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Dan Holohans Heating Help Page 4

THE 25 STEPS IN THE REPAIR OF A VACUUM STEAM SYSTEM 1. Long ago, a Dead Man decided to heat a big building with a steam system. Since it was such a big building, and since pipe, valves and fittings have never been cheap, the Dead Man decided to use a vacuum pump to suck the air from the system. By doing this, he got to undersize every pipe, valve and fitting in that building. He also got to arm a heating land mine for you to step on long after he was dead. If nothing else, this Dead Men had a sense of humor. 2. At the beginning of time, Mother Nature decided that thermostatic radiator traps should last about 10 years. This is because thermostatic traps have these flexible metal bellows that open and close a gazillion times a year. Traps do the best they can, but they can't last forever. When they die, they usually do so in the wide-open position because this is the position that will cause you the most grief. 3. Mother Nature also decided that most human beings will be either too cheap, too dumb, or too lazy to repair thermostatic radiator traps. She, too, has a sense of humor. 4. Without thermostatic radiator traps the condensate gets hotter and hotter. Vacuum pumps don't like really hot condensate because they have a tough time pumping it. This is because, in a vacuum, water boils at a lower temperature than it does under atmospheric conditions. Since vacuum pumps produce vacuum (hence the name!), they have a good reason to be concerned about really hot condensate. 5. The red-hot condensate slugs into the vacuum pump, and flashes into great puffs of expansive steam as it hits the pump's impeller thereby causing the pump to run dry. In a few hours, the pump's mechanical seal cracks and spews lava-like condensate across the boiler room floor. 6. The building superintendent shuts off the vacuum pump, and opens the bypass line in hopes that the condensate will be smart enough to return to the boiler by itself. The superintendent has no problem making this decision because he sees no need for any system component he doesn't fully understand. He would bypass the boiler if he could. 7. The condensate now takes its sweet time returning to the boiler. It may make it back by next June if all goes well. Meanwhile, there's a lot of water hammer in the building, which everyone considers normal. It's a steam system, right? 8. The boiler goes off on low water. 9. The superintendent curses under his breath, and then adds raw, icy water to the boiler. If he's a real superintendent, he'll also convince management they should hire a plumber to install an automatic water feeder. With this device, he figures he'll have one less thing to do, and the tenants just might stop banging on his door in the middle of the night. 10. The automatic water feeder does its job, maintaining a safe, minimum water line inside the boiler. The boiler continues to run, sending steam up toward the undersized pipes that continue to hold back the condensate. At some point, the condensate stacks up high enough in the system to build the pressure it needs to return. When it does, it floods the boiler. 11. The superintendent curses the automatic water feeder. He drains water from the boiler and goes back to bed. Within minutes, the cycle begins again, and before you can say "oxygen corrosion" the boiler grows holes big enough to toss a tomcat through. 12. You get hired to install a new boiler. Naturally, you chose a modern, highly efficient, low-water-content steam boiler. Thirty seconds after you start it up, it goes off on low-water. The superintendent calls you a bad name in a foreign language and your sphincter muscle begins to do the mambo. 13. You call the boiler manufacturer for help. They send a guy in a suit and he tells you to add a boiler feed pump. He doesn't explain to you who will pay for this new boiler-feed pump, so you approach management and try to get them to fork over some more cash. 14. Because you are so persuasive, management goes along with you, but only after several weeks of haggling that leave you with an "at-cost" installation. You're delighted, though, that they've decided to stop the lawsuit. For now. 15. At this point, you're still bypassing the vacuum pump, bringing all your returns directly to the boiler-feed pump's receiver. But since every pipe, valve, and fitting in the building is undersized, the steam distribution stinks. You, of course, respond by raising the steam pressure. The urge to do this is greater than the sex drive. All this accomplishes, however, is to make the tenants who were already getting heat open their windows. The cold folks remain cold. Finally, you get it through your thick skull that the Dead Man must have had a reason for installing that vacuum pump in the first place. Maybe you should fix it. 16. You squeeze a few more bucks out of management and get the vacuum pump repaired, but you don't do anything about the thermostatic radiator traps. It was tough enough convincing management they needed that boiler-feed pump. You don't want to be the guy who tells them they now need to have their traps fixed. Life's too short. 17. Because the steam traps are still broken, the newly repaired vacuum pump fails after two weeks. 18. You go to the supply house, buy the biggest float & thermostatic trap they have and install it at the inlet to the vacuum pump. You figure that this single trap will take the place of the hundreds of traps throughout the building. You do this in defiance of logic, mechanical history and physics. You do it because it's cheap. 