Wrong voltage from Carlin 40200 primary control to t-stat


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Old 01-18-13, 03:42 PM
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Wrong voltage from Carlin 40200 primary control to t-stat

The system:Crown KSZ125 oil fired steam boiler
Carlin EZ-1HP oil burner
Carlin P98022 Burner motor
Carlin 40200-02 Primary control http://www.carlincombustion.com/products/40200.htm
For reference, top to bottom in the linked schematic, the contacts are T1, T2, F1, F2
Heat only, single pipe steam radiators, no AC

Current T-stat: Honeywell, unknown model #, looks similar to T86116 2002
Wires: 3 conductor, 18 AWG, red, white & green

Current wiring: White Ė unused both ends
Green goes from T1 on primary control to W on t-stat ( I know, Iíve very shockingly discovered the previous owner wasnít much for color codes on previous projects)
Red goes from T2 on primary control to R on t-stat
On the t-stat, Rc is jumpered to R
There is also a pair of 16 AWG wires going from T1 and T2 on the primary control to a Honeywell 4006A aquastat on the boiler

Basically what is happening is ever since I bought the house, I havenít been able to program the thermostat. I can set the temp up or down manually but if I try to do anything else, the thermostat goes dark, nothing on the display. Left alone for several minutes it will eventually come back to life. I tried replacing the batteries, 3 AA, but no dice, still happens. I assumed the t-stat was going bad and bought myself a Nest 2.0 to replace it. Wired up the Nest (Red to Rh, Green to W1). It connected to wifi just fine, updated all its software, detected the wires properly, then spit out error code E24 (no power to Rh wire detected). I followed thru their troubleshooting and ended up with a multimeter taking voltage (wires unplugged at t-stat) across red and green for 14Vac. At one point it dropped to just north of zero (I was still on the contacts) then came back to 14Vac after maybe 30 seconds. I tried to measure the same voltage across T1 and T2 downstairs (with both the nest installed but not functioning and the old t-stat reinstalled and operating just as before) and it just jumps constantly, the only numbers I can make out as it goes through are 30V, 4V, 0.1V and OL. Iíve noted from the wiring diagram for the primary control that these 2 pins should be putting out 24Vac to the t-stat. As a verification, T1 to ground and T2 to ground both read 7.4Vac, so that confirms the 14Vac I saw upstairs. F1 to ground and F2 to ground each read 12Vac. F1 to F2 read 0Vac. T1 or T2 to F1 or F2 all read in the 28-29Vac range.

Any idea why I am getting 14Vac to the t-stat instead of the 24Vac I expect? Does it have something to do with the aquastat also pulling off those terminals? I was thinking maybe transformer problem but as best I can tell, the transformer is in the primary control. Is the primary control likely faulty? Do I maybe need a 24Vac common up to the thermostat and if so, where would I pick that up. Iím contemplating pulling a new t-stat wire through the wall to get rid of the really old one and rule out any possible shorts in the wire so getting more conductors for common or anything else needed won't be an issue.
 
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Old 01-18-13, 05:12 PM
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Do you also have a coil in this boiler to provide domestic hot water? If not, then is there a hot-water space heating loop in addition to the steam?

The 4006A aquastat is probably used to heat the water in the boiler for the domestic water heater coil. The wall thermostat is merely a switch in parallel with the aquastat switch. Since these are switches any reading across the T1 and T2 terminals are going to change depending on whether or not the two switches are open or closed. Quite literally, the voltage across these two terminals is almost meaningless.

The F1 and F2 terminals are for the flame scanner and should not be considered.

You MAY have a damaged transformer internal to the control and that would be the reason why you are experiencing a low voltage situation.
 
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Old 01-18-13, 05:34 PM
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Yes, the boiler also provides hot water as the inlet to a GE SE50M12AAH water heater. Personally I'd never seen a boiler used as a pre-heater for an electric water heater before this. Maybe not the most cost effective solution given the current oil prices vs. kWh prices, but it does get the job done.

So without knowing whether the t-stat and aquastat switches are open or closed, should I disconnect both to take a reading across T1 and T2? Without either switch attached I would think it should be the straight up 24Vac the schematic suggests unless something is wrong.

Based on the schematic, I didn't think touching F1 or F2 would be wise either but figured I'd throw all the info I had up here.
 
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Old 01-18-13, 07:43 PM
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Disconnect all the wires from T1 and T2. Check voltage from each terminal by itself to the case of the control. Be sure the meter is set for AC volts. One terminal to case should read about 24 volts and the other terminal to case should read close to zero volts. Don't be too alarmed if neither terminal to case voltage is proper as it could be the secondary of the control transformer is not "grounded".
 
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Old 01-19-13, 05:28 AM
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The best way for you to install the nest on this system is using a separate transformer and relay.
 
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Old 01-24-13, 07:07 PM
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So looking at a 24V down step transformer, there are 2 leads for 120V on one side (easy enough) and 2 for 24V on the other. How would this hook in to the nest? I'm guessing one 24V lead would go to C but not sure what if anything to do with the other. I would think it would need to go somewhere to complete the circuit
 
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Old 01-24-13, 07:23 PM
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Old 01-25-13, 04:34 AM
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Wow! Thanks for the quick response with the wiring diagram.

My only confusion is why the relay is required in this circuit. It looks like you've drawn in a SPST normally open relay. My understanding is SPST is basically used for electrical isolation between in this case, the t-stat circuitry and the primary control on the furnace. Why is that isolation required when it looks like from the 40200 schematic I linked to earlier, the T1 & T2 connections are also a 24V circuit?

Could I hook W directly to T2, and T1 to the common on the transformer in lieu of using a relay? Just thinking of keep it simple, and fewer parts used means fewer parts that can potentially fail in the future.
 
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Old 01-25-13, 05:17 PM
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The reason why is because you could potentially do harm to the aquastat if you do not use the relay. Take my advice or don't.... I only do this every day.
 
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Old 01-26-13, 09:02 AM
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No need to get defensive. After all, from the documentation/receipts left by the previous owner, the last guy who set up the system also only does this every day, and yet here I am with a thermostat that doesn't get sufficient power.

I just like to understand why I'm doing something before I do it, especially when electricity is involved. In my experience relays typically isolate systems working at different voltages, so I wasn't really thinking of using one like you set up to eliminate potential for surges, overvoltages etc. to delicate equipment.

I didn't intend to insult your expertise and I do appreciate the help you have provided. I am going to see what I can find for relays locally and hopefully have a fully operational thermostat again by the end of the weekend.
 
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Old 03-05-14, 11:54 AM
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Carlin 40200 and Nest 2.0

Mick,
I have the exact same setup and just purchased the nest... What was the final result? My nest works with w and r hooked up to t1 and t2 but doesn't charge properly and I het the same error message on the nest! I have a third wire hooked up to nothing running from my Carlin and to the thermostat... Both end are just capped.
Help and thank you in advance!
 
 

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