Boiler short cycling on pressure

Levie

Hi I have a steam boiler that is short cycling on pressure it builds pressure up to 2 PSI and loses that pressure within 2 minutes 
I replaced main vent on the long supply side and have a very short supply side with no vents

All radiators are getting hot and I don't see any steam leaking out of the vents

Steam builds up in the system in about 15 minutes and then starts cycling on and off every 2 minutes

I would imagine a 2-minute cycle on and off is way too short and cannot figure out what else could be wrong.
The only thing I didn't do yet was fill up the boiler to make sure there were no leaks higher than the water line.

Am I missing anything is there any other reason a boiler would short cycle like that?

EBEBRATT-Ed

If everything is heating well it could be the boiler is grossly oversized.

Has it always run like this or is this a new problem?

Check all the radiator valve to see if they are all open.

Are you sure the boiler is cycling on pressure and not low water or some other safety?

What kind of control is operating the boiler? Thermostat?

If a thermostat you may be able to change the cycles/hour or the heat anticipation.


Check the EDR of the radiation and compare it to the boiler name plate.

Jamie Hall

My first guess is that the boiler is much too big. That's also my second guess.

The rapid fall in pressure when the burner stops is perfectly normal -- I'd expect a smaller system such as yours to drop very quickly. That's not indicative of a leak.

Levie

I did not do a calculation to figure out sizing yet I should probably do that next

The system has a lot of fin type steam radiators not sure how to calculate load on those
The boiler is definitely cycling only on pressure as the water is way above the low water cutoff and the pressure gauge is quite on target for the on and off cycles

I don't think a thermostat can fix that

What is the normal cycle of time if it was a sized correctly?

Levie

"The rapid fall in pressure when the burner stops is perfectly normal -- I'd expect a smaller system such as yours to drop very quickly. That's not indicative of a leak."

How long is it usually off for?
Can you give me some reference to understand what is normal and what is not

Tnx

EBEBRATT-Ed

@Levie

We can't help without more information as I posted above. Your there we are not we don't have x ray vision.

What controls the boiler a thermostat? Or a Heat timer control? House or apartment building? New boiler install or existing? Has it always operated this way?

Levie

Thermostat. it's a regular house. it's an existing boiler but not that old looks like someone replaced it a couple years back homeowner just bought the property so I'm not sure how long this is going on for
Thx

Jamie Hall

Levie said:

"The rapid fall in pressure when the burner stops is perfectly normal -- I'd expect a smaller system such as yours to drop very quickly. That's not indicative of a leak."

How long is it usually off for?
Can you give me some reference to understand what is normal and what is not

Tnx

As @EBEBRATT-Ed says, without a lot more detail we really can't get much further. There really isn't a "normal". However, if it is cycling on pressure one can get a rather good idea as to just how much to big the boiler is by waiting until it's cycling stably -- that is, the cycles are all about the same -- and then timing a few. If you take the total time from the beginning of one cycle -- when the boiler turns back on -- to the beginning of the next and divide that by the length of one on cycle -- from when the boiler turns on to when it turns off -- you will have a pretty good estimate of how much too big the boiler is.

Levie

Thanks everyone it's really amazing how fast people respond on this form
One thing I forgot to mention is that I changed out 6 radiator vents that were in the general area of thermostat and noticed that all of them were Gorton c so I replaced them with #4 I seem to think a lot of this house might have very large vents on the radiators can that contribute to fast cycles?
Thx

Levie

Obviously I'm going to do a calculation to figure out how oversized the boiler is

Jamie Hall

Large vents on the radiators really have very little effect on the boiler cycling once everything is pretty well warmed up. The cause of the cycling is simply that the boiler is producing more steam than the radiators can condense, and vents don't change that.

Levie

I really want to understand this one so if there is too much steam and not enough condensing radiators why does that cause rapid pressure drop and cycling?
Thanks learning new things everyday

Jamie Hall

What happens is this. Steam is put out by the boiler, and travels (remarkably quickly -- quite a breeze in those pipes!) to the radiators. In the radiators it cools and at the same time heats the radiator -- but as it cools, it condenses back to water -- and a given amount of steam shrinks to only 1 : 1700th size! That happens as quickly as the radiator can put the heat into the room.

Now however, if the boiler keeps putting out more steam than the radiators can condense -- is too big -- the only thing that can happen is that the steam pressure has to rise, just like blowing up a balloon. The only way to stop that is to turn the boiler off -- which is what the pressure control, the pressuretrol, does. There's no point to making more steam if there isn't anywhere for it to go! But, of course, as soon as the boiler stops the extra steam is condensed... so the pressure will drop.

And so on. Pressure drops low enough, boiler fires up again, produces another shot of steam, pressure rises, boiler shuts off... rinse and repeat!

It really isn't as inefficient as it might sound.

Hap_Hazzard

The output from fin-tube convectors is surprisingly high, but most installers don't know that, so they put them on supply pipes that aren't big enough, and you usually get only partial filling and/or lots of steam hammer. I used to have a nine foot one with two rows of 2" pipe with 4" fins. That's more than 100 ft² of steam, and they put it on a 1" copper runout. Surprisingly it didn't bang, but it didn't heat worth a damn either.

I'll try to find the formula for calculating the EDR, but I'll need to know the pipe and fin sizes and how it's piped.

Levie

Thanks Jamie that was well explained and thanks all for helping this was super useful

ethicalpaul

> What is the normal cycle of time if it was a sized correctly?

It should run the boiler during the entire call for heat from the thermostat, then shut off the boiler until the next call for heat. No pressure should really build in a properly sized system (although even in a proper system you can get it to build for example from a long setback of like 5-10 degrees)

I can count on 1 hand the number of correctly sized boilers I've seen described on this board in 3 years. Of course, the folks with proper boilers are less likely to come to the board for help so it's a bit of a selection bias there