Single pipe steam. Inlet side hot. Vent side cold. Should radiator fins all heat up at same time?

Sounds like you may need some new vents. Also you may need some boiler room maintenance.

To be clear about how the radiators actually heat. if you have a cold start, that means everything is room temperature, the boiler and pipes and radiators above the water line will be filled with AIR. Air and steam don't mix. If you could see it in a bottle, and the steam was Red and the air was Blue, it would look like a Red gas (the steam) would have a line where the Steam ends and the air, a Blue gas, starts. Just like vegetable Oil and tap Water in a glass are separate.

Now you start the burner and put your hand on the header pipe where it exits the boiler. It will stay cool to the touch until the boiler starts to make steam. Then you will feel a rapid change in temperature as the steam reaches that point in the piping. Now move quickly down the main pipe away from the boiler and feel that pipe. Cool to the touch while it is pushing air out of the way. Then very hot when the steam gets to that point. Quickly move further down the pipe and feel the cool pipe and then it becomes a hot pipe.

This keeps happening until all the air is out of the pipes and radiators. If your vents are not letting the air out at a balanced rate, so each room gets heated at about the same time, then your vents need to be changed to vents that will let the air out slower on the radiators that get hot too fast and let the air out faster on the radiators the don't get hot fast enough.

That is easier said than done in some homes, on some systems. 

Wait a little longer to see if everything works itself out.

Another thing to remember is that most steam systems were installed when Coal was King in the heating industry. Coal boilers did not have thermostats. They had people that put coal in then by hand. So the fire did not turn on and off like a gas or oil burner does today. The heat was always on. More in the winter because you might put 3 shovel fulls in the boiler every few hours. Less in the spring and fall, because you might even let the flame actually die out in the afternoon so the house doesn't overheat. You learned how to keep up with the weather when you operated a coal fired boiler.

I haven't had one break off that I couldn't get out. If you're a car guy you should be OK but if in doubt, call a pro. It's certainly easier than a bolt breaking off in a head. Even not a very good plumber should be able to handle it. You might have to tell them which brand of vent to use, or buy it yourself and have them install it so they don't buy some garbage.

Pretty much. You might get a little heat on the inlet pipe, and a few — maybe two or three — sections — at the end of a really long run when the pressure got to 2 pounds or so.

And as has been said, 20 minutes from a cold start it just getting going. Give it a little time.

Wouldnt hurt to first: 1) boil the vents in white vinegar for about five (5) minutes and see if that resolves issues before spending significant amount of money on replacing all vents, 2) It's a good idea to periodically boil vents anyway as preventive maintenance. Lastly, I have one pipe steam that works excellent mainly due to routine annual maintenance ( by qualified tech) and occasional inspections / boiling of vents

Regards,

RTW

☝️☝️☝️ All those taps above should be NPT I'm sure @STEAM DOCTOR implied

<shrug> my radiators do not even out. The heat travels across them during a call for heat and it takes some time to do that. As I said, it depends on the venting.

Here's a faster one than mine. You can time yours.

BR#2 needs the packing nut on the valve tightened (a little). Just until the hissing stops.

BR#3 and DR may need a faster vent.

Vents should be hand snug. Careful use of teflon tape (dont get any in port to vent) is recommended.

On startup (pipe cold) check to see if main vent is emitting air. Once the pipe is hot the vent should be closed.

As @Grallert says:

The goal is not to heat the radiators up completely. The goal is to satisfy the thermostat. 

Sorry, I meant they don't "even out" without filling with steam. If they completely fill with steam, yes they even out at full heat.

I'm sure my vents are working 🙃

@CoachBoilermaker , some radiators will behave like that if there were made without push-nipple connections at the top of the radiator sections. This does confuse some folks. Post some pics of your rads so we can have a look.

There is no predicted time, so the answer is, the appropriate amount for how your system is set up. That could be minutes, an hour, or never.

It’s akin to asking how big is a box.

There are tons of variables that influence it. Boiler output to system size, vent size, main venting size, style of radiator, size of radiator.

I have a boiler that is sized to radiation with zero pickup so mine heat really slowly, slower typically equates to greater comfort as it flattens out the swings between firings. If you have an oversized boiler with higher output it might go faster, or if it’s drastically oversized and bangs off the pressure safety like crazy it could go slower.

And finally, I’ve done a lot with my system and quite intimate with it, I couldn’t tell you how long mine take because I’ve simply never timed them. I can’t really think of a reason it matters. I have aggressive main venting to fill the mains first, that takes about 3 minutes on a warm system. After that all the radiators have steams available to them and heat at the rate needed to keep a particular room at the temperature I desire. Some are much faster than others, in general the upstairs goes slower to keep it cooler which is what the family likes. My master bedroom is 8 sections and I’m not sure if the 4th section has heated…ever. I just don’t need heat there so it’s set up to go very slow. When it’s mild I don’t think it ever heats past the first section.

You are trying to satisfy the thermostat not fill radiators. Adjust venting to make things even as you want and satisfy the thermostat. As Chris said above, his radiators are oversized for his house so fully filling them would overheat the house, even when it’s at or below design temp.

In order to make it easier for all of the folks here to be on the same page, including the seasoned professionals all the way up to the first time steam system owner, I would like to clarify the terms that may be misunderstood when I see them used incorrectly. This is not to be demeaning in any way, I just believe that when you know the correct term for something, it is easier for all those reading this conversation to eliminate any confusion.

When @CoachBoilermaker uses the term Fin or Fins, I believe they are actually referring to a radiator section. The term Fin usually refers to the thin sheet metal sections of a convector or a baseboard radiator that increases the heating surface area of a piece of pipe that is holding the hot steam or water as shown here:

In these photos above, the left photo is a copper pipe with aluminum fins that are enclosed in a metal housing designed to cause air currents to flow across the fins and pipe to cause convection heat to enter the room.  Item #6 are the Fins.  

On the right photo is a cast iron radiator that has no fins.  There are 16 sections that make up that radiator. 14 intermediate sections and 2 end sections with legs so the radiator can be installed as a free standing radiator in the room that will be heated by the radiator.

You have two spaces were the vents are hissing and the radiators are fully hot?. I would consider changing these vents. The vents should stop venting when confronted with steam. Hold a cold spoon up to the vents in question and see if they are allowing steam to pass. You'll likely see condensation on the cold surface indicating a stuck or failed vent.

If the other radiators are not satisficing the thermostat it's time to have a look at those. Are they venting air? Sometimes a little tap with the handle of a screw drive or something will loosen up the stuck mechanism inside. Try this with vents that fail to close when they see steam. This is often a process and requires some measure of patience, even for a pro. If you replace any vents the the balancing process starts over, takes a while sometimes. You'll get it. Give it time.

Note that even if a radiator section is steam-hot at the top, there may still be air near the bottom of the section that is keeping the vent open. Steam will tend to rise to the top of the radiator, especially if the sections are connected across the top; cooler air will tend to sink the bottom.

What you should never see in a system that is working correctly is steam (technically water vapor since it will condense as soon as it escapes) coming out of an air vent.