You have options when it comes to the tubing you bury or staple up on a radiant job. You may choose to go with a particular supplier and never question the type of tubing they provide. That's a legitimate choice, if you trust the suppler.
Nowadays, most suppliers deal with some sort of PEX tubing. PEX is an acronym for polyethylene, crosslinked. It's very good stuff from what I've seen. It's also made to last about 200 years, but you and I will never know for sure if that's true because we will both have joined the ranks of the Dead Men by then. PEX is, however, the material you'll find all over Europe. The Europeans have been doing hydronic radiant heating with PEX since the early Seventies and there's a lot more of it there than there is here. PEX has a great track record and has reached the point where it's almost considered a commodity item by many heating contractors and plumbers.
To avoid having it become a commodity (which is the kiss of death for any manufacturer), PEX manufacturers have been taking the time to spell out the differences between their products. And there are differences in the way different companies make PEX. Knowing this can help you make the right decision when you're shopping for a PEX supplier. Here's the deal.
Most manufacturers start with a material called HDPE, which stands for High-Density Polyethylene. HDPE evolved from polyethylene, a material discovered by accident by a group of British scientists during the 1930s. Polyethylene played a huge part in World War II because it gave the British the ability to carry radar on their airplanes for the first time in history. This was during the Battle of Britain. The German pilots didn't have radar (and they didn't know that the British did) so the British were able to kick the stuffing out of the Nazis by shooting at them from behind the clouds. But that's a story for another day.
PEX differs from HDPE in that PEX has this special three-dimensional link between the molecules. Scientists call this stuff a "macro-molecule." What that means is that if you hold a 200' coil of PEX pipe in your hands, you're actually holding only a few molecules. Pretty strange, eh? It's this network of macromolecules that gives PEX such a fine memory for its original shape. If you kink PEX pipe, you can just heat it up and it will always return to the shape in which it was first crosslinked.
The linking of the molecules happens during the manufacturing process, and how a manufacturer chooses to make that happen affects the properties of the final product. The manufacturer can't link all the molecules together because that would make the PEX too brittle. On the other hand, if they link too few molecules the material won't be any better than HDPE, from whence it came. They have to find just the right combination of linked and non-linked molecules. There are different ways to get where they need to go, and from what I've learned, some ways seem better than others. Here are the principal methods manufacturers are using today to make PEX:
Engel-method PEX (also known as PEX-A) This is the stuff Tomas Engel brought into the world. Engel was the guy who invented PEX. He is a European scientist and he had nothing to do with the radiant heating industry when he invented PEX. I heard him speak at a meeting in Sweden in 1990 and was fascinated by his story. But that, too, is a story for another day. What's important for you to know is that Engel-method PEX gets crosslinked while it's still in its melted form. The manufacturers do this by adding peroxide to the mix and then applying a tremendous amount of pressure and temperature to the liquid. What comes squirting out of the machine is PEX that's as clear as glass. As it cools, it takes on a darker color. The Engel method gives the pipe an essentially uniform distribution of the crosslinking sites throughout the material. From what I've read in independent scientific papers that have come from Europe, this is the best way to make PEX. It takes a while longer to do it this way, and this sort of PEX may cost a bit more.
Irradiation-method PEX (also known as PEX-C) Irradiated PEX starts out as straight polyethylene tubing. As with other methods of making straight polyethylene, the tubing takes on a definite form during the extrusion process. With the irradiation method, the crosslinking takes place in a second process when the manufacturer runs the tubing through an electron beam cannon. The beam gets the polyethylene molecules so excited that they crosslink.
This method can sometimes result in a less uniform crosslinking of the material, however, especially if the pipe is larger than 1" in diameter. This happens because the thicker pipe requires a higher dose of radiation. If it's not done properly, the outer layer of irradiated PEX can become brittle, but this is not a concern in a well-controlled process. If the pipe maker is experienced there is no need for concern with this sort of PEX when used on a radiant heating system. Buy in the mainstream and you'll do fine.
Silane-method PEX (also known as PEX-B) The big difference between this method and the other two methods is that with the Engel and Irradiation methods, the crosslinking consists of a bond between carbon molecules. With the Silane method the crosslinking takes place across silicon and oxygen molecules. These links are weaker than the carbon-carbon links that result from the other methods, and this may have an effect on the long-term chemical stability of the material. If we live long enough, we'll probably find out.
