Help with an old thermostat


I have a old boiler furnace. I'm having an issue where the boiler periodically wont come on and the house temperature will continue to drop. If I turn the thermostat up, the boiler will kick on and will maintain the set temp for some time but will eventually fail to kick on again. Based on the fact that I can get it to kick on by adjusting the thermostat, I'm assuming the issue is thermostat related but I'm not sure if the thermostat itself of the sensor...or maybe I''m totally off base.
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On 5 Apr 2007 19:25:23 -0700, snipped-for-privacy@hotmail.com wrote:

And then to get it to go on, you turn it up even higher, or you turn it down and turn it back up again? Or do both work?

OR the sensor? The sensor in the thermostat, right? I might, but very few people would try to repair the thermostat, so if you mean a sensor in the thermostat, you might as well replace the thermostat.
...or maybe I''m

Another way to verify this is to note that that the thermostat connects two wires together. You can take the stat off the wall, and if you figure out which two wires they are, you can touch them together and see if the boiler goes on immediately. If you only have two wires (no AC) it will be easy to tell which two wires they are.
When the stat is totally disconnected, you can also use an ohmmeter on the matching screws of the thermostat and see if the stat goes from infinite resistance to near zero resistance as you adjust the thermostat near the room temperature.
It's possible that the stat works but is wrong as to temp. If that is the only problem then you could just learn to add 3 degrees or whatever from the setting you have it at. This is especially relevant to my first question above. If you turn the stat to warmer, say to 76, but later on turn it back down to 70, the house might get down to 66 or 67, because the stat is miscalibrated and it thinks 66 is 70 and 67 is 71. If that is the case, just set it to 74, and it won't think it is 74 until it is actually 70, which for this example is what you want.
Also, make sure the thermostat isn't getting a warm air breeze or a lot of sunlight, so it's warmer than the rest of the house. Normally they are put where this won't happen, but who knows.
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Is there any way you can get your hands on another thermostat just to test it? Or just buy a new one.... Anything is possible, but it does sound like a thermostat problem to me. If the thermostat is a modern digital type, they can get flakey. Of course be sure the batteries are good. If it's the old Honeywell type with the mercury bubble inside, they MUST be level, but the contacts may be failing, and of course, make sure the mercury did not separate (unlikely but possible).
wrote:

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On Thu, 05 Apr 2007 23:33:16 -0500, Freebird33@hot_ail.com wrote:

Is this like milk separating from cream? I have digital now and I know I will never need to know this, but I'm curious.
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From your description, it sounds like it could be a bad thermostat, but "old" is a relative term. Over the years there have been a number of types of thermostats and controls used on boilers. The most common type today, is what Honeywell calls a "series 80" which is low voltage two wire. There have been series 10 through series 90 thermostats and controls used over the years. The first thing you need to do is identify what you have. Remove the thermostat from its base, and if it's for heating only, note how many wires it has and if they are low voltage or line voltage. Once that's determined it would be easier to solve your problem

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On Apr 5, 10:25 pm, snipped-for-privacy@hotmail.com wrote:

I have a gas furnace but had the same problem. We had an old mechanical thermostat that just wouldn't hold a constant temperature.
I don't really know what goes wrong with those old thermostats but it almost seems like they just stiffen up over time and don't respond to temperature changes fast enough anymore. My old thermostat would also show that it was 68 degrees in the house when I could *feel* that it was about 55 at best. Once it did finally kick on, it would run full blast for hours trying to catch up, and then would overshoot the target temp because again it was not responding fast enough to the temperature change. But then when it kicked off, it would stay off all day again. Sounds like the same problem you're having.
Went out and bought a $25 digital programmable thermostat and the problem was solved. Good news is thermostats are so cheap and easy to install that you could just buy a new one as an easy diagnostic. Took me 20 minutes to install it and required nothing but a screwdriver.
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