19. Your BIG F&T does nothing for the steam distribution. Nor does it keep the condensate from getting hotter than hotter. All it does is vomit steamy buckets of condensate into the vacuum pump. The pump screams in protest. You bite the bullet, go back to management and give them the bad news about their radiator traps. There's no way they can get around it. They'll have to have them repaired. 20. Five years later, management makes up its mind and listens to your advice. They hire you to do the trap replacement, which you do because you need the money, and because time has made you forget the pain. A good vacuum system repair can be a lot like childbirth - always less painful in retrospect. 21. You start up your refurbished system and learn (to your horror) that when your new steam traps close you're winding up with a greater natural vacuum on the supply side of the system than the pump can pull on the return side of the traps. Hey, who knew? As a result, the condensate stays in the radiators. It also floods the boiler when it does eventually return. Just as it did before. This bad thing is happening to you because you forgot to add the equalizing line between the vacuum pump and the boiler. You would have known about this equalizer line had you read the instructions that came with the vacuum pump, but no one knows where those instructions are anymore. 22. You get advice from some old-timer, and you add the equalizing line. You try again. As things now stand, you're returning your condensate to the vacuum pump, which is pulling a nice even vacuum all the way back to your boiler on start-up. The vacuum pump dumps its collected condensate into the boiler-feed pump, which waits until the boiler's pump control senses a need for water. The controller then starts the pump. Life is good. All seems right with the world. 23. But it's not. Since the vacuum pump is pulling a vacuum all the way back to the boiler, and since the boiler-feed pump is vented to the atmosphere, the vacuum in the boiler opens the feed-pump's check valve and sucks all the water out of the receiver. The boiler floods and you're now right back where you started. 24. You get some more advice from the old-timer. He tells you to add one more component to the mix. It's a motorized valve, and you pipe it into the discharge line of the boiler-feed pump. The valve keeps the vacuum in the boiler from sucking on the boiler-feed pump's receiver. When the boiler's pump controller calls for water, it signals the motorized valve to open. The valve opens, trips an internal end-switch, and starts the boiler-feed pump. Just enough water enters the boiler, and everyone's happy again. The traps work. The vacuum pump works. The boiler-feed pump works. The system works. 25. You get paid. With your pint-size profit, you buy as much beer as you can. Like to learn more? Visit us at and check out the Books & More section. The Lost Art of Steam Heating has a whole chapter on vacuum systems. If you have questions, you can get answers by clicking on the Steam Problems? button. You can also bring your questions to the Wall, our popular bulletin board. Knowledge is power, and there's no reason why you should ever feel alone.

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THE INS AND OUTS OF MONOFLO® TEES Bell & Gossett sells a special fitting called the Monoflo tee. You'll find these in thousands of heating systems, and they've been there for decades. If you ever have to move pipes around within one of these systems here are a few tips that should save you some time. The rings go between the risers. One side of each Monoflo tee has a red ring. That ring should always be between the risers that lead to the radiator. This means that if you're using two Monoflo tees, they'd be facing in opposite directions. If you have a radiator that's not heating as it should, check the position of the tees. If they're facing the wrong way, the radiator won't heat well. And check your circulator too. Circulators sometimes wind up going in backwards. Alternate up and down. If you have an upfeed and a downfeed radiator next to each other, the Monoflo tees should go like this: First tee (a standard tee) goes to the upfeed radiator. Second tee (a Monoflo) goes to the downfeed radiator. Third tee (a Monoflo) comes from the upfeed radiator. Fourth tee (a Monoflo) comes from the downfeed radiator. In other words, you alternate the up and down connections. That produces more resistance to flow along the main and nudges more hot water into the radiators. If you remove a radiator, don't seal the branches. If you cap the pipes that used to lead to the radiators, all the water will go through the run of the Monoflo tee. That increases the overall system pressure drop and slows the flow of water to the entire system. If you remove a radiator, remove the Monoflo tees as well. Or if it's easier for you, just connect the two branches with a short length of copper tubing. That way, the water that used to go to the radiator will still have a place to go. On downfeed radiation, keep the temperature low to start. Cold water is heavier than hot water. If you drain a downfeed Monoflo system, and you're having a tough time getting it to circulate again, try lowering the water temperature. This brings the density of the hot water in the main closer to the density of the cold water in the radiators and helps to get things moving. It's an old-timer's trick, and it works! If air is a problem on start-up, raise the static pressure until you've cleared it. More air will dissolve in water that's under pressure. If you're having difficulty getting rid of air on start-up, try raising the static fill pressure. The higher pressure drives free air into solution and makes it easier for the air to get to your air separator. Once you've