Old Gas Furnace Issues

[grissom]

I have a home built in the late 50's that has a natural gas down draft furnace (General Electric) that I believe is original to the home

We are having intermittent problems which seems to become more frequent.

1. Turn on the thermostat to the desired temp and it comes on and heats the house, turns off when temp is reached. Sometimes it will run perfectly for the whole morning, however recently it has started short cycling after about running for a hour

2. When the heater has stopped and we turn off the thermostat - sometimes the heater blower will attempt to restart itself even though the thermostat is off.

There is a switch in the furnace closet that I have to turn off to keep it from wanting to start again

The thermostat is a Honeywell dial type - maybe original also

I have cleaned the air filter which is a washable type mat

I have also vacuumed around the blower area

Anyone have experience with these old furnaces?

 

[Shiftless]

Your furnace is 65 years old!
I too live in CA and replaced a 40 year old gas furnace 2 months ago when it was having similar problems.

Better check yours to see if your heat exchanger is cracked too.

As you probably already know, CA has already banned new gas furnaces and water heaters starting in 2030. It will be legal to repair old ones, you just can’t install new ones. personally, I bet this law will be postponed or greatly modified by that time, but nevertheless, I feel a lot better with relatively new equipment going into the future.

Unless you have a pretty big house and pretty poor insulation with pretty leaky windows, your 120,000 BTU furnace is probably too big anyway. Your house will be more comfortable with a smaller furnace running longer in between cycles. I downgraded from a 100,000 BTU to a 2 stage 80,000/60,000 BTU high efficiency condensing unit and had the company get rid of our old asbestos covered ductwork and install fiberglass insulated (R-6) ducts. Big improvement.
(Our vintage 1950 house is 2600 sq. ft. and only insulated in the attic)

 

[Bert_]

Fan limit, or airflow

The fan is turned on when it senses a certain amount of heat.

 

[HoosierBuddy]

I had that happen on a furnace of about that same vintage at a rental one time. Short cycle...basically fire up, shut down, fire up, shut down, fire up, shut down.....maybe 4 times a minute.

The landlord took a couple of weeks to have someone check it out. I think it was the high limit thermostat was bad (kicking out on an overheat condition)...which could also indicate a stopped up flue rather than a bad safety.

Anyway...that month's gas bill was 3 times as high was we'd ever seen.

Speaking of which....that furnace of yours is probably plated at 80% efficiency, but I'd estimate it's more like 60% by now. You could likely cut your gas bills in almost half if you replaced it with a modern furnace, and get a lot more comfort if you replace your old thrermostat (I assume with a furnace that old you are probably still using a tilt style mercury t-stat?).

I did that in the home we bought 25 years ago. Replace the old "80%" original furnace with a modern furnace with a digital t-stat. Cut the gas bills dramatically. Way more comfortable, because the temp stayed +-.5 degrees rather than +/- 2.5 degrees.

Good luck!

 

[Stelzer]

Easy fixes first. Check air filter and confirm it's clean and facing right direction.
Next is flame sensor, which is a 5 minute fix at most, and can solve short cycling very quickly. Remove flame sensor, sand with emery cloth or fine grit paper, wipe with alcohol, reinstall. Carbon builds up on the end of the flame sensor. Very quick, cheap & effective way of correcting some short cycling issues.
At the very least, even if cleaning of the flame sensor doesn't solve your short cycling, it will make your furnace run better and longer.

 

[fitter30]

There are three little levers on the round dial of the fan control. Middle lever hold dial move lever cw just 5° higher. Wait five minutes after burner shuts off. You will see the dial turn ccw till left lever shuts fan off then residual heat will turn dial cw then it shouldn't get to the middle lever but if it does lower temp setting left lever 5° hold dial.
Try it again.

 

[gmcgeo]

also check your door switch, make sure it is not going bad

 

[Bert_]

also check your door switch, make sure it is not going bad

Easy fixes first. Check air filter and confirm it's clean and facing right direction.
Next is flame sensor, which is a 5 minute fix at most, and can solve short cycling very quickly. Remove flame sensor, sand with emery cloth or fine grit paper, wipe with alcohol, reinstall. Carbon builds up on the end of the flame sensor. Very quick, cheap & effective way of correcting some short cycling issues.
At the very least, even if cleaning of the flame sensor doesn't solve your short cycling, it will make your furnace run better and longer.

His furnace will have neither of these. No door switch on anything that age. It will have a standing pilot and thermocouple

 

[gmcgeo]

His furnace will have neither of these. No door switch on anything that age. It will have a standing pilot and thermocouple

Some did. and i can not rule it out due to not being able to see the model of this furnace.

 

[mike93lx]

Wow. I agree on the sizing and efficiency. I bet some air sealing and a good load calc would let you drop down the size of that unit considerably.

Can't imagine gas is cheap in CA...payback might be really solid

 

[lilredex]

Had a similar issue with my 1958 furnace. A dirty filter caused it to cycle on the high limit switch......new filter fixed that. Did rough calculations a while back and a new furnace would take 28 years to payback investment. Ever see a new furnace last that long?