I hope all this chemistry isn't making your head spin. I think it's good for you to know the differences between these materials even though they all go under the name PEX. Let's face it, you're the one who's going to be putting the tubing under the floor.
In fairness, though, I have to tell you that none of these methods is a bad method; The American Society for Testing and Materials (ASTM) approves all three methods. However, in a world among "equals," it seems to me that some methods are more "equal" than others. Don't think of PEX as a commodity item. There is a difference.
And that brings us to this last one:
PEX/Aluminum/PEX I first saw this tubing in 1991 at the big ISH fair in Frankfurt, Germany. I was walking past one of the booths in the radiant heating building when a salesman handed me what looked like a plastic cane. He didn't pick me out of the crowd; he was giving canes to everyone who passed. I thought the kids would like it so I took it and twirled it like Charlie Chaplin as I continued down the aisle. But then it suddenly occurred to me that what I was twirling was hydronic radiant heat tubing (this stuff was so new to me in 1991). I walked back to the booth and asked about it. They found someone who could speak pretty good English and he explained that this tubing had an aluminum core that was sandwiched by two layers of PEX. "The aluminum keeps out the oxygen," the representative told me. "And when you bend it, it stays bent!" This I could see as I looked at the cane. "When you put it down on the floor, it doesn't bounce back up at you." He gestured by flailing his arms.
When you put it down, it stays down. And therein lies the benefit of PEX/Aluminum/PEX. That, and the fact that it doesn't expand as much as straight PEX. Less expansion means less noise if you're using PEX on a staple-up job.
Like to learn more?
What you just read comes from my book, Hydronic Radiant Heating - A practical guide for the nonengineer installer. You can check it out in the Books & More section at www.HeatingHelp.com.
THINKING LIKE AIR
THINKING LIKE AIR
The house had been sitting on this old Connecticut street for more than 100 years. Children, grown and long gone, had once skipped across its wide lawn and watched in wonder as the new cars clattered by. Grownups sat in the shade of the latticed porch and waved off the summer heat with paper fans. If you close your eyes, you can still see them there.
In the winter, they sat inside the house by the ornate, cast-iron radiators that ringed the room with warmth. Down in the basement, a behemoth of a coal-fired boiler heated water. The warmed water became buoyant and rose upward toward the radiators. It displaced the denser cold water in the radiators, and sent it tumbling downward through the large pipes to the boiler where it too would be heated.
Systems such as these depend on natural convection. Hot water rises; cold water falls. It's a simple principle, but to make the system work well and heat evenly without circulators or controls the Dead Men had to be very specific in the way they laid out a job. They had to size and angle the pipes just so. If they made a mistake, the convective currents wouldn't happen and the house would be cold.
For more than 100 winters, the gravity-hot-water heating system in this old house had warmed the families who lived there because the Dead Men had done a superb job. But now, the new owner of the house was changing things. He was leaving the old radiators with their raised metalwork; they were far too beautiful to remove. Most of the old piping would also stay.
There would be a new boiler, of course. There would also be a circulator and zone valves, and controls. The new boiler would have a bypass line to protect it from condensed flue gasses. There would also be an air separator.
The new boiler would also have an additional zone because the owners had decided to extend the old house so they could have a larger kitchen. Their architect had done a good job of matching the addition to the original house. The heating engineer now had to figure out the best way to warm this new space.
After much consideration, the engineer decided to use two, recessed floor-convectors in front of the wide kitchen windows. He would also use a kick-space heater beneath the sink cabinet on the other side of the kitchen. He calculated the heat loss of the new kitchen very carefully and sized the convectors and the kick-space heater very accurately.
The architect liked the engineer's choice in heaters because the recessed floor-convectors had attractive brass grates that fit well with the home's Victorian character. The kick-space heater, well, it would be practically invisible under the cabinet.
The contractor installed the new boiler, made the changes to the system and piped the new zone. The general contractor finished work on the home and everyone waited for winter to arrive.
And arrive it did. That first winter was much colder than normal and the weather put the new kitchen zone to the test almost immediately.
It didn't work.
The homeowners called the heating contractor to complain. The temperature hovered around 62 degrees, even though they had the thermostat set for 72 degrees. The contractor went to the job and checked things out. Everything seemed to be working properly, except that the kitchen was too cold. He called the engineer.