got the system going, lower the static pressure again. This is important because if you continue to operate at the higher pressure, your compression tank may not be large enough for the system. Your relief valve will pop. Pitch the main and the radiators up in the direction of flow. This advice goes back to the original installation books of the 1930s. The pitch makes it easier to get rid of air on start-up. Check those pipes. They can sag as years go by and that can give you fits when you're trying to get the system going again. If you're having problems, always check the pitch. Use the right amount of tees. Radiators above the main usually work with one tee, and that tee should be on the return side. Radiators below the main always need two tees, and those tees should be the width of the radiator apart. And keep in mind these rules apply to convectors and freestanding cast-iron radiators. The folks who invented the Monoflo fittings never imagined you'd be running 50 feet of copper baseboard from two tees piped six inches apart. The long run of baseboard presents too much pressure drop along the branch. The water responds by taking the path of least resistance along the run. The result? A cold radiator. And it looks just like an air problem! If you have long runs of baseboard, run them as a separate zone. Put your circulator on the supply, pumping away from the compression tank. When you pump away from the compression tank, the circulator adds its pressure to the system's static fill pressure. That drives air bubbles into solution and makes it much easier for you get rid of the air that appears when you start the burner. Usually, you'll find you won't have to bleed the radiators when you pump away. If you'd like to learn more about this, get a copy of my book, Pumping Away. You'll find it in the Books & More section at Want to learn more about Monoflo tees? Go to and click on the Hot Water Q&A button. Once you get there, click on the Diverter-Tee-Hot-Water-Heating button and that will take you to Chapter Three of my book, How Come?. There's lots of solid information there that can help you solve tough problems, and it's absolutely free!

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WHY COMPRESSION TANKS WATERLOG When hot water heating was new (and this goes back to the turn of the century) the Dead Men installed gravity systems because Homer Thrush had not yet invented the circulator. Up in the attic of these homes you'll usually find an expansion tank that was open to the atmosphere. To fill these systems, you'd have to carry buckets of water from the well to the attic and empty them into the system. Imagine that! After a while, though, (and after city water became available) someone came up with the idea of using a ball cock in the open tank to keep the tank filled. Then, if the tanks waterlogged, the excess water just ran out onto the roof through an overflow pipe. No problem. Some of those early gravity systems didn't have attic tanks. The Dead Men would, instead, connect their cast-iron radiators across the bottom push nipples and leave a portion of each radiator filled with air. The collected air in the tops of all the radiators became their "expansion tank." This is sure to have you scratching your head if you've never been on one of these jobs. You'll spend a lot of time wandering through that house, looking for a tank that's not there. After the circulator came along, the pipes got smaller and there was less expansion going on because there was less water in the system. The Dead Men abandoned those open expansion tanks in the attic in favor of closed steel compression tanks, which they usually hooked up to the butt end of a flow-control valve down in the basement. And for a time, there was peace in the valley. But before long, the Dead Men leaned that, almost without exception, the compression tanks would lose their air cushions and fill with water. No one was quite sure why this was happening. They'd "solve" the problem by making it their habit to drain several gallons of water from every tank on every visit. This led to the common practice of draining all tanks, whether or not they needed draining. Generations were raised on this belief. You see a steel tank you drain it. Case closed. And so it was for years. But then came the diaphragm tank. Diaphragm tanks don't need to be drained, but they do lose their air pressure over time. That's because the diaphragm is made of rubber, and rubber is a semi-permeable membrane. Gasses will pass through the rubber and into the water at the rate of about 1-psi per year, which is why you should always check the air pressure before you throw away one of these tanks. But let's get back to those steel compression tanks. The air sits on the water like cheese on a pizza. When the circulator runs, it can't add any water to the tank because the circulator is operating within a closed system. For the circulator to add water to the tank, the circulator would first have to remove some water from the pipes. And if it did that, there would be an empty space in the pipes where the water used to be, and that's simply not possible. In a similar way, when the circulator runs, it can't take any water out of the tank and put it into the pipes because the pipes are already filled with water. Since the circulator can neither add nor remove water from that compression tank there won't be any pumped flow in the line going up to that steel compression tank. With no flow, the water in the tank will be cooler than the water in the pipes because the water in the pipes is passing through the boiler. If you'd like a more detailed explanation of how circulators work in a closed heating system get a copy