Ducane furnace can't keep up

[Jadnashua]

Most homes have a furnace (or boiler) that is way bigger than needed...so, depending on what you add, the existing one might actually work better (once it is working properly in the first place)! Back when energy was cheap, the cost wasn't high to use a larger unit than needed. Today, that's folly, as bigger units cost more to both buy and their life is shortened when they use short cycles. One reason the heat exchanger may be shot is if it is oversized, and doesn't need to run longer cycles...that prevents it from burning out all of the condensation, and in a burner, that tends to be somewhat acidic, not counting the fact that the moisture can rust things even when pure.

 

[Dana]

What Jim said, most hot air furnaces are ridiculously oversized for their actual design loads, and yours probably is too.

Sometimes they're even oversized for the ducts they're hooked up to, yielding lower air flow, running the heat exchanger temperatures close to the overtemp limits, which is another way to burn 'em out quick.

Since you have a heating history on this place, using wintertime temperatures only, run a fuel use heat load calculation to determine what the actual heat load of the house is, including the duct losses (since they can't be subtracted out of the fuel use number.) Then compare the nameplate BTU output to your calculated 99% heat load. Anything over 1.4x is going to be sub-optimal overkill, but most hot air furnaces out there have burners with output more than 2x, many more than 4x the the actual design load, as if they were expecting a cold snap that dropped to -150F or lower or something

With a heat load calculation in hand you're in a better position to make rational decisions about the replacement equipment.

The "75" in the Ducane CMPEO75u3 model number probably indicates a 75,000 BTU/hr (input) furnace, which at 80% efficiency would deliver 60,000 BTU/hr out, at 92% efficiency about 70K. That's enough heat to keep my ~ 2400' 2x4 framed 1920s antique at 70F indoors with outdoor temperatures down to about -40F to -50F or so. This document indicates that the CMP series comes in 50K, 75K, 100K, and 125K BTU ratings. Don't know how much house you have or what it's air leakage levels or R-values are, but a 75K furnace is probably way oversized for most houses in your area (even those houses that currently have 100K furnaces installed.)

A fuel use heat load calculation will probably tell all you need to know about sizing up a replacement unit. Even if you doubled the size of the house in the major rennovation, if you're building to IRC 2015 code minimum and fixing bunch of deficiencies of the older part of the house it'll still be oversized. Sometimes the total heat load goes DOWN with a major renovation, due to better air tightness, better windows and higher R values on the do-over/addition, even if the total amount of conditioned space goes up substantially.

 

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[Spfrancis]

Thanks for the help. So I'm going to find out if the heat exchanger is still part of any warranty period. I don't believe that the furnace is more than 15 years old. I'm not the original purchaser of the unit, so I'm not sure how that works for trying to get the parts covered under the warranty.

 

[Dana]

The serial number usually encodes the date of manufacture, which is what many manufacturers would go on for warranty issues when there is no other supporting documentation of when it went into service. If the original owner filed the warranty registration, the restration date would start the clock. Most furnaces come with a 10 year warranty (even though in normal service a 20-25 year service life it typical), so if it's 15 years old it's unlikely that anything is currently covered under a normal warranty. If there was a product safety recall on the heat exchanger, maybe. but that's pretty rare.

 

[Spfrancis]

So looking at the serial number it looks like it was 34th week 2002. So a tad over 15 years. It does sound like Ducane has a possible 20 year warranty on the heat exchanger: "Warranty registration is for new equipment installations only. Equipment must be registered within 60 days of installation. Or, if you purchased a new home, the equipment must be registered within 60 days of closing (or your home purchase was more than 60 days ago and equipment was not registered online for extended coverage). Your warranty defaults to a 5-Year Limited Warranty on parts and 20-Year Limited Warranty on heat exchangers on unregistered equipment." There is a sliver of hope.

 

[Dana]

A pretty good sliver at that, given that even if the unit was never registered it's still within the warranty period for the HX.

Most warranties cover only the components, not the labor, so it's worth getting a budgetary estimate from the HVAC contractor for what it takes do do the HX. Sometimes it's cheaper/better to replace the equipment with something new, fully warranteed, higher efficiency (especially when state subsidies are available) and right sized for the actual load rather than a 1:1 BTU replacement. Most houses in the mid-Alantic or further south can be served well with a ~40K-in / 38K-out 2-stage condensing furnace costing less than $1500 for the unit. Even in NY/New-England most 2000' or smaller homes would use a unit that small (or smaller.) Knowing your actual load numbers and oversize factors allows you to make better informed and better overall decisions on the repair vs. replace, and if replace, with what. Don't just leave it up to the contractors, which have a pretty lousy track record, on average, always estimating/guessing to the high side of reality, since the cost of up-sizing gas furnace burners is so small. The comfort consequences of oversizing furnaces is real.

A complicating factor can be if there is also an oversized air conditioner requiring more cfm than the blower on a smaller, right-sized furnace would deliver.

 

[Spfrancis]

So a new piece of information from a second HVAC company that came out today. The guy did some analysis and put up a manometer up to the pressure switch. It sounds like he jumped the PS for a test, and the burner stayed on. He started to test WC, and noticed that it was dropping too low. The one thing he noticed that when the water pump came on, then the WC value went back up. He crawled around behind, and was able to see that the 2 ports that drain into the pump was coming in from the side. The pump was mounted about 3 feet off of the ground. He took the pump and moved it to the ground, and he had the 2 ports(AC, Furnace) drain from the top. What he was thinking was that the PVC was filling up with water where it came into the pump. When he did this change, it started to drain better, and the PVC wouldn't get filled with water. He said that he let it run for 30 minutes, and it did not cut off, and the WC value stayed within an acceptable value. This was a pretty cool, that he was able to figure that out. I would have probably never figured that our. It really irks me that the other company did very little troubleshooting, and decided that a new heat exchanger was needed, and was really pushing for a new furnace. The reality is that I would have paid for the labor for that company to change out the heat exchanger, and it would not have fixed the problem. It is pretty frustrating that they didn't do much troubleshooting, and came up with a heat exchanger assessment. I see that there is a guy who has a youtube about the "heat exchanger scam" Oh well, I'm hoping that this will be the end of this problem. Just in time, as my wife's family starts showing up at the place Saturday for a 4-5 dray stay. I think that Tuesday/Wednesday was going to be dropping into the low of the mid 20's. Thanks for all the help from folks on the forum.