Old LWCO McDonnell # 67.....labeled 120 VAC....can it do 24VAC safely?

JUGHNE

My son has a boiler in his "new" house in Chicago.
The boiler is Weil Mclain E-4 series 3...... 105,000/84,000 BTUH.
Maybe 265 EDR rated.

First question: any idea of age, has 2 2" risers.
He has flooded the boiler up to the mains and it is holding water.

Then has M&M #67 LWCO. It is labeled for pilot duty of 115 VAC.
The wiring diagram on the steel plate attached to the boiler shows the LWCO disconnecting the entire 115 circuit.
The 24 vac transformer is mounted on the ceiling above and from there the 24 vac
is routed thru the Tstat, LWCO and pressure control.
I understand why this happened as no 115 comes down to the boiler.

It was probably this way for 50 years. Is there any reason the LWCO should kill all the 115 power to the boiler??

Also any guess on the age of the #67? It has "Doing One Thing Well" on the label.
It does have a ball valve type flush valve....maybe upgraded in the past.


I believe this is as old as the boiler and have encouraged him to replace the complete LWCO.

Perhaps a little advice from Wallies, other than Dad, would convince him.
TIA

EBEBRATT-Ed

I am in favor of safety's killing the 120 volt but my only reason is 120 volt wiring is just a better class of wiring than the class 2 24 volt stuff......but that"s just me I have lost that battle years ago.

I think MM recommends replacement after 10 years??. I know some on here have rebuilt them, but I have seen too many all crusted up I would replace it. They are $$$$ but cheaper than a new boiler

retiredguy

If you would like to rebuild that control and I would if it's clean inside and you don't break off one the many bolts holding the float assembly in the control look for McDonnell Miller model 67 parts which I believe is 6667 or SA-67-2 or as @EDEBRATT-ED said, just replace the whole control. Sometimes rebuilding is harder than replacing especially if you have never done it.

Gilmorrie

From a safety point of view, there is no reason why a 120-V switch can't be used for 24V. But, a switch is also rated for amperage, so check that too. AC ratings are usually different than DC ratings for switches.

ethicalpaul

I offer this just as a data point of interest because this topic is one I have my thoughts on as well.

My old Utica has a 24 volt LWCO but my brand new just purchased little Peerless has a 120v LWCO that kills all the power.

I have no idea why this seems to be the more modern way but I assume it has to do with a couple catastrophes and more than a couple lawyers.

Precaud

ethicalpaul said:

...but my brand new just purchased little Peerless has a 120v LWCO that kills all the power

Isn't that thing installed yet?

ethicalpaul

Not yet!

JUGHNE

Paul, in some cases the boiler installer can not/should not do 120 VAC wiring. In my son's case the transformer is remotely installed, probably by an electrician.
Then boiler installer deals with 24V only.
I have seen this in other installs also.

But your new boiler needs 120 to the boiler as that is where the transformer is located. But why the 120 LWCO, IDK.

The only remote possibility I can think of is that 120 contacts may be more reliable.
Being a reuse type guy, I used an old working fan relay for my boiler relay. Thinking it was heavy duty and would last forever.
What happened it was switching only 24 volts and not always making positive contact. Tried another one and was getting the same results. Contacts looked good on both. 24 supply is from a 75 VA transformer that is barely loaded.
Gave up and installed a RIB.

mattmia2

120v high current contacts are designed to arc and clean themselves as they operate. If you use them to switch low voltage, low current, they will form a layer of oxide that won't conduct.

JUGHNE

Thanks, that seems to make sense.

How about the RIB's that are used for multiple purposes?

OTOH, many of these LWCO's have been working for years.
Perhaps it is the physical hammering of the float that insures good contacts?

ratio

https://www.dropbox.com/s/mwjja8uhao4yemh/ElectricalContactsPrinciplesandApplicationsSecondEditionByPaulGSlade-1.pdf

Fascinating read!

EBEBRATT-Ed

They only list two switches for a #67 cutoff. 1 is a millivolt switch and the other one is 120 volt.

So I would think the 120 volt switch is the one used for 24 volt

retiredguy

Wow, what a good discussion on the wiring of a low water cut-off. Since I am (or was) schooled on commercial and industrial boilers using most any type of fuel my observation may not be too helpful but here goes. Wiring the low water cut-off to the 120 volt supply would shut off everything (all power) while wiring the same control to a 24 volt circuit would only shut off the control part. so, if a relay stuck closed due to stuck/burned contacts or failure of the relay itself, you could still possibly power part of the burner system which may not shut off the burner.

Which type of voltage shut-off would be better would probably be to shut off the primary voltage which is the 120 volts as long as the amperage across the contacts is less than the controls contact rating.

In my line of work, most every operating control had to have a back-up device that did the same job. There had to be 2 low water cut-offs and both could not be mounted on the same side of the boiler or had to be piped to a separate boiler tapping depending on the boiler code in that area.

