L8124 always sensing a call for heat?

wobano

I have an L8124 Aquastat. I have radient heat, tankless coil, and 4 home heating zones with Taco zone vales (but only 1 circ). I am fairly well versed with the HI/LOW/DIFF settings, wiring, etc...and I am convinced I have a malfunction of some kind.

My LO setting seems to not function. The boiler is behaving as though there is always a call for heat (i.e. it ignores the LO setting and is always firing up to HI limit - as soon as the internal water temp drops to HI limit -10, it fires up again. I have disconnected ALL thermostats in the house and STILL this is occurring. So it is as though the TT contacts are receiving some voltage making the system 'think' there is a call for heat when there is not. I do not know how to test the TT terminals but I know the white wire that returns to the TT terminals (from the zone valves) is carrying 24v when a thermostat is on...0v when off/disconnected. If I throw a multimeter ACROSS the two TT terminals during a call for heat - I get erroneous readings...around 0.1volt. The white wire T terminal carries about 6 volts when I put the black multimeter line on a grounded metal surface.

Needless to say this is wasting oil.

Robert O'Brien

Is the relay pulled in? Does it drop out when a wire is removed from TT?

HVACNUT

With the wires off of TT, or no heat demand from any zone, the reading across TT should be 24v. If there is a heat demand, the reading should be 0v. Your 0.1v reading is basically 0.
How old is it? It's been known to happen. Short in the high limit circuit. I've also seen the L8124 lose the high limit completely and just cycle off the low side regardless of heat demand. 
I would recommend an upgrade, but since there's a tankless coil, there would be no benefit. 
You wrote "Needless to say this is wasting oil."
Everything about a boiler with a tankless coil and the need to maintain a minimum temperature is a waste of oil.

Jamie Hall

Your 0.1 V across T-T isn't erroneous; that's simply showing that the contacts on something connected to it are closed, or the two wires going from whatever to it are shorted. T-T isn't looking for a voltage; it's interested only in whether it's a closed circuit or an open circuit.

Question is -- what's closed? Most likely the aquastat -- but I would test before throwing parts at it.

Power off, boiler cool.

First, disconnect both wires at T-T and test between the two terminals. Should be an open circuit. If not, there's a problem in the control.

Second, put the wires back on there, but disconnect the other end of the two wires (probably at the aquastat). Test again across T-T. Again, should be an open circuit. If not, the wires are shorted.

Third, check across the two terminals to which those wires were connected. Should be an open circuit. If not, problem in the device to which the wires were connected.

wobano

Thanks a ton gentlemen...hopefully you can guide me a tad more...

Robert O'Brien said:

Is the relay pulled in? Does it drop out when a wire is removed from TT?

Yes, relay pulls in, and drops out if either wire is removed from TT

HVACNUT said:

With the wires off of TT, or no heat demand from any zone, the reading across TT should be 24v.

Jamie Hall said:

First, disconnect both wires at T-T and test between the two terminals. Should be an open circuit.

Aren't these two pieces of advice in contrast to one another? I assume HVACNUT's advice is with power-on, and Jamie's advice is with power-off? Then it makes sense.

RESULTS:
If I remove the two wires from TT, boiler powered on, I get 24v across TT. So, I then connected back the two wires to TT, and disconnected them from their other end.

On the other end, the green goes to the end-stop circuit which travels through the 4 TACO zones valves I have. The white goes to the voltage dropper (regulator?) that takes 120v down to 24v (this device also has a screw for the little red wires that goes OUT with 24v to the various thermostats in my house).

With the wires off the other end of the Aquastat, I STILL get 24v (boiler powered on). So the L8124 is good as far I understand your replies. With boiler powered OFF, I get 0v across TT.

So I this leads me here:

Jamie Hall said:

Third, check across the two terminals to which those wires were connected. Should be an open circuit. If not, problem in the device to which the wires were connected.

If I disconnect the green and white wire from the voltage dropper (see pic), and test across the wires (leading back to the aquastat) with boiler powered on, I get 24v, powered off, I get 0v.

Testing across the two screws in the voltage dropper, I get 24v (makes sense). But i dont know what else to test?

It still seems to me that there is some wonky votage getting to the Aquastat that makes it "think" there is a call for heat...even just a few volts...to close the relay.

Jamie Hall

All three tests I mentioned are to be done with the entire system powered off. As I mentioned, if there is a closed circuit -- for any reason -- between T and T, the control should cause the boiler to run. This is not a matter of voltage -- your voltage measurements are interesting, but not useful. If you have a multimeter, set it to read continuity or ohms, not volts. Turn the power off to the entire system. Now perform the three tests I mentioned.

The correct term for what you refer to as a voltage dropper is a transformer,

STEVEusaPA

If you disconnect T-T and the boiler runs when the temperature is above aquastat's LL setting, the aquastat isn't sensing temperature properly and/or needs to be replaced.
If that's not the case, and the 2 wires going to T-T (disconnected, power off to the transformer) are showing continuity (with no thermostats calling for heat), then something is wrong with the wiring (short) and/or, one of the zone valves end switches, or a thermostat and/or it's base.

EBEBRATT-Ed

@wobano

What @STEVEusaPA said

Sounds like you have a zone valve end switch stuck closed.

You need to find which zone is keeping the boiler running. Could be the thermostat for that zone, bad zone valve or bad end switch. Find the zone that is keeping t & T closed and it will lead you to the problem

Shane_2

Did you switch any of the 4 thermostats to a smart thermostat recently?

Ctoilman

Bad zone valve.  With power off see if terminals 2 and 3 have continuity.....none should.

