What happens below the cut-in pressure?

Brian_74

Maybe it's the frigid temps (we just made it to 8°F) but just realized that I don't know what the pressutrol is doing when the pressure is below the cut in.



Just to be sure I'm using the right terms: on an additive pressutrol, the cut-out pressure is the sum of the cut-in plus the differential. The cut-in is set on the outside; the differential requires removing the cover of the pressutrol. The pressutrol turns off the burner when the cut-out pressure is reached. When the pressure is between the cut-in and cut-out pressures, it lets the burner fire if the thermostat calls for heat. What does it do when the pressure is below the cut-in?

Joe V_2

This sounds like a trick question

The switch on the pressuretrol should be made at cut in and below and if the t-stat calls for heat, you get heat.

Brian_74

That's what I thought

But then why mess with the two adjustments if there's really only one thing to adjust? In other words, why doesn't the pressutrol have one setting and it's the cut-out?

Jamie Hall

Because...

the differential is, or can be, a useful adjustment.  Assuming there is some adjustment range in there, changing the differential will change the length of time between on and the next on (essentially, it varies the pulse width).  Depending on the specific system, you may get slightly more efficient operation with a longer pulse width -- or a shorter one.

Brian_74

Does this mean

that the cut-in pressure is what starts the cycle clock?

Rod

Pressuretrol vs Thermostat

Hi Brian - I'm going to just run over the basics as I think you're seeing it all as just one circuit. There are two control circuits to the burner involved, both of which are are connected separately to the burner.

One control circuit is the thermostat circuit. When the room temperature is below the thermostat's setting, the thermostat's "switch" is turned ON which sends a signal to the burner to turn ON.  When the thermostat's setting is satisfied the thermostat's "Switch" is tuned OFF which turns off the burner.

The second control circuit is the "safety" circuit.  This circuit has several controls (switches) which are wired in series so that if any one of the switches is OPEN, the circuit is not complete and the burner will not turn on.   These controls are the Service Switch (the "light switch" usually mounted in a red box connected to the boiler) , the Pressuretrol and the LWCO .

Pressuretrol - The Pressuretrol just responds to pressure.  If the boiler pressure is below the Cut In pressure, the switch is ON and will stay on until the Cut Off pressure is reached. It will not turn ON again until the pressure drops below the set Cut In pressure.



With the thermostat turned off and the boiler cold, as you can see from the above the pressuretrol having no pressure is below the Cut In setting an so the switch is "ON".  When the thermostat is turned ON (providing the other safety switches are closed) the burner will turn on and won't shut off until either the Cut Off pressure is reached OR the thermostat setting is satisfied. If the Cut OFF pressure is reached first then the burner will turn off and not turn on again till the pressure drops to the Cut In setting which closes the Safety circuit and the burner turns on again. This ON -OFF cycle continues until the thermostat setting is satisfied.  I know you already know all this but I thought if it was written out it might help put things in better perspective.



- Rod

Jamie Hall

Two completely separate cycles

Rod's explanation of the two separate cycles is vvery nice -- thank you!  I was referring to the cycling of the burner alone, keeping the steam production rate equal (very nearly) to the system condensing rate.  Superimposed on that is the thermostat turning on and off; it's frequency is set (for the ones which have a cycles per hour setting) by the cycles per hour, and the pulse width is varied by how long it takes for the space to return to the set temperature after the thermostat calls for heat -- this keeping the average heat output of the system equal (again, very nearly) to the rate of heat loss of the space.

Brian_74

Aaah, I think I get it.

Thanks, Rod. I think I get it now. Just to be sure, here's what I think happens. If the pressuretrol had only one setting (cut off), then it would shut off the burner at say 1 psi (if the thermostat were still calling for heat and the LWCO said it was ok). But only seconds later, the pressure would drop below that cut off point and so the burner would go back on. The differential is what tells the boiler to wait a bit before starting the burner back up (and assuming the thermostat is still calling for heat, and LWCO says it's ok). Is that right?

Brian_74

My mistake, Jamie

I was trying to see if I understood you (I didn't). Sorry that I confused matters, but it looks like it led to the right answer.