I really enjoy reading all your posts on the subjects discussed since I rarely got to enjoy that type of banter since I mostly worked alone or with an inexperienced helper that I was trying to teach. Keep the discussions coming for the education they provide.

606Bungalow

Sorry it took me so long to put up the photos. I'm very surprised no one has asked for them yet.
Here is the LWCO in question.
Based on the crud built up on the bottom outlet, I probably need to replace the whole thing.
As far as the wiring is concerned, the boiler ran all winter while I was working on the house (we moved in in July) with no problems or strange noises. I tested the LWCO this winter and it worked on the 24 VAC wiring.
This last week I worked on the boiler a little bit and after installing a new thermostat and new 24 VAC wiring the LWCO tested fine again.

I'm not sure how old it is, or how old the boiler is.
The house was built in 1912.

ChrisJ

mattmia2 said:

120v high current contacts are designed to arc and clean themselves as they operate. If you use them to switch low voltage, low current, they will form a layer of oxide that won't conduct.

JUGHNE

I think Matt's theory has some fact to it.
By the time I read Ratio's paper, which is very intensive BTW, I'll just buy an new relay.....which I did.


606, there is probably a good chance the inside of the LWCO looks a lot like the drain port.
Is there a date code on the top of the round section between the body and the switch? If so tell us what it is and we can tell you the year.
They started putting date codes there after 1972.

If you replace you do not want the manual reset option.

ChrisJ

JUGHNE said:

I think Matt's theory has some fact to it.
By the time I read Ratio's paper, which is very intensive BTW, I'll just buy an new relay.....which I did.


606, there is probably a good chance the inside of the LWCO looks a lot like the drain port.
Is there a date code on the top of the round section between the body and the switch? If so tell us what it is and we can tell you the year.
They started putting date codes there after 1972.

If you replace you do not want the manual reset option.

I'm sorry, but with no disrespect towards Matt I'd need some outside sources quoted because it goes completely against everything I understand about switch contacts.

I've never seen arcing improve any contact, this is why ignition points have a capacitor and why we put diodes across relay contacts that may arc etc. Arcing destroys them. I believe there's also a grease used on very high current switches to reduce arcing.

mattmia2

24vac controlling a .5a relay coil isn't exactly low voltage or current. It isn't high power, but it is a lot more than something that is doing electronic signaling that might be 5v or 12v at something like 10 ma. That is where you get in to problems unless the contacts are specially made for small signals.

JUGHNE

All of this discussion because I repurpose/recycle things.
But it advances the "banter" as retired guy says.

In the meantime back to the LWCO.......to me it looks time for a change.....

ChrisJ

JUGHNE said:

All of this discussion because I repurpose/recycle things.
But it advances the "banter" as retired guy says.

In the meantime back to the LWCO.......to me it looks time for a change.....

Yes,
To a Hydrolevel Safgard.

The voltage on this matters because you're powering a circuit.

retiredguy

Here's one more; years ago Hydrolevel Safegard had problems with the controls we used causing nuisance shutdowns due to distillation in the probe reservoir. The answer at that time was to blow down the control a couple times a day or to pipe them as close to the boiler as possible. At a later date, they fixed that problem by increasing the voltage on the probe. During that time no decent boiler company recommended that control but instead relied on McDonnell Miller for all the low water cut offs until the fixed the problem.

@mattmia2 on a boiler 24volts was considered low voltage and almost every low voltage device utilized 24 volts. Today that is probably all changed but the new way is something I do not have to learn.

ChrisJ

retiredguy said:

Here's one more; years ago Hydrolevel Safegard had problems with the controls we used causing nuisance shutdowns due to distillation in the probe reservoir. The answer at that time was to blow down the control a couple times a day or to pipe them as close to the boiler as possible. At a later date, they fixed that problem by increasing the voltage on the probe. During that time no decent boiler company recommended that control but instead relied on McDonnell Miller for all the low water cut offs until the fixed the problem.

@mattmia2 on a boiler 24volts was considered low voltage and almost every low voltage device utilized 24 volts. Today that is probably all changed but the new way is something I do not have to learn.

Matt is talking about electronics in general, not really boilers so much. A phonograph cartridge has typically 0.001V coming from it and FM tuners use as little as 0.0000012 volts to the antenna. That's not a typo.

Though I would've thought millivolt systems were considered "low voltage" in the boiler world and 24 was normal?

mattmia2

It is all low energy in that generally not enough current is available in a fault to cause serious injury to a human or easily start a fire but the way contacts work with different amounts of low power is very different.

ratio

Depends on who you're talking to; gotta go by context. With my Sparky cap on, high voltage is 480, low voltage is 120 to 240. With my Controls guy cap on, low voltage is 24 VAC, high voltage is everything else. Linepersons, well, high voltage to them is something else entirely!