Caleffi 504 Aercal

Second question is, I thought this is BSP to BSP, I will confirm tomorrow when I receive a G 1/2" BSPP tap, and a thread gauge. I also note that the caleffi 504 is pre-knurled, which suggests the manufacturer is expecting it to be hemped/sealed at the threads. So if it's BSP to BSP why does it bind after 4 turns? It would seem surprising to me if they tapped everything NPT parallel since other radiator importers do not do this. But I guess I need the thread gauge to know for sure what it is.

Actually there may be a 3rd option as the interior appears that it may be milled to take a 15mm compression olive, you definitely cannot see this in the photo in your post, but the bevel is there in the units I have. Not sure if that's intentional as I can't find any documentation on the approved connection methods. Still waiting on the thread gauge and tap.

Hudson Reed did not supply the thread specs, and I even tried asking them for it but never received a clear answer. However today arrived the thread gauge. The threads fit nicely in the 55 degree G gauge, but I don't have a straight NPTS to compare fit with, and it's so small. However the 1/2" G tap was more helpful. I threaded it into the radiator by hand, it went in nice and smooth, then suddenly at that 4th turn became immediately stuck. I used pliers to turn it every so slightly, less than 1/16th of a turn, and it went back to threading in clean by hand the rest of the way. Radiator was factory new, never used and never had a valve in that port. I tested some other ports and found sometimes they could be sticky as well. So I think it is a thread quality issue. I definitely was not cutting metal with the tap, just chasing the threads and had resistance only at that one point. So I think it's a thread quality issue on the radiator.

Now the 504 Aercal threads in nicely be hand, and it looks like there is plenty of room to compress the fiber washer and get the valve facing straight up. If I can't for one of the radiators, I might try to get a thicker washer or use a different kind of washer. I think it's it's worth the extra 30 seconds or so to put the Loctite 55 cord on the the threads, since that would likely reduce the severity of any leak to develop, but also provide a backup seal. I've noticed that the o-rings on some of the blanks and tend to need tightening up after a few years, but you wouldn't want to do that with the Aercal since it would become crooked, so that would probably require depressurizing the radiator and installing a new washer.

I think that Loctite 545 is probably a choice of last resort for me. It would be difficult to remove the valve since the paint on the radiators could be damaged by a torch. I do think it's worth being able to change the valve, particularly when used in the US since it's not that easy to source them. Even in Europe most suppliers I tried had them backordered, so if there were a 6 month lead time for a replacement or parts, I would probably want to stick on another valve in the mean time. Moreover, it is sometimes useful to remove the valve. It might be necessary to unmount the radiator if there is something above it, the valve could become stained from discharge mist, or we may want to replumb the radiator differently in the future to solve a plumbing problem since these radiators can return from the top.

Overall I'm very happy with this valve. It looks great, and it has the redundant hygroscopic seal to guard against leaks in the float mechanism that are a risk with other auto bleeder designs. The chrome finishes matches the other radiator hardware and looks nicely finished out, though it is not a true mirror finish, it looks good. They would not start at all in any of the NPT female fittings I have, so you might want to get an adapter for this application. A straight male thread and a tapered female is not a combination I would want to use since the straight thread would bottom out in the taper. Germany is where I was able to find stock most easily. If Amazon Germany gets them back in stock, it may become fairly easy to import them. Let's see how it goes.

Thank Matt, I think the blue Loctite would be adequate here because there shouldn't be much risk of backout, but if you only have 1 or 2 threads engaged, it's not much contact maybe that is why the red is used? For my situation, I think I'd probably prefer the blue if there is no other way to stop spiral leaks. If it's just about leaks though, I see there is a product loctite 565 which is designed for pipes and has a 45lb*ft torque spec for disassembly. Feels a bit like overkill when teflon tape does a fine job on similar projects with tapered threads, but I see how straight thread design has benefits.

You're absolutely right you can get the disc caps on other valves, what I did not find was one that threaded in at a right angle. A chrome street elbow could get me to a straight valve, but it didn't look as nice in my opinion, it doesn't look like the right style for this type of radiator and the valve comes up tall above the radiator. Plus they are surprisingly expensive and can be scratched up during installation if not careful. We're putting in some more of these for about 20 total radiators.

Now I could be wrong, but it is my assumption that the value of the hygroscopic safety cap, is that when the float leaks, it will seal the vent and not vent air, and that it will leak much more slowly. TRVs also can leak. But if I have 20 radiators on 4 floors, it's a headache to get everything bled and balanced. Ideally if everything is working great, we can close the caps, then just open then for a little while if needed to purge the radiator. Even tenants could probably do this, but I'm not sure I'd want them messing with the coin bleeders, because those can leak a lot of water if they do something silly.