Thermostat replacement

Before we start to worry about that old Honeywell thermostat — which is likely to be far more reliable than anything you could buy today, by the way — we need to be sure what it is we are trying to do.

The first, and probably most important question, is how is the space comfort in the house? Does the temperature hold fairly constant, or does it go up and down more than you would like? If the space temperature is comfortable, changing the thermostat isn't particularly reasonable — if it aint't broke, don't fix it!

Now on cycles. You mention that the system "cycles more than it should" and has "long run times". These are actually opposite conditions. If the thermostat is what is controlling the cycles and run times, there are ways — even with that old one — to get shorter run times. There are also ways to get fewer cycles. Unhappily, you can't do both. Your house needs a certain amount of heat to stay warm. You can do this either with shorter bursts of heat more often, or longer bursts of heat less often. It's a compromise act — the idea is to get the thermostat set so that the temperature of the space holds reasonably constant, whatever combination of "how often" and "how long" that takes.

Now if it is the boiler that is cycling — the boiler turns on and off usually on pressure — while the thermostat is still calling for heat, that is a very different problem and changing the thermostat isn't going to make a difference at all.

So we sort of need to know if the boiler is turning on and off — but your comment on pressure suggests that it isn't. And, incidentally, NOT seeing pressure while the boiler is running is a good thing, provided the radiators are warm. You don't need much pressure to get nice warm heat from steam!

Maybe you need to take the cover off and adjust the anticipator setting. Is this a mercury thermostat? IIRC they have a variable resistor heating element near the mercury glass bulb that heats up the bimetallic coil. If that is set to a slower heating setting the burner should run longer.

Hmm. No, that's one of the REALLY simple thermostats! And, in my view anyway, your "on" cycles are a little short for steam — and a little frequent. Again, in my view, you might be well served to replace it with almost any one of the battery powered modern digital ones — they're available Amazon of at the Big Box home stores. It would take the same two wires, but they do need a battery (there are ones which don't — but with just two wires your life will be much simpler with a battery powered one) and is easy enough to hook up.

Something like this

New! Honeywell Home Non-Programmable Thermostat, Single-Stage, 1 Heat/1 Cool Compatibility, X1N, White - Amazon.com

would probably do just fine.

@geemalar , you say "The system will run, I’ll get little heat from the 2 closest radiators to the thermostat and then shut down. 5-10 minutes go by and the system cycles again."

If only those first two rads are getting steam, the problem isn't the thermostat. It is the steam distribution in your system.

First thing I'd check is your main vents. If they are missing or inadequate, the radiators further from the boiler will not heat well. Main vents allow the steam leaving the boiler to clear the air from the mains quickly. When the steam reaches them, they close, and now every T joint where a radiator is connected has steam in it. So the steam rises to all the radiators at the same time, and the boiler doesn't have to run as long to get heat to all the radiators. This can save quite a bit of fuel.

Take some pics of your boiler, the ends of your steam mains and one or two radiators, and post them here. Let's have a look at this.

Puzzling to me, too. Where is the water level on the sight gauge?

That boiler looks brand new. It couldn't be a millivolt gas valve and you have AC power to the boiler. That thermostat you have has a snap switch in it where the contacts are only rated for 100mA at 24VAC. It is probably in series with controls that are drawing far more than that, unless it is just driving a relay. If it is driving a relay that relay itself may draw more than 100mA.

We would need to see the circuit diagram.

There are so many inexpensive thermostats out there it seems like a no brainer at this point to get one and swap it in.

If you wanted to tinker in the meantime, you could remove the two wires from the thermostat and twist them together and make sure there aren't some defective controls on the boiler causing the early shutdown. Flip that boiler switch off by the boiler before doing that, twist the thermostat wires together and then go down and flip the switch back on and see what's what.

@geemalar Said: "It’s as if it’s cycling on its own, the heat isn’t sufficient enough to trip the thermostat. But it must be the thermostat as the system hasn’t generated any pressure to trip the Ptrol."

With the photo of the boiler having a power switch connecting it to the 120v. house power I know exactly what your problem is.

You have the wrong thermostat. that thermostat is from the older boiler that your existing boiler replaced. You can not expect a millivolt thermostat heat anticipator to operate properly when 24 VAC is applied to that thermostat. You need a different thermostat. Just about any thermostat rated at 24 VAC will do what you need without the short cycles you are experiencing with the T822? thermostat you have now.

Replace the thermostat and set it to 1 cycle per hour and see of that solves your problem. This would be a good one to try: https://www.supplyhouse.com/Honeywell-Home-TH1100DV1000-PRO-1000-Non-Programmable-Heat-Only-Vertical-Thermostat

Or you could try the one that @Jamie Hall suggested above.

This has been a problem since the new boiler was installed, Your installer didn't do you any favor by keeping that old thermostat.