Hello all,
I’m in the process of changing our heating system in the house I’m remodeling from baseboard fin-tube to radiant heat installed under the subfloor. I will be installing two runs of 1/2″ pex tubing in each joist cavity (16″o.c) and have foil faced insulation beneath that. There are going to be 6 zones in the house total. It’s my understanding that each run of pex should not exceed 300′ lineal. Since I have so much 3/4″ copper laying around from taking out the baseboard heat (about 400′), I thought I would do most of the runs with the copper and essetially make long manifolds out of it. Seems to me that I will use much less pex then too. I used the 3/4″ copper to supply each zone and another 3/4″ copper for the return. As I was running the copper to the zone I installed a 3/4″ x 1/2″ x 3/4″ Tee to accomodate each loop of the zone The largest zone in the house has 3 loops of pex coming off the 3/4″ copper supply and return. I am concerned about proper water flow in the system and removing all the air from the system when I fill it up. Just about all of the copper and pex is going to be covered with ceiling material when the house is finished so I may need to create access panels if need be.
Question are:
1. 300 feet per pex run sound right?
2. Do I need to install shut off valves for every loop in every zone or can I count on completely filling up each zone by flushing it with a garden hose hooked up at the boiler and turning off all other zones? (each 3/4″ return line has a boiler drain valve just above the boiler) I was going to let it run till no more air came out. I am concerned that there may be an air lock in a part of a zone that normal filling might not get rid of.
3. Given the size of the manifolds I made up, do I need to worry that some parts of a zone may get more hot water that other parts, maybe the part that is furthest from the boiler? Will the water take the path of least resistance and not feed some parts of a zone very well?
4. Should I look into replacing the boiler with a more efficient hot water heater instead, since I am no longer going to need water that is around 200 degrees?
Sorry if this isn’t very clear…….Let me know if you need more info and thanks for any help.
Jim
Replies
just trying to bump this up so someone might read it....lol
just bumping to front again....
I just caught up to this thread. There is a bunch to know about RFH. David Thomas may jump in and lend some experience. I went to http://www.ipexinc.com, and got a lot of info from their charts and walk through example.
I am designing a house for myself that will have both forced air and radiant floor heat. I have an unanswered concern about water idle in hundreds of feet of tubing during the summer season, if using the domestic water heater system as the source. I may pipe one lead of line from the return manifold to the powder room toilet, so that each flush changes some of the loop water . PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Paul,
Consider using a heat exchanger on the water heater. That would allow you to keep the RFH system closed, and the domestic (obviously) open, with everything running off the same water heater.
Not sure of your climate or how you use your house, but having the RFH system closed would also allow you to add glycol if you are inclined to do so.
Mongo, thanks. I am trying to learn alot about the options and systems, and will certainly go to three or four companies for their help, ultimate design and input. BUT, that said, getting field input carries a big stick to help seperate the sales pitch from the reality.
I want to supplement just a few rooms (the where we live rooms) and not the whole house. In fact by use of french doors, air lock foyer etc, we will not need to condition the upstairs or a few rooms on first floor unless the kids or friends visit. Any thing you share is appreciated. I have the 2" thick tech manual from Wirsbo to digest as well. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Mongo, forgot to add: location could be Alabama (with little to no local help available), or Virginia where builders "just don't do that much around here". So, I will do it myself in either case. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
While the safety of open systems is debated, I would want a closed heating system. I would not want to be a 'case study' if the microbiologists prove that open systems grow Legioneers or somesuch.
It does not add that much to the averall cost, and there is even some cost offset in not needing to use bronze circulators (assuming you use O2 barrier tubing).
Beyond safety, it is worth considering whether an open system would cause condensation problems in the summer if your floors hit the dew point.
CS, thanks. Do you have a recommended water heater/heat exchanger package? I have also considered a dedicated water heater for the RFH system, expansion tank, air eliminator, fill valve, pressure guage etc.
The smallest of the high eff heaters is probably still more capacity than I need, but will get the load calculated before any decision.
Any models, makes you like will be appreciated.
Did you see my post to David Thomas? I asked about using cpvc pipe for RFH, and aluminum flashing for staple up plates. I could make an easy press to preform the aluminum into grooved panels. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
How many BTUH do you need?
The most popular HWH for heating would have to be the very efficient Polaris, though I hear Bradford White and some others have competing models. Standard water heaters are notoriously inefficent. Better to get a more efficient unit, and use it for all your water heating needs (leverage your investment). You can put the space heating circuits on the other side of a plate-type heat exchanger. The DHW side needs a bronze circulator, while iron circs can be used on the closed side.
