modify L408B Vaporstat to "break" on pressure?

Binnacle

Does anyone know if a bit of soldering / clipping / gluing can be applied to a Honeywell L408B so it behaves as a L408A would?  B's are easy to come by, but A's are looking scarce.



After reading Gerry Gills comments on the new J models it's clear that older-is-better when it comes to Vaporstats.

BobC

Relay

You could use a relay that would actuate when the stat makes on pressure rise and put the relay contacts where the vaporstats would usually be. That would work but I don't know how happy the safety Gods would be with it.



Bob

MDNLansing

Voltage Relay

You would use a voltage sensing relay for this. They come in normally open, or normally closed, then reverse when voltage is applied to the contacts.

Binnacle

but a relay needs power

I guess I'll have to keep my eye out for a L408A.



Boiler control is a Honeywell Thermopile millivolt setup.  The design is absolute genius, with the 500 mV that runs the thermostat and gas valve generated by pilot flame heat.



My house was the only one on the block with heat after Hurricane Sandy.  This feature I intend to keep.



Thank you for the idea though.  Much appreciated.

BobC

That is a problem

You could add a 24 v transformer to power it all but that would mean it requires power to work. If you wanted to get tricky you could use a dc relay and back it up with a battery so you could use it if the power went out.



Bob

Binnacle

perhaps

The pressuretrol cranked all the way to minimum is working ok, so I can afford to be patient while searching for a L408A.  If it looks hopeless after a few months I may come back to this idea, but I love the simplicity of the millivolt system and want to retain as much of that as possible.



My guess is that professional plumbers would be leery of installing mercury switches, so hopefully competition for the old-style L408A won't be too intense.



Thank you for the idea.

MDNLansing

DC

I'm with BobC. You could use a 9 vdc transformer and a battery in an emergency. You could also use a 110 vac setup running off a computer UPS. Thermopiles just won't generate the voltage needed for this.



That thermopile idea is brilliant. I've never heard of a setup like this, akthough I'm not a steam pro. Is this something common, or is it a one off?

Binnacle

all good things come to those who are patient

I'm pretty sure that I can snag a L408A if I'm patient, so that's plan A.



Was referring to BobC's DC battery idea as a good plan B above--or I could just surrender and buy the inferior L408J.



UPS--forget it.  My UPSs did a wonderful job for the first 45 minutes after the lights went out.  But for the next ten days until PSE&G restored our power the UPS was a just a big ugly doorstop.



Interestingly Thermopile systems are commonplace and condemned by many as obsolete energy wasters.  Some folks subvert them by installing fancy intelligent (and largely pointless) thermostats and running the millivolt control via a 24VAC-powered relay.  I'll happily keep paying $3/month for the pilot light so I can be warm the next time the Atlantic surges 18 feet and floods every power substation in the NY metro area (not to mention half of downtown).  And my super-simple years-old Honeywell thermostat (with a mercury switch, just like he L408A Vaporstat) works great, is perfetly reliable and doesn't attempt to waste my time or distract me.



Even more interesting is that Honeywell doesn't appear to have promoted this most excellent feature of not requiring external power.  It's well worth having even for the more likely scenario of two-days without power due to an ice storm taking down a few overhead lines.

Mike Kusiak_2

L408A

Keep an eye on Ebay, as they do turn up occasionally. You can enter a search and they will email you when one becomes available for sale.



I would not use anything but a mercury switch with a millivolt system. With such a low voltage, high current circuit, contact resistance becomes critical. With the typical dry contacts of a microswitch, any oxidation can increase the resistance to a point where the system may become unreliable.



Stick with what you have until you can find a proper mercury switch control

Binnacle

-

Thanks for the info, especially in regard to the value of mercury switches in millivolt systems.  Hadn't thought of contact corrosion as a problem, but it makes perfect sense.

Mike Kusiak_2

L608A

Also, keep your eye out for the L608A model. This is the same as the L408A, but has a mercury switch with both NO and NC contacts (SPDT). A number of these have sold on Ebay recently for less than $100

Binnacle

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For sure.  Only reason I did not mention it here is that no L608B exists and the thread was originally about hacking a L408B to be a L408A.  I half-wonder if the mercury bulb can be reversed, but since it's a precision instrument doing so would certainly throw off  the calibration.  I'll focus on obtaining a proper A unit for now.

Mike Kusiak_2

B to A conversion

Looking at the photos of the A and B model, the only difference appears to be that the position of the mercury switch capsule is reversed end for end. So perhaps it is possible to convert?