of my book Pumping Away (and other really cool piping options for hydronic systems). You'll find it in the Books & More section at Okay, now for a bit of science. Gasses will dissolve in liquids in proportion to the pressure and temperature. As water cools, it absorbs air. When water gets hot, it releases air. Now, since the water in the compression tank is relatively cool (compared to the water in the pipes), it will absorb some of the air that's inside the compression tank. That's only natural. Here's what happens next. The hot water in the pipes rises by buoyancy into the tank as the cold water in the tank sinks down into the pipes. These flows pass each other in the single line that connects the compression tank to the system piping. We call this "gravity circulation" and it has nothing at all to do with the circulator. It's just a natural phenomenon. Now watch this. The air that got absorbed up there in the tank eventually winds up in the piping because of this gravity circulation, and once that air-laden water comes up to temperature, the air gets released as bubbles. Those bubbles get pumped out to some radiator where they settle out of the flow because the water's velocity out there in the system is usually less than it is closer to the circulator's discharge. Someone winds up venting the radiator. That makes the system pressure drop. The fill valve senses this and opens, allowing fresh water to enter the system. This water goes up into the tank because that's the only place that can accommodate it. Remember that the pipes are already filled with water. So every time this happens, there's a little less air in the compression tank, and a little more water. That's how steel compression tanks get waterlogged. The way to prevent it is to use a special fitting that's designed to stop gravity circulation between the tank and the system piping. Bell & Gossett's Airtrol Tank Fitting is a good example of what I'm talking about. These things have been around for more than 50 years and they work well. Sometimes, those "old-fashioned" devices deserve a second look, especially if you're spending a lot of time draining steel compression tanks.

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CONSIDERING MODULAR STEAM BOILERS? Modular steam boiler systems can look very attractive when it comes time to replace an old steam-heating boiler. The people who sell these systems promise ease of installation and energy savings. But how you pipe those boilers - both individually and then together - will either make or break the job. Here are a few points to keep in mind if you're thinking modular: First, steam is dynamic. It can move at 60 miles per hour in a steam heating system and still be within the normal design range. As it moves, steam loses pressure though friction and condensation. How much pressure it loses depends on the pipe size and configuration, whether or not you've insulated the pipes, and the boiler's Gross load. Because steam is dynamic (always moving, and always dropping in pressure), the pressure at any two points in the system will never be the same. This is true even after the system has heated from one end to the other. I mention this because it's easy to confuse steam with compressed air. You might think that once you get some pressure at the boiler, that same pressure will be everywhere in the system, as it would with compressed air. But that's not the way steam works. Steam can condense; it's always losing pressure. As it whips its way through the common header, the steam can affect the water lines in the individual boilers. A mere one-ounce difference in steam pressure equals a 1-3/4-inch difference in water column, and depending on how you pipe the system, this can present you with a real challenge when you're trying to balance your boiler water lines. And that's why you should never interconnect modular boilers on their condensate-return side. Here, forget modular systems for a minute and imagine you just specified two big boilers to provide heat for an old elementary school. One boiler is a stand-by to the other. You plan to have the superintendent alternate the boilers once each week during the heating season. Now, if you look in any steam-heating text, you'll see that when you have two big boilers such as these you have to pipe each one with an independent condensate-return line. If you connect both of those big boilers to a single boiler feed pump through a common condensate-return manifold, the water from the feed pump won't know where to go. It can't possibly enter both boilers equally because the boiler that's firing will be under a slightly greater pressure than the boiler that's not firing. The water lines between those two supposedly "equalized" boilers will never be the same. The feed water will enter the boiler that's off at a greater rate then it enters the boiler that's on. The "on" boiler will then shut off on low water, and at that point, the steam pressure will drop, and the boilers will equalize. The "on" boiler will restart, and then stop again as it loses more water to the "off" boiler. Short-cycling such as this kills your combustion efficiency. But worse than that, after a number of feed cycles, both boilers will probably flood. And as the water level gets closer to the exit nozzle, the steam will carry boiler water into the header and possibly into the system piping. That leads to wet steam, unbalanced distribution, water hammer, and high fuel bills. Check any good steam-heating text, and you'll see that to make the two big boilers work together you have to either provide each boiler with either an independent feed pump or a motorized valve. Each pump or motorized valve takes its orders