Capping black iron radiator pipe with PVC

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gadolphus32

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I removed a hot-water cast iron radiator in my attic and need to cap the pipes, which are 1-inch black iron. As detailed in this thread, I can't get a black iron or galvanized cap to screw on, apparently because of issues with the threads on the pipes.

I was able to get PVC caps to go on and they seem to work (by which I mean they don't leak when the system is on) but I'm wondering whether I can trust them long-term.

I know PVC is rated only up to 140 degrees and circa 250 PSI. I don't know what temperature my boiler (installed in 1949) runs at, or how to check it -- but I don't think it is super hot because I can touch my radiators without burning myself, so I assume it's closer to 140 degrees than to 200.

I also don't know what pressure the system runs at because the gauge is broken, but from what I gather, a hot water radiator system should not be more than 30 PSI at the high end. I'm optimistic that the fact that the pressure on this system is well below the PVC maximum can help make up for the possibility that the temperature slightly exceeds the maximum.

Would you sleep comfortably with PVC caps on your radiator pipes? (If not, would you feel much better with CPVC, since it has a higher temperature and pressure rating? I haven't tried CPVC caps but I'm hopeful that they'd fit on, too.) And is this up to code? I can't find anything in the code about combining PVC with black iron; all I'm aware of are the maximum temperature and pressure ratings for PVC.
 
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I removed a hot-water cast iron radiator in my attic and need to cap the pipes, which are 1-inch black iron. As detailed in this thread, I can't get a black iron or galvanized cap to screw on, apparently because of issues with the threads on the pipes.

I was able to get PVC caps to go on and they seem to work (by which I mean they don't leak when the system is on) but I'm wondering whether I can trust them long-term.

I know PVC is rated only up to 140 degrees and circa 250 PSI. I don't know what temperature my boiler (installed in 1949) runs at, or how to check it -- but I don't think it is super hot because I can touch my radiators without burning myself, so I assume it's closer to 140 degrees than to 200.

I also don't know what pressure the system runs at because the gauge is broken, but from what I gather, a hot water radiator system should not be more than 30 PSI at the high end. I'm optimistic that the fact that the pressure on this system is well below the PVC maximum can help make up for the possibility that the temperature slightly exceeds the maximum.

Would you sleep comfortably with PVC caps on your radiator pipes? (If not, would you feel much better with CPVC, since it has a higher temperature and pressure rating? I haven't tried CPVC caps but I'm hopeful that they'd fit on, too.) And is this up to code? I can't find anything in the code about combining PVC with black iron; all I'm aware of are the maximum temperature and pressure ratings for PVC.
 
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I would get a pipe die and run it over the threads on the black iron pipe until the galvanized caps fit. You can either rent one from a tool rental place or buy a pipe die set from Harbor Freight. It sounds like the threads weren't cut deep enough on the pipe to fit the caps. Pipe threads are tapered so you should be able to make them fit with 1-2 turns of the die after it gets tight. Use cutting oil and back up the pipe with a wrench.
https://m.harborfreight.com/38-in-2-in-ratcheting-pipe-threader-set-62353.html
 

Jadnashua

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PVC comes in more than one type and usage. Most of it is not designed for pressure, and, that that is, is designed for cold water supply. I would not want to rely on a PVC cap to hold on that pipe. If you can remove that black iron pipe, you could then put a plug into the T or elbow where it was removed from, getting those pipe stubs out of the way in the process. Again, I'd not use PVC for this.
 

gadolphus32

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Thanks for the responses. I can't fit caps onto any of the other 1-inch pipes in my system (there is some abandoned pipe elsewhere that I tried capping, and the caps also seem just a tiny bit too small for it), so I'm thinking the threading that they used when they installed the pipes is the problem. For that reason, trying to take out the problem pipes and attach a fitting further downstream probably wouldn't work in this case.

