Cycling time with vapor system

motordiscord628

so I have my main vent installed now (loooong story with photos on this board) and now my system is running at no more than 8oz of pressure. Now I have a new issue - I never actually trigger the vaporstat. With the main vent I’m seeing the boiler get up to about 5-13” of water and just shut down. Cycle of about 2-3 minutes on, 3-5 minutes off. The steam is getting to radiators just fine but I don’t like the cycle times. The vaporstat is a new old stock mercury vaporstat. The thermostat is a museum piece (photo included) and the temps are pretty much dead on. There was always a plan to replace the thermostat but I’m not sure what’s going on with the cycling, unless it’s normal?

ethicalpaul

If you're sure the vaporstat isn't cycling it on pressure, then it's possible I guess that the thermostat is cycling it, but that sure seems unlikely to me.

Just to make sure it's not the vaporstat, try turning up the thermostat to like 5 degrees above the current temperature and see if it still cycles. If it does, it would tend to rule out the thermostat.

Or even better, jumper over the thermostat wires to take it out of the equation entirely. But don't forget to remove the jumper after the test haha

PS: if the temps are pretty much dead on, why is the thermostat set for 67 but it's 74 in there? Did you just lower it before the pic? Or perhaps you took the photo in the Summer 😅

mattmia2

What happened with that thermostat before? I forget exactly how it is wired but old series honeywell thermostats had a separate power source for the anticipator or something like that. /you have to be careful connecting it ti a modern boiler with t-t.

EBEBRATT-Ed

Agree with @mattmia2

I don't know what kind of burner control you have but I don't think you can use and old 3 wire stat to run a 2 wire control, you can however run a 3 wire control with a 2 wire stat.

You have to find out which control is shutting the boiler down.

Thermostat

Low water cut off

pressure control

Jamie Hall

That's not right. You can check the vapourstat very easily, though — watch the mercury bulb tilt.

I'm pretty sure that thermostat is two wire, and the anticipator — like most if not all Honeywells — is powered by R and W and only needs two wires from the boiler.

mattmia2

I'd disconnect one of the t-stat wires before trying the jumper test just in case there is something weird about it that might damage the t-stat if it is jumpered.

109A_5

My guess is excessive heat anticipation, maybe there is a way to adjust it.

109A_5

Hello motordiscord628,

I believe your Model 77 Thermostat is a Honeywell Series 10 complaint device. Probably not directly compatible with newer systems, as far as the Heat Anticipator is concerned. If your system is connected to the W and R terminals of the Model 77 Thermostat the current is always going through the Anticipator heater resistor during a call for heat like the more modern round Honeywell T87F would.

With a Series 10 Thermostat it appears the Anticipator heater was only used (energized) near the end of the heating cycle (B terminal opens, heating the resistor) to end the heating cycle sooner. I think the heat generated in the Model 77 may be greater than that of a round Honeywell T87F. So in your case the Anticipator heater resistor is energized for the whole call for heat, so the call ends sooner than what is considered a normal runtime, due to the extra heat.

http://www.hvac.amickracing.com/Controls/Heating%20Control%20Handbook-Honeywell.pdf

Using one set of contacts in a Fan Center or a R8845U Universal Switching Relay for contact Number 1 and an isolated contact Number 2 controls the heating equipment the Model 77 Thermostat may work as intended.

One possible relay addition for better Model 77 Thermostat performance. The trick is will the heater resistance hold the relay engaged when the B terminal opens on a temperature rise.

EBEBRATT-Ed

Like I said you can't use a series 10 three wire stat to drive a two wire control