The engineer showed up on the job and went over the mechanical aspects of the system. He could find no fault. He checked his heat-loss calculations, and they seemed fine. He turned to the contractor and said, "Well, what are you going to do?" This bewildered the contractor because he thought the engineer was in charge.
"What do you mean what am I going to do?" the contractor asked.
"What do you mean what do I mean?" the engineer said. "You obviously did something wrong. My calculations are correct. The heaters are sized properly. There must be something wrong with your installation."
"But you designed the installation!" the contractor protested.
"I know I did," the engineer shot back. "And it's a good design. You must have made a mistake in the installation."
"Where? Do you see anything here that deviates from your plans and specs?" the contractor asked in frustration.
"There must be something stuck in the pipes," the engineer said. "Did you flush the system? Have you checked the flow rate? The system is operating as though it's not getting enough flow. You should check your work. My design is correct." And with that, the engineer left.
The homeowner wanted to know why it was still cold in his new kitchen, and with the engineer gone, the contractor was left holding the bag. He decided the engineer might be right about something being stuck in the pipes. These things happen. So he flushed the system and installed flow meters. The flow meters showed the flow to be what it was supposed to be. The contractor called the engineer and after some more frustrating conversation they both began thinking about the movement of air rather than the movement of water. The pipes could deliver hot water to the recessed convectors and the kick-space heater, but unless the air could remove the BTUs from those hot surfaces, the kitchen would remain cold. They finally got together and used their imaginations to see how the air would move in a room served by heaters such as these.
This is what they reasoned:
The recessed floor-convectors work by convection. These units heat the air above their elements and cause it to rise. The cooler air along the floor will then fall into the convector's recessed trough and be heated. It, in turn, will rise and create a convective movement of air within the room. The movement of air will be very similar to the movement of water in the ancient gravity hot-water system that had served the old house for so many years. The principle was the same.
Now the kick-space heater also worked on convection, but it had a fan. It used that fan to draw in the air near the floor. It heated the air, and then it kicked it back into the room - right at floor level. The heated air rose toward the ceiling and tumbled the rest of the air in the room in a second convective current.
And it was then that the contractor and the engineer began to consider what happens to the recessed floor-convectors when the kick-space heater is shooting hot air across the floor? How can warm air rise out of the recessed convector if there's no cooler air to replace it?
It occurred to the contractor and the engineer that these two types of heaters will never get along within the same zone. Each unit is looking for something different and the problem will always crop up on the coldest days of the year because that's when both units are needed. Until they began to use their imaginations, the engineer and the contractor couldn't see those air currents fighting each other. Once they opened their eyes, though, the solution was easy.
They drilled some holes in the bottom of the recessed convector. That allowed air to circulate up from the basement and the problem was solved.
Seeing with your mind's eye
Whenever I write a book I do my best to paint word pictures so that you'll be able to see what I see on a problem job. I'm not an engineer so I have to work harder to explain the principles of engineering. This is the sort of "imagineering" that I try to bring to all my books. I invite you to check them out in the Books & More section of www.HeatingHelp.com.
And while you're there, get yourself a cool tee shirt!
HOW STEAM TRAPS CAN BITE YOU
HOW STEAM TRAPS CAN BITE YOU
In the beginning there was one-pipe steam, and one-pipe steam was pretty simple. The steam traveled up the pipe; the condensate fell down the pipe. Size and pitch the pipes properly, don't over-fire the boiler, keep the wet returns clean, and you were in pretty good shape. But as buildings grew larger, one-pipe steam became impractical because there was just too much condensate draining down against the steam. The Dead Men got around this for a while by using a system that sent the steam up to the attic through an express riser. From there, steam flowed downward toward the radiators, taking the condensate with it. That's called parallel flow, by the way, and it's a lot easier to deal with than counterflow.
In the early days of heating, rich complained because the one-pipe steam vents on their radiators often spit dirty water on their curtains and wallpaper. As a result of those complaints, the Dead Men invented what we today call Vapor heating. Vapor works with very low pressure (usually only about eight ounces or so) and two pipes at each radiator instead of one. The steam enters the top of the radiator through one pipe, and the condensate drains from the bottom on the radiator through the other pipe. The steam pushes the air through the radiator and shoves it out of the system through a big main vent, which is somewhere near the end of the dry return down in the boiler room. If that vent spit dirty water, the water went on the basement floor rather than on the curtains and that made the rich folks happy.