Running a die over the threads was suggested to me but the good ones seem pricey and I figured at that point I might as well just hire a plumber. But if you guys think I can get the job done with a cheap Harbor Freight die set, that is probably the best solution. Then I'll have it on hand in case I run into a similar problem with my pipes in the future.

As for PVC, I guess that is just risky, especially for fittings up in the attic. Leaks on the third story of a house are a nasty thing (which I learned the hard way when I first discovered the problem with the radiator that I removed).
 

hj

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IF the PVC caps go on the pipes, there should be no reason the steel caps cannot do the same. They ALL have the same thread. Putting a pvc cap on a steel pipe CAN cause it to crack someday.
 

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Running a die over the threads was suggested to me but the good ones seem pricey and I figured at that point I might as well just hire a plumber. But if you guys think I can get the job done with a cheap Harbor Freight die set, that is probably the best solution. Then I'll have it on hand in case I run into a similar problem with my pipes in the future.
A cheap die set would do the job well. To keep that useable for the next job, store it well oiled. I don't see a 1 inch NPT die there. What is the part number?

I see a 1 inch NPT (11.5 tpi) die for 20.65 including shipping "New 1" -11-1/2 NPT Taper Pipe Die Right Hand" on an auction site.

"US Stock 1" - 11 1/2 NPT Taper Pipe Die 1 - 11 1/2 TPI" is $14.84 with shipping.

You need a big adjustable locking pliers to hold either of those unless you get a matching holder.

"Hi Quality 1" -11-1/2 NPT Pipe Die Nut Hex Rethread Chaser 2-1/8Hex Carbon Steel" for $17.95 is probably the one I would go for, since the hex shape is easier to grasp with a wrench or a locking pliers. Chaser or Rethread -- to chase a thread is to clean it up. They are saying that this die is not good enough to cut new threads into a pipe from scratch, but it suitable for cleaning up existing threads. That is what you will be doing.

" npt" die "11 1/2" was my search term into an auction site.

Be careful to not cross thread. I like to start the die by hand, and I initially turn CCW. I note where the die drops, on each of a couple rotations. Then I switch to CW a little bit CCW from that point. Once the thread has started by hand, I use the tool to get cutting. Use a little oil with the cut. For slow use, the oil is not critical. It could be motor oil, 3 in 1, kerosene or vegetable oil. Also, after you cut for a bit, back off some, and continue. This cleans out cuttings.

Remember this is tapered. So if you keep cutting until the pipe extends through the die, you will have easiest insertion into your cap.
 
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gadolphus32

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Thanks all for the continued advice. A die set seems like the best solution. Before I buy one, though, just one last question. Would a compression fitting like this do the trick as well? I'd use it to couple the existing pipe with new black iron pipe, which I could then cap.

I know the die is a better solution, but the compression fittings are not that expensive, would be much easier to install and would not leave me worrying about breaking a connection below the floor when I am running the die. (I am not super confident in my ability to hold back sufficiently when running the dies.)

IF the PVC caps go on the pipes, there should be no reason the steel caps cannot do the same. They ALL have the same thread.

I know this is theoretically how it should work. But the metal caps just won't go on. I tried black iron, galvanized and brass. They all feel a little too small for the pipe, no matter how carefully I try to screw them on, how level they are, etc. I think my pipe threads are a little off and the PVC has enough flexibility to work with them, whereas the metal caps don't.
 

Reach4

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Would a compression fitting like this do the trick as well? I'd use it to couple the existing pipe with new black iron pipe, which I could then cap.
Probably. That Dresser-type coupling requires the outside of the pipe to be in reasonably good condition. It will also look hokey.

I don't know how many years the PVC cap would survive. Rather than PVC, which is supposed to deteriorate over time with hot, how about CPVC? http://www.nibco.com/Industrial-Pla...117-3-Thread-Cap-FPT-Corzan-CPVC-Schedule-80/ It looks hard to find in 1 inch. I would go for one of the dies.
 