HVACNUT

It's a bad aquastat. And I'm stealing "voltage dropper".

wobano

Thanks gents! The hunt continues with odd results....

Ctoilman said:

Bad zone valve.  With power off see if terminals 2 and 3 have continuity.....none should.

ALL did! Power off to the whole system, all home thermostats OFF...and I found that ALL my 4 Taco zone valves had continuity between terminals 2 and 3.

Shane_2 said:

Did you switch any of the 4 thermostats to a smart thermostat recently?

Negative

STEVEusaPA said:

If you disconnect T-T and the boiler runs when the temperature is above aquastat's LL setting, the aquastat isn't sensing temperature properly and/or needs to be replaced.
If that's not the case, and the 2 wires going to T-T (disconnected, power off to the transformer) are showing continuity (with no thermostats calling for heat), then something is wrong with the wiring (short) and/or, one of the zone valves end switches, or a thermostat and/or it's base.

It's the second case, here...with power off, TT disconnected, the 2 wires off of TT are showing continuity.

So we are closer...so now how to i go about isolating the wiring issue? Again, all 4 zone valves showed continuity between 2 + 3.

Many thanks everyone - VERY informative so far.

STEVEusaPA

Just to be clear, you are checking the 2 wires that were on T-T, not T-T on the aquastat?
I'd make sure there was no power to the transformer. Sometimes that's on it's own power.
The only way all 2's and 3's would show continuity (with no power on the transformer) is a short. Look for a thermostat wire with a pinched staple, or laying/touching something hot.
I'd take all the 2's and 3's off and apart until you break continuity. If you can't, you might have to start re-wiring.

Jamie Hall

The zone valves are -- or rather I should say should be -- wired in parallel. Therefore, if you leave the wiring connected and any one of them is closed, all of them will show continuity. The only way you can check them is individually with at least one of the wires -- preferably 3 (2 is common to the end switch and the motor) -- disconnected.

If, after checking the zone switches that way, individually, and you wire them all back up and still find continuity, you then know that it's something in the wiring -- such as a short in any one of the zone valve leads. I rather hope not, as the only way to track that down is to seperate the zone valve leads from each other and from whatever they connect to, and check each lead pair individually and then whatever it is they connect to.

wobano

STEVEusaPA said:

I'd take all the 2's and 3's off and apart until you break continuity. If you can't, you might have to start re-wiring.

FIXED! I literally just removed the first Taco zone valve head (1st of the 4), and as I was about to disconnect it at 2+3...suddenly the continuity went away. I put the zone head back on and low and behold - once i powered everything back on, the burner is now behaving correctly (firing as it should with LOW/HI aquastat control. All 4 zone heads no longer have 2+3 continuity...

Some sort of short with this zone head not being properly placed on?

wobano

To clarify (in case it helps someone else) - when I removed this one zone valve head, I was holding it in my hand (still wired up) and moved the manual override lever fully down and then back up. This "might" have disengaged the sticky end stop switch - or maybe something else mechanical in there that had gotten stuck or jammed?

STEVEusaPA

It could've been hanging up in the body or the end switch. If it does it again, power off to transformer, switch heads with another zone. Then if it happens with the same head, it's the head. If it happens again to the original zone, it's the body. That's a non sophisticated way to determine if it's the head or the body.
BTW, doesn't completely rule out a short. You may have disturbed (fixed) it by moving stuff around. May act up again when things get hot.

EdTheHeaterMan

Also, you should not remove the Zone Valve actuator with power applied to terminals 1 and 2. Without the spring tension of the valve (the part connected to the pipes), the wax inside the element may expand past its designed stopping point and leak out. (wax expands inside a piston in the actuator to force the valve open using hydraulic pressure) If that happens you can throw that actuator away. AND... It says so in the instructions, just because!


If you look at the diagram you will see the internal electrical circuit of the T T terminals in the upper right corner, and understand why you should get a voltage reading with no call for heat, and "no voltage" or 0.0 reading with a call for heat.

Also, consider that the call for heat on your control comes from end switches in the actuator of each zone valve. unlike the diagram that shows only one zone where the thermostat is connected directly to the L8124

EdTheHeaterMan

For further clarification, I broke the circuits down to make them easier to follow. This is because Taco
Zone Valves have a back for letting the smoke out of transformers Voltage droppers. I like that term too @HVACNUT

Sometimes Professional plumbers gett his wrong ... so don't feel as though are any less able to solve this.

Take each portion of the circuit by itself one at a time.

First is the internal world of the zone valve actuator The Blue circuit "REPLACES" the thermostat connection to the T T terminals on the L8124.
The Red circuit is the thermostat to open the zone valve You can clearly see that there are 2 transformers in that diagram one for the valves and one inside the control.

As long as you wire each valve in the same exact manner, you won't go wrong. ! always goes to the thermostat and the rest will fall in place.

The diagram shows the other valves. See how all the # 1 terminals go to the thermostat on each zone?

All the #2 terminals are connected to each other and the common side of the Transformer

All the other guys are giving you great advice. I also wanted to share this because sometimes a second explanation makes the light bulb ignite in the brain. I took the liberty of remaining the transformer on the diagrams because I really like that name.

I hope this helps with your diagnosis

All the best.

Yours Truly,
Mr.Ed

STEVEusaPA

I thought the problem was solved

EdTheHeaterMan

STEVEusaPA said:

I thought the problem was solved

Looks like he was still clarifying earlier today. And I did not like the fact that the zone valve actuator was not on the valve and the power was still connected.

Also, I wanted to type "Voltage Dropper" on the diagram