Radiator vents hiss periodically after system is fully pressurized. Why?

Joe43

I'm a home owner with a pretty good understanding of my steam systems. I have 2 old (1970s?) residential steam systems with 5 single pipe radiators on each.



I test the systems by turning thermostat all the way up.



As system starts coming up to pressure, radiator vents hiss as expected.



Once at full pressure and radiators are fully heated the vents click off.



The boiler continues to cycle off and on to maintain pressure.



Periodically the vents on several (maybe all) of the radiators open up and hiss for about 3 seconds and turn back off. This keeps happening. My question is why!



As I understand it, once the radiators are full of steam and the vents have stopped hissing and the boiler stays up to pressure, steam in the radiators condense to water which runs back to the boiler. As the steam condenses, new steam comes up to replace it. So there should always be steam in the radiators, without the need for vent releases. So why would the vents release periodically?



A second question - even when the vents click off, if I hold a cold piece of glass over the vents, they all show a very small amount of steam being released. Is this by design? This happens even with new vents. Is this a problem?



The boilers need to be manually filled once every 2 weeks or so in the cold New England winter, say 20 degrees average temperature. Is this acceptable or am I loosing too much water?



Thanks in advance for your help.

Paul48

Pressure

What pressures are the boilers running at?

Joe43

Pressure

One has Cut In of .5 and a Diff of 1. The other has 2 and 1. I've gotten mixed recommendations. One guy suggested 3 Cut In  and 2 Diff. What is right? The forums here seem to suggest .5 Cut In and 1 Diff. Thanks.

Paul48

Pressure

.5 and 1 maximum. That will shut the boiler down at 1.5 psi, and even that is higher than it needs to be.

nicholas bonham-carter

hissing vents

my theory is that if the vents hiss, there is inadequate main venting in the system, along with over-pressure. if you put a good low pressure gauge [0-3 psi, gaugestore.com] on next to the other code-required, but useless 0-30 psi gauge, you will know what the pressure is, and will know also how much extra gas you have been buying to force the air out of the system.

one extra thing the vents have to do is to let the air back into the system. when the burner cycles, the steam begins to condense, quickly forming an intense vacuum. this is relieved by air coming back in through the vents. in some ways, there is greater capacity needed to handle this than letting the air out.

show us a picture of your main vents, and we can tell you what you need.--nbc