An alternative is to use a small condensing boiler, and put the DHW on the other side of the exchanger (an indirect tank). HTP has just added 2 small wall mount modulating/condensing boilers rated for 10-50KBTU and 15-80KBTU. Good for homes with very low heat loss. Optional integrated outdoor reset controls set the proper heating water supply temperature right at the boiler for max efficiency. This is some very cool stuff. http://www.htproducts.com/literature/lp-60.pdf
I have never seen CPVC used in heating applications. My understanding is that is is not rated for that use. I have heard that it is very porous to O2. Something was reported about how it may leach byproducts of clorine that can build up in a closed system. Generally the stuff is relatively fragile and brittle. Also more labor than PEX, due to the bending radius and all of the required fittings. Anyways, I would suggest O2 barrier PEX on a closed system.
You can use aluminum flashing for plates if you can figure out a way to bend it efficiently. Some of the package suppliers sell "form it yourself" plates, but they are made of a softer (possibly annealed?) aluminum material.
CS, nothing sized yet, spent hours on the house design, but still looking for the right lot, and a builder who will cooperate with what segments I will do. Rough math from layout indicates 1000 ft of pipe needed , keeping all loops to 200 ft or less, to provide the spacing.
I have preached Polaris to many clients, and so far, no problems. But there are some units out there that are dying early in life. I have been talking with AO Smith re their Cyclone. It has anodes that can be changed to keep the tank safe from rust thru, but the unit is reported to be loud. Hopefully, they will make it more home owner friendly in near future. I would like to have one for evaluation. Also like the muscle of a Bork, though about 80% vs the 94% of Polaris and Cyclone.
There is always the hesitation with systems that are not done in the area. Prospective buyers are afraid they are not able to operate the homes if I get too far off the beaten path. Current house has been a project lab for 28 years, so whoever buys from us, will need a little savvy to fly it. I have written an operations manual for the systems.
I will check out the site you provided, tks.
Re bending aluminum, I was in the aluminum business for 32 years, so forming it won't be a big challenge. I have some flashing left over, so will make a pipe /groove press, and see what I get. May mave to anneal it in the oven.
Heard a number of builders here only put copper in their houses. "Pex droops too much" is the local call.
Just wondering about the cpvc. Will end up with O 2 barrier pex I guess. Thanks for your help.Energy Consultant and author of Practical Energy Cost Reduction for the Home
"Heard a number of builders here only put copper in their houses. "Pex droops too much" is the local call. Just wondering about the cpvc. Will end up with O 2 barrier pex I guess. Thanks for your help."
I still like copper for potable water. Old school, I guess. The fact that PEX supports a 'bio film' makes me a little uncomfortable, even though no health risk has been proven. Even so, I would use PEX in a poor access situation on regularly used supply lines. Around here, PEX does not even have code acceptance for potable water yet. PEX has obvious advantages for low-temp radiant heating.
Would you use copper if building a house today, of cpvc?
I have used both, and Flowguard Gold has never failed me.
PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
I would use copper at this time. It's proven, and I sort of enjoy working with the stuff. I have this cool bender to minimize the number of fittings.
I lack faith in CPVC. It's brittle, and just seems too fragile to me. I just used a bunch of it for a temporary reroute, and it was great for that. Can bend it somewhat, which can be an advantage.
Hi:
I'm just learning about hydronic heating installations, but know a fair bit about piping, so here goes:
From what you've described, it sounds like you have a number of "zones" (i.e. loops of 1/2" PEX) supplied by a long distribution header (what you called a "long manifold") of 3/4" copper, and returning via a similar header. If the zones are not of identical length, and you do not use a valve to throttle the ones with the lowest pressure drop (i.e. the shortest length), the flow through the shortest loops will be higher than the flow through the longer loops- the ones which are heating a larger area- so the effect is in the opposite direction to what you want (i.e. you need less, rather than more, flow through the shortest loops). The ratio of the flows is related to the ratio of the SQUARES of the lengths, so even a relatively small increase in length per loop will cause a fair bit of flow difference without a valve to compensate. The pressure drop resulting from flow in the PEX will probably overwhelm the pressure drop in the headers themselves so that the relatively minor difference in the length of header the water has to travel through to get to each PEX loop won't matter much in terms of flow distribution.
