Whats wrong with my ancient gas furnace?
stanleyj2
| Posted in Energy, Heating & Insulation on
Don’t Laugh but I live in a 100 year old home with two very old furnaces with Honeywell programmable thermostats that worked great until today. One is still working fine. I am not currently interested in one of the new fuel efficient models. The problem is with the oldest one. The burners are not lighting nor is the blower coming on even when I change the blower from the automatic setting to the constant on position. The blower motor was replaced about 1 year ago. The circuit breaker is on. It is supposed to get very cold this weekend and I need to fix the problem quickly. Where should I start looking?
Thanks,
Stan
Replies
Probably a bad transformer in the furnace.
Are you changing from auto to manual on the thermostat or a manual switch on the furnace itself?
I am changing from auto fan to manual fan on the thermostat. I have since checked the blower motor by wiring it directly to an outlet and it runs fine. Electricity is reaching the furnace (via voltage tester check) and there is a transformer mounted on the electrical box inside the furnace. The blower motor is not receiving electricity from the furnace.
Thanks,
Stan
Yeah, if the blower doesn't come on when you switch "fan" from "auto" to "on", then likely the the transformer has gone bad, or the wire to the thermostat has been cut/broken somehow. If the thermostat is loose on the wall or some such it may be simply that a wire has broken off the terminal.
If ignorance is bliss why aren't more people
happy?
To add what Dan hit on
Do an ohm check of the thermostat wires.
I had the transformer checked and it is putting out 23 volts.
Stan
Then it's probably a broken wire somewhere. If you meter between the wires at the thermostat you should find two that have about the same 23 volts between them. If not, there's a wire broken somewhere. (Even if you measure that voltage at the thermostat, there could still be a broken wire, but that's for the next round.)
If ignorance is bliss why aren't more people
happy?
"Honeywell programmable thermostats that worked great until today"
Is there a dead battery inthe thermostat?
There is a relay in the circuit with the blower motor that appears to be bad. On older furnaces the blower in the heating cycle is controlled through the fan/imit switch when the temperature gets to the set point it turns the fan on and when you turn the fan switch on the tstat to ON the motor is energized through the indoor fan relay. From what you describe there are other problems with the burners not lighting as well. It could also be a bad thermostat, the burners not lighting and the fan not coming on with both be indicators of this as well. I do not how familiar you are with troubleshooting and using a meter to isolate problems, but if you are I would suggest you first try and jumper the terminals on the tstat, if you put a jumper between the R term. and the W term and it comes on it is the tstat. Hope this helps
The relay operates the fan in AC and "fan-on" modes. The fan is operated by the fan/limit switch (which parallels the relay contacts) in heating mode.Assuming a "single point of failure", the relay wouldn't be it -- it's not needed at all for simple heating mode with no "fan" switch on the thermostat.If indeed this is a multifailure situation, and the fan relay has indeed "failed", most likely it's simply rattled out of its socket -- a push on the top of it will reseat it and restore the connections.But this does bring up one point: Ignoring the failure of the "fan-on" switch, the fan/limit switch is another likely point of failure for the heating system.
If ignorance is bliss why aren't more people
happy?
Depending on the vintage of the furnace, most relays are soldered in place. I have a Rheem 90+ with a bad blower relay and it was not plugged in. It's Robert Shaw module and, while the relay has 2 pairs of contacts, they only used 1 pair. That, added to the fact that the furnace is heating a very small house, lead to the relay's untimely demise from short cycling.
"I cut this piece four times and it's still too short."
Units as old as this one purports to be generally have a fairly large (roughly 2x2x3 inches) relay in a socket. Sometimes there's a retainer spring to hold the relay in place, sometimes not.A 90+ furnace is an entirely different beast.
If ignorance is bliss why aren't more people
happy?
I didn't see that the furnace was 60 years old until a later post. They were definitely different back then.
"I cut this piece four times and it's still too short."
"The relay operates the fan in AC and "fan-on" modes. The fan is operated by the fan/limit switch (which parallels the relay contacts) in heating mode."It varies.I have a pair of 79 Bryant furnaces. The downdraft the blower is controlled by the bonnet fan/limit thermostat. The updraft has the fan is controled by a time delay relay to kick on after a delay from when heat is called for.