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Jadnashua

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The hassle with a plastic cap is that on a tapered pipe thread, the tighter you make it, you end up stretching the plastic. Go too far, and it will split immediately, but the plastic doesn't really like being in tension and it can eventually let go later. It works better in compression.
 

hj

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quot; I don't know how many years the PVC cap would survive.

I had one "survive" exactly 6 hours until after everyone left the site and then it broke flooded the building. I cannot believe this person is having this much trouble with a simple cap. I would really like to see what they are doing incorrectly.
 

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A plastic cap on a pressurized line is asking for problems...don't do it when there are better alternatives, or you care if it fails.
 

gadolphus32

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I cannot believe this person is having this much trouble with a simple cap. I would really like to see what they are doing incorrectly.

I ended up calling a professional plumber, who confirmed that the pipes I am dealing with are slightly larger than 1 inch for whatever reason. On top of that, one (but not both) of the radiator pipes was reverse threaded. To solve the issue the plumber took my old fittings to a local supply company to have them manufacture custom couplings. One end of the couplings will be matched with the fittings so that it fits my weird pipes, and the other end will be standard 1-inch.

The plumber, who is not a young guy, told me he had never seen radiator pipes that were not a standard size, or a reverse-threaded one. I guess I am just lucky (or not, because a project that I thought would cost about $3 -- the cost of a couple of caps for my radiator pipes -- will now cost hundreds of dollars, but at least I no longer feel like an idiot for not being able to get caps on the pipes mysefl).
 

gadolphus32

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I ended up calling a professional plumber, who confirmed that the pipes I am dealing with are slightly larger than 1 inch for whatever reason. (And yes, he tried 1.25-inches, which is way too big -- so the measurements are somewhere between 1 and 1.25.) On top of that, one (but not both) of the radiator pipes was reverse threaded. To solve the issue the plumber took my old fittings to a local supply company to have them manufacture custom couplings. One end of the couplings will be matched with the fittings so that it fits my weird pipes, and the other end will be standard 1-inch.

The plumber, who is not a young guy, told me he had never seen radiator pipes that were not a standard size, or a reverse-threaded one. I guess I am just lucky (or not, because a project that I thought would cost about $3 -- the cost of a couple of caps for my radiator pipes -- will now cost hundreds of dollars, but at least I no longer feel like an idiot for not being able to get caps on the pipes mysefl).
 

hj

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That makes absolutey no sense. Someone would have to have made the oversized pipe, AND FITTINGS for it, and putting a left hand thread on it would have compounded the cost even more. IF all that were true, the PVC caps would NOT have screwed onto the pipes.
 

gadolphus32

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That makes absolutey no sense. Someone would have to have made the oversized pipe, AND FITTINGS for it, and putting a left hand thread on it would have compounded the cost even more. IF all that were true, the PVC caps would NOT have screwed onto the pipes.

Yep, well, it's true. I wish it weren't because if in the future I need to replace valves on other radiators in my house I'm probably going to have to deal with similar problems. If you're ever in upstate New York and would like to stop by and see the pipes for yourself, I'd be happy to show them to you.

One theory I have is that rather than the pipes having been deliberately oversized, what happened is that they froze at some point and that is why they are slightly larger. They were in the attic, which was uninsulated, and so if water was left up there without the system running it would have been easy enough for them to freeze. The PVC caps presumably screwed on because they are a little flexible and could handle the extra width.

I think they did the left-hand thread on the one pipe because it would have allowed them to screw into two fittings simultaneously, by the way. With a reverse-threaded fitting on one end and a normal one on the other, turning the pipe in one direction would screw it into both fittings. For whatever reason they must have preferred that approach over using a union.
 

gadolphus32

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I suspect this may have been a left right nipple, used as an alternative to a union.

Yes, that makes sense. One side of the nipple (the side going into the radiator) is right-threaded. Only the other side, which meets the pipe coming out of the floor (which goes I don't know where exactly because it's all behind walls or floors) is left-threaded.
 
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