When you work with two-pipe steam radiators you do have a challenge, though. These systems are set up like ladders and each radiator is like a rung on that ladder. One of the ladder's uprights is a supply main; the other is the return main. The Dead Men had to come up with a way of stopping the steam from scooting from one side of the ladder to the other. This was important because, for steam to move, they needed a point of high pressure and a point of low pressure. If the steam could find a shortcut through any of the radiators, it would pressurize the return side of the ladder and the air wouldn't be able to get out of the other radiators. The result would be no heat.
So the Dead Men came up with all sorts of neat devices that would stop the steam from getting through the radiators. They used orifices, and little check valves, and tiny steel balls, and water seals, and whatnot. And then they came up with the idea of the thermostatic radiator trap and that idea stuck around for a long time.
A thermostatic radiator trap is an automatic valve that responds to temperature. It's normally open. It lets the air get by. It closes to steam. It opens to condensate. It sets up the points of pressure and no pressure in the system. It fails after a number of years, and it usually fails in the open position.
Steam traps trap steam, so there's little or no pressure on their outlet side. That gave the Dead Men another challenge because, with their one-pipe systems, they had been using the "leftover" steam pressure at the end of the main to help put the water back into the boiler. If they sized their pipes properly, all they had to do was allow for a vertical distance of 28 inches between the boiler water line and the bottom of the lowest steam main. They called this the "A" Dimension. The water would stack in that vertical space and its weight would combine with the leftover steam pressure. Those two forces were enough to overcome the pressure inside the boiler and get the condensate back where it belonged, which was inside the boiler.
Steam traps changed all that. With no pressure on the returns, the Dead Men had only the weight of the returning condensate to help them get the water back into the boiler. A column of water that's 28 inches high exerts a force of 1 psi. So on those two-pipe Vapor systems, they'd need at least 30 inches of vertical space between the boiler water line and the lowest return line for every pound of pressure inside the boiler (that extra two inches is for pipe friction losses). If the boiler ran at 2 psi, they'd need 60 inches of vertical height. I call this the "B" Dimension. If the boiler ran at 5 psi, they'd need 150 inches. The trouble was that those old basements weren't deep enough, and that's why they sized the piping for the lowest possible pressure drop.
They also invented a device called a Boiler Return Trap, which helped if the boiler pressure went too high. The Return Trap contains a float-operated, double-seated valve. It works with two check valves in the return. If the returning condensate didn't have enough pressure to get into the boiler, the condensate would flow into the Boiler Return Trap instead. When it got high enough inside the Return Trap, the float in the Return Trap would rise and open a steam line. The steam would shoot into the Return Trap, combine with the weight of the condensate, shut the outboard check valve, open the inboard check valve, and put the condensate back into the boiler. It was a very simple, mechanical device that's designed to last a very long time. If you see one, the best thing you can do is leave it along – if it's working.
But you may not know that it's not working until it's too late. Here's why. Let's say you get called to look at an old Vapor system because the fuel bills are very high, there's water hammer everywhere, and most of the radiators never get hot. You suspect, and rightly so, that the thermostatic steam traps on the radiators aren't functioning. No one has ever worked on them. You tell the owners that you're going to have to repair all those traps. You give a price and they give you the go-ahead to start the work.
But once you get the traps fixed, the system still doesn't work. Condensate won't return to the boiler. The boiler takes on fresh water through an automatic water feeder. The boiler floods during its off cycles. There's still no heat in most of the building, and you think the traps you installed are bad.
You know what's probably going on here? The Boiler Return Trap isn't working, and it hasn't worked for years. It seemed like it was working before, but that's only because the steam traps on the radiators were bad. They were allowing steam pressure into the returns. That pressure was causing all those problems they were having, sure, but it was also allowing the condensate to return to the boiler.
But once you fixed the thermostatic steam traps, there was no longer any pressure in the return lines to push the water back into the boiler. If the Boiler Return Trap was working, it could have helped. But it wasn't working, and you didn't know that because the broken steam traps were masking the problem.
So now, without the Boiler Return Trap, the condensate backs into the returns and blocks the one main air vent. Air can't get out of the system so the radiators stay cold. Without the Boiler Return Trap, the condensate stays in the return lines. It won't go back to the boiler so the boiler goes off on low water. Then the automatic water feeder does what it's supposed to do, and when the system shuts down the returning condensate floods the boiler.
You think it's the traps that you installed, but it's not.
