furnace questions

[master53yoda]

is the fan motor pulsing with the burners firing or what is it that is pulsing.

Normal operation for the older furnace is for the thermostat to send a signal form the W terminal to the gas valve and fire the burners. In heating that is all that the thermostat does on a gas furnace. after the heat exchanger heats up the fan limit combination switch turns on the fan. the fan stays on until the thermostat shuts off the burner and the heat exchanger cools off. In a 70's vintage system there would not have been any electronic controls within the furnace unless they were added later. the AC system would energize the fan from the G terminal on the thermostat through a relay in the furnace. Many times this was an added component. i hope this helps a little in understanding how the furnace actually works.

[LCHIEN]

if your heater is indeed that old get a CO detector if you don't replace it or have it checked.

I usually get a burnt smell the first time the heater is started but not thereafter. My son started his heater last week and it apparently set off the smoke alarm from that bburnt smell. I imagine its dust and stuff that settles on the burner during the off season and gets burned up first use.

[woodturner]

is the fan motor pulsing with the burners firing or what is it that is pulsing.

Normal operation for the older furnace is for the thermostat to send a signal form the W terminal to the gas valve and fire the burners. ...
In a 70's vintage system there would not have been any electronic controls within the furnace unless they were added later.

Good points - and the thermostat can be ruled out by manually "closing the switch" at the thermostat. Check the wiring diagram, but IIRC the red wire is the 24V supply - so if you connect the white wire (W) with the red wire (R) the furnace should run. If the furnace runs normally, it's the thermostat that is bad. If it still pulses, it's either the furnace or the wiring between the thermostat and the furnace.

I'm a little confused by the vintage and the description. I understood this to be a glow plug/igniter type high efficiency furnace, but I thought those came out in the 1980's. If my recollection is correct and this furnace has a glow plug or igniter, it must be a more recent furnace.

Anyway, if it has an igniter, they are a common source of problems.

One common ignition sequence for a furnace is:
1. Exhaust blower starts
2. Glow plug starts
3. Gas valve opens
4. Wait 15 seconds or so
5. Thermocouple or other heat sensor is tested to verify burner operation - and shut down the furnace if thermocouple voltage is not within range.
6. When plenum temperature rises, turn on ventilation blower.

So it could be the temperature sensor is bad - thermocouples are a common failure point.

Best advice I can give, though, it to find the installation manual for the furnace. If you cannot find it on the web, call the manufacturer - they will often email you manuals for older furnaces. The manual should have a troubleshooting chart and sequence of operation that will help you diagnose the problem.

[annunaki]

Warning

Toolguy1000 is right. Be careful. Back in the early 90's I would sometimes sleep over at my lady's house on Long Island NY. She had forced air system and the master bedroom was upstairs in the cottage attic. During the winter I would wake up groggy and my feet would ache. While getting ready to move to Florida, and prepping the cottage for rental, we had the heating system checked out, and a hole was discovered that allowed fumes to enter the ductwork. The headaches and aching feet were a sign of poisoning. Like everything else, as things age, they wear out.
This reminded me of the old VW Beetle heater that had a heat exchanger wrapped around the exhaust system. Once a small hole wore through from moisture, fumes would be included with the heat - not good!!

[vaking]

You should be able to rule out thermostat easily. There are 2 wires that you can connect together directly and this should turn on the furnace bypassing the thermostat altogether. If the furnace does not come on steady with wires shorted together - thermostat isn't the one to blame.

[siliconbauhaus]

Thanks for all the replies.

I'm going to have a look at it tomorrow am and see if I can find a model number and such. I do know it's a Bryant.

When I put the thermostat on I had (from memory) 2 red and 2 white wires (definitely 2 red) they were marked with stickers and I just connected them to the same letter on the back of the t'stat.

When I look at the water heater and furnace I have to wonder who it was that installed them as the copper pipe was crudely bent by hand and really annoys me looking at it. ( I like neat )

[Mr__Bill]

In addition to the info on the outside of the furnace, there will be some on the inside many of the old furnaces that I have seen have a bunch of guts hanging off of the front of the fire box that should have make model and other info on it.