Some of the systems I've seen on websites and TV have had basic flow indicators and throttling valves so you can observe and adjust the flow through the zones to give the heat balance you want. Others have put thermostatic valves on some of the zones to help with the heat balancing. But you won't get away with a balanced system without using throttling valves on each zone. That's why most systems I've seen bring each PEX loop all the way back to the "manifold", so the flowmeter and/or thottling/thermostatic valves will all be located in one place for easy balancing. Some designs also require more than one circulator pump.
Hope this helps-
molten,
thanks for the good info.
From what you've described, it sounds like you have a number of "zones" (i.e. loops of 1/2" PEX) supplied by a long distribution header (what you called a "long manifold") of 3/4" copper, and returning via a similar header.
I'm not sure I described it accurately. Each zone in the house (6 total) has it's own 3/4" distribution header and similar return header. Each distribution header has at least 2, some having 3 loops of pex coming off them at various points to feed that part of the zone.
As I'm answering the replies that I've received so far......I'm thinking now that between (1) putting a valve on each PEX loop, (2) having each zone controlled by a seperate thermostat and (3) having the ability to isolate each loop of PEX in each zone to fill and bleed air, I should be able to control the heat in the house quite effectively.
Thanks,
Jim
Jim,
moltenmetal gives some good advice regarding the desire to be able to control, or balance, the flow through not just the manifold, but through individual loops off each manifold.
Question are: Very basic answers follow:
1. 300 feet per pex run sound right? A common number tossed about is 330' max for a half-inch loop. That said, while there are several reasons to limit the length of the loop, one you need to be concerned with is flow resistance, or head pressure, that the circulating pump has to overcome in order to get the water to flow through the loop so the BTUs can be delivered. Ideally I recomend shorter, 180-250'. So, don't just be concerned with the length of the PEX, be aware of the total length of tubing after the circulating pump...copper and pex. Normally not a huge deal, just rying to make you aware.
2. Do I need to install shut off valves for every loop in every zone or can I count on completely filling up each zone by flushing it with a garden hose hooked up at the boiler and turning off all other zones? (each 3/4" return line has a boiler drain valve just above the boiler) I was going to let it run till no more air came out. I am concerned that there may be an air lock in a part of a zone that normal filling might not get rid of. It is advantageous to have control over the flow through each loop on the manifold (for balancing, maintenance, etc), as well as through each manifold itself (to isolate for maintenance, etc). Both for filling and burping air, and for balancing the system afterwards. When done with the initial filling, it's nice to have an air bleed valve at a high spot on the system that can bleed residual air, as well as a water makeup valve, about 12psi or so, that will add water to the system when the system pressure drops.
3. Given the size of the manifolds I made up, do I need to worry that some parts of a zone may get more hot water that other parts, maybe the part that is furthest from the boiler? Will the water take the path of least resistance and not feed some parts of a zone very well? In general, and in theory, if the loops are all the same length, the system will be balanced by virtue of each loop having the same length, and thus, the same flow, or resistance to flow, characteristics. That said, a loop feeding a corner room with two exterior walls would need to deliver more BTUs than a same-length loop that delivers heat to a hallway located within the centeral portion of the house. So yes, it's nice to have flow valves so you can tweak the volume of water flowing through the individual loops. In theory.<g>
4. Should I look into replacing the boiler with a more efficient hot water heater instead, since I am no longer going to need water that is around 200 degrees? RFH obviously needs a lower delivered water temp than other hydronic systems. Boilers won;t do well delivering 120 degree water, though they can be brought down to the 160-180 range or so. Use a mixing valve to add cold water to the boiler water to drop it down to the desired temp, which shoud be around 110-120 degrees or so. Don;t forget the requirements for your domestic water.
Mongo, question for you. I have three pex loops in the basement slab operating and am now installing staple up under some rooms on the ground and first floor. The manifolds, pumps, heat source et al are in the basement. Are there any tricks to getting the air out of staple up pex on the upper floor? or will the pump frorce it out?
An ex-boat builder treading water!
Mongo,
Thanks for the reply,
When done with the initial filling, it's nice to have an air bleed valve at a high spot on the system that can bleed residual air, as well as a water makeup valve, about 12psi or so, that will add water to the system when the system pressure drops.
