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Millivolt Vaporstat

We have a gas fired single pipe steam boiler about 25 years old which seems to be still running fine (knock wood). Recently I put a 0-3 psi pressure gage on the pipe leading to the pressuretrol and watched a boiler cycle on a reasonably mild day here in NJ (about 30 deg F outside). Pressuretrol was set at 0.5 and diff of 1, per guidance here. Pressure rose to 2.3 psi before the cut out stopped the boiler, but I actually think the thermostat may have been satisfied since the boiler did not come on when the pressure fell below 0.5 psi. I know that these devices are not all that precise but I'm thinking of replacing the pressuretrol with a vaporstat. The gas valve, pressuretrols, LWC and thermostat run off a millivolt generator currently and I like it (due to the availability during power outages).

Looking at discussion threads from the past I see that the Honeywell L 408A 1132 seems to be the recommended item (from a thread in 2004). It doesn't seem to be available any more. Does anyone know where I can buy one or what a current replacement model might be which will run on my current millivolt system. I intend to keep the second pressuretrol (set at 5 and 1) as the backup.


One final comment: I recently solved a pretty drastic downstairs / upstairs temperature differential annoyance (about 5 to 7 degrees hotter on second floor than on first floor where the t-stat is) by first adding bigger vents the steam mains (they are mostly insulated except for the elbows and Tee's), adjusting/replacing vents on the radiators (faster downstairs, slower upstairs) , and then finally changing the 25+ year old thermostat with a pretty simple and inexpensive new one. I'm pretty sure the thermostat was the main culprit. We now have nice pretty even temps (maybe 1 deg difference but in an old house...). Funny thing is the temperature difference kind of crept up on us gradually over the past 2-3 years slowly till this year it was just too much and I started an earnest investigation. I'm sure the venting changes helped, but they did not really make much difference until I changed the thermostat. I just wanted to pass along my experience, since I see a lot of recommendations about insulating the mains, main vents, correct size radiator vents on threads about heating problems, and not often a thought about the thermostat. I'm pretty sure in my case the t-stat was not working properly.

But, still interested in the vaporstat model and availability.


  • Jamie Hall
    Jamie Hall Member Posts: 15,858
    I can't think why the current model -- L408J1009 -- wouldn't work. L408A was the mercury bulb type, which had somewhat better switching characteristics for millivolt systems -- but since mercury is now thought of as Really Scary Stuff you can't get them any more. But the 408J should be OK.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Steamhead
    Steamhead Member Posts: 14,476
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
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