Help With Main Venting

JSR

We have recently bought a house in Michigan with steam heat. We are replacing a leaking boiler this spring. I have read Dan’s books and am interested in understanding the system better and tuning it up for peak performance, especially the main venting.

We have a two-pipe system with three 2” steam mains rising off of the header. For what it is worth, I have attached a pic of the near boiler piping.

One of the steam mains supplies steam to approximately 50% of the total radiation load; each of the other two mains supplies steam to approximately 25% of the total load. All of the attached radiators have Hoffman 17C steam traps.

The system has three return mains. These are dry returns until they reach the boiler and drop down into a wet return.

One of the return mains does not collect condensate from any of the radiators, but rather connects to each of the three steam mains near the end of their runs. These are just simple pipe connections (e.g. elbow, tee); there are no steam traps at or near the connections. This return main has a Hoffman Main Vent Air Valve No. 75 on it where it drops into the wet return. I have attached a pic of where it connects to one of the steam mains.

I assume the purpose of this return is to drain whatever condensate forms in the steam mains. It seems to me, however, that steam is going into this return because where it connects to the steam mains it gets just as hot as those steam mains. In fact, after the boiler runs for a while, this return gets “steam hot” all the way back to the boiler.

Is this a usual or normal setup for a return main? Should there be F&T traps at these connections? How fast should I vent this return? Does the Hoffman No. 75 seem adequate?

One of the other return mains collects condensate from approximately 25% of the radiation load and has a Hoffman Vacuum Vent No. 76A where it drops into the wet return.

The third return main collects condensate from the remaining 75% of the radiation load and has a Hoffman Vacuum Vent No. 16A on it where it drops into the wet return.

I guess the venting on these returns makes some sense because, according to Gerry Gill’s venting capacity charts, the 16A vents almost three times as much air as the 76.

I’d like to replace all of these main vents when I replace the boiler. Does anyone have any thoughts or suggestions on how to size the new vents and what vents to use?

Also, I am thinking of replacing the guts of the Hoffman 17C steam traps on our 11 radiators because they are almost 20 years old and I believe at least two have failed. Should I replace them with something with more venting capacity?

I am not sure I am asking the right questions, but any insight or help any of you have in helping me tweak this system would be appreciated. Thanks.

Jeff

Jamie Hall

Haven't I seen this one before? Well, anyway. You can replace all three of the main vents with larger ones. You may not have to, though, if they are working. Opinions differ as to how much venting is enough.

You neither need nor want F&T traps on that system. Don't do it. It's just fine as it is (except that you will want to insulate that hot "return").

There's no harm to replacing all the traps -- but there's no real advantage, either. If there are traps which you are sure have failed, yes they should be replaced. Otherwise, if it ain't broke, don't fix it. The system I care for has quite a number of traps -- all Hoffmans -- and is 80 years old. All original traps... works fine.

nicholas bonham-carter

How is the system working now? Any cold radiators may indicate a trap problem. Any nonfunctioning traps can have new elements installed from Barnes and Jones, (sailah).
Insulating the pipes would certainly help with efficiency, and as for venting, you could use Hoffman 76 vacuum vents, as it must have been an early vacuum system.
Try to find out why the present boiler has failed, and if because of steam or water leaks, then fix them.
Measure all the radiators to calculate the EDR of the replacement boiler, Make sure all the instructions are followed. The equalizer should be at the other end of the header, in order to catch as much water coming up with the steam as possible. I don't think a check valve will be needed, as long as the pressure is kept low with a vaporstat. try to avoid the Cycleguard LWCO, which shuts off the boiler every 10 minutes or so. Clean the boiler exactly as the maker specifies, through the skimming port.
this will be a nice summer project!--NBC

JSR

Thanks Jamie. Can you or someone explain why I don't need F&T traps on the steam mains in my system where they connect to the return that is just dedicated to the steam mains. That return is clearly getting steam in it. Everything I have read seems to indicate that steam in the returns is not good. Also, all of the two-pipe diagrams I have seen show F&T traps at the end of the steam mains. Maybe that only applies where the mains connect to returns that are also servicing the radiators??? The system seems to work well enough, so I understand that the "if it ain't broke, don't fix" it rule is likely applicable here. I am just trying to understand how this system works. Any thoughts?

Also, thanks NBC. Thanks for the equalizer advice (and your other points about the new boiler)! Great points. I can't find any steam or water leaks in the system, other than the one or ones in the boiler itself. (I can see water dripping out underneath.) I suspect the previous owner just neglected the boiler for so long it just gave up the ghost. I am planning to insulate all pipes after the new boiler install.

Jeff

Abracadabra

Just FYI, you have asbestos insulation in bad shape there. Be careful.

JSR

Thanks, Abracadabra. I am aware and being careful.

JSR

One other question. If I am getting steam all the way back to the vent on the return main dedicated to the steam mains, could I be losing steam through that vent? If so, how do I solve that problem?

Jamie Hall

When is a return a return? That long return to which you are referring -- which has no other returns going into it -- is, as you have figured, a way for the condensate from the steam mains (defined in a strict sense as the pipes going from the boiler to the last runout) to get back to the boiler. In your setup, it's well above the water line, and therefore it's dry. And needs to be vented. Having steam get into it isn't a problem, since there are no other returns connected to it. If it is insulated -- as it should be -- you will lose very little heat from it. The vent(s) at the end, where it drops and becomes wet, should close when steam hits them, so there should be no loss of steam through them. If the vents don't close on steam, then they should be replaced, of course.

Now -- you don't want an F&T on that, as that would change to pressures in it, and the condensate in there would not return to the boiler as well.

I would point out that some two pipe systems have dry returns -- which serve radiators -- which also serve the steam mains through crossover traps at the ends of the steam mains. However, they will also have a wet return -- at or near floor level -- which is connected by a drip to the ends of the steam mains and to the ends of the dry returns, which are isolated from each other by the water seal in the wet return.

JSR

Thanks, Jamie! Great explanation!