I've been able to fix a few furnaces based on the trouble code thing. Pretty handy. This one has me stymied. Will not respond to a 'turn on and heat, GDit' command. Displayed no trouble code, did the off-on move on the toggle power switch, the code it showed was the 'no trouble codes to report.' In the past I've seen a code of 'low voltage fuse out.' There's a 3 amp, light blue spade fust that protects the 24V DC operationg system. It looks good, I think I'll buy a new one and swap it anyway.

Got ahead of myself I bit, I would swap it as no current is going up to the thermostat. The existing thermostat looked old, that style hasn't been sold in years, I've had luck in the past just putting in a new one. I did so - nothing. No display came up even. So I checked for current - none. The red wire should show 24v when tested with the green wire. It did not, I tried every other combination just in case - no voltage. Holy crap I want to fix this. The lady of the house, gorgeous old home in Bezerkletown, insists on paying me $75 an hour.

I checked the circuit breakers of course, AC is getting to the unit. The trouble code lights did light up as they were supposed to when I toggled the switch.

__________________
1986 300SDL, 362K
1984 300D, 138K

Where did you check for current for the thermostat, at the furnace, where the thermostat wiring begins its run, or at the thermostat? If the latter, I would check at the furnace as well, in order to ensure that the wiring to the thermostat didn’t become a rodent appetizer somewhere along its run...

Is there just one thermostat?

You're testing the system wrong.

HVAC is 24V AC. Not DC. If you have your meter on DC, you won't read anything. Depending on how the control board is set up, you may not read anything anyway by reading from Red to any of the colored wires.

The Red, Green, White, and Yellow wires are what control various functions. Your power "Feed" is the red wire. Shorting it to Green turns on the fan. Shorting it to Yellow turns on the A/C compressor. Shorting it to White turns on the heat.

You say the A/C works. That rules out the transformer and power wiring running to the thermostat.

Try jumpering red to white at the control board and see if the heat comes on. If it doesn't, you have a fault in the control board, probably broken solder joints on the back of the circuit board. If it does, remove the jumper and install it between red and white at the thermostat base. The heat should come on like it did when you jumper it at the board. If it doesn't, the wiring is bad between the furnace and the thermostat. If it does, the thermostat is bad.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Both replies sound like useful advice, thanks. I can check it tomorrow or the next day. Good thing for balmy weather. I had read that some furnaces are 24ac and others 24dc. I know the fuse looks like a DC fuse - spade variety - but what do I know, might work for AC as well.

Low v ac weirds me out. I have a little Garder-Bender multimeter I like quite a bit - has two ac settings, like many - 500 and 200. I wasn't sure how to ground it so I tried the neutral wire in an extension cord - works well on light boxes when there's no neutral and you're looking for which is hot - old wiring, not color coded. No ac reading that way also checked for ac on every two wire combo, I tried every combination in dc as well (no extension cord), - nothing. One ass-u-mes that 24ac would show on the 200 volt scale but ... I know red is supposed to be live wire but I also gather that one never knows.

When I said AC is getting to the unit, that was poorly worded. I should have said 120. This house has no AC, the yellow wire is a stubby. Not sure if you thought I meant air conditioning. I'm not sure if AC in caps is for cold air making while ac in lower case is current, might be able to find the etiquette on that.

__________________
1986 300SDL, 362K
1984 300D, 138K

The latter. I agree, I should check at the furnace. I love finding an obvious source of a problem.

You know, I didn't ask if there was another, and it is a two story house. I guess I assumed the lady would tell me. Will inquire.

__________________
1986 300SDL, 362K
1984 300D, 138K

I hate (really hate) to admit that I knew that once and forgot about it. True enough, if I short 'em and get heat that would tell me something.

__________________
1986 300SDL, 362K
1984 300D, 138K

Since no A/C in the house (BTW, electricity at a source is usually referred to as "power" or spelled out as "240V" or such to eliminate confusion with Air conditioning), your wiring should be blissfully simple at the thermostat. Red, White, Green.

