Cold radiator with shared take off and return (2 pipe)

JoeChan

I have a curious cold radiator problem on starting up this season. The cold one is on the second floor, and it is at the end of the main. The curious thing is that it shares a take off and return with the radiator in the next room, branch in the middle under the floor in between the two radiator. The two radiator is about 2ft apart. Let's call the one that heats up "radiator A", and the cold one the "radiator B". Radiator B isn't completely cold, but it takes 45min or so to slowly heat up, so in practice, the thermostat has usually stopped calling.

Usually, I attribute this type of behavior to broken steam trap up stream. However, I can't come up with a theory on which one is broken.

- One explanation could be that radiator A's trap is broken, so it feeds the steam into the return. However, I can't see that is the case, because they share a take off and return with the branch points ~1ft from the radiator. I'd expect both to heat up together for a bit, and then radiator B stalls. I don't see that. B stays cold pretty until A is 3/4 heated. Furthermore, radiator A's steam trap doesn't get hot for a good 30min. I'd expect a radiator with a broken steam trap to be pretty hot.

- Another explanation could be that another radiator further up stream is broken. But if that's the case, both A and B will be affected, which is not the case here.

A few more details about the system. My system has a differential loop, so it only runs on 6oz. The main has two runs separated by a wet return. The affected radiator is at the end of the long run with 6 radiators. Looking at the layout of the pipes, the shared take of looks like it is in the middle of the radiator A & B, while the shared return looks a bit closer to radiator B. There are also steam trap in the basement between the main and return at the end of each of the runs.

Thanks for any tips.

BobC

Was this ok last year?

The feeds to both radiators would have to be just about level for this to work because they are are so close. Is it possible something shifted and you have a little water trapped in the short connector to the problem radiator?

Bob

JoeChan

Things worked fine last season, and we had all the steam traps in the house replaced back in late 2011. Those two were always the last to heat up, but they did heat up together, that is, until now.

The only other thing I can think of is that some stuff got into the trap of the cold one so that the air drains very slowly.

Jamie Hall

That steam trap between the main and the return is called a crossover trap, and functions as a main vent for the main. Your main vent or vents, then, will be close to the boiler at theend of the dry return. A common setup.

When you have two radiators close together like that, and one doesn't work and the other does, the first thing I would check is to make sure the valve on the cold one is open (don't laugh -- it happens!). The second thing I would be suspicious of is the trap -- they can and sometimes do fail closed.

That is assuming that the lines are all pitched reasonably well, but it sounds as though you have that checked.

JoeChan

Yes, I checked the valve multiple times already.

Fred

Every once and a while, even though the valve seems like it opens and closes properly, the internal disc has detached and may be stuck. Only way to check is to disconnect the rad or take the bonnet off the top of the valve. Does the pipe get hot up to the valve? Are you sure the radiator vent isn't stuck closed?

JoeChan

When the radiator is cold, the pipe up to the valve also stays cold. If I crank the heat way up, then the radiator slowly heats up to about half way across the top (takes over an hour). The bottom third near the trap never heats up. The radiator has 6 sections.

Fred

Is the radiator that gets hot larger than the one that stays cold? If so, I suspect the steam is taking the path of least resistance and maybe creating a vacuum, stealing all the steam from that other rad. I suspect that trap has failed, allowing that to happen. Even though a common feed supplies both rads, it can happen. Before you cycle the boiler on again, shut the valve off on the radiator that gets hot and see the the cold one heats up.

JoeChan

They are exactly the same size (6 sections), same model, down to the steam traps. I'll try shutting it off next time, if it's possible. It has one of those valves that has a handle rather than a nob, so it's pretty hard to get it out of the full on position.

As far as the trap goes, even if it fails, I assume that the broken trap will get hot when steam passes through it, right? I'm not seeing that at all. The trap of that hot radiator is stone cold pretty much all the way towards the end of the cycle.

Fizz

It's possible there is too much radiation attached to supply run. You said there are 6 radiators on the long run, did you open any of the valves on the other radiators more than they were set. I had similar problem with my one rad and eventually had to repipe another radiator from the overloaded line to another line. All works well now. Try closing or reducing steam supply valves on one or 2 of other radiators to see if it makes a difference. If so, you probably have too much edr attached to line.

Jamie Hall

A trap may fail either closed or open. If the trap is failed open, the return will get hot -- steam hot, not hot water hot, when the radiator is pretty well full (it may be hard to tell without a thermometer -- I like the little IR guns). A trap which is failed closed, however, will prevent steam from getting into the radiator at all; you will get very little heat from the radiator, and that only after a very long time.

Which is what I think I'm hearing?

JoeChan

Your description of failing closed matches my symptom.

As for Fizz's suggestion on number of radiator, the runs of this system has been getting smaller over the years. This run with 6 radiators used to have 7, one more beyond this cold run. That was removed a couple of years ago when we move the bathroom. The other run had a total of 2 radiators removed in the last 30 years. So the boiler is probably a bit oversized compared to the original system. However, we have also tagged on the 2 hot water loop and a small radiant loop for the kitchen. So they probably all even out.

Jamie Hall said:

A trap may fail either closed or open. If the trap is failed open, the return will get hot -- steam hot, not hot water hot, when the radiator is pretty well full (it may be hard to tell without a thermometer -- I like the little IR guns). A trap which is failed closed, however, will prevent steam from getting into the radiator at all; you will get very little heat from the radiator, and that only after a very long time.

Which is what I think I'm hearing?

Jamie Hall

Take it apart and see! I had a radiator trap freeze once (don't ask -- this place can get mighty cold under certain conditions) and couldn't get any heat in the room... had to use a hair drier on it.

JoeChan

I'm here to report back: problem solved, but the cause isn't anything in the heating system at all. The root cause is that our gas supply line is quite clogged with rust. Thus the gas pressure is way off.

We found out of this problem because our gas stove almost shut itself off when the boiler came on. We called the gas company and they immediately sent a crew out to put a new gas line in. The heating problem immediately went away. According to the gas guy, this is very common at the start of the heating season, when the increased gas consumption really stir things up. This condition is apparently a safety hazard.

Jamie Hall

Thank you for reporting back! It's really helpful to all when people do. And I'd never have figured it for a gas supply problem...