I could really use some help!!

A couple of weeks ago out nest thermostat read, "e24, no power from Rh". We got our nest last summer and has worked both heating and cooling. Suddenly, and perhaps more that coincidentally, it stopped working when we changed it from heat to cool.

I have a 1989 Lennox G12 gas furnace. It doesn't have a circuit board, but more a terminal with R W Y G T terminals. R W and G have 1 wire each input. Y has 0 input. T has 2 input wires. R W Y and G are all output to the thermostat. The Y and T terminals output to the outside unit.

The first thing I tried, after much research, was to connect a second output wire to the T terminal as a "common wire" and connected it with the thermostat. The thermostat had power and fan would come on, but not the outside unit.

At some point along the way the transformer in the furnace blew so I replaced it.

Right now I can get:
R-Y - 26v
R-G - 26v
R-B - 26v
R-W - 13v

these readings are for at the thermostat and at the terminals.

Another problem is when I connect the blue wire as the common wire, I can only get 13v to the outside unit. However, if I don't connect the blue wire as the common wire, I get the, "e24" error again and the thermostat says it doesn't have power (Voc~8).

I have replaced the contactor, capacitor, and fan capacitor.

When I have the common wire attached and the thermostat has power, I can manually press the contactor and the outside unit starts up and blows cold air.

The problem is for some reason I cannot get power to both my thermostat and 24v to energize the contactor at the same time.

Any ideas would be appreciated!

Thanks!

 

Any pics of the wiring Dia?does the heat work now? was it all working before you replaced the transformer?
Geo

 

Thanks for the response.

Before I got the error, everything was working fine. No problems. When I first got the error, the thermostat did not have power, so nothing would work. If I took out the Y wire from the thermostat the heat would come on, but not the AC.

I was working with the wiring at the thermostat with the power still on (oops) and saw a spark and heard a pop. Then there was a burning smell. I go no volts to the thermostat anymore. I burned up the transformer, but got a new one and restored power.

I didn't test the voltage at the thermostat right after the error, so I'm not sure what it read before the transformer blew.

Here is a picture of the wiring as it was before I changed anything...

 

OK,

So it seems that everything works inside and out if I jump R-Y and R-G like the picture illustrates.

The problem is I am only getting 13 volts R-W...thermostat and at the terminal. And that's where my thermostat (Nest) gets power from.

Ideas?

 

Will the heat run when you jump R-W at the stat? Do you have a C terminal connected at both ends?
That is what should be powering the Nest,check between R-C for 24 volts, when you are reading 13 volts the stat must be calling for heat.
Geo

 

First , turn the power off . Check and / or re terminate the Tstat wiring . Disconnect and document the wiring going to the outside condenser unit . Disconnect and document the wiring at the Tstat .

Make sure none of the bare ends of the low voltage wiring is touching anything . Turn the power back on .

At the Tstat location , test for 24 VAC from Common to Red .

If good ( 24 VAC ) , terminate the Red , Green and Yellow Tstat wires together . Did the furnace fan come on ?

At the furnace , measure from Common to Yellow wire or terminal . Did you get 24 VAC ?

Turn off the power , connect the Common from the condenser and the Yellow to the condenser , to the furnace wiring . ( To the common and Yellow , at the furnace . )

Turn the power back on . Did the furnace fan restart ? Did the outside condenser unit start ? Are you starting to get cool air ?

Let us know when you get to this point .

Personally , I am still running a mercury bulb Tstat & I am quite happy with it . And have no plans to replace it . If it ain't broke , do not fix it .

God bless
Wyr

 

You can't test the system voltages like you are since you are measuring the voltage drop thru the nest.

The thermostat steals voltage from the A/C and heating systems.
When in heat mode and calling for heat there will be 0 volts between R and W.
When not calling for heat there should be something between R and W. That voltage can vary depending on the charge state of the thermostat. 13v could be normal.

In my opinion..... the nest should be used with the Common wire connected.

 

It look's like you have a spare blue wire in that Tstat cable ,connect it to the C on the stat and the common at the furnace.the measure between R-C at the stat ,it should be 24VAC.
Geo

 

Thank you for all the replies!

First of all, the furnace does not have a Common terminal. It has R W Y G and T. The T is going to the outside unit contactor.

I have tried connecting the blue wire to the T at the furnace and C at the thermostat. This allows the thermostat to work and the fan turns on, but it steals(???) voltage from the contactor and it will not have enough voltage left to pull the contact, so the AC will not come on outside.

Right now I've got the R-Y and R-G and it will run the fan and AC. Getting cold air with that.

I do not get heat when I jump R-W.

I have also connected to a new thermostat, so it's something other than the thermostat at this point.

Thanks again for the replies!

 

It looks like Lennox still called 24VAC common a T terminal 2 generations after the G12.





Is your new transformer a 40VA transformer?

 

T is the common for the control circuit,Houston has a good point on VA size of transformer, with T connected to C at the Tstat and with the wires disconnected at the condenser Contactor and a call for cooling what voltage do you get between those 2 conductors?try the same between C -W at the control board with a call for heat.
Geo

 

T=C Thanks!

The transformer is 40Va.

I'm getting 13v at the contactor (disconnected) when calling for cooling.

Getting 27v at the circuit C-W when calling for heating. The furnace/fan did start up.

Fan comes on in both cases, but not getting enough volts (13v) to the contactor.

It seems that when I attach the common wire to the thermostat it is stealing ~13v out to the contactor (which comes from the same control terminal).

 

If you eliminate the Tsat wiring from the control board jumper Y,G,R will the cooling run?and try the same for heating W,R,beware there will be a delay before the blower starts in the heat mode,does anything at all happen ie? purge motor start?
Geo

 

Both cool and heat work completely when jumpered at the circuit board.

 

I would make these connections at the thermostat location next to prove it isn't the stat wire. (But it sounds like a stat problem)

 

They work at the thermostat too, but only if the C wire is not connected. If the C wire is connected, the contactor does not have enough voltage to make the connection.

Neither thermostat (nest or previous digital) I have had has had a common wire connected. For some reason neither thermostat will work without the common wire now.

The T circuit terminal only had a wire going out to the contactor and it has ~26 volts and works, but the thermostat doesn't work. When I connect the C wire to the thermostat and to the T terminal the themostat works and the fan starts when cooling is called, but the contactor only has 13v and will not connect so the AC does not start.

The problem I'm having at this point is getting the thermostat to work and enough voltage to the contactor.

 

In post # 16 the common should not be used in that test anyway,This may be a stretch but is it possible you have 2 transformers somehow wired in? What voltage do you get between R&C at the stat?What is the digital stat you have tried?
Geo

 

Just one transformer

R-C at thermostat is ~27v

other thermostat
Honeywell 1-Week Programmable Thermostat-RTH221B - The Home Depot

 

There is no reason that I can see why the digital won't work as there is no C connection required,you did install fresh batteries.

 

What is the make and model of the outdoor unit?

Some pictures of the wire connections outside and inside of the furnace control box may help reveal the problem.

 

Will the Nest power up with only R&C connected to it?
Geo