Changing L408B vaporstat to normally closed?

dbrant

Hi all,
I recently wanted to change out my pressuretrol for a vaporstat, and was able to get a hold of a L408B (with a mercury switch) for a great price. Unfortunately I didn't realize that the L408B is normally open, which is the opposite of what I need.

However, I've had the following idea:
The mercury bulb inside the vaporstat is secured in a metal clamp, but can be easily popped out. It wouldn't be too difficult to flip around the orientation of the bulb so that the mercury falls the other way, and the switch becomes reversed (normally closed).

How stupid is the above idea, and is there anything in principle that would make this not work?

Alan (California Radiant) Forbes

https://forum.heatinghelp.com/discussion/148295/modify-l408b-vaporstat-to-quot-break-quot-on-pressure

EdTheHeaterMan

Accuracy may be compromised. In theory, it will work. You just can't go by the number on the scale. The proper control part number should end with A. L408A. In the Honeywell numbering system, The B stands for open on fall or decrease. A means open on rise or increase. If you are not sure what you want to do then, you can purchase a "C" type control that has a SPDT switching arrangement.

I recommend getting the proper control. But I would do it in my own home. You know "Do what I say not what I do!" ...I have been a parent too long!

Yours truly,
Mr.Ed

Jamie Hall

Another possibility -- without having to tamper with the vapourstat -- is to use it to control a relay. Use an SPDT simple relay, 24 volt coil, normally closed. Wire the coil and a power source through the vapourstat. Wire the vapourstat through the normally close contacts on the relay. Now when the vapourstat does its thing and closes, it will power the coil and open the contacts.

If you do do this, it's not fail safe, and you should also have a pressuretrol, wired in the usual way, set for a higher pressure, in the safety circuit as well (which isn't a bad idea anyway...).

SteamingatMohawk

The liability and risk averse landlord has to speak up. From those points of view, I believe modifying equipment is an unnecessary risk that could have serious consequences if something ever goes wrong.

The other point of view is that you, or someone, will eventually sell the house or have work done on the system. By modifying the equipment you introduce unknown unknowns in the operation of the system.

dbrant

Thanks, folks!
I flipped the mercury bulb, and it seems to be performing perfectly well, but I will definitely take the suggestion of installing the pressuretrol alongside as a backup.

SteamingatMohawk

After looking at sketches of the L408 on the Honeywell web site, I can see it appears to be a minimal task to turn the mercury switch around, reversing how it operates, but not knowing if any internal adjustments need to be made to get it to work. If I was smarter, I would have looked before I leaped.

However, that being said, I remain liability/risk averse in my recommendations.

EBEBRATT-Ed

@SteamingatMohawk

I agree with your liability concerns and normally wouldn't recommend monkeying with a safety control. If he install another pressure control as a back up as a few posters suggested he is fine iMHO

SteamingatMohawk

@EDEBRATT-Ed Thanks, you are the first to agree with me on that point.

@dbrant I suggest that if you do do the mod to do a bench test to verify the setpoints and repeatibility before you put into actual use.

EBEBRATT-Ed

@SteamingatMohawk

Well @Jamie Hall mentioned a second pressure control as well as using a relay to reverse the switching action which is what I would have done. Only because if somebody else comes in to change the control with the mercury bulb flipped (not likely because mercury is no longer) it would be an issue

SteamingatMohawk

That's one of my long term concerns with home -made mods.

retiredguy

Remember this, an altered control has no liability protection. If there is an accident with an altered part, regardless of who made the change, gives the insurance company a reason to not cover the loss. Insurance companies would hire experts to go over that piece of equipment looking for any altered device or any other reason to deny the claim. I say, go buy the correct control and remove the altered control. . Do not leave it installed unless you want to assume the liability.

SteamingatMohawk

I no longer stand alone on the risk/liability front! Thank you.

retiredguy

Let me explain one reason why I am so risk adverse. In the mid 1970's I had just gotten to a hospital in New Castle, Pa and entered the boiler room when the down boiler (cold) in a 2 boiler job exploded. It was a Keeler high pressure steam boiler with an S T Johnson FDC burner (very old). It was a furnace explosion (natural gas) and not a boiler explosion (failure of the pressure vessel). Being the service man of record and with my presence in the boiler room, the company I worked for and me in particular, was charged with the explosion and the resulting damage. The total cost of the physical damage was about $100,000.00 . When the insurance company wrote up the cause of the explosion and who was responsible they indicated that I was or should have been in charge and could have prevented that from happening. The actual problem was that an "in-house" electrician had replaced a 4 pole double throw gas/oil changeover switch that they said I supplied. That was a lie since I still had the switch in my truck. The electrician had installed an old switch that he had in his shop that had broken contacts and was not the correct switch for that burner nor was it wired correctly. There were 8 prople in the boiler room but luckily there were no lasting injuries. It could have been worse if the operating boiler had ruptured it's pressure vessel. After that event I became extra careful with anything that I serviced. True story