Pressuretrol Cut In

RuffinoVino

If you have the cut in as low as the setting screw will allow(<.5), will this in anyway effect the boiler to turn on when the thermostat calls for heat during a normal cycle? Meaning, if the boiler ran for an estimated 25 mins until the thermostat was satisfied can the very low cut in cause the next cycle call for heat to be delayed because the pressure in the boiler remains above the cut in of .5?

EdTheHeaterMan

How is this question any different that the other? The pressure control has no affect on the burner. The burner you have will heat all the radiators without ever getting to a half pound.

Your system needs to actually build pressure before the pressure switches do anything.

dabrakeman

Likely not but it could. I think it best to test the pressuretrol if going for the lowest setting. Crannk the thermostat up high enough to force the boiler to shutoff on pressure via the pressuretrol. Then verify that it comes back on before the thermostat is satisfied. If you ahve a low pressure (3psi or 5 psi) gage you can also verify what the actual cutout is. Seems it is often higher than the 1.5psi that you 0.5 setting with the 1.0 differential would theoretically command.

RuffinoVino

Well from a logical standpoint of the cut in means at .5 lbs of pressure the boiler will be allowed to turn on then the boiler must generate some pressure correct?

KC_Jones

When the pressutrol cuts it off, you will drop to zero in less than a minute. Most likely seconds. Mine never gets above 4 ounces and when the call for heat ends, it's at zero in about 5 seconds.

RuffinoVino

Ok thanks kc that makes it more clear.

RuffinoVino

KC that almost immediate pressure drop occurs even when the thermostat being satisfied turns the boiler off rather than the pressuretrol?

109A_5

Hello @RuffinoVino,

Using the example of;
Cut-In = 0.5
Differential = 1
the Cut-Out = 1.5

I think you are missing the point. Cut-in means 'Cut BACK in' or return to being turned back On (the switch is closed), it is the pressure that the when the pressure is at or drops back below (less than) 0.5 PSI the Boiler is free to operate.

So if you start with a cold Boiler and everything is at local atmospheric pressure or Zero PSI as far as the equipment is concerned. The Boiler will run until it gets to the Cut-Out or 1.5 PSI (Cut-in plus the Differential) The Boiler will not come back on until the Cut-in pressure is at or less than 0.5 LBS.

So if the Cut-Out pressure is achieved the Boiler will be off while the pressure is in the range between 1.5 and 0.5. Once below 0.5 it can come on again.

The Differential is actually the hysteresis of the switch and when it all starts at 0 PSI the switch is closed and the Boiler's burner should work.

If the pressure never gets to 1.5 PSI nothing happens as far as the Pressuretrol is concerned, the switch remains closed and the burner is free to operate as the thermostat commands.

RuffinoVino

Hi 109A_5, 
So you're saying the cut in setting will not matter or engage if the cut out was not reached?

EdTheHeaterMan

RuffinoVino said:

Hi 109A_5, 
So you're saying the cut in setting will not matter or engage if the cut out was not reached?

EXACTLY CORRECT

If you don't turn the switch OFF, then you can't turn a switch that is already ON... "MORE ON"

So on a 0.5 cut in plus a 1.0 diff, the switch will turn OFF when you reach 1.6 PSI
(Higher than 1.5). If you never get above 1.5 to turn OFF the switch, then the switch will still be ON. If the pressure is at 1.3 when the thermostat is satisfied, then the pressure switch is still ON. If the thermostat calls for heat before the pressure drops too much(like for example the pressure drop to 0.7 psi), the burner will still come back ON because the pressure switch is still ON.

RuffinoVino

So if what your saying is correct then at what pressure point does steam start to leave the boiler to the mains? It seems like there has to be some component that regulates this and your sure the pressuretrol cut in does not control a normal heat cycle?

EdTheHeaterMan

RuffinoVino said:

So if what your saying is correct then at what pressure point does steam start to leave the boiler to the mains? It seems like there has to be some component that regulates this and your sure the pressuretrol cut in does not control a normal heat cycle?

Steam leaves the boiler as soon as the pressure is higher than the air pressure in the system. The amount of pressure does not matter. It just needs to be higher. Ig yher air pressure in the radiator is zero on a pressure gauge and the steam is at 0.002 PSI, that pressure is higher than the air pressure in the pipes.

ratio

The pressuretrol is open on rise. The cut out point is the pressure that the contacts open & shut off the burner. The cut in point is the pressure when those contacts close to allow the burner to restart. Note that the steam pressure is not controlled by the pressuretrol, it is merely limited by it. If amount of steam your boiler produces is well matched to the amount of steam your system can use (condense), your system will heat without ever showing any pressure. Depending on how well piped & maintained it is, it will do that silently & evenly as well.

EdTheHeaterMan

Now the pipes and elbows cause friction loss to that steam pressure... As the steam moves thru the pipes and fittings, that friction will use up some of the steam pressure energy of that steam. So, to move further down the line, the pressure must increase to overcome that friction... to maybe 0.009, then to 0.01. then to 0.05 then to 0.1, then to 0.15 PSI.... and eventually all the way up to 1.0 then 1.1 then 1.2 PSI. This will keep getting higher and higher until the radiators heat the room enough to satisfy the thermostat.

RuffinoVino

Thanks to each of you for breaking it down to such simplicity that now I fully understand it! 
In another note, with adequate main venting do you want to use larger radiator vents to get heat as fast as possible thru Al the radiators or should you do each of them as slow as possible (evenly balanced) to avoid over shooting the set temperature after the boiler shuts off?

109A_5

Hello @RuffinoVino,
Again but a bit different; I have one of those very few and rare examples Boiler SQ. FT = 300 and the EDR = 347. So my radiators will dissipate the heat faster than the Boiler can get it to the radiators. My pressure never goes over 2 inches of Water Column (2 WC = 1.156 Oz). 16 Oz = 1 PSI for a point of reference so my pressure is always WAY less than 1.5 PSI. My Pressuretrol never trips (switch shutting off). Regardless I have it set as low as it will go Cut-in = 0.5 and the Differential = 1. By the way 27.7 inches of Water Column = 1 PSI.

In other cases (with oversized residential boilers) there is no usefulness making any higher pressure (assuming your system can and will) so the Pressuretrol should be set for the minimums.

So in my case the Pressuretrol is just a safety device (as I believe it was originally intended to be by the manufacture) in case the steam main plugs up or something bizarre happens that would allow the boiler pressure to rise significantly.

Jamie Hall

Being a belt and braces kind of fellow, I prefer to see systems have two pressure control devices -- one, a vapourstat for better accuracy and precision, to keep the pressure in the operating range (below 8 ounces for a vapour system; 1.5 psi for most others), and a second one, usually a pressuretrol, as a safety at a higher pressure (3 psi cutout) to protect the entire system from overpressure. It would be good if that second one was manual reset as well, as that protects against the latent failure condition of the vapourstat failing and not being noticed.

SteamingatMohawk

Hey, Jamie, are braces the same as suspenders? I'm just an old baby boomer.

Jamie Hall

Just the same!