What if it isn't possible to have an air bleed valve at the high spot on the system. If it's going to be under the 2nd story floor and in the ceiling of the main level, I won't have access to it unless I make access. Is this absolutely necessary to do, or is there another way around this. I was thinking that if I put valves on each loop of each zone I could essentially shut off everything else except the loop of pex that has the air in it. By forcing water into it, would that be sufficient to get the air out? I'm hoping that I wouldn't have to do this very much. Doesn't sound like much fun.
Thanks,
Jim
To both Jim and Bob,
I prefer to have a manual bleed valve at each manifold. This allows you to burp out any air within the loops on that manifold. The burping is done after the initial fill, and it may, though usually isn't required, to be done at Fall start-up. I've only had to burp a system once after intial installation, and that was a system where the 12psi make-up valve froze and didn't admit water when neccessary. Replaced the make-up, and I've heard of no problems since. I think that was about 6 years ago.
If for some reason you ever got so much air into a loop that the air bubble locked out any flow, hitch up a hose and force more water into the loop to move the bubble along, then burp out the air.
The manifold bleed valve that I've used has a small key that's used to open and close the valve. Fill the system, run the system, manually open the bleed, and air gurgles out until there's no more air, then water shoots out in a small stream.
Again, those manifold bleeds are manually operated.
Back at the boiler, near the expansion tank, include another air valve that will automatically removes entrapped air from the system. Works 24/7. Where the expansion tank typically hangs below a horizontal pipe, the air valve is mounted high, towards the high point in the boiler's plumbing.
I don't mind having manifolds for the first floor located in the basement, but I put them up, at or slightly above eye level, up closer to the ceiling than the floor. This puts the manifold about 24"-36" below the tubing. Not a big deal.
For upper floors, I prefer to hide the manifold in a second floor closet or cabinet, somewhere above the second floor platform. Makes it much easier to remove air when neccessary, and saves a bunch of PEX.
Plus, having the second floor manifold up at the second floor platform requires you to only extend the copper manifold supply and return tubing from the basement, through the first floor walls, and up into the second floor platform. If the second floor supply manifold were located in the basement, then you'd have a PEX supply and return for each second floor loop running from the basement up to the second floor. That could easily add between 25' and 50' to the length of each loop that heats the second floor. A waste of materials (8 loops means 200' to 400' more tubing) as well as added load on the circulating pumps.
It's easier to protect and insulate two coppper lines buried in walls than it is to take care of 16 runs of PEX. Not a design killer...but it's just not neccessary.
Others may do it differently, but I've not ever had a problem with bleeds at these locations.
I'm into this heavily as well, and something that comes up again and again is the requirement for a room-by-room heat loss calculation that takes windows, doors, R-value of walls and ceilings, % of exterior versus interior walls, design delta (how many degrees difference between your coldest day and what you want the air temp to be inside). Maybe you've done one of these or are following the experiences of someone else with similar house type--climate.
The various research I've been doing indicates that it is pretty easy to miscalculate exactly how your system should be designed, and you can end up with a very expensive and hard to change icebox...or oven. I think that's why the pros do a pretty sophisticated heat loss calculation and then combine that with design and construction techniques that have been proven successful.
Having said that, I recognize how difficult it is to find a pro that is really up on the lastest techniques in a somewhat small town rural type area. Around here (SE MN) I even had a devil of a time finding someone who would blow cellulose instead of FG in my attic. So in many areas you're almost forced to DIY if you want to apply modern technique.
I think that using your left over copper for extended manifolds is a GREAT idea. I think you'll also want to install balance valves on every loop. You might want to re-consider (if it's not already too late) the necessity of SIX zones. Thay's alot of pumps or zone valves, and may introduce un-needed complexity.
One thing no one has mentioned yet is that the length of the loop also has a realtionship to how fast you'll suck the BTU's out of the water to meet design temperatures. If it's too long and your BTU requirement is high, the water may cool too much at the end of its run to be totally effective.
My direct experience is limited to a partailly DIY'd garage slab, and I used a pre-fab fill/expansion tank/air eliminator unit from one of the Vermont RFH companies. It also included in and out thermometers, pressure guage, pressure relief valve and all the various fittings, a properly sized pump, relay and slab sensor, valves etc...with alot of it pre-assmbled. Really made the whole fill and maintenance process a breeze, and now I have a pattern to model from once I do MY house with RFH.
There are a number of pros on this BB who will be able to provide real life advice, provided they're not too busy out doing this for a living!
By all means let us know how it turns out and clue us in to what you learn along the way.
Johnnyd,
thanks for the reply and advice.