 

[gmcgeo]

Wow. I agree on the sizing and efficiency. I bet some air sealing and a good load calc would let you drop down the size of that unit considerably.

Can't imagine gas is cheap in CA...payback might be really solid

Im afraid to ask what the price of propane is this aug. and im in pa.

 

[jlv03]

Had a similar issue with my 1958 furnace. A dirty filter caused it to cycle on the high limit switch......new filter fixed that. Did rough calculations a while back and a new furnace would take 28 years to payback investment. Ever see a new furnace last that long?

I replaced a 24 year old, still functional, furnace because I needed to relocate the A/C condenser and the old A/C was leaking anyway.

Probably to the shock and horror of many on the internet, the old furnace was over 90% efficiency (plastic pipe for exhaust). The unit rarely had problems, and, when it did, the parts were inexpensive. I get not ripping out old and still functional just for "new", but I have a hard time believing that the new stuff won't last at least a decade or two.

 

[HoosierBuddy]

Had a similar issue with my 1958 furnace. A dirty filter caused it to cycle on the high limit switch......new filter fixed that. Did rough calculations a while back and a new furnace would take 28 years to payback investment. Ever see a new furnace last that long?

Yeah. No. It'll be a lot faster than that.

I assume you are basing that on a plate on the furnace that shows an 80% efficiency. Most if not all of them came with a same rating plate showing exactly 80%....I'd say that was printed up by the marketing department and not based on any real analysis.

Here's how I would look at it.

120,000 BTU/HR X .3 Inefficiency X 1 DT/1,000,000 BTU X $8/DTH = $0.288 per hour burner on time wasted.

Replace with

90,000 BTU/HR X .05 Inefficiency X 1DT/1,000,000 BTU X $8/DTH = $0.036 per hour burner on time wasted.

So...every hour your burner is on, you are throwing a quarter out the window.


The correct way to do this is obviously the other way. I'll skip typing up the math, but the % savings on gas is 24%. And, I'd actually be shocked if it wasn't a lot more than that. When I changed my old 1960 unit out and replaced the mercury t-stat with a digital one, my gas bills dropped by 40%. That old 200,000/hr furnace was plated at 80% efficient. Marketing hype. Nothing more.

My payback was much faster because I changed out my own furnace...but it still should be a reasonable period, unless they are throwing in an ac unit, etc. in your replacement quote.

 

[Bert_]

I did the programmable thermostat thing for a few years. Never did like it.

I put in a salvaged mercury thermostat and leave it at one temperature all winter. Gas usage is actually a little less. I assume because it runs longer cycles.

 

[PoorUB]

Yeah. No. It'll be a lot faster than that.

I assume you are basing that on a plate on the furnace that shows an 80% efficiency. Most if not all of them came with a same rating plate showing exactly 80%....I'd say that was printed up by the marketing department and not based on any real analysis.

Here's how I would look at it.

120,000 BTU/HR X .3 Inefficiency X 1 DT/1,000,000 BTU X $8/DTH = $0.288 per hour burner on time wasted.

Replace with

90,000 BTU/HR X .05 Inefficiency X 1DT/1,000,000 BTU X $8/DTH = $0.036 per hour burner on time wasted.

So...every hour your burner is on, you are throwing a quarter out the window.


The correct way to do this is obviously the other way. I'll skip typing up the math, but the % savings on gas is 24%. And, I'd actually be shocked if it wasn't a lot more than that. When I changed my old 1960 unit out and replaced the mercury t-stat with a digital one, my gas bills dropped by 40%. That old 200,000/hr furnace was plated at 80% efficient. Marketing hype. Nothing more.

My payback was much faster because I changed out my own furnace...but it still should be a reasonable period, unless they are throwing in an ac unit, etc. in your replacement quote.

Keep in mind house is in northern California. I doubt the furnace gets much serious use and is probably oversized.

I got thinking about it earlier today when I read this post. I have an Excel spreadsheet of the last three years of natural gas usage. I also have a gas water heater so I averaged out the water heater use in the summer, multiplied that by 12 and subtracted it from the total usage.

I use roughly 1,300 therm a year total, about 300 of it is for water, so that leaves 1,000 therm for heat. On a bad year I paid a $1 therm so $1,000 a year for heat, some years it has been 30 cents!. Going from an 80% to a 95% theoretically would save 15% or $150 a year. Shops are asking $5,000 and up to swap out a furnace so $5,000 / $150 = 33 years! Even if I could get it done for $2500 it would still be roughly 16 years. Even if I go with your 24% it comes loosely to 10 years on a $2,500 change out, or 20 years on a $5,000 change out.

 

[Jackfre]

I would not assume an 80% or even close to 80 efficiency rating for that furnace. The gas codes changed in ‘92 requiring a min of 78%. Prior to that those old furnaces ran in the high 60’s and went down from there. The gas industry fought like he** to prevent having to meet even 78%. One of the main reasons was the difficulty with the venting and condensation in the lower efficiency units. Net/net you can gain probably a 25-30% gain in efficiency meaning yo can dramatically reduce the size of the unit. There are a lot of lazy contractors who will look at the current rating plate and put todays match in. Get through the season an look into another unit. You will be happy you did.