out a water sample to have tested. In the past, I used Rhomar for both methods. The below values are a sample of what one boiler manufacturer requires.
1. Hardness: Between 5 and 12 grains per gallon.
2. pH: Between 6.5 and 8.5. Water with pH levels below 7 is acidic and increases the corrosion rate; levels above 7 are basic and can cause scale buildup on the heat exchangers. Scaling creates hot spots on the heat exchanger, poor heat transfer and a potential premature death of the heat exchanger.
It’s important to note that pH is logarithmic, meaning that a pH level of 6 is 10 times more acidic than a pH level of 7. And a pH level of 5 is 10 times more acidic than a 6 and 100 times more acidic than a 7. Little changes in this value make big differences.
3. Total dissolved solids: Less than 350 ppm and ideally less for proper operation of electronic low-water cut-offs.
4. Chlorine concentration: Less than 150 ppm.
After I lost those three pumps, I changed everything I was doing regarding the quality of the fill/makeup water in the systems I designed and installed. Nothing changes if nothing changes.
• Water test. As mentioned previously.
• System integrity. I never take the owner’s word as to whether there are leaks in the system. Once I open or drain the system, I test the water. I examine the circulators, the screen on the fill valve, and other components. Those checks usually paint a clear picture of the water’s condition and whether I should take a closer look.
If I suspect a leak, I pressure-test the system or the places that are most likely to leak, and then repair those leaks as necessary.
• System components. I had been using Y-strainers or filter ball valves. Both are great products, but they’re simply not enough with the new boilers and new circulators. Dirt separators, magnetic separators and black iron oxide sludge filters are now part and parcel of these systems.
Without them, your new variable-speed circulator with the ECM permanent magnet motor will serve that purpose and fail. The magnet in the ECM pumps attracts and collects iron oxide. These items are no longer options; they are necessities.
• System flush. All I do on this step is flush the system with fill water to purge any contaminants before cleaning.
• System cleaning. Many quality products are available for this step. I’ve used Fernox F3, Sentinel X300 and Rhomar 9100. I either inject the aerosol or pump in the fluid, depending on which product I chose, while following the manufacturer’s recommendation on the amount to use.
Next, I bring the system up to normal operating temperature, which aids in the cleaning process, and run the pump(s) for about four to six hours, sometimes longer. Then flush the system for as long as it takes until the water runs clear.
• System protection. The system is now leak-free, cleaned, filled and purged of air. At this point, I add an approved multimetal corrosion inhibitor such as Fernox F1, Sentinel X100 or Rhomar 922. Make sure the product you’re using is compatible with the boiler’s heat exchanger, and you use the recommended amount. Again, you’ll either be injecting an aerosol through a boiler drain or using a transfer pump to pump it in via a boiler drain.
I’ve been playing it safe for a while now when it comes to mod-cons. Let me explain. I read a lot and I listen a lot to those in the industry who are smarter than I am. Those folks are easy to find. If you’re the smartest guy in the room, you need to find another room.
So, by playing it safe, I mean that I do my homework, follow the tried and tested rules, keep my eyes and ears open, and make every attempt to remain quiet unless what I want to share will be helpful to someone. I hope this was for you.