from the individual boilers' pump controllers. Piped this way, feed water can flow only into the boiler that needs it, and never into a boiler that's off. Now change things around a bit. Instead of using two big boilers let's use, say, five little boilers. We'll connect them all together on the condensate-return side, and we'll feed them with a single boiler feed pump that takes its direction from a single pump controller. We'll mount that controller somewhere in the middle of the group of boilers. What do you think? What's going to happen when the feed pump comes on? Will the laws of physics change just because we're using small boilers instead of big boilers? Will the water suddenly gain the intelligence required to flow only into the boilers that need it? Or will the water simply enter the "off" boilers because that's where the pressure is lowest? I think you're making a big mistake if you interconnect those returns on your modular systems. Each boiler should have a pump controller and a motorized valve with an end switch that will start and stop the feed pump. Without those segregated condensate-return lines your individual boiler water lines will be bouncing like the cylinders in a V-8 engine. Now, here's something else to consider, and this is on the supply side, not the return. The water inside an "on" boiler will naturally be hotter than the water inside an "off" boiler. No surprise there, right? So you shouldn't be surprised when the steam from your "on" boilers travels through the common header and condenses inside your "off" boilers. Where there is relatively cold water, steam will condense. There's no getting around that. This is a common cause of boiler flooding in any steam system that has more than one boiler. It's the reason why when you open those steam-heating textbooks you'll see either a gate valve or a check valve on the main supply leaving each boiler. If you don't find these valves on the job (because they can be real big and very expensive!), you'll probably find a "spill trap" in the header equalizer. This trap (usually a 3/4-inch F&T) sits at a point in the equalizer that's slightly higher than the operating level. As steam condenses in the "off" boiler, the excess boiler water will open the trap and spill back to the boiler-feed pump. The ASME Boiler Code shows this as an acceptable solution to this common problem. You would be wise to specify a spill trap in the equalizer of each modular boiler, or a check valve in the steam supply line of each boiler. And it wouldn't hurt to specify both because boiler-water level is critical when you consider the limited amount of steam-disengaging space inside a small, modern steam boiler. A rising water level can cause boiler water to carry-over with the steam. It winds up in the header, and leads to wet steam. The installation diagrams some modular steam boiler manufacturers publish also show low-water cutoffs on some, but not all, of the boilers. I think every steam boiler needs a low-water cutoff. If you provide motorized valves on your return piping, your pump controllers on the modular boilers will perform the low-water cutoff function. Individual near-boiler piping is equally important but often overlooked in modular systems. If you installed just one of the small boilers you find in a modular system, you'd have to pipe it with a proper header, an equalizer, and, if it were a gravity return system, a Hartford Loop (you won't need Hartford Loops with your pumped return lines). If you didn't pipe the boiler this way, you would be violating the boiler manufacturer's piping instructions. Boiler manufacturers make a big deal out of these instructions (as they should!). Nowadays, you have to consider the near-boiler piping to be a part of the boiler because it acts as a steam separator. The exit velocities of modern steam boilers are far too high to produce dry steam without help from the near-boiler piping. The near-boiler piping rules that apply to an individual boiler should also apply when you pipe that little boiler into a modular system. You'll be making a mistake if you connect the steam supply directly out of the boiler into a common header without benefit of individual headers and equalizers. You'll get the best results from a modular steam boiler system if you pipe each boiler as though it were standing alone (with a proper header and equalizer), and then feed off each individual boiler header into a common drop-header. The term "drop-header" comes from the way the individual boiler supplies approach this header. The supply tapping from each boiler header rises to a point higher than the drop-header, and then takes a two-elbow turn to connect into the top of the drop-header. If you size the drop header to handle the entire load of the system at the proper velocity your modular boiler system will deliver the driest steam possible to the building. Supply steam to the building through one or more mains, which you'll take from the top of the drop-header at a point between the last supply from the modular boilers and the end of the drop-header. To keep any carry-over water from bouncing up into the mains, connect them not closer than 12 inches from the end of the drop-header. Drip the end of the drop header through an F&T trap back to the boiler-feed pump. This F&T trap is usually very small because if you size and pipe everything properly, it won't have to handle much carry-over or condensate. Not every steam boiler manufacturer goes to the trouble of incorporating these design elements into their modular systems. I suppose they're concerned about first costs, but I would never recommend a modular steam boiler system without insisting that it be piped this way.

