70's Heil-Quaker Gas furnace high temps
I just bought a house about two months ago and have been monitoring/tinkering with the furnace for a few weeks now. It is around a 71 (only date on the book) Heil horizontal nat. gas. So the problem goes like this: 1. T-stat closes 2. Gas valve opens / burner lights 3. Fan controller slowly rises in regards to temp reading (seems very slow to me) 4. Fan finally kicks on at 110* 5. As soon as the fan turns on the temp rapidly rises on the controller to the point where it hits the high limit. 6. By this time the T-stat has been satisfied anyway so the fan runs until the heat ex changer has cooled down enough and shuts off. The filter is brand new. All vents are fully open. I have replaced the fan/limit and changed the fan speed from low to high in an attempt to lower the heat ex changer temps. This has worked to a degree, but the temps still rapidly rise and settle around 180-190 (the limit was set at 200 before so that's where I put the new one, seems high though?). I believe the furnace is rated for a temp rise of 70-100 so this puts it around 110-120. It seems like there is not enough air flow to keep the heat ex changer cooler while the burner is on or there is too much flame/heat being generated. Could the regulator be set too high a pressure or is the orifice too large a size? It also seems there is a flaw in this design with the fan/limit controller. There is no way for the hot air the naturally flow over the heat ex changer and to the fan controller. Thus why it takes so long for it to sense the heat, but it seems that by that time it is almost overheated and can't cool quick enough. Or is this being exaggerated by the high heat being generated by the ex-changer? I should note there was an in duct humidifier that allowed me to look inside the furnace and there are no obstructions to the air flow. I currently have no way to check the regulator pressure and i'm just looking for some suggestions before calling a tech. The furnace seems solid and I wouldn't mind keeping it going since it doesn't have any of the electronic garbage on it. Get it checked by a pro, since the heat exchanger might be leaking exhaust into the supply air. Older furnaces by their very design can be quite dangerous - they lack the safety features found on more modern units. It's probably only 55-70% efficient and oversized, so the cost of keeping that furnace might exceed that of a new model over the next 10+ years. I currently have no way to check the regulator pressure and i'm just looking for some suggestions before calling a tech. If you insist on not calling a tech: Purchase a CO alarm with a digital display which reads low levels. (most retail alarms only go off above 70ppm - by that time you're already poisoned ) Google clock the gas meter. Do not adjust the gas pressure yourself if it's overfired. Measure the temperature rise.
You are correct that horizontal furnaces often have fan an limit switches that don't sense the temperature accurately or rapidly. For that reason they often use a fan switch with a special heater circuit. The heater uses 24 VAC power and is switched on with the main burner. The heater causes the fan switch to start, and it's commonly called a sure start fan switch. Why did you change out the fan switch? It sounds like the heater might have burned out on the old fan switch and you might have purchased the wrong fan switch when you replaced it. Di you have some left over wires when the fan switch was replaced? If so, those might be the power for the heater. Personally I'm mnot as negative on older furnaces as Muggle. Pretty much all furnaces are engineered to extract as much heat from the combustion gasses as is practical without causing condensation in the flue --- that means that all gravity vented furnaces are about 75% efficient when the burner is operating. There are stand by losses from pilot lights and other issues that reduce that figure somewhat. QAnd it's true that old furnaces can have safety issues and deserve annual maintenance by a qualified person to avoid possible hazards. Thanks for the replies. The temp rise is between 110-120 as I said. The manual calls for 70-100 so it is definitely out of range. As for a CO detector I installed one as soon as we moved in and it has a display that show the current ppm and stores the peak ppm reached unless it is reset. To date the peak is 0 ppm. Once I realized the issues with the furnace I even held the detector right at the shortest vent with no registered CO by the detector. The fan controller that was in the furnace was a Honeywell 8 Fan/Limit with no heater. I replaced it with the same. There are no extra wires going to it and none are listed on the schematics in the manual. It would seem likely that it may have been changed out before to the wrong one. Are the heated controllers still available? While it is old it has proven to be quite durable given the condition it was running in for whatever amount of time. It does have an aux limit switch in addition to the honeywell fan/limit which I have verified to work correctly. I feel once tuned properly it should last for some time, but I will replace it if necessary. Do you have a/c? Maybe the coil is plugged. What kind of filter are you using? Some of the pleated allergen filters can be very restrictive to airflow. Clocking the meter is the best way to determine if it's overfired - best of all, you can do it yourself. Is the blower motor belt or direct drive? After verifying that it's not overfired, increasing the speed might be an option. --------------------------------- With respect to the integrity of the heat exchanger... For the co level in the supply air to be above 30 ppm (even alarms with digital displays don't read below 30), the heat exchanger would have to have very large openings and there would have to be a major combustion problem to begin with. A properly operating furnace can easily have less than 30 ppm in the exhaust. CO alarms, while useful, can not alert you to a latent defect which could cause CO poisoning or a house fire down the road. (kind of like using a smoke detector to find faulty wiring before it causes an electrical fire) For that reason, it's a good idea to get the unit checked out. Well that makes sense. The filter is a cheapo 4 pack filter I bought in the hopes of being the most free flowing. No A/C. Direct drive blower now set to the highest speed. The meter clocking I will have to do this weekend when its feasible to shut off the water heater and garage heat. Would this be a better Fan controller? - https://customer.honeywell.com/en-US...d=L4064W1098/U It sounds more and more like I need to get a tech out here. Given the area I'm in I need to know all the info I can get so I can make sure the tech is knowledgeable enough to know what they are doing. We don't have the brightest people around here.... Something I forgot to mention is that the input on the rating plate may not be correct if your altitude is above 2000 ft. At higher altitudes, furnaces should be derated. Seattlepioneer could probably provide better info.
That fan switch does not have a sure start feature. If you have the old fan switch, post the make and model of it. That might allow us to see if it has a sure start feature. Why and when was the fan/limit switch replaced? Unfortunately, your efforts illustrate the limitations of DIY repairs when someone starts making changes without understanding what they are doing. You appear to have made so many changes that almost anything could be happening at this point.
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