Boiler wiring Q: Boiler operates with 120v switch OFF???

Motorapido

My gas boiler operates when the master shutoff 120v switch is turned off. Why? At first I thought the switch was faulty and had failed in the ON position, so I put in a new switch. In testing the old switch, I get continuity with the switch on but no continuity with the switch off. I put in the new switch anyway (after first testing the new switch for continuity in the ON position). Same thing. Boiler will fire on call for heat from thermostat even with the 120 volt master switch OFF. Next, I disconnected the wires from the switch poles for a final test, which absolutely guarantees that no 120 power is getting into the boiler circuitry. Same thing. Boiler fires on call for heat. Using a voltage on/off tester, the light lit up when placed near the thermostat wire where it enters the boiler body. I assume that it is a 24 volt ac wire.

Now, here is a clue. About 40 years ago, a prior owner had central air added to the house. When I trace the thermostat wire out of the boiler room, I can see where it feeds up through the floor to go up through the wall to the thermostat. In the basement, it is joined there with a wire nut and that run of thermostat wire then travels over to where the air handler is for the central air (in a crawl space, where I cannot easily get in to find where it terminates.

Could it be that some numbskull is using the same 24v ac transformer that tells the air handler when to turn on/off to send 24v power over to the boiler when the thermostat calls for heat? I thought you were supposed to have a separate 24v ac transformer for the central air and a separate one for the boiler. If somebody decided to share the transformer between both the air handler and the boiler, but still decided to run 120 power into the master switch to the boiler (maybe to trick an inspector), would that explain why my boiler fires even when the 120v ac input is removed from the boiler? I have not yet opened the service panel for the boiler (Peerless -- about 12 years old), to test to see if the 24v ac transformer inside the boiler is connected to the gas valve, or if it is getting incoming 120v power, but not connected to the gas valve in the boiler.

Is my detective work good? If so, could I simply remove the 120v ac incoming power to the boiler and just switch the 24v ac power coming in through the thermostat wire? I have a hunch I would still be in violation of code, but at least I could switch off the boiler at the boiler itself.

To all you experts out there, please advise, and thanks in advance.

EBEBRATT-Ed

Many different possibilities. You didn't mention if it is steam or hot water boiler

1. You could have a millivolt gas valve that generates it's own power so it doesn't need 24 volts or 120 volts to operate. The 120 volts may operate your circulator pump if it's a water boiler
2. Yes the air conditioning and the boiler could be wired to 1 24 volt transformer. This is not uncommon
3. Sounds like your switch is not shutting off the 24 volts to your boiler.


Need some pictures and more information to be able to help

JUGHNE

The good thing is that your heating tstat does shut off the fire.....so you probably do not have a stuck open gas valve.

There may not have enough conductors to run the AC and heat so some creative cobbling is called upon.

Or your tstat may have an Rh and Rc connection that would be not jumped and that would allow you to have 2 separate 24 volt power sources.
or
You could just have a 24 volt relay mounted at your boiler that runs a relay.
The relay would close on a call for heat and start the boiler provided you have a 24 volt source in the boiler.
Or as ED said you might have an old millivolt system that generates it's own control power. Different animal.

Jamie Hall

Whoa. Back up a bit here. Whatever the reason, you have an unsafe -- not to mention illegal -- situation here. Stop, take a deep breath, and figure out -- with the help of an electrician if you have to -- a)what does that master switch control, if anything? and b)what circuits power the 120 vac or 24 vac at the boiler, and where is the master switch for that circuit, and which circuit breaker/fuse is it on.

Once you do that, you may need to do some rewiring to get things safe and at least vaguely resembling code. Power to the boiler must be on its own, individually fused or circuit breaker equipped circuit, and must have its own, conspicuously labelled master cut off switch, both of which do nothing else except control your heating and cooling appliances (they can be separate circuits, in which case both of them need to be individually fused and have master cut offs).

Note that all this does not apply if you do not have any 120vac or 24vac at the boiler -- but your comment on the voltage tester tells me that you do.

EBEBRATT-Ed

@Jamie Hall we don't know what the 120 volt controls (if anything) this could be an oil to gas conversion where the 120 volt from the old oil burner was left there unused and the 24 volt power for the boiler could be from the AC unit. or it could be a gas fired millivolt.... seen plenty of those installed with no service switch.

