Main blower won't stop running

[deltasmith]

I recently moved into a rental home where the main ventilation blower runs 24/7. I asked the landlord about this and he thought that this behavior was by design. It's not a design that I appreciate (when the furnace cuts out, the vents blow air that feels uncomfortably cool because the fan keeps running; the incessant fan has to be hurting my electric bill; I wake up in the morning with chapped lips, etc), but he might be right - maybe the behavior is by design - I'm not familiar with this kind of system so was hoping for some insight from someone who is.

This is a combination gas furnace + AC system, both served by a single ventilation fan. The furnace is a Ruud UGDG 10NBRJR and the l0gic board appears to be Utec 1012-831. I wish I could find a manual for one or both.

Here's what I've discovered on my own so far:

• The system has two limit switches: both have continuity (closed) when the system is cool. Disconnecting a lead from either limit switch has no effect: the main blower still runs as soon as I close the door switch, even without the thermostat calling for heat.
• There are four different speed taps to the blower (red, yellow, blue, black). There are two relays on the logic board. One relay has no wires attached, the other has black attached to a terminal labeled "cool" and yellow attached to terminal labeled "heat".
• If I disconnect the "cool"/black connection, the blower almost behaves the way I would hope for, but not quite. That is to say: it doesn't run when the thermostat isn't calling for heat, it starts running 10 or 20 seconds after the burners ignite, but after the furnace gets up to some limit temperature, the burners AND the blower cut out with this configuration (the draft inducer keeps running for a few minutes though, then the blower runs and blows cool air, then it stops and the igniter comes on, then the gas burner ignites and the cycle repeats).
• If I leave cool/black connected and disconnect heat/yellow, the blower still runs even when I don't want it to (when there is no call for heat from the thermostat).
• With both cool and heat wires connected to the blower motor in the way I first found them, if I disconnect all of the thermostat wires from the logic board (approximately six or seven screw terminals), the blower still runs as soon as I bump the door switch.

I'm wondering if the system is deliberately wired to always run the fan, and if I could change the behavior by moving some of the fan wires from their current location on relay #1 to one of the unused terminals on relay #2? I'm not comfortable trying this experiment without a deeper understanding of the electrical design and with no schematics or manuals available from Rheem/Ruud, I'm at a bit of a loss. As far as the landlord is concerned, the system is behaving the way it is designed so I have nothing legitimate to complain about. I'd like to keep a good relationship with my landlord, but would also like to find a way to persuade him that this is not reasonable behavior and he should have the system addressed in some way. Any insights appreciated.

 

[DonL]

I did not see a model number for your T-Stat.

It may be wired wrong. Or not set for your system type. Check the Dip switch on the board.


Good Luck.