Old Thermostat (Honeywell T8085A)

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Old 01-05-09, 01:33 AM
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Old Thermostat (Honeywell T8085A)

Hello all.

I've got an old Honeywell T8085A that's been faithful for about 22 years. A couple weeks ago, it's started to misbehave a bit. It will work pretty well for about half-day, then it will start to act up.

Here's what's happening: The stat will trip, the furnace ignites, and then about 45 seconds later, the stat will drop out. I watched the little mercury tube in there move after the 45 seconds (or so). This is supposed to move when the temp increases, but the fan hadn't even had a chance to come on yet. When the stat dropped out, the burner had been on for 45 seconds, so then the fan kicked in to cool down the furnace. The furnace unit appears to be functioning properly.

So, there is some kind of an adjustment inside. It looks like a wire-wound resistor with a slider-contact that can be adjusted. I imagine this is to adjust the "sensitivity" of the stat?

Where should that be set?

And, since this thing is turning itself off before any temp change, does that mean it's toast? Or can something be done to fix it? Will that adjustment help?

thanks

Dana
 
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Old 01-05-09, 10:09 AM
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The resistor is the "heat anticipator." It's a small, adjustable heater that makes the thermostat react to air temperature changes more rapidly.

That it suddenly is shutting off the furnace way too early suggests someone has changed the setting --- do you have children in the house? That's just the kind of thing I would have done as a kid!

If the setting is to one extreme, reduce the setting by moving the arm more to the middle. If you move it too far, the furnace wont turn on often enough and you'll get temperature swings you'll find to be annoying.

You can measure the AC amperage that flows when the furnace is on, the correct way to make this adjustment, but winnging it will probably work fine for you.
 
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Old 01-05-09, 10:13 AM
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You could try cleaning it up a bit by using some spray contact cleaner, but I wouldn't put a lot of time into it. Honeywell lists it as obsolete, so getting any parts would be difficult. Might be looking for a new programmable model.
 
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Old 01-05-09, 10:40 AM
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Originally Posted by lacruiser View Post
:

And, since this thing is turning itself off before any temp change, does that mean it's toast? Or can something be done to fix it? Will that adjustment help?

thanks

Dana
:USAF:

Time to retire the old Stat, This one has served it's time. 22 years is long enough; spend $50-$75 on a new one.
Remember to dispose of the old one properly, it contains mercury.
 
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Old 01-05-09, 12:03 PM
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Originally Posted by Mr Happy View Post
:USAF:

Time to retire the old Stat, This one has served it's time. 22 years is long enough; spend $50-$75 on a new one.
Remember to dispose of the old one properly, it contains mercury.
Yeah, I know it's definitely past it's prime.

However, I take pleasure in coaxing the maximum utility out of old stuff. I guess it's my contrary nature, and a small protest to our society's tendency to simply toss a part into the landfill rather than try and get it back into working condition. I've been able to keep tons of stuff going way past the point where most folks would have given up.

Including my furnace. I had a problem a couple years back, and I was told by a couple techs that I should just replace the whole unit, that it's old and not so efficient, etc. Well, I found the problem and fixed it for absolutely nothing. It works just fine now. And since I live in a temperate climate and the gas bills aren't too bad, it was the smart move, economically.

The funny thing is, the guy who installed my furnace 23 years ago asked if we wanted the Rheem that we went with, or one of the new "high-tech" and more efficient ones. He said that he couldn't recommend the newer ones, because they were problematic. I like high tech, but it often comes at the cost of reliability. I really prefer good solid stuff that works forever, rather than the latest and greatest and newest.

We just bought (at considerable expense) a brand-new pair of Whirlpool top-of-the-line Duet washer/dryer. The washer works great, but the dryer is giving us all kinds of trouble. Turns out the old vent ducting is too restrictive (too much backpressure) for the new high-tech machines to work. (they blow through twice as much air as the old low-tech machines) The back pressure causes the burner flame to re-shape, tripping the high-limit switch, so the dryer never warms up. The duct is only 10 feet long, and is not restrictive. I have no idea what we're going to do at this point. I'm not the only one with this problem.

So, if it's a possibility to easily fix this old stat, I'd prefer to do that. Don't get me wrong, I'm not going to make a career out of trying to fix this thing. But if I can easily do so, I'd like to try.

So, in that vein, what tends to wear out on these things? The mercury tube is attached to a coil spring that's obviously the temperature-sensing device. Does that part just go bonkers after awhile? I can't imagine why. It was pretty weird watching it just move the mercury tube even though there was no temp change going on at the time.

Also, could someone please tell me what the adjustment does? It's a non-linear scale from 1.2 to .10. Where should this be set, usually?

thanks

Dana
 
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Old 01-05-09, 12:25 PM
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That thermostat will probably be good for additional decades of use. The idea that it needs to be replaced because it's twenty years old is ridiculous.

The heat anticipator allows the thermostat to match up with the furnace. Ideally it's set by measuring the AC amperage that flows when the furnace is operating.

But you can adjust it by gradually zeroing in on an acceptable number of furnace cycles per hour and/or temperature swings.

Too few furnace cycles --- the temperature gets uncomfortably cool some of the time. Too many (that's what you have now) and the heater in the thermostat keeps it too high to turn the furnace on.

Personally, I have a Honeywell clock/thermostat I installed in 1985 that has a mercury bulb for the thermostat as does yours. The clock quit operating years ago, but the thermostat portion will no doubt work as long as I want to use it.

I have a much newer Honeywell digital thermostat that's been sitting around for several years --- I use it as a clock. I really haven't had a reason to spend the 20 minutes that it would take to replace my existing thermostat.
 
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Old 01-05-09, 01:09 PM
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Originally Posted by lacruiser View Post
So, in that vein, what tends to wear out on these things? The mercury tube is attached to a coil spring that's obviously the temperature-sensing device. Does that part just go bonkers after awhile? I can't imagine why. It was pretty weird watching it just move the mercury tube even though there was no temp change going on at the time.

Also, could someone please tell me what the adjustment does? It's a non-linear scale from 1.2 to .10. Where should this be set, usually?

thanks

Dana
OK, Make sure the T-stat is level, I would start there, Then take approximately 4' of #18 or #16 ga wire and an amp probe, wrap the wire around the clamp of the probe 10 times. Then use the 2 free ends of the wire and place one on the R terminal and the other on W while the unit is in heat, whatever that reading is what the anticipator should be set at.

Now the coil that the mercury bulb sets on, that coil expands and contracts with heat (temp changes) and tilts the switch to make the contacts open and close either in heating or cooling mode. Sometimes (Especially after 22+ yrs) there is a collection of dust and crud that can build up on the coil, use a can of compressed air (dust off) this should help.

I was a firm believer in the 80's on the T-87, and other Honeywell T models, now I am a believer in the new technology, due in part to energy savings with a set back electronic stat.


Hope this helps, GOOD LUCK with the antique
 
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