Bill, over here on the other ocean

[siliconbauhaus]

Updated everything on the first page. Thanks to all who have replied so far.

[siliconbauhaus]

Another update.

Took the run capacitor to a motor specialist in Martinsburg WV as everywhere local was closed. They tested it and it's ok.

Had a look at the whole thing today and here's the situation:

Under the fuse box is another box with a transformer in it. I opened it up as the 24v lines were going into it. I find that 2 pairs of the lines are connected together with wire nuts red to white and white to red. The other line was just sitting there with the end's stripped off. I look at the terminals on the transformer and the red and white ones have the screw backed out. I tested the terminal and there was no power on that side.

Of the 3 lines, 2 go over to the furnace. One is just hanging there coiled up behind the furnace and the other is connected to the board inside the furnace. The other wire went off towards the ac unit so I can only assume that was where it was going.

I don't seem to be getting any power from the 24v line coming from the transformer under the fuse panel. I checked the main 110 line going into the furnace via a fused switch. The fuse had popped so I replaced it.

I had the missus switch on the ac and the fan would start but not keep running. It cycled a few times before I could get her to hear me to switch it off. I went the the fuse box and it popped the fuse there.

Just for laughs, I replaced the fuse and asked her to turn it on again and I saw the smoke build up in the fuse and pop it went.

I'm knowledgeable enough on electrical work to do most things but this one has me stumped so I guess I'm going to have to bite the bullet and call someone in. I will certainly be annoyed if it's something simple I overlooked.

[woodturner]

Just for laughs, I replaced the fuse and asked her to turn it on again and I saw the smoke build up in the fuse and pop it went.

Are you getting power from the transformer before the fuse blows?

I don't see how the fan can be cycling if the fuse is blown, that doesn't make sense.

If the fuse is blowing, though, that suggests there is a short or that something is drawing much more current than expected. Is the fuse the correct size? Sometimes prior owners will replace a fuse with a lower value, then next season when the furnace has been sitting for a few months and needs a little extra current to get going, it will blow the (smaller) fuse.

One other thing to try is lubricating the blower motor. Particularly on older equipment there are often oil ports on the motor and blower bearings. If these have dried out, that will increase the power the blower motor pulls, maybe enough to blow the fuse.

So my suggestions would be:
1. Lubricate the blower motor
2. Check the fuse size
3. Look for any other signs of overheating or something drawing more current
4. Check if the transformer supplies power when the fuse is good

[cgallery]

Does the tstat have a "Fan On" and "Fan Auto" switch? If yes, what happens when you switch to "Fan On" with the tstat not calling for heat?

Assuming that works and the fan comes on, what happens when you leave the "Fan On" switch where it is, and then adjust the tstat to call for heat?

Also, any chance you have the original tstat that you could reinstall and test?

[siliconbauhaus]

The guy who does the maintenance for the apartments where my parents live had his hvac guy call me last night. (My parents live next to him so he knows them well).

The guy came to the house around 7pm and looked it over. He pulled the motor and it was on it's last legs. He took off and about an hour later came back with a new unit and installed it.

I got charged $150 for the whole thing (including a 1hp motor) so I'm very happy and the family is warm again.

[cgallery]

When I replaced the blower motor on my furnace, I purchased one w/ sealed ball bearings (old one had oil ports, I assume some sort of bushings on each side). I think it was 2/3-HP or something. The motor cost me about $100. Putting it in took a couple of hours.

So I think you did very well at $150.

Good job!

[Mr__Bill]

I am glad you found the right person to service the furnace and have heat back in the house. He is a person to put on your Christmas card list.



Bill, on the Sunny Oregon Coast

[siliconbauhaus]

Need to find out what beer he likes as he'll get a case for xmas.

Now all I need to do is find a sparky as good as this bloke and I'm set.