My house has a very early high-efficiency furnace, sold by Sears in the early '80s. We do not have central air. The furnace operates the way it always has over the 5 years we've owned the house, but a couple of friends recently pointed out that it's not acting as it should. I'm trying to narrow down the cause(s).

First, the blower kicks on instantly when the thermostat turns on the furnace. I've been told that the blower should not come on until after the burners have started to warm the heat exchanger, but this is not what happens; blower start-up is instantaneous.

While the furnace is operating, it always seems as if the airspeed in the ducts is faster/louder than it should be. The wiring is set up so that the blower should be running on low speed, but it sure is moving a lot of air. It's a 50k btu furnace in a small house (600 square foot first floor, no heat upstairs) and has only four 6" ducts coming off of it, so perhaps it simply moves more air than it should for so few ducts, even on low speed.

Lastly, when the thermostat shuts the furnace off, the blower shuts off instantaneously too, rather than continuing to operate until the exchanger cools off as I understand it should. There is always a several-second delay of silence, then the blower (but not the burners) kicks back on for something like 15-30 seconds -- I assume this is the limit switch doing its job.

So there you go. Yesterday I made a copy of the wiring diagram and traced every wire on it, crossing off each as I went. All of the connections inside the furnace are as they should be. The only discrepancy between the diagram and what I actually have is the way the thermostat is hooked up. The diagram shows a 3-wire connection to the thermostat; red, white and green. but it appears that the installer used the old 2-lead wire that was already threaded through the walls up to the thermostat; the red and white are correct, the green was omitted. I don't know what that third wire is for, so I don't know whether it's relevant. Should I be looking at the fan relay? The limit switch? Can anyone point me in the right direction?

labman

Your are right, the blower should come on at low speed after the the furnace heats, and stay on until it cools. The red/white pair is enough to control the heat. The green is for the fan only option and is high speed. It sounds to me like the white wire is somehow also connected to the G terminal at the furnace. Thus, the blower comes on high speed as soon as the thermostat calls for heat. Sounds like something, maybe even on the control board is jumpered that shouldn't be. There should be 2 relays, one for each speed. If you can identify the high speed one, you might try disconnecting it and see if that solves the problem.

jon1270

Thanks, labman.

I found the installation manual, left behind in the house by the previous owner. It contains a note, in the section that covers the limit switch, saying that "the fan comes on as soon as the thermostat calls for heat," so perhaps that is actually working the way it should. I can't see that the thermostat connection is shorted to the green terminal, and I didn't find any jumpers that weren't on the factory diagram.

There's only one "fan relay" on the diagram, though it's got eight terminals and has wires going to both the high and low speed taps on the blower so it's probably, actually, multiple relays cast into the same hunk of plastic. I went ahead and removed the violet lead from the relay, which is the wire meant to work with the nonexistent AC, and that made no difference in the fan speed. I'm thinking that the real reason the fan seems overactive has more to do with the limited ductwork that is connected to the furnace. There are only four ducts, making use of the original gravity-furnace registers in this 1930s house.

I recently had to engineer the dust collection system for my little woodshop, so I now know just enough about flow rates and static pressure to be dangerous. Dust collection, though, deals with much higher volumes and speeds. Anyhow, I looked in the technical support manual for the furnace and it has blower performance data for static pressures of .1 to .5 inches W.C. I don't know what pressure HVAC guys usually aim to hit, but even if this was on the high end of the range described in the manual, at 0.5 in W.C., the blower would be moving 690 CFM, or 172.5 CFM through each of the four 6" ducts, requiring an airspeed of 879 feet per second. Does this sound high for HVAC? Could it be that the static pressure is significantly higher than it should be, cutting down the CFMs and generating all the noise?

labman

I just don't have enough experience to say what the norms are for HVAC. Too small of ducts do make for higher air speed and more noise. Likely the old gravity system had huge ducts. There should have been plenty of room for larger ducts.

When the blower comes back on after the heat goes off, is it the same high speed, or a gentler speed? The 8 terminal relay sounds like 2 regular ones cast into the same plastic block. 2 of the 4 coil terminals should be connected to the common side of the transformer. The high speed one should be connected to the G terminal and the low speed one to the limit switch. 2 of the 4 power terminals should be connected to the power feed. The other 2 each to one blower coil. Is the violet the high speed blower coil? If not, you might try removing the lead to the high speed coil and see what happens.

jon1270

Blower speed is always the same. As far as I can see, the relay is correctly wired. Yes, the violet wire goes to the high speed tap.

Thanks so much for the thoughts.