Ghost in the machine?

[xxpaulcpxx]

I've got a REALY old gas furnace - heat only style - it may be 40 years old. Yes, the heat exchanger is cracked, yes it will be replaced sometime soon. I've talked to this forum before about this unit - I was looking at what it would take to run this unit in "Fan Only" mode. The answer was that a "Fan Center" would have to be added. I even got a lead on where to get one. Never followed up on that yet.

Funny thing though... I noticed that the furnace it running - and it is only running the fan! I never noticed this last year as I has it unplugged throughout the summer. It seems that now on a hot day it will kick on by itself. It only has two wires to the thermostat, and it is turned off while it is running.

I'm thinking that it is hot enough up there to fool it into thinking that gas is heating up the exchanger. Could this be a feature? How hot does it have to be to trip a temprature sensor normally?

[beenthere]

Depends on the type of fan limit switch, could be 115, 125, 150.

[simpleman]

Either that are somebody already push in the little white button..

[simpleman]

Or was it pulled?

[xxpaulcpxx]

<WhooshOverMyHead>Little White Button</WhooshOverMyHead>

Yep, no idea what that is.

Well, mabey when I replace this old tank I'll repurpose it as an automatic attic vent

[steamer]

Yep the higher temps are telling your fan and limit control to turn the fan on, the limit control "simpleman" is refering to is a silver oval type fan and limit mounted probably above the burner compartment and it has a number dail with temp settings on it setting for on /temp and for off/temp, if it gets above the off temp on a hot day it will turn the helix in the limit control and engage the fan to run, there usually is a white manual fan button on these types of limits,(push for constant fan on, pull out for auto mode), but if the furnace is very very old, it probably wont have this type of fan and limit control, hard to say.

If you can see this type of limit, bump up the off temp a little higher for summer months, just remember to set it back down in the winter months, so your dishcarging all of the heat out to the home vs staying in the exchanger, but in this case as you said the Heat exchanger is allready cracked, so replace the damn thing before ya kill somebody,lol.

[lennox dude]

I agree with all about the fan limit.

But....

LOL

Unplug it?!?!?

LOL again.

Make sure when your new furnace goes in that the circuit feeding it is dedicated to the furnace and is properly ground and has a safety switch/disconnect. This is very important for the new furnace circuit boards: PROPER GROUNDING!!!

Have a good one!!!

[control_noob]

Originally posted by steamer
Yep the higher temps are telling your fan and limit control to turn the fan on, the limit control "simpleman" is refering to is a silver oval type fan and limit mounted probably above the burner compartment and it has a number dail with temp settings on it setting for on /temp and for off/temp, if it gets above the off temp on a hot day it will turn the helix in the limit control and engage the fan to run, there usually is a white manual fan button on these types of limits,(push for constant fan on, pull out for auto mode), but if the furnace is very very old, it probably wont have this type of fan and limit control, hard to say.

If you can see this type of limit, bump up the off temp a little higher for summer months, just remember to set it back down in the winter months, so your dishcarging all of the heat out to the home vs staying in the exchanger, but in this case as you said the Heat exchanger is allready cracked, so replace the damn thing before ya kill somebody,lol.

Wow Steamer, sure is a good thing we don't give DIYers step-by-step instruction on this site or else your post could be misconstrued...

[steamer]

Lol,I got carried away,Heat exchanger cracked , end the story right there, replace the thing.