Thermostat mystery with steam heat??

David_46

So in the spring, I bought a home in RI and inherited a steam system built in the 1920's. I bought "We Got Steam Heat" to educate myself, and actually made some significant improvements, but there is one area I just can't figure out?

The old owners installed a one zone central AC system a while back, and in the process installed a new honeywell "round style" thermostat in the kitchen, and cut the wire to the original thermo in the living room. It also looks like they disconnected the AC line to the transformer on the boiler. The new thermostat wire was hooked up in series between the low water cutoff and the gas valve on the boiler. The thermostat has never worked right, sometimes when you turn on the heat, it will fire right up and work fine, sometimes there is a delay of a few minutes to hours, and sometimes the boiler just never fires.

In troubleshooting this, I noticed something. If I connect the red and white thermo wires (either at the thermostat or at the splice location at the boiler) it fires right up, every single time. "HA!" I thought, "The thermostat is bad!". So I bought a new one (Honeywell T87) hooked it up, turned on the heat and... nothing! I tried it with and without the jumper connecting Rh and Rc, but it still won't kick on. Connect the wires directly though, and she fires right up!

So here's my question... everyone I talk to seems to think the system/thermostat needs 24vac, but to me it just looks like a simple switch? There is NO power hooked up to the boiler at all, but when I connect the wires, boom, heat is on. My thought is that the thermostat is designed to accept 24vac, and without it, there is too much internal resistance to complete the circuit and turn on the boiler. So my question is, should it have 24v? Should it just use the transformer from the AC unit, and if so, why wouldn't the jumper work? And if so, why does it work with 0 volts just fine? Finally, is there a "simple switch" type thermostat I should be using instead (and where do I find one)?

Thanks all, the wife thinks I'm a half genius at this point, I'd like the full genius status :) BTW we did have some local guys come in a couple days ago, first thing the service guy said was "Wow... I have no idea what any of this does! What is this, steam?" and he kept his hands in his pockets the whole time! At least he was honest about it, and didn't charge me!

missafossa

thermostat

it sounds like you have a millivolt heating system. It generates it own current.if you can close the cicuit anywhere near the boiler and get it going and there are no power wires any where thats how they work.millivolt depends on the conductivity of the wire to send the current all the way to the t-stat. tighten and or burnish the wires. seperate rc and rh and make sure the stat you have is compatab ls compatable with millivolt systems.

Scott Larabee

Hmmm...A millivolt steam system! I have never seen a gas steam boiler because in my area #1 there's not too many steam systems, and #2 there is no natural gas, just propane, so gas heat is not common. That said,this type of system would be perfect. No power? Still have heat!

If this is indeed a millivolt system, the thermostat must be a millivolt thermostat as was stated in the last post! Change out the thermostat for the correct type, and the problem should be fixed. Then this winter, when the power goes out, enjoy your heat!

Scott

David_46

Veeeery interesting!! I had never even heard of a millivolt system, but it makes perfect sense. The new thermo is 24vac heating and cooling, so that's going back tomorrow, and I'll start a hunt for the right one asap. You know, the guy that did come by mentioned that the thermocouple on the pilot was generating current (which makes sense to me), that must be what's powering the system! Huh!

You know, while I'm at it I think I'll relocate the thermostat to the old location in the living room... never made sense to me why it was reinstalled in the kitchen, and if the living room is where the original design said it should go, who am I to argue?

Wife thinks I'm a genius, plus I get +10 Macho points for showing her how I can turn on the heat by shorting the thermo wires! Thanks guys!

Scott Larabee

You may need to use a seperate thermostat for the A/C. I don't think there are any thermostats that will handle a millivolt boiler, and a 24 volt A/C. You could use a relay with the current thermostat, using dry contacts from the relay to the boiler, but then if the power went off, you would be without heat (just like most of us!), at least until you touched the wires together!

I just re-read the original post. I noticed you said the transformer on the boiler had been disconnected. Have you tried unhooking the thermostat wires from the gas valve, and jumping out the valve with no wires? It almost sounds like someone cobbed things up when they hooked up the A/C. This may be a 24 volt system that is getting it's power from the A/C transformer.

Would it be possible to post some pics of the gas valve? it may help determine just what you have. Aside from that, you may need to find a good gas tech. At this time, it doesn't much matter if they understand steam. They just need to understand how a gas boiler operates, and the control system.

Scott

David_46

That was my thought too, looks like a separate thermostat is the way to go, and is a good excuse to relocate.

And the transformer was disconnected entirely from mains power! In addition, nothing is connected to it on the boiler. I can jump the gas valve wires and everything starts up, so I think it is a 24V system that was converted at some point to milivolt with a new gas valve, and the guys that did the work were just sloppy and left the transformer and wiring connected to the boiler. It's about par for the course, the rest of their work was pretty sloppy as well (old thermostat wire was just left wrapped around the gas pipe for instance).

After doing a bit more research now, I'm sure it's milivolt. The thermocouple powers the system from an always-on pilot, there is no external power source, etc. I'll post up some pics if the milivolt thermo doesn't cut it though.

Thanks again for all the help!
-David

Bill W@Honeywell

Millivolts...

ANY VisionPro programmable or FocusPro Non-programmable will work with Millivolt systems. The new T87K&N will not.
They were designated with a "S" in the part number. TS87 stats, VS gas valves. TS87 is obsolete.