So the next time you run into Vapor Heating, look at the whole system, and not just the steam traps. You can fix a Boiler Return Trap. You can also replace it with a boiler-feed pump if you're more comfortable going that way.
Like to learn more?
My book, The Lost Art of Steam Heating has an entire chapter on old Vapor systems, with lots of cutaway diagrams of these components. You can get check it out at the Books & More section of www.HeatingHelp.com. And while you're there, take a look at The Lost Art of Steam Heating Companion. That book has the original specs for a lot of these old devices.
The more you know, the better you look!
M&M SAFETY RECALL
IMPORTANT SAFETY RECALL
I posted this message on the Wall at www.HeatingHelp.com yesterday:
"I just got word that M&M is recalling all PS-804 control head assemblies. The PS-804 is their foam-compensating probe type low-water cutoff. The recall applies only to this control, and only to the ones built between August 1999 and January 2000.
Here are the date codes: 99H, 99J, 99K, 99L, 99M, 00A.
"They are NOT recalling any other probe-type controls, only the foam-compensating model.
"The majority of these controls were packaged with Burnham boilers."
Mike Gordon, Burnham's Director of Engineering was kind enough to follow up almost immediately with the following post, which I'm sending you with his permission. This is important stuff and I urge you to take action if you've installed any of these controls.
"M&M has advised Burnham that they have found a defect in the PS804 probe low water cutoff. The PS804 is a foam compensating control and should not be confused with the standard, high volume PS801 and P802 controls.
"This defect may cause the burner contacts to stay closed even though a low water condition exists in the boiler. This is an unsafe condition!
"A very, very small number of the PS804 controls have demonstrated this problem. However, as a precaution, M&M has initiated a mandatory recall of all PS804's. This recall has been filed with the Consumer Product Safety Commission. Accordingly, everyone involved in the chain of distribution of the controls is legally obligated to assist in the recovery of the controls.
"The PS804 control was optional on Burnham V7 oil-fired steam boilers (M&M model PS804-120) and Independence gas fired steam boilers (M&M model PS804-24) manufactured between August 1999 and February 2000. This control was NOT used on all Burnham V7's and Independence boilers manufactured during this period.
Burnham is sending a formal announcement to all of our distributors who sold these controls. In addition, M&M is sending a announcement to all Burnham distributors that sold PS804 equipped boilers and M&M reps. The M&M announcement will detail the recall program and identify, by distributor, how many boilers with the PS804 control were sold to that specific distributor. It is the obligation of the distributor to notify the installing contractor and the contractor to replace the control as soon as possible.
M&M is sending a replacement kit to the distributor for each PS804 control sold by that distributor.
"The kit will include:
1) A replacement low water cutoff. For the V7, the replacement is the PS801 control. For the Independence, the replacement control is the PS802.
2) Instructions on how the replacemnt control should be installed
3) A return mailing label.
4) A labor reimbursement form
"In the next few days, the replacement kits will be arriving at distributors. Contractors who may have installed V7 or Independence boilers in the time period noted above, should contact their Burnham distributor. The distributor can assist in determining if a certain boiler does in fact have a PS804 low water cutoff. If so, the distributor will provide the proper replacement kit an no charge.
"The contractor should arrange to visit all installations that they installed with PS804 low water cutoffs. The PS804 must be replaced with the proper control supplied in the kit. Place the old PS804 control in the box used to supply the new control and affix the mailing label.
"Fill out the labor reimbursement claim and place it in the box with the control. Mail the control back to M&M. Note, the labor reimbursement claim must be filled out fully to get labor payment. The boiler serial number and the old PS804 control are absolutely required to process the claim. No exceptions. Once processed, M&M will mail a labor payment directly to the contractor.
"M&M will accept all reasonable labor charges to replace the PS804 control. This is a very serious problem and your assistance is essential in located every control and assuring safe installations.
"On behalf of Burnham, please accept my apology for the inconvenience and frustration caused by our problem."
Director of Engineering
And my thanks go to both Burnham and McDonnell & Miller for reacting to this problem so quickly and so responsibly.
UNDERSTANDING PRIMARY/SECONDARY PUMPING
Primary/secondary pumping has become pretty popular nowadays, especially with boiler manufacturers. They love it because it offers a simple way to protect their boilers against low-temperature return water and the resulting flue-gas condensation that low-temperature water causes. This is a legitimate concern because more and more of us are involved with radiant heating where the water returning from the system can be as low as 90 degrees. It's also a concern when you add a modern boiler to an old gravity hot water system. There's nothing worse than hitting a hot boiler with cold return water.