No doubt this needs to be checked out but there may be nothing wrong. Could be a millivolt gas valve with the 120v runs the circulators.

He hasen't provided us with enough information.

Fred

Is this the switch at the boiler? Is there another switch, either at the top of the stairs or elsewhere in the basement that might have been wired around that master switch and directly to the boiler, then back to the master switch, after the terminals on the boiler or transformer? if so, it may be providing power to the boiler even when when the master switch is in the off position. It is clearly wired incorrectly. Was this boiler/wiring installed by a professional or a homeowner?

Motorapido

Boiler is gas, steam, Peerless 63 series, one-pipe, in an 85-year-old house. Boiler is about 10/12 years old. Need to run the serial number to find out. I will do more testing and tracing wires tomorrow and also post pics. What do I look for to identify a millivolt gas valve versus one that requires 24v ac? Also, I have an automatic damper that closes the flue when the boiler turns off. When I have the 120v ac power line disconnected from the boiler, that automatic damper continues to open when the thermostat calls for heat and just before the boiler fires, and it closes immediately when the boiler finishes its heating cycle -- again that damper motor operates when the 120v ac power is disconnected at the boiler switch. The switch I am referring to is mounted on the boiler exterior on the side with the sight glass and the pressuretrol.

Jamie Hall

EBEBRATT-Ed said:

@Jamie Hall we don't know what the 120 volt controls (if anything) this could be an oil to gas conversion where the 120 volt from the old oil burner was left there unused and the 24 volt power for the boiler could be from the AC unit. or it could be a gas fired millivolt.... seen plenty of those installed with no service switch.

No doubt this needs to be checked out but there may be nothing wrong. Could be a millivolt gas valve with the 120v runs the circulators.

He hasen't provided us with enough information.

True enough, true enough. I just get really nervous when I encounter a switch which is supposed to shut something off and doesn't -- and I can't find a switch which does.

Steamhead

If that Peerless has a stack damper as the OP says, it's not a millivolt system. I'll bet someone bypassed the switch somehow.

JUGHNE

If the AC air handler is not handy, you could find the breaker for the handler. If breakers not labeled you can put the tstat fan switch to on and switch breakers until it is off.
With the AH breaker off you can see if the boiler fires from the tstat.

There could be a transformer anywhere that powers the tstat, but most likely is in the air handler.

The transformer that came with the boiler would not be hiding very well, usually easy to find.

Motorapido

Steamhead said:

If that Peerless has a stack damper as the OP says, it's not a millivolt system. I'll bet someone bypassed the switch somehow.

But stack dampers work on 24v ac, so if I have 24v ac entering my boiler controls from the thermostat transformer, couldn't that 24v ac be actuating both the gas valve and the stack damper?

Over the weekend, I will remove the access panel, and make notes and take photos of where the 120v ac power connects inside the controls area of the boiler. I will see if the transformer inside the controls area of the boiler is wired to the gas valve and the stack damper, or if the incoming 24v ac wire off the thermostat transformer that is on the other side of the basement at the AC unit is wired to the gas valve and stack damper. Maybe somebody wired the 120v ac incoming power to the transformer inside the boiler, but then did not hook the output from the boiler transformer to the gas valve and stack damper, and instead just wired the incoming 24v ac feed from the thermostat to the gas valve and stack damper.

EBEBRATT-Ed

Chances are after the boiler was installed someone changed the thermostat or AC or air handler wiring and used the AC transformer to run both cooling and heating so now the 120v at the boiler is no longer needed.

Sounds like they "forgot" to put in a 24v service switch for the boiler at that time jmho

Motorapido

SteamRoller

Just by way of example, at my house we have one-pipe steam, a Utica PEG-187 boiler that was installed in 1983, and it is a millivolt system.

The standing pilot heats a thermocouple which powers the gas valve, opening it when the thermostat calls for heat.

There's also a wall switch with 120v that powers the water auto-fill, but the previous homeowner didn't mark the switch and the wiring wasn't obvious at first. From its position at the top of the basement stairs, it seemed like it was a boiler shutoff switch.

When we first moved in, my father-in-law, a mechanical engineer by trade, shut the switch off, but the boiler didn't shut off. He eventually shut off all the power in the house (I wasn't home...) and marveled at the boiler still running... THEN he went and started searching the internet for what it might be...

It's nice to have heat when the power is out.