System Pressure During Boiler Cycling

BKJohnny · February 2020

Trying to get the controls working as efficiently as possible for the single pipe steam system on our 16 unit apartment building. The boiler is oversized, so cycling is inevitable (1,875 sq. ft. EDR for the convectors, 2,700 sq. ft. net EDR for the boiler). The pressure doesn't reach the high pressure limit until the system is uniformly hot, so I think everything is working correctly, but the oversized boiler is producing more steam than the convectors can condense.

I have the Pressuretrol set to cut in at 0.5 PSI and out at 1.5 PSI. At those settings, the system pressure drops all the way to 0 and the vents open each time the boiler cycles. The system calls for the burner to come on at 0.5 PSI as it should, but there's a delay between the call and the burner firing as the burner goes through its startup and purge. Should I adjust the cut in/cut out pressure up so that pressure is maintained throughout the cycle? I remember reading in Dan's book that allowing the vents to open during cycling might be a good thing, as it would allow any air trapped behind a vent that closed on pressure rather than temperature to escape, but I don't think that's an issue in this case.

I'd appreicate your thoughts!

Jamie Hall · February 2020

On a big boiler like that, it's hard to avoid having the pressure drop to 0 during the post purge/pre purge cycle. You can try setting the cutin pressure higher (without setting the cutout pressure higher -- depending on the type of pressuretrol you have there are various ways to do this).

On the other hand, on a one pipe system such as yours, I think Dan's got a very good point in letting the pressure drop to reopen the vents -- if they need to. On the other hand, I wouldn't set the delay too long -- there's a lot to be said in terms of response for the fact that the boiler won't have gone much off the boil and all the piping is still hot.

JohnNY · February 2020

I think you're assuming the pressure drops to zero during that downtime part of the cycle. And if the vents are hot, they will be closed. Either way, as you're describing it, none of this sounds terrible especially since you're talking about a boiler oversized by about 50%. My approach to this would be to add a 0-5 psi gauge or smaller and watch the pressure then. I might also be tempted to downfire the boiler by 10%.

exqheat · February 2020

I use a delay after fire time to thermostat satisfaction for the pressure to drop for vent opening, and to allow the heat to fill the room. This makes for more even comfort and savings on fuel use.

EBEBRATT-Ed · February 2020

@BKJohnny

I would leave it as is. Raising the pressure will do no good. The fact that you said everything gets heat before it builds pressure is a condition everyone should strive for

ScottSecor · February 2020

I did not notice whether you have a atmospheric burner or a power burner but I suspect the latter. If I'm correct, I too would recommend you reduce the firing rate and thereby get slightly longer run times and perhaps provide slightly more even heat to the building. Keep in mind that the burner will have a minimum input rating.

We work on a lot of power burners and have similar issues often, especially with a long pre-purge and post purge. Lately it seems that every commercial steam boiler we service is over-sized for the load and we struggle with the cycle times of the burner (especially with a mild winter here in NJ).

System Pressure During Boiler Cycling

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