HVAC mags

[OSECS]

Our HVAC first floor system seems to be constantly running. I turned the thermostat switch to the off position, but it's still running. Any thoughts as to what's happening ??

[trainnvain]

Thermostat is shot.
Even if it's digital, it still runs on a mercury spring.
Just not sending a low voltage signal back to the boiler/furnace to shut off.

Obviously, check thermostat connections first.

[OSECS]

Thermostat is shot.
Even if it's digital, it still runs on a mercury spring.
Just not sending a low voltage signal back to the boiler/furnace to shut off.

Obviously, check thermostat connections first.

Never replaced one of those. Is it DIY ?

[trainnvain]

Never replaced one of those. Is it DIY ?

You bet, just match the colors and it's all low-volt.
Snap off the top, find the two main baseplate screws, unscrew, unscrew and reconnect all terminals and you're set.

[OSECS]

Thanks man, my Saturday project dance card is now filled.

[Snow Dog]

Thermostat is shot.
Even if it's digital, it still runs on a mercury spring.
Just not sending a low voltage signal back to the boiler/furnace to shut off.

Obviously, check thermostat connections first.Fail. Electronic thermostats use a thermistor (temperature sensitive resistor) to measure temperature.

Some HVAC systems are set up to always run the fan even if it's not heating or cooling. The wiring to the thermostat might be 2-wire (heat, common), 4-wire (heat, cool, fan, common) or more if you have 2 stage heating or cooling. The control module on the HVAC unit has (typically) jumpers to configure the operation and might ignore the fan switch (I assume it's off) on the thermostat.

[OSECS]

Right now the thermostat unit is in the off position, but the fan is still running. It's been on constant run for a couple days I believe. That's unusual.

I have no clue as to whether the system is multi stage or not. It was originally installed in the late 80's early 90's I believe. Natural gas furnace with a central air conditioning system.

[marmot mb]

As you say thats unusual for the fan to run 24 / 7, then I assume your furnace is not wired to normally do that.

it is likely the fan temperature/ limit switch on the furnace.

This device operates as follows. when the burner is engaged by the thermostat the flame comes on and generates heat in the heat exchangers.

the fan limit switch waits until the heat exchangers are at a certain temperature to turn on the fan. (otherwise it would blow cold air at full RPM)

Ther furnace then runs until the thermostat is satisfied. The burners then shut off.

The fan limit switch then continues to run the fan to strip away remaining generated heat. Once the set point is met on the fan limit switch the fan will turn off. This portion of you fan limit switch is A. stuck, find it give it a loving wack with a screw driver. B. is fuckered. replace.

hope this helps.

[OSECS]

So I didn't swap out the existing thermostat. I've been monitoring the situation is little closer. This is what I'm finding: The thermostat is an older analog unit, original to the late 80's early 90's system.

In the heat mode the the fan runs constantly (don't remember it doing that in previous years) Once the programmed temp. has been met. I seem to remember it cycling off once programmed temp. is met in the past.

In the AC mode the fan will shut off once the desired/set temp. is reached.

If I turn the unit to the off position (between Heat and AC) the fan continues to run, blowing cool air. Again this seems completely wrong or why else would there be an off position between heat and cold ??

There's a separate fan switch which can be in the "auto" or "on" positions We always have the switch in the auto position.

[OSECS]

Just a little Sunday bump

[Core Shot]

Open the cover to your tstat and disconnect all the wires. If the fan still runs, you have issues with the furnace.
if the fan stops running, you need a t-stat.

It could be your auto/on fan switch broken on the tstat.

you can also re-boot your furnace by flipping the swtch off and back on (that is the switch in the furnace room, usually right near the furnace. it looks like a regular light switch and will cut the power to the whole unit).

sounds like its in your furnace, but getting it fixed for less than $100 wont be easy unless you know someone smarter and handier than you that can do some diagnostics.
good news is, it should not be too expensve in parts.

other good news is that heating season is almost over, and having your fan run 24/7 aint that big a deal.

[MMP]

my ac compressor is running and the pipe running to the air handler is iced over but the fan wont run, it's 100 degrees out, and I'm getting irritable, please help!! I checked circuit breakers. i turned the T-stat up so the compressor would turn off and maybe when thawed start blowing air again, but i have no idea if that's the right thing to do or in any way useful.

[OSECS]

my ac compressor is running and the pipe running to the air handler is iced over but the fan wont run, it's 100 degrees out, and I'm getting irritable, please help!! I checked circuit breakers. i turned the T-stat up so the compressor would turn off and maybe when thawed start blowing air again, but i have no idea if that's the right thing to do or in any way useful.

Just stayed in a hotel that had individual units in each room. It did exactly what you're describing. Can't remember what the tech who came and fixed it said, but they replaced the whole unit. Fairly common problem. You need to arrange a service call.

[jgb@etree]

my ac compressor is running and the pipe running to the air handler is iced over but the fan wont run, it's 100 degrees out, and I'm getting irritable, please help!! I checked circuit breakers. i turned the T-stat up so the compressor would turn off and maybe when thawed start blowing air again, but i have no idea if that's the right thing to do or in any way useful.

You're doing the right thing, but I dunno what up with your fan not running.

I had the same issue the first couple of times I turned the AC on a couple of years ago. Had the Carrier guy out to the house, and it was just a dirty air filter (it really didn't look that bad to me).

His instructions in thye event that it ever happened again (luckily, it hasn't) was:

1. Turn the AC off, but turn the fan on (fan helps thaw things out)
2. Replace the air filter
3. Wait for the system to thaw and flip the AC back on.

He didn't charge me a dime for the visit since it literally took him 5 mins to diagnose & fix, and it worked so it's worth trying if you can get the fan going. Dunno if it's possible, but maybe the fan is iced up too?

I'd swap out (or completely pull if you don't have a clean new filter) the filter, turn it off until it thaws, and try it again.

Hot as fuck out today. Hope you get the AC back on soon!

[Pow4Brains]

The pipe is iced over because the fan is not running. This is weird because they should be interlocked and one should not run without the other, unless of course, the motor is toast. If you have the means to check if you have voltage at the motor start there.

[MMP]

i think the blower motor is toast. I took the cover off and the circuit board is powered, i turned off the breaker to try to cycle the power & see if it would reset, it didn't. I don't have a meter, but have a guy that works for me coming to look tomorrow. tried banging the motor to see if i could get it fired up. nope. it's hot, i'm unhappy, and obviously much too soft in general. [/pity party]

thanks for the advice

edit [pity party] the only beer in my fridge is a michelob light ultra [/pity party]


edit: blower motor circuit board is toast. shouldn't be a big deal to replace. It's cooler today, and I washed the sand out of my vag.