Problems with Low Voltage Wiring

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Old 05-01-10, 08:18 PM
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Problems with Low Voltage Wiring

Ok, sorry in advance for the tidal wave of info, but I want you guys to have all the facts about the situation...

History:
We have a Heil oil furnace with a Heil straight-AC added on. Furnace is 36yrs old, compressor and coils are 3.5yrs old. Both have had seasonal maint every year. 2 weeks ago, the heat started short-cycling, without warming the house. My guess was a bad cap on the circ fan. Called service tech, he turns system on, fan humms, he spins fan and it runs fine. The diagnosis is a bad cap (I agree). While replacing the cap, he screws up the wiring and pops the fuse and breaker (the furnace is on a 20Amp CB but has a local 30Amp glass fuse, for some reason). Fuse replaced, CB reset, cap in place, system runs fine. Two days later, same problem. Tech comes back, says "whoops, bad fan motor too." I agree (the fan motor is 36yrs old). Replaced fan motor, tested by switching tstat to heat/cool/fan_on and it runs much better than old one.

Current problem:
It was hot the next few days and so we only ran the AC. Then it colled back down and we noticed that the heat runs all the time when the tstat is set to heat mode, whether its calling for heat or not. I replaced the tstat with a new digital one (we wanted to anyway) with the same results. I have checked the tstat functions disconnected from the base and it works perfectly. I checked the LV wiring in the unit and its wired correctly (I used to service HVAC equip). The problem is we're getting some strange voltages which are causing the heat to run all the time. Below is a description of the wiring path and my test results.

Wiring Path:
Transformer terminal 1 to tstat R;
tstat G to fan relay terminal 1;
fan relay terminal 2 to tranny terminal 2;
tstat Y to compressor 1;
compressor 2 to tranny 2;
tstat W to burner 1;
burner 2 to tstat 2;
(all of this makes sense, power gots to tstat, it swites power back down to relay terms 1, terms 2 go back to tranny... this is how it should be...)

Now the problem: the voltages (at the tstat base)
R-G = 35 (correct)
R-Y = 35 (correct)
R-W = 60 (WTF? should be 35)
G-Y = 0 (correct)
G-W = 35 (should be 0)
Y-W = 35 (should be 0)

Its obvious why the heat runs all the time when the tstat is in heat mode: the G-W are bridged and trigger both relays without ever touching R.

More interestingness: I reversed the LV terminals on the burner relay; in other words instead of W to Burner1 and Burner2 to Tranny2, I did W to Burner 2 and Burner 1 to tranny 2. Now I get the same voltages as above, except R-W is 0 volts and we get no heat no matter what, although the fan comes on properly when the tstat calls for heat.

So it looks like the burner relay has voltage of its own somehow? And I either get 60 or 0 volts, depending on how I phase it with the main tranny? I didnt think the burner should have voltage, I thought it was just a dumb relay like the fan and comp (not including comp protect circ).

Either way, does anyone know whats wrong here? I used to service HVAC, but never oil/gas, just elec and HP. I really appreciate any input and thanks for reading all this, I know its a novel!
 
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Old 05-02-10, 02:06 PM
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On your handler's low volt terminal strip that the stat wires hook to, do you show 24 volts from W to C(common?, even when the stat is not calling for heat?

And when the blower is blowing this heat, when you don't want it to - is actual heat being blown out the registers continously, without a reduction in heat temp? The reason I ask this, is to confirm that it is not simply residual heat left from the heat mode that the blower keeps pushing out. IOW -the house would get to 90 degrees inside if you did not intervene and shut it off?
 
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Old 05-02-10, 08:47 PM
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@ecman51

Thanks for taking the time to read all this and help me troubleshoot. In the original post I mentioned all the voltage readings at the tstat base, and they are the same at the block in the handler. The W-R (common hot) voltage is 60 volts, which is strange considering its a 24-volt tranny thats running around 30-35 volts between the terminals, and 15-17 to ground from each terminal. When I switch the tstat from "off" to "heat", room temp 74, set point 68, the oil burner and blower both engage, even though the relay in the tstat has not clicked in to connect the W and G to the common hot (R). The house will eventually become a new circle of hell if I let it run like that.

Edit: Sorry, I didnt think about your question enough. The voltage between the W and the common (Terminal 2 on the tranny, not labled C but what would usually be labled C) is 7 volts, which is the same as W-Ground...
 
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Old 05-03-10, 08:17 AM
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Everything is strange about those readings. 60 is odd. 35 is odd. 7 is odd.

Can you disconnect load wires from load side of transformer to only expose transformer load wires by themselves?, and test across those two wires?, to see what you have? I've never ran into anything that somehow causes 24-35 volts to become 60 volts at one location, and 7 at another location, if one is absolutely only testing across low voltage/common wires - and not getting mixed up and testing across say contactor-like relays that energize 120 volts.
 
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