Red is 24V, White is Heat, Green is Fan. Unless the common wire was brought up to the thermostat, you can't test voltage at the thermostat base. You can short the wires together to turn the system on.

The Automotive fuse in the 24V circuit was chosen because ATC fuses are rated at 32V and already have the 1/4" spade terminals to make attaching standard wires to them super easy. Don't mistake that as a DC source. *ALL* HVAC wiring is AC voltage, with the exception of communication wires for inverter systems or power-pile wiring from old "millivolt" gas systems.

Start with the basics. Jumper out the red and white terminals at the control board and see if the system fires up. If it does, you know you have thermostat or wiring issues. If it doesn't, you have furnace issues.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Yes, you are correct. Red, white, and green. The blue and yellow were stubbies.

Also good to know about 24ac. Do you know about detecting it with a multi meter? You would think it would show on the 200 volt setting, the lowest on this one.

__________________
1986 300SDL, 362K
1984 300D, 138K

You have to have a complete circuit to read voltage. If there is no "Common" wire at the thermostat, there's not a way to read the AC voltage present. You can do it between R and C on the control board.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Going there today, I'll try that.

__________________
1986 300SDL, 362K
1984 300D, 138K

Saints be praised, I finally did get 24, 25 vAC between red/green, red/white.

I learned something useful: the furnace trouble code does not alert to faulty condensate pump. It was full to the brim, I saw a black and white thermostat looking wire pair leading away from it, sort of indicating a shut off switch. Drained it and I was back in business. I had found a vid alerting me to that possibility.

Only makes sense it's not in the trouble code readout - the pump is not a component of the furnace, it's an occasional accessory. This house has on small basement section with concrete floor and 7 foot celing height. A large door gives access to the rest of the crawl space, the furnace is in the first room off from that. I have to wonder if the condensate would drain into the earth just fine there. But oh well, I'm guessing I should replace it just because.

The lady of the house paid me well FWIW.

__________________
1986 300SDL, 362K
1984 300D, 138K

Most condensate pumps have a safety switch to shut off the furnace/air handler if they overfill. Typically, the red wire is broken going to the thermostat so that the whole system goes down if the pump overfills.

If the pump is overfilled and it wasn't caused by being unplugged, it should be replaced. They don't last forever. You might want to put a note on the furnace door to check the pump if the furnace seems dead, could save you a call-back.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I think I will get a new one. I'm telling you though, if it was my house I'd let it drain into the dirt for a while and see what it's like. I'm guessing a pint to a quart a month max. It pumps the stuff outside to generally wetter dirt on the same level, about 10 feet away.

Oh well, life is funny. I appreciate your help here, as in many matters (plumbing and electric) I'm not a journeyman furnace guy but I can do large parts of the work. Just need to know my limitations and the important saftey items in the code.

My specialty was finish carpentry and I actually do that work now and then.

__________________
1986 300SDL, 362K
1984 300D, 138K

You guess wrong. Do the stoichiometry on the fuel to combustion product water. I did this decades ago .... IIRC it was about gallon of water to gallon of LP. Perhaps more. O being 16 to C being 12. I never encountered any pump, but the gravity drain line has to be clear and sloped or the unit senses the back up (via pressure) and shuts down to prevent asphyxiating the inhabitants. That water you want to evap on the ground is at saturation with the combustion products CO2 aplenty and some CO . that woman would do well to spend her $ on someone qualified.

^^^This.

Depending on the runtime and BTU rating, that furnace can produce several gallons of condensate per day. Not only is that condensate laden with combustion products, but it's acidic as a result of the dissolved CO2. Once the water evaporates away, that stuff is released back into the atmosphere where it will be recirculated through the furnace and blown into the house.

Not only that, but the water will raise the humidity in that enclosed room, possibly to the point of encouraging mold growth. Did I mention that moist soils encourage the release of Radon gas?

That condensate pump is there for a reason. It's required by code anywhere that the furnace cannot gravity drain outside the structure. It isn't legal to drain into sump pits anymore either for the same reasons. Replace the pump, they're not expensive at Home Depot and the like and should hold up an easy 5-10 years if not longer.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)