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Calab 03-03-2011 08:00 PM

Thermostat issues on old furnace - questions before I replace thermostat.
We have an old Airco forced air gas furnace, model #HBS2-100/90. We've been having a few issues lately, so I'm looking for some opinions. I haven't found a date stamp on the furnace, so I'm assuming that it's at least 20 years old.

The furnace has two wires going to the thermostat (red & black). There are more wires available in the wall, but are not connected at either end.

Our thermostat was a battery powered 7 day programmable unit and did the job, although I didn't really like it. Recently we were finding that the heat was staying on longer than expected and the house was warmer than we had the thermostat set. After new batteries and trying a few different programming settings we decided that the thermostat was failing.

At this point we installed a mechanical (mercury switch) Honeywell thermostat that our father had stored in the garage. With this installed, the burners come on for about 45 seconds, they shut off then the blower comes on for about a minute, then shuts off. This repeats every couple of minutes. Definitely NOT what we want.

I just jumpered the red/black wires today and the burners and blower ran the whole time without shutting down, so the furnace isn't overheating or anything like that.

At this point I figure we just need to buy a new thermostat. Since I'm doing this I'd like to use one of the wires to allow the blower fan to be controlled by the switch on the thermostat - currently I need to open the furnace and push in a button. I'd also like to run a 24v(?) line from the furnace to power the thermostat instead of depending on batteries.

Sounds doable so far, right?

Now my questions...
- On the inside of the mechanical thermostat there is a dial, or heat anticipator. Adjusting this didn't seem to make any difference to our issues. Would an issue at the furnace affect this and throw the mechanical thermostat out of whack? Would the same have happened with the original electronic thermostat?

- There are no terminals on the fan controller in my furnace. It's just a metal box with Honeywell stamped on it and three wires coming out (Red, Black, Blue). Is there a simple way to add wiring to this to control the fan from my thermostat fan switch?

- Finally, to bring the 24 volts up from the furnace, what wires would I use? There is a black box attached to the side of an electrical box inside the furnace. I assume that this is where the power goes from 120v to 24v. The wires coming out of here are black and yellow. The yellow wire here connects to the gas valve and to the black wire going up to the thermostat.

I can provide photo's or other info if necessary... just not sure what is important and what isn't.

Can someone provide some help?

beenthere 03-04-2011 04:04 AM

Will need pics to know what you have to do.

Grampa Bud 03-04-2011 06:51 AM

Be careful about those wires in your furnace. The "Red, Black, and Blue" ones are 120vac windings coming off your multispeed blower motor. There should be a 4th and/or a 5th wire, probably white that is nuetral.
To run your furnace off of 24vac you will need a small 120/24vac 40va transformer -not the furnace- to power the stat as you have already confirmed by your own test "twisted the red and black wires together" that the furnace only needs a dry closure across those leads to operate. A stat with a battery in it does one of to things 1. It backs up the microprocesser memory in case of a line voltage failure for up to a year. Usually it is only a 1.5 volt AA battery or sometimes two 1.5 volt AAA batteries. 2. The battery could be powering the stat microprocesser on its own and in this case it has an operational life of over a year so there is really no need to jury-rig a 120vac-1.5vdc power supplyto run your stat. If you want a separate circuit to enable a higher blower speed for blower-only operation, well that's a different picture. The blower control leads from the stat simply provide a dry closure. DO NOT CONNECT THEM TO THE FURNACE BLOWER LEADS. Unless you want to see a puff of smoke where your stat used to be. You will need a 120vac x 24vac-40va transformer, a SPDT 24vac relay with contacts rated for the max amperage draw of your blower to accomplish connecting the blower only function of your stat, but what do you do for your A/C?? Instead of trying to reinvent the wheel you could just get a new high-efficiency furnace with a 6-8 wire hookup for the stat that will let all the electronics in the furnace select heat, A/C, blower, with an electronic 7 day programmable stat to provide time of day, temperature, day of week, and a gazillion other things at a press of a button.

Calab 03-04-2011 10:28 AM

Thanks for the reply's guys...

Just a few points...

- I've a good understanding of wiring and won't do something like connecting motor leads to the thermostat.

- There is no A/C here. We plan on getting a high efficiency furnace and A/C installed eventually, but the funds are not there for it now.

- I've picked up an electronic 5/2 thermostat. No power lead, so it runs only from batteries. That ends my running power to the t-stat, but I'd still like to control the fan from the t-stat.

I'll be posting some photo's shortly. Manufacturers label, various connections. I'll try and keep it clear. Lots of wiring to move around.

how 03-04-2011 10:44 AM

The thermostat with the mercury bulb can be watched to help diagnose what is going on with the burners coming on & off. Is the mercury in the bulb rocking back & forth every 45 seconds (turning the gas on & off) or is it consistently calling for heat (mercury on the left hand side of the bulb) but the gas is coming on intermitantly. If the mercury is rocking back and forth on its own without being affected by an alternative source of temp then check the stat anticipater setting. You'd want to try moving the slider control on the anticipator to a less sensitive position.
(Example moving it from .25 to .6). check for any tiny breaks in the anticipater wire or windings.
Because of the coincidence of two thermostat problems I would double check your 24 V thermostat line voltage.

There are ways to slav off your furnace transformer to run an isolation 24/110 relay for a remote blower but that's a bit of trickier info for the web to communcate.

Calab 03-04-2011 10:47 AM

Here are the photos... Sorry about the size.

Calab 03-04-2011 10:51 AM


Originally Posted by how (Post 602201)
If the mercury is rocking back and forth on its own without being affected by an alternative source of temp then check the stat anticipater setting.

There are ways to slav off your furnace transformer to run an isolation 24/110 relay for a remote blower but that's a bit of trickier info for the web to communcate.

I've made adjustments to the anticipater without any real change in the furnace operation. The current thermostat is used and been sitting in a garage for a while - not even sure why it was taken out - so I don't trust it.

I was hoping that the furnace had a connection for remotely controlling the fan already and that it just wasn't hooked up. I'm not going to be adding relays/etc. if that is what it needs. I can live with the control in the furnace if need be. Hopefully we'll have this furnace replaced in a year or so anyhow.

Thanks guys!

how 03-04-2011 11:28 AM

Thats a great work horse of a furnace. Judging from the picture of the controls & the vent limit switch its a 1984 vintage or newer. Take care of it and you can expect another 12 -18 years on the exchanger despite what you'll hear from furnace sellers. All the parts on it are easely available, cheap and the diagnostics are simple.
No way to remote control the blower here except with a 24V/120V relay or a huge 120Volt line & switch up to the location of your choice.
Be gentle with the plastic summer fan switch. No twisting or it snaps off.

beenthere 03-04-2011 08:59 PM

Afraid you would have to add a fan control relay to control it from your thermostat.

Calab 03-05-2011 02:10 PM

Just an update... New thermostat is in and works great. Everything is back working as it should.

I'll do without the fan control at the t-stat. Not worth the effort to get working.

Thanks all!

han0115 02-18-2012 08:15 AM

Hi. I was wondering what thermostat you bought

hvac5646 02-18-2012 08:43 AM

IO, you are getting quit a bit of flue vapors staining the burner vestibule. Older standing pilot furnaces would do that, but i would suggest getting it checked. You could be blowing fumes into the living area when the fan comes on.

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