You got it backwards. A downdraft unit uses a timer, since a thermal switch wouldn't work (given that heat rises). I've got the same setup, only with Williamson units.
If ignorance is bliss why aren't more people
happy?
I had to go check it out.Both of mine have timers.When I was in grad school I had a mobile home with a FHA oil furnace.Now it had two thermostats in the heat exchanger. One at the top and one at the bottom. And one was in series with the other one and with the thermostat. And one also had a set of contacts for the blower.Can't remember the details of how they where wired. But when it called for heat it would heat up the exchanger and start the fan and cut off the burner. Then as the air was blowing it would warm up the 2nd stage and cool of the first one so that if the thermostat was still calling for heat that the burner would fire again and the the air moving the first thermostat would not interrup the burner so it would run continously until the thermostat was satisfied.
If you are able to do furnace work yourself and decide to replace one or both furnaces, look at yahoo shopping and punch in whatever brand of furnace you want. There are places that will ship directly to you.
Thanks to all for the replies. I checked all the things that you mentioned and finally had to call in my HVAC man. The bad news was that my furnace is dead and I will have to bite the bullet. It lasted almost 60 years. The hvac company recommends a Trane 90% efficiency in either the single or variable stage versions. Does anyone have experience with either?
Thanks,
Stan
I find it hard to believe that anything short of a bad heat exchanger can cause an old-fashioned GFA furnace to be unrepairable. Thermocouples, valves, relays, transformers, limit switches, motors are all standard design, and no single part is more than about $100.
If ignorance is bliss why aren't more people
happy?
I would echo DanH's comment and say it sounds fishy that both the heat exchanger (which is the only real component who's failure would cause you to have to scrap the furnace) and some other control component would fail at precisely the same time.
If you will permit me to play devil's advocate for a moment - is it conceivable to you that the HVAC guy has something to gain by pronouncing the furnace dead?
I will tell you this - a high-efficiency furnace that is even one quarter as reliable as your old one doesn't exist in the residential market. ie. you'll be lucky to get 15 years out of it, and besides that you will find that it will require much more frequent repair.
Is this the first time you've had a problem with this furnace?
From the vintage I'm guessing that the thermostat controls the burner, then when the temp rises from the burner being on the fan kicks in via a thermostatic switch in the plenum? So if that is the case try bypassing the th'stat altogether by jumpering the two terminals that lead to the thermostat right at the valve itself. If still nothing then tap the valve with a screwdriver handle while jumpering. I experienced this problem and fix with a previous house.
Of course, the upside of a new furnace is that you will be trading long-term reliability for efficiency, a cleaner environment and lower fuel bills.
Good luck - Brian.
I decided to have my old furnace repaired and the work was finished today. My old furnace is not efficient but it is rugged and I can perform many of the repairs myself. The hvac man replaced the old three piece valve, regulator, and safety with a combination standing pilot gas valve and re-piped to the 100% shut off. The unit works great and I am glad to save it. Thanks for everyone's input.
Stan
So why wasn't the fan switch working?
If ignorance is bliss why aren't more people
happy?
I don't have a clue.
Stan
Sounds like i'm a little late to the party with this for a response, but...
Have you checked to see if you have some type of switch after the fan, inline in the duct for the exhaust? A big Peerless unit that I'm familiar with has a switch on a flapper in the exhaust line.
My unit starts the draft inducing blower, but the flapper in the duct must swing open to prove that there's exhaust flow, BEFORE it'll energize the ignition and light the main burner.
I'm wondering if your problem is an exhaust-flow-prove switch.
keep us up on what you find!
I don't think your typical 60-year-old furnace has a draft inducing fan.
If ignorance is bliss why aren't more people
happy?
I had a similar problem with our (old) gas furnace a few years ago. The draft inducing fan was fine, but the vacuum switch that verified operation was leaky/flaky. Replaced the inducing fan (expensive) at recommendation of repairman and it did nothing.
Replaced the vacuum switch (cheap) and it works like a champ.....