I haven't done any "real" calculations on the design of the system. I figured that if I have the capability oversupplying the zones and each one is controlled by a seperate thermostat I should be able to adjust it in the future to fit our needs. I've already got six zones coming off the boiler as it sits now. That's what was in the house previously used for baseboard heat. I cut off all the 3/4" copper just outside the utility/boiler room. All the zone valves, drain valves and piping is already in place for this setup. It looks very "complex" around the boiler now. Like a tree badly in need of pruning. I thought I would just go off what is already there.
so the planned setup would be..... Six zones total in the house. Each zone has at least 2 pex loops, a couple of the zones have 3 loops. Each zone is supplied by 3/4" copper. There is a single Taco pump on the existing boiler and each zone has an existing valve on it controlled by different thermostats.
I don't believe that I'll have any loops over 250' so there shouldn't be a problem with the water getting too cold by the end of the run. The entire house is going to to get foam insulation with FB to fill in.
What is a balancing valve? Can it just be an ordinary valve that I adjust to control amount of flow in each loop? My plan was to install a valve at the start of each pex run and adjust them control the temp properly. The basement is going to remain unfinished for a while so access to the valves shouldn't be a problem. After that, I'll have to create some sort of access panel for them. If I get this system tweaked right before the basement is finished, I shouldn't have to ever remove the access panel. Thanks again for the help. I'm going to check out the other site now.
Jim
Interesting project!
What is a balancing valve? Can it just be an ordinary valve that I adjust to control amount of flow in each loop?
Yes, they should be ball valves, but you might want to look into flow meters (my Rehau manifold has these and they're nice to reference as you do your tweaking).
More importantly, if you use your exisiting boiler, you'll for sure need a mixing valve so you can control the temperature of the water going into the tubes. They work by mixing return water with hoter boiler water. Remember that you're looking for a supply temp of 110 - 120 degrees, return temp of 80 - 90 degrees. Max floor temp of 85 degrees. These are the rules of thumb I've heard anyway.
I may use your "long manifold" method myself. seems like it would simplify runs to distant rooms.
Hope you get enough replies to keep this thread on top for awhile.
I don't know a whole lot about this, though I'm learning fast. One thing I do know is that a large overcapacity in the boiler is not a good thing. A burner which is always running on a short cycle will not run efficiently or have a long and happy life.
Ron
Your chimney will have a shorter life too.
Paul, That was Molten who mentioned high temps. Maybe he's used to temps that melt metal, but I'm just guessing maybe he made a typo using C insteadof F, - that's how I read it anyway.
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Nope- the numbers I used matched the units. CPVC PIPE is good for 80 C (176 F) for continuous industrial duty (i.e. at least 10 yr life) , but creeps rapidly at 90 C (194 F) and turns to spaghetti rapidly at 100 C (212 F). You're right that your household hot water for showers etc. won't generally go much beyond 60 C (140 F), but some parts of your radiant hot water system will definitely go beyond 80 C unless you take special precautions to prevent it. Since anything I want in my house is for a 40 yr life minimum, I'd be personally happier keeping CPVC out of the radiant heating system. As I said, PEX is a different animal, being cross-linked, so it has more creep resistance.
In the radiant, we mix down to about 110 to 114°F. Don't set the boiler to more than 140°F.
Use Pex and copper - never had the CPVC
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You might try reading through the threads on this forum...not nearly as active as BT, but there are three or four pros that contribute lots of valuable information.
http://www.rpa-info.com/forums/forumdisplay.cgi?action=topics&forum=Public+Bulletin+Board&number=24
This isn't my area of expertise but I have worked closely with my heating engineers on a couple of jobs, so a couple of things stand out.
You shouldn't be assuming anything but should do some calculations.
The max delivery lenghth is 250' practicall. That includes your juryrigged copper. SAounds like you are using older used copper for this. Copper has a lifespan - maybe forty years. You are adding to the number of joints and elbows which reduce flow and add to chance for leaks.
You need a mixing valve. Radiant under a wood floor needs to be a bit cooler than what you were supplying to fin radiators.
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Taunton University of Knowledge FHB Campus at Breaktime.
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Copper has a lifespan - maybe forty years.
What is the expected lifespan of pex???????
Good question, but I'm sure that it is longer than used copper.
Wirsbo PEX is regarded as the best of all these cross linked ploys. I can show you places that are ten years in operation so far with no squirts
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Galvanized steel pipe has a lifespan of 40 years. Copper, used or not has to be a lot longer. As long as you clean the ends well and use new fittings old lengths of pipe should be no problem. For domestic water applications of course...nothing corrosive.
