Rethinking 1-pipe steam venting strategy

Abracadabra

I'm doing a complete revent tomorrow of a third building. 19 unit building, 4 rads per unit.  I'll be changing out 76 vents.  On the first 2 buildings I did last month, an typical  venting strategy has caused me some balancing problems.  I'm using MOM vents on the rads and Gorton 2s and 1s as needed on the mains. Yes, I quickly vent all the mains, the longest mains with multiple G2s.  Mains fill with steam within 4 minutes. Just about as fast with an open pipe at the vent. But  for 1-pipe systems with this many radiators the typical venting idea that most rads get 5s, larger rads get 6s, far/cold rads get C and small rads or rads in rooms with the thermostat get 4s, and maybe far/cold/rads on the third floor might need to be stepped up one step to vent the riser, the amount of radiator venting exceeds the venting available at the mains.







Just as an example, I added up the main venting on one building. total venting on the mains is 5 G2 and 2 G1. This is 6.16 CFM at 1 oz.

The 76 vents on the radiators vent at 7.216 CFM at 1 oz. And if you calculate the radiator venting at 3oz  it comes out to 21.702 CFM.  I believe that this is too much venting on the radiators. I have decided to try a more conservative (aka slower) venting strategy on the radiators on this third building. All radiators will be getting MOM #4 with the following exceptions. Add 1 size for 3rd floor rads.  Add 1 size for "very large rads".  That's it. On this building I'm doing tomorrow, that give me total radiator venting of 2.08 CFM at 1oz and 10.6CFM at 3oz.

With the additional "backpressure" I should be able to get steam to go where I want it to, not where it wants to.

Any comments?

Thanks!

nicholas bonham-carter

radiator venting suggestions

my choice would be hoffman 40's on all radiators, with the idea of monitoring the performance for a few days, and maybe if one or two are slow to melt the butter, or trip the lascar data-logger, then do a step up in capacity.

the slower the radiation venting, the more likely the mains fill all the way before steam begins to rise into the radiators. i certainly know how difficult the job of making the whole place more or less even in temperature can be, because even if you can get all the rads hot at the same time, different parts of a building will lose heat at different rates.--nbc

Dave in QCA

On the right track

I think that your strategy may work.  If this is the same building that you have been working on with that one main that is very long and slow to get steam, VERY slow rad venting may prove to be helpful.  MOM #4 is a VERY slow vent, probably slower than I would pick.  The worst thing that can happen is that you find out the venting simply is to slow, and once the main vents close you system pressure runs up fairly quickly and you shut down on high pressure.  If that happens, then you know that you need venting just a tad bit faster.



My own preference is the same as Nicholas'.  The old Hoffman #40 is a good old slow workhorse and it happens to vent at a rate about half way between the MOM 4 and the MOM 5.  It is a long lasting and durable vent and is not subject to tampering or adjustment by the tenants



On the 1 oz scale, Your MOM 4 vents at .028, the Hoffman #40 is .042, the MOM 5 is .100, MOM 6 is .150, MOM C is .283



I would be wary about venting the rooms with the thermostats or sensors with a slower vent.  This could cause your building to overheat.  Ideally you want the thermostat room to be exactly the same temperature as the rest of the building.  If you venting causes it to be either cooler or warmer, that will give you problems.



The reason that venting is much slower than the rate that the boiler produces steam is because that vast majority of the steam that enters a cold radiator condenses to water, about 700 times smaller in volume, making room for more steam.  As the radiator comes up to temperature, usually a section at a time, the steam will move into the next section, thus needing a little bit of venting to allow the steam to push out more air, little by little as the radiator heats up.

Abracadabra

venting

Dave,







Yes, this is that building that's hardly getting any steam at the last few rads. I was actually hoping to use something between a MOM #4 and an MOM #5.  I'd prefer to not have to go back and replace all the new MOMs.  I'm not worried about shutting down on high pressure.  Burner is hi/lo fire and vaporstat is set to 6oz cut-in and 12oz cut out.  I can lower the cutin if needed. The owners have invested about $1600 in new vents.  I can get replacement orfices for the MOMs for  about 90 cents. If the MOM #4 prove to be too slow for some I was thinking of drilling the orfice with a bit somewhere between a #4 and a #5. A Watts SV lands somewhere between a #4 and a #5, but I'm hesitant to use a Watts as I've had a few small problems with them in the past. Also, not worried about venting the unit with the thermostat too slow.  The building is controlled by a unique system that measures temps across 7 different units and averages them to come up with a "average building temp".  http://www.rdcontrolsystems.com/steam-200-series.php Basically a multi-zone thermostat. I can access the control remotely and have written an app for it that will take readings every 15 minutes and allow me to graph the apt temps.  I can easily use that data to create a graph of the differential temp of each unit from the calculated average.  When I find one that's above the "building average" I'll slow down it's vents.

Dave in QCA

makes good sense

Your ability to reuse the vents that you have already purchsed by changing out the ports makes good sense and you will save a chunk of change.  I think in your case you'll be OK using the #4.  The #5 may prove to be too fast for the large rads and for the top floor, but trial and error will determine if that is the case, or hopefully, will determine that it is "just right".