INDUCED DRAFT BLOWER RUNS CONTINUOUSLY

[Plumber69]

first thing i would of done is jump the air switch

 

[Thomas G]

Dear Stuff,

So much going on in parallel that I have not had time to get back to you until now. I disconnected the direct power feed from the safety switch to the HUM terminal as you suggested. IDB has stopped running continuously and the AC and heat cycles both work as they are supposed to. Schematic on the inside of the furnace cover shows humidifier hooked up to the HUM terminal, not a direct power feed as you surmised.

My humidifier is but mine is not hooked up to that terminal though. I suspect the power feed is supposed to go to that mystery transformer at the top of my picture. I have not traced out all the wires yet but will do so and figure out what is going on there. For now though I think I'm good.

I want to offer my sincere thanks for your willingness to help me out and for your advice. Who knows how long it would have taken me to stumble on this on my own, or if I hired a tech if he would have just unplugged the wire or kept replacing parts until he ran out. Your posts were greatly appreciated. If you find yourself in Akron Ohio let me know. The beers are on me.

Best regards
Tom

 

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[ThatSeattleGuy]

Sorry to revive an ancient thread, but this is what comes up first in the Google searches for "induced draft blower won't shut off". Which is what my issue is...

I have a 1980's-era Lennox G16 75K BTU gas furnace that's still running strong - it's been a real workhorse. (Although, true, it doesn't have to work all that hard in Seattle, compared to MN where I was from). Lately, however, the induced draft blower sometimes (not always) keeps running past the main blower fan shutoff. I'm assuming there's a stuck relay somewhere, since if I bang on the furnace, it almost always immediately cuts off.

My question...where is that relay? Is it in the RobertShaw ignition control (a universal 780-745 that's about 12 years old IIRC). Or is it external, somewhere else in the furnace? Just don't want to go replacing the pricey ignition control if it's just a small separate relay that's the issue. Thanks for any tips to a newbie!

TSG

 

[Fitter30]

Proabl tripped on flame failure three times and lockout with prepurge running. To reset kill power for a second

 

[ThatSeattleGuy]

Proabl tripped on flame failure three times and lockout with prepurge running. To reset kill power for a second

No, like I said, it's mostly running fine. Maybe only about 20% of the cycles does the blower not shut off; the other 80% of the cycles it's fine.

It's not a lockout issue.

 

[Fitter30]

Tap on the board with a screwdriver handel could be a stuck relay with no power on w.

 

[ThatSeattleGuy]

Tap on the board with a screwdriver handel could be a stuck relay with no power on w.

Fitter, I know you mean well, but you're not reading my original question. I already said I could tap around the control area to shut the motor off, and so I already know it's stuck relay. You repeating that doesn't provide new information.

What I asked - and what I hope someone else will still answer - is whether that induced draft relay is in the Robertshaw ignition control box, or external to it - somewhere else in the furnace. That's the question I'm asking.

 

[Fitter30]

If theres no lock out and no power on w it's the board . Surprised that the idb is on without a call for heat pressure switch made me board doesn't lock out because contacts shouldn't be made.

 

[ThatSeattleGuy]

What I asked - and what I hope someone else will still answer - is whether that induced draft relay is in the Robertshaw ignition control box, or external to it - somewhere else in the furnace. That's the question I'm asking.

>sigh< I found the manual online here: http://www.completeheating.ca/images/pdf/LennoxG16.pdf
It's clear from that (page 4/figure 5) that the relay is in the lower control box, near the motor - not in the RobertShaw ignition control box.
Sometimes it's best to answer your own questions, I guess?

 

[Dana]

>sigh< I found the manual online here: http://www.completeheating.ca/images/pdf/LennoxG16.pdf
It's clear from that (page 4/figure 5) that the relay is in the lower control box, near the motor - not in the RobertShaw ignition control box.
Sometimes it's best to answer your own questions, I guess?

That's true, especially when dealing with equipment that was installed during the Reagan era. Often times the manuals aren't even online, and the board-level spares to swap out might not be available. At some point it will be worth just moving on to something newer, more efficient and more reliable.

When you do move, note that a 75K-in/60K-out furnace is sub-optimally oversized for all but the largest and/or leakiest homes in Seattle, which takes a toll on comfort. The 99% outside design temps in King County are in mid-20s F, and the 99% design heat loads for a typical 2500' 1980s house would be less than 40,000 BTU/hr, many are under 30,000 BTU/hr. The output of your furnace is a whopping 61,000 BTU/hr, so even when it's 25F outside it's duty cycle might still be only 50%. From a comfort point of view that means you get the warm (and often noisy) flush of heat during a cycle followed by a long drafty cooling off period. Sometimes the cycles aren't quite long enough to fully heat the rooms at the end of the duct runs.

Per ASHRAE's recommendations the furnace output would be no more than 1.4x the load at the 99% outside design temp. That way when it's 25F outside the furnace is running a (1/1.4 = ) 71% duty cycle- the on-cycles are more than 2x as long as the off cycles, and it will be at a lower cfm (=quieter, with less wind chill), and the off cycles can be brief. Yet that's still enough of an oversize factor to fully cover the load during low single-digits or even 0F weather, rare in King County (but it happens). To those unaccustomed to right-sized equipment the furnace may seem like it's "struggling to keep up" when it's in the teens or cooler outside, but the overall comfort levels in the house will in fact be higher at all temperatures (until it's actually losing ground, which for an exactly 1.4x oversized furnace in Seattle would begin at around 0F outdoors.) Sub-0F cold snaps in Seattle are as brief as they are rare, so this is really the right thing to do. Most of those rare sub-0F hours in a decade happen in the pre-dawn hours when most people are still cozy in bed.

To get a handle on the actual design heat load for the house, run a fuel-use against degree-day data type load calculation, which uses the existing furnace to actually measure the heat load, including all of the parasitic duct losses, etc. Upsizing the furnace by 1.2-1.4x from the 99% temperature bin using a fuel-use derived heat load calculation will never leave you cold in a location as cool but temperate as Seattle (though it can in warmer sunnier climes.)

Many HVAC "pros" will tell you you're crazy to install a furnace half the size as the original, but in many homes doing just that is ESSENTIAL to making the house truly comfortable. But there is more to comfort than merely correctly sizing the HVAC- most houses of that era leak copious air, have no foundation insulation, and have gaps in the wall & /or attic insulation. While contemplating the next furnace swap it's useful to fix those deficiencies of the home's thermal envelope, beginning with AIR SEALING beginnig with the biggest leaks first (which might not be obvious), followed by even more air sealing, and only then rectifying the insulation issues. (The exception is foundation/basement walls- they're a huge air & heat leak that often adds up to 25% of the total heating bill, but often an even bigger factor in the comfort problems. A 2 story house that's cool/cold on the first floor and warm on the second floor almost always has huge basement air leaks, as well as air leaks out the top floor ceiling plane.)

As the building envelope issues get fixed, the heat load also drops, further reducing the size of the furnace. If the furnace gets swapped out before the rest of those issues are dealt with, size it no more than 1x the calculated heat load- most of the time even 0.9x will work out just fine, as long as you really follow through with the rest of the plan.