There are different grades of copper and different water conditions around the country. I've seen copper develope pinholes after ten or twelve years. Sure makes owners unhappy!
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Copper in a closed heating system should last indefinitely.
I guess there would be fewer abrasives running through it.
I was just thinking, most of these pinholes I have ever seen develop are within anbout four or five inches of an elbow. I'm speculating that the turbulence must be greatest there. Also a god reason to cuyt back six inches when re-using copper lengths.
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That's exactly what I've seen too. Copper seems to fail most readily in areas where there's erosion or high local velocities which strip off the passive film. But in a properly-designed closed-loop heating system it should last a very long time.
So should PEX, provided you keep the temperatures low enough. The big issue with these plastic materials is "creep"- their tendency to change shape over time. The cross-linked stuff like PEX is creep resistant, but not totally immune. It has been used in Europe for a long time, but I don't know about 40 yrs. CPVC, on the other hand, is NOT immune to creep. It's good to ~ 80 C in industrial service, but go to 90 or 100 C even for a short time and it turns into spaghetti and creeps all over the place.
Not just the abrasives, but the lack of O2 to support corrosion reactions. Also, even if filled with acidic water, any reaction with the acids will happen once, then stop.
Potable water provides an endless supply of O2, plus optional abrasives and acid.
Perhaps it's partly a Maine thing. My mother lives in Maine, and her well tested at 5.2
The stuff tasted tart!
Edited 2/25/2004 3:04:16 PM ET by csnow
Lime, manganese, sulpur, iron,,you nameit.
Uranium and arsenic have made the news big time this past week. We'll be getting the big test done next week on ours
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We have had about 3 pin hole attacks in a 10 ft area of copper pipe in our 28 year old house. I think it was a bad piece of pipe , cut into sections and used in one area. All have been very small pin holes, with good cross section thickness of wall material.
Pex seems like a good idea, and I don't know much about it, but have read some reports that indicate a propensity to support bacteria growth.
One poster said cpvc creeps at 100deg C. That's boiling, who has 100 c in their house? PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
My thinking tends to believing that excess solder globules inside the pipe near a joint really add the the spot velocity of the fluid.
Has anyone else considered this??
SamT
Even in my dreams, I'm not enough of a fluids mechanic to have thought about that.
But you may be right
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Sam,
That is correct. If the pipes aren't deburred, or if too much solder is used, it can indeed wreak havok on laminar flow through a pipe.
If the plumber did a truly horrible job, as the fluid worked its way around the globules of solder, the increased spot velocity of the flowing water would result in a decreased localized pressure inside the tubing (think Bernoulli)...potentially causing the tubing to collapse inward...which in turn would create an enormous vacuum, causing the house to collapse with most of the houset being sucked into the tubing.
Oh, the horrors...
That must be why plumbers get paid so much, because the potential downside of a sloppy soldering job is just too great.
Edited 2/26/2004 3:26:28 PM ET by Mongo
Turbulent flow and electrochemical action together can result in localized cavitation in the copper which in time can lead to pinhole perforations. Poor quality control during manufacture (extrusion and alloying) of the pipe can also contribute. Both these forces are known to cause cavitation in boat propellers (aluminum alloys).
I would tend to think that the use of mildly corrosive fluxes used in bygone days might contribute, as well as the heating/overheating that ocurrs during the soldering process. This might also explain the perforations you guys have noticed close to fittings.
The heating will degrade the metal not only by the obvious acceleration of the reaction with oxygen but also by the metalurgical changes that take place within the copper. In fact, (IIRC from metalurgy classes from years ago) the hardening (by heat treating? etc.) of copper, to the degree that the ancients did it, is something that modern science has been unable to do. The art has been lost. The person who figures out how they did that would likely find himself wealthy.
So if you figure it out tell me first.
-Brian.
BTW - Predicting the long-term durability of something is difficult to scientifically quantify. One of the measurements of the stability of a material in contact with water is ion migration- the amount of the components of the parent substance that are most easily lost to the water. In terms of absolute ion migration Engels method produced PEX is more stable than copper pipe. In purely scientific terms then, PEX is more inert than copper.
Also - The biofilm on PEX might be of concern in areas where a sufficient chlorine residual to keep biological action in check is not continuosly maintained in the water, such as a DIY well or cistern arrangement.