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Radiant heat and wood floors More and more radiant systems going into existing homes as retrofit projects. On many of these jobs the tubing winds up getting attached to the underside of the wood floor. On other jobs, the tubing goes down on top of a subfloor, beneath the finish wood. The tubes may be in concrete, or they may just be sandwiched between the subfloor and the finished floor. To avoid problems, there are certain rules you should follow when you're working with any radiant floor that involves wood. Here they are. The wider the boards, the greater the chance for trouble. Try to stick with boards that are no wider than three inches. Wide wood is much more likely to warp warp. I've been on jobs where the floor warped so much it looked like it was made out of corduroy (they all had wide boards). Keep this in mind – especially if you're doing retrofit work. Use mechanical humidity control. Ideally, the relative humidity in a radiantly heated home that has wood floors should be no more than 50 percent. "Without this constant humidity, you must live with the cracks in the wood," according to the experts at the National Oak Flooring Manufacturers Association. Understand this before you begin, and don't be surprised when the cracks show up if the humidity isn't what it should be. Realize that the seeds of the damage on any job will be planted during construction. If you're placing wood over a concrete floor containing radiant tubing keep in mind that it takes time for the water to leave the concrete. And as the concrete dries, the moisture will leave it and enter the wood. As a precaution, tape a square of clear, plastic sheeting over the concrete floor and watch it carefully for moisture. Don't let the carpenters install the wood flooring over that new heating system until the plastic proves there's no moisture left in the concrete. Provide for heat and ventilation during construction. You have to do this because the painters and plasterers are adding gallons of moisture to the indoor environment. If you don't get rid of it, all that water is going to wind up in the wood floor. The damage won't show up until you turn on the system. And by that time, the other guys will be long gone. Buy a moisture detector. Spend a hundred bucks or so and get your own. It's a great tool to have if you're doing radiant heating systems. Stick the detector in the wood and you'll immediately know what's going on. Keep a log of the finish wood's moisture content as the job progresses. You're aiming for 6% moisture, at most, before you turn on the heating system under that new wood floor. Run the heating system for about five days before you let the carpenters install the finish wood. This will help dry out the wood. I remember visiting a famous movie actor's home in Park City, Utah while it was still under construction. The finish wood sat in the corner on a pallet as the radiant floor ran wild and uncontrolled in the subfloor's gypsum concrete. It must have been 90 degrees F in that house and they hadn't even installed all of the windows yet! This was in late October, by the way. That wood was pretty dry before they nailed it in place! If you find you can't run the heat beforehand, lay down an 8-mil polyethylene vapor barrier between the concrete or gypsum-concrete and the wood. That should keep the wood from sucking up the water. Avoid using any paper containing tar or horsehair under the wood floor. These things can give off an odor when the heat comes on. And the smell can stay with you for a long, long time. Use red resin paper instead, and if you're doing a retrofit, under-the-floor job drill up through the floor to check for that tarpaper or horsehair. Never make a wood floor hotter than 85 degrees F at its surface. Even if rugs are going down. Too much heat isn't good for the wood. Consider using a setpoint control to monitor the wood's surface temperature instead of an air-temperature thermostat. You may have to have some sort of supplemental heat in the room if an 85-degree surface won't get the job done on those really cold days. Whether or not you'll need this depends, of course, on the room's heat loss and the size of the floor. Summertime means more humidity. After that first summer, when the system you installed kicks in, the finish wood just might develop small fissures. That's normal. Just know that it's liable to happen. And about those small gaps that may appear in the floor. This can happen when there's a urethane finish on the floor and the floor is made from nonlaminated solid wood. The finish actually "glues" the individual boards together, and as the wood expands and then contracts, the contraction will localize itself and wind up as a gap. This is so common with hardwood floors (whether they have hydronic radiant heat under them or not) that the hardwood flooring industry even has a name for it. They call the phenomenon, "panelization." Be aware of it before you start the job. Learn more about wood. Go right to the source and learn more from the wood-floor associations. These folks provide brochures some very interesting brochures. The more you know, the better it gets! National Oak Flooring Manufacturers' Association, Inc. PO Box 3009 Memphis, TN 38173-0009 Telephone: (901) 526-5016 The Hardwood Council PO Box 525 Oakmont, PA 15139 Telephone: (412) 281-4980 Like to learn more? Get a copy of my book, "Hydronic Radiant Heating – A primer for the nonengineer installer." I adapted what you just read from that book. Check it out at Books & More the next time you visit us at