[timaishu]

Hello,

My moms house has an original day&night ac/furnace system and it as an issue where the blower fan doesn't turn off. It continues to run when the thermostat is set to off, and when the set temperature for the heater or the ac is reached.

From my research, it sounds like its a relay or maybe the thermostat? Hoping you guys can point me in the right direction. I feel this should be fairly easy as its not 100% of the time. Sometimes it works properly. Its usually either working properly and continues to work properly for a good period of time, or it works incorrectly for a period of time.

Here are some pictures of what I have.

 

[user_12345a]

That's an old furnace, 1980s, possibly even 70s. Please get it thoroughly inspected and checked. heat exchanger likely cracked by now. don't like the flex gas line into the cabinet, but it may be legal where you are.

The refrigerant line could use some insulation.

The most likely cause is a stuck heating fan relay. Could also be a short between R and G in the thermostat cable - disconnect G wire at furnace to rule that out.

Most furnaces from this era use mechanical controls, yours has a circuit board.

Check blower access panel for a schematic.

 

[timaishu]

That's an old furnace, 1980s, possibly even 70s. Please get it thoroughly inspected and checked. heat exchanger likely cracked by now. don't like the flex gas line into the cabinet, but it may be legal where you are.

The refrigerant line could use some insulation.

The most likely cause is a stuck heating fan relay. Could also be a short between R and G in the thermostat cable - disconnect G wire at furnace to rule that out.

Most furnaces from this era use mechanical controls, yours has a circuit board.

Check blower access panel for a schematic.

Yeah it used to have a mechanical thermostat, but it was upgraded at some point around the year 2000 I believe.

 

[user_12345a]

If you disconnect the wire connected to G on the circuit board, tape it off, restore power and the furnace fan still runs, you know the problem is inside the furnace itself.

 

[timaishu]

If you disconnect the wire connected to G on the circuit board, tape it off, restore power and the furnace fan still runs, you know the problem is inside the furnace itself.

Thanks. Test complete, unplugged the ground at the thermostat and the blower turned on automatically as it was.

 

[user_12345a]

it's not a ground. you have to do it at the furnace to rule out a short in the wiring.

You'll have to get to the board to do it, manual shows it enclosed in a metal box, blower compartment.

If that doesn't work, disconnect the blower lead connected to LO on the board, re-power the furnace and see if it still runs. The board is very likely to be bad.

Based on the serial it could very well be a 1979 and finding a oem. replacement board if it's that may be a challenge. May be able to find an aftermarket board by icm or something. This furnace is only 60-70% efficient so unless in a warm climate*, replacement may be the best option given that repairs are needed.

*I suspect that you are because of the heating capacity of this furnace relative to a/c coil size.

 

[roughneck]

That's not actually a ground, it's a 24V supply back to the unit to turn the blower on and off.
Disconnect the thermostat at the unit and see if the blower still turns on. If so it's something to do with the unit itself. Something that old probably has a fan/limit control you'll have to troubleshoot.

 

[timaishu]

Thanks guys. Yes, I live in southern California. The heater is rarely used, mostly just AC. Though its been a cold December, so we have been using the heater for the last few days and it heats up the house quite nicely.

I am having trouble finding where the wires from the thermostat actually come in the cabinet. I would guess I will find it under the lower panel but its stuck in place.

 

[timaishu]

Okay nevermind, grabbed a big screw driver and pried it out. I found all the wiring and what looks to be the blower.

 

[user_12345a]

i think there's a electrical box that you have to open to get to the board. check for a filter too.

Something that old probably has a fan/limit control you'll have to troubleshoot.

No fan/limit, its got a timer board and limit. likely a bad board.

 

[timaishu]

Sorry for spamming my own thread, but I cannot find any edit button for my post..

I unlocked the green wire from the unit, the one labeled GC. And the blower still turned on.

Here are some pictures.

 

[user_12345a]

Disconnect the wire connected to the lo terminal on the board, restore power.

You're likely in need of a new board. If you can find one it may set you back $100+, but cheaper than letting the fan run all the time.

relay sticking or issue in the circuitry. it's designed so that the heat fan relay is normally closed, that way the furnace doesn't overheat if the timing circuit fails.

 

[timaishu]

Here it is:

I feel you are right. There is a part number on it, 302075-302

Might be best to try and find a replacement board and swap it out.

 

[user_12345a]

did you try disconnecting the lo wire?


icm271 seems to be the available replacement. the only oem carrier stuff seems to be used on ebay.

found this just by searching; http://www.air1supply.com/302075-304

Before you go buying parts look up the model of the furnace and see if the heat exchanger is prone to splitting in a certain area. if u find nothing online shine a flashlight into the cells from the burner openings, pull the blower, crawl into the blower compartment and take a good look at the exchanger for cracks.

The blower is likely to need cleaning anyhow.

Not comfortable doing this? it may be time to call someone out to look this entire furnace over before throwing parts at it. this thing is old, make sure it's still safe.

 

[timaishu]

Yes, unplugging the lo wire caused the blower to not turn on.

 

[user_12345a]

so change the board after verifying that it's safe. try to find it online or better yet at a local place that sells to anyone. (small appliance repair companies that also do some hvac will be more likely to sell to any DIY're)

edit:

looking at the pictures again, seems you disconnected the wrong t-stat wire.

one of the Gs is jumped to r on the board, it's the other G that deals with the fan. See the j1 between R and Gh?

 

[timaishu]

so change the board after verifying that it's safe. try to find it online or better yet at a local place that sells to anyone. (small appliance repair companies that also do some hvac will be more likely to sell to any DIY're)

edit:

looking at the pictures again, seems you disconnected the wrong t-stat wire.

one of the Gs is jumped to r on the board, it's the other G that deals with the fan. See the j1 between R and Gh?

The one I disconnected is the green wire on GC, was that correct or no?

 

[user_12345a]

yes.

.,

 

[a.man1302]

The replacement for that board will cost u right around $100... heat fan relay tends to get stuck... I have had to replace 3 in the last 2 weeks.

 

[user_12345a]

The replacement for that board will cost u right around $100... heat fan relay tends to get stuck... I have had to replace 3 in the last 2 weeks.

do you still see a lot of these furnaces?