Primary/secondary pumping lets you use small, inline circulators - even on large commercial jobs - and that's a real plus. And in a multiple-boiler system, this simple method of piping gives you a way to lower boiler standby losses and save fuel. When piped properly, no water will flow through an "off" boiler when its secondary circulator stops. And when the water stops flowing, the standby losses practically disappear.
There's nothing complicated about primary/secondary pumping. It all comes down to what happens when water flows through a tee. If water goes in, it has to come out. That's common sense. But how it comes out makes all the difference in the world. With primary/secondary, you have to set the tees that lead to the secondary circuit no more than 12 inches apart. As the primary flow enters the first of the two tees, it "looks" ahead, and then makes a choice. It can either go straight for 12 inches and be past these two tees leading to the branch circuit, or it can divert through the first tee's branch and flow through the entire secondary circuit. It becomes a question of which is the easier way.
Now, imagine yourself as the water in that primary main. What would you do? If the secondary circuit's circulator were off, wouldn't you choose to flow straight across those 12 inches of straight pipe? I know I would. That's the path of least resistance. This is why that 12-inch maximum spacing is so important. If you place the tees too far apart, primary water will begin to see the secondary circuit as a path of lesser resistance and begin to flow that way.
It's pretty simple when you get right down to it. When the secondary circulator is off, no water will flow through the secondary circuit because the 12-inch "gap" between the tees in the primary main is the path of least resistance. And it doesn't matter how large or small the primary and secondary circulators are either. They operate independently because they're hydraulically disconnected. You size each circulator for the flow rate and pressure drop needs of only the circuit it serves. That's why you usually wind up with lots of small inline circulators instead of one or two large, base-mounted pumps.
The compression tank belongs in the primary main, as does the air separator and the fill valve. Make sure you install your primary circulator so that it pumps away from the compression tank. That way, you can take advantage of the primary circulator's full differential pressure. This makes it much easier for you to get rid of any system air that works its way into the piping. Oh, and if you put full-port ball valves in the common piping between the primary and secondary circuits, you'll be able to get the air out of the system a lot quicker as well.
The secondary circulators use the common piping between the primary and secondary circuits as their "compression tank." Always pipe your secondary circulators so they pump away from the primary loop and toward the radiation.
On most jobs, you'll connect your secondary circuits in a manifold off the two primary-to-secondary tees. Pipe the manifold the same way you would if you were connecting it to a boiler. If one or more of your secondary zones are going to serve a radiant heat zone, use a two-, three-, or four-way valve to mix the water returning from your zone into the hot supply water from the primary circuit. Pipe your secondary circulator on the radiation side of the two-, three-, or four-way valve. Use flow-control valves to stop gravity circulation from the primary to the secondary circuits. I didn't always preach this but I've leaned from experience that it pays to have those flow-control valves in place. And depending on the piping configuration, you might need them on both the supply and return sides of the secondary circuits to stop gravity circulation.
We call the system I've been describing "one-pipe, primary/secondary pumping." The primary circulator moves the boiler water through the primary circuit. When the flow of cooler water returns from the secondary circuit, the hot primary water, which jumped the "gap" between the two tees mixes with the return water. The hotter water instantly raises the returning water's temperature and protects the boiler from flue-gas condensation and thermal shock.
If every zone in a one-pipe, primary/secondary system should call at the same time, however unlikely this might be, the primary circuit's temperature will drop by its full design temperature drop. In large commercial systems, this can lower the supply water temperature to the secondary circuits at the end of your primary loop.
To get around this potential problem, you can use two-pipe, primary/secondary pumping instead. With this system, you pipe the primary circuit as a two-pipe direct- or reverse-return system. This gives you a way of delivering the same water temperature to each secondary circuit. You make your connection from the primary to the secondary circuit from a crossover "tunnel" that drops below the level of the primary supply and return mains. The drop in the pipe makes it easier to get rid of the air.
Balancing is more of a challenge in a two-pipe, primary/secondary system. You have to look carefully at the flow rate needs of each circuit and "tunnel" and make sure you're delivering the right flow to those secondary circuits. This type of system usually calls for some thoughtful engineering, so take you time. If you get stuck, bring your questions to the Wall, which you'll find at www.HeatingHelp.com. There are some very bright people there!