Hi everyone,

I posted on here before about my consideration to get a new furnace and what one of the contractors here in suburban MA suggested. I am at a fork in the road and would love your opinion. Here is the full story --

I purchased the home August of last year. Home inspector found no problems with heating/cooling forced air natural gas system, but I suspect the previous homeowner may have had issues prior to selling. I think so because this past winter, the northeast experienced a lot of snow, but more importantly, we experienced some periods of significant cold temps during which my furnace began acting up, i.e. the burners would turn off, but the fan would keep blowing cold/lukewarm air. The furnace did NOT act up in the months leading up to the cold weather - just mostly during the cold winter months. For example, I tried replicating the problem now (it is April and I turned the furnace to 68 degrees), and the furnace is functioning fine.

Contractor #1 said I am experiencing overheating problems due to the small size of my return duct. I look at my return duct in the basement and it does admittedly look relatively small compared to my friend's. If I go with contractor #1's suggestions, I will have to replace my furnace system, drill holes to basement wall for furnace flue, install a basement ceiling PVC to carry condensate to basement sick, and most likely install a chimney liner since my water heater currently connects to the chimney along with my furnace flue. My question is, do you think he is right? The cold winter months would demand more heat from my furnace, so it would be on more frequently. Also, is it possible to drill a hole to basement wall for my current water heater flue, as opposed to doing this 'liner'? The former seems less involved and efficient compared to the latter. My water heater is from 2013, just unsure if liner or basement wall flue option is dependent on what model water heater I have.

Contractor #2 astutely noticed 4 red light flashes on my indicator box inside the furnace and figured out that it was due to high limit switch being open. At the time, I didn't know what these switches were, but after reading more about it online, I am now wondering if replacing these switches upon his recommendation would even solve the problem if the issue is overheating and not faulty switches. If I go with Contractor #2's plan, I would need to know if the switches were indeed faulty or if the switches just tripped (aka doing its job right). The fact that my furnace's switches only 'tripped' or became faulty during the cold winter months leads me to believe Contractor #1 more....

Some specs on my furnace: AMERICAN STANDARD (I think it is from 1992, so roughly 23 yo!). Contractor #2 said, upon opening the furnace door, that the inside looks very clean and no major problems, and that I should/could ride out the furnace until it broke. I'm hesitant to do so, since the furnace this past winter made my house 40-50 degrees F at times because we just couldn't turn the furnace on when it was 0-10 degrees outside.

Thanks!

 

Forced air furnaces have about a 20 year life expectancy depending much upon how it was installed and how it is maintained. If your furnace has minimally sized return air ducting, and I strongly suspect it does, then a shorter lifespan would be more likely than a longer lifespan.

Tripping of high limit switches is indicative of restricted return air, either because of too small of ducts or because of too restrictive (or dirty) filters.

One thing in your favor is that most likely the present furnace is significantly larger, in BTUs/Hour output than what you need and installing a new 90+% furnace of the proper size might minimize the duct issues to the point where they are no longer an issue. The FIRST thing you need to do is a rough heat loss calculation on the home as it is and then a final calculation including all energy saving measures that you plan to implement. This will allow you to properly size the furnace to the actual heat loss of the house.

 

Now that I think about it though, why can't it be the sensors/switches at fault? Perhaps every time the furnace is called for long periods of use, it overheats and then the sensors/switches only 'become faulty' when the heat is high inside the furnace? Or is that illogical since if a sensor/switch is faulty, then it will be faulty regardless of the temperature inside the furnace?

 

I doubt the switches are faulty. At 23 years you need to be thinking about a new one. As mentioned not enough air, filter, I have seen this happen when the blower is about to die.

 

Try contractor #3 or 4 or ....

It is good to know that the high limit switch cut out but next is to figure out wh y. The previous owners probably did not live with 20 years of cold winters. Admittedly an old furnace but it might still be OK. Maybe, as mentioned, the filters are dirty. Or maybe the blower is running slow. You need someone to investigate. Or get a new furnace - but make sure they do a proper load calculation and see if the ducts need re done.

 

First off, the heat exchanger needs to be thoroughly checked. If it's been cycling on limit for years, chances are that it's cracked by now.

Secondly, the temperature rise needs and fuel input needs to be checked.

Many things can cause a furnace to cycle on limit and you need to find out why before changing the furnace. A dirty a/c coil or blower wheel could do it. Overfiring could do it. Same goes for undersized ducts.

Sometimes increasing the fan speed alone can resolve the problem.

Cycling on limit is a lousy reason to change a working furnace. High fuel bills, a dangerous condition, or lots of breakdowns justify replacement.

 

I think the blower is already on high speed, and truth be told, it does look like my six-burner furnace currently is being fed with a smaller-than-necessary return duct. The duct itself looks like its 8 inches or at least less than a ruler in diameter. I'm not too familiar with the insides of the furnace, but after having one or two contractors look into it, it does seem like my furnace insides (at least what's visible) looks operational and good. Burners have blue flames, no soot, etc. If I were to look at the heat exchanger condition, I would need to take out a significant portion of the furnace to inspect it. Also, the air coming out of the vents do not have any distinct combustion smells that you would normally expect if the heat exchanger was cracked, so I think that part should be fine despite 20 years of usage on this furnace. Hence, due to the age alone, I think it may be time to get a new one - we bought the home last year and do plan to live in it for the next 10-15 years. The only thing I am annoyed with is the complications that come with installing a High Efficiency furnace - drilling holes to the basement sides and putting in an aluminum chimney liner at additional cost since my water heater is direct vent / not power vent (apparently they are moving water heaters to power vents only moving forward....from what I've been told).

 

Post a few pictures of this furnace and the ductwork. With six burners the BTU output of that furnace must be around 120,000 BTUs/hour and THAT is likely way more heat than you need, possibly twice as much. Couple that with way undersized ductwork and you have a complex problem.

Please post the pictures as well as the BTU rating from the nameplate inside the burner compartment. Give an approximate size of the house and what the lowest temperatures were that you you experienced this last winter. Also did the furnace cycle on and off during the coldest weather and approximately how long were the on and off cycles.

 

if the furnace is <90% efficient the heat exchanger can be inspected by pulling the blower.