If there’s one thing I have learned about Michigan, it’s that the weather does whatever it wants. This past Spring we were warming up in the 80s and 90s and then had a cold snap down into the 30s at night.
I’m usually the first one up in the morning and when I went downstairs it felt cooler than it should have been. A quick glance at the thermostat told me it was 63 when it should have been 68. The thermostat showed me it was set to heat and the fan was running. Putting my hand over a nearby vent told me my morning wasn’t going to go as planned – the air blowing out was cold when it should have felt warm.
Ok, so we have a Trane XR80 furnace and it’s honestly served us well for a number of years. We had this happen one other time quite a few years ago and it was due to my not changing the air filter soon enough, it was filthy, not enough air was flowing through and a high limit switch had tripped. While I recalled it, I really didn’t think that could be the case because I had changed the air filter with a new one a few weeks earlier.
One thing I like about the XR80 is that Trane put a small peep hole with a clear plastic lens over it so you can look straight in and see the error code that the control board flahes when something is wrong. So your’s truly who is fatter and stiffer than he should be got down in the crawl space, went over, looked and it was flashing its red diagnostic LED four rapid times, paused, and then repeated that code over and over.
So, I extracted myself and went to look up what four flashes meant on the Internet – well it signifies that the high limit switch had tripped. The high limit switches are located on the heat exchanger and are important safety devices to not allow the gas furnace to fire up, or keep burning, because things are too hot. Instead, the control board keeps running the fan trying to cool things down. If you didn’t have them, things could get too hot and a fire could start – they are definitely important.
On the XR80, you remove the lower door, take out the retaining screw holding the panel above the door in place and then remove the access panel to get to the switches. Sure enough, the left high limit switch was tripped – it pops out and to reset it you push it back in.
The usual cause of a high limit switch tripping this is a dirty filter – but my Filtrete MERV 15 filter was brand new and these things are huge – 16x25x4 – you’d think air could flow pretty easily. When we moved in the original filter to the old long-gone furnace was one of those electrostatic filters that zapped the dust … into smaller dust … in theory. I was never impressed and I think that is why they are gone. You can pull the whole old electrical plate cartridge out and replace it with a huge 16x25x4 pleated filter, which is what we did.
So, my best guess at that point was that the high limit switch had tripped before I changed the filter and we just didn’t notice it. Sounds good, right? Well, it sounded good at the time too but the next morning I came down and it had tripped again — with the brand new filter installed. It wasn’t due to the old filter.
Ok, something was fishy. Yes, the limit switches can fail but I had my doubts – they usually last a long time. Before you go crazy ripping things apart, always, always, always ask yourself “what changed?” I can’t stress this enough – this question can be worth its weight in gold.
As best as I could recall, the old filter was dirty but it was not filthy. I change my filters three times a year now. What changed was the filter. I had replaced an old Filtrete filter with a new Filtrete filter of the same model rated to MERV 12. That’s what changed – I had changed the filter so decided to investigate further because I wasn’t really sure.
I called up one of my best friends, John, who is an amazing car mechanic and picks up all kinds of knowledge through his own experience and talking to others. I told him what I just experienced.
John asked me “Do you have a high efficiency furnace designed for those allergen grade filters?” No – I had a normal furnce but I had been using Filtretes instead of Honeywell’s own brand of 16x25x4 filters for a few years but the Filtrete was a finer filter that claimed to even remove viruses and was hospital grade. I even remember thinking “finer is better” when I put them in. Finer is not always better as it turns out.
What John had learned from a fellow in the trades is that these super-fine filters sometimes restrict air flow too much in a furnace not built for them and could cause the high limit switches to trip because not enough air was flowing across the heat exchanger to cool it within it’s specifications.
To be honest, that made perfect sense to me. I couldn’t help but wonder if there was something a tad different about this one filter so I replaced it with a new one – yes, I replaced a new filter with a new filter. I just happened to have a spare MERV12 filter there.
Guess what? It worked. For rest of the cold spell, no more problems. All I can guess is that MERV12 is just a hair too fine for my furnace and that the one particular filter just happened to restrict the air too much – new or not it went in the trash after a few days.
With the next change iterval that also coincides with cold air coming in Michigan, I am changing to a slightly less restrictive Filtrete filter that has a MERV 11 rating that still is supposed to stop allergens. It ought to work with fewer surprises – time will tell and I know what to look for next time.
So, I thought I would pass this along in case someone needs to reset a limit switch in a XR80 and/or rethink running a really restrictive filter in a furnace not designed for it. I hope this helps you out.
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