cycle time for steam furnace

T. Riley

It's an old system running with cut-out pressure of about a pound of steam. Large residence. Radiators heat well
at the ends of the system. Vastly oversized boiler with one
of two gas inlets shut off for years. Firing rate checks out to be about about half the rated boiler input. EDR is
about twice what I would expect for the heat load. What would be an expected cycle time on the coldest day of the year?

nail_17

re: cycle time

Okay, here goes,

What we know,

Cut-out pressure of about a pound of steam.

Firing rate checks out to be about about half the rated boiler input.

EDR is about twice what I would expect for the heat load.

Firing rate = .5 ( half ) / 2 x EDR = .25 1/X = 4X

2 x EDR / Firing rate = .5 ( half ) = 4X

HUH?

The above is probably not even correct, I'm geussing
that some 'ole timer said "I can fire this building
on 12 oz. steam, turned off one burner and cut the
pressure from 2# to 1#, which would ( nearly ) double
the velocity of the steam.

This is very likely since you had stated that,

Radiators heat well at the ends of the system.

Net effect being .5 rate * 2x velocity = 1

Could be exactly the same as is was intended,
yet more efficient.

The gentle folk here about explain this at great length.

Look at threads on "vapor" or "vaporstat" or
"lower pressure increases velocity".

Counter to what someone would assume to be true,
yet it is.

Cheers

Fred Harwood

Cycle time

What kind of thermostat. How many rooms/floors/zones. Approx size/length of mains.

And, what is current cycle time, and what mains vents do you have?

With the above info, we can give better estimates.

T. Riley

cycle time

Hi Fred:

The system is two pipe gravity return steam. Thermostat is Honeywell "honeypot" type. Two 3" steam mains with an F & T trap at ends. Main vent is 3/4" open hole at top of dry return in boiler room. Heated space is approx. 5,500 sq. ft. (approx. 18 rooms).

Thanks,
Tom Riley

T. Riley

cycle time

Nail 17:

Thanks for your comments.

Tom

scrook_2

in other words...

1. The boiler is currently fired at about 1/2 its max rated firing rate?

2. The current calculated heat loss of the structure is about 1/2 the currently connected radiation (EDR)?

So what is the measured firing rate? What is the nameplate input? (Also: what is the measured flue gas temperature (is it condensing in its flue?), the measured steady state efficiency, and what brand and model boiler?)

What is the connected radiation's EDR? How much greater is the gross output of the boiler (as calculated from measured input and efficiency) than the connected radiation's EDR?

What is the calculated structure heat loss and by what method?

Steamhead

Tom, I think that's a Vapor system

These were the Cadillac of heating in their day, and they usually ran on less than a pound of pressure.

Have you noticed any manufacturer's info on the system's hardware that could help us identify it? Look on radiator inlet & outlet fittings, traps, and any devices piped near the boiler. Post some pics if you can, we love to see this old stuff!

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Fred Harwood

Cycle time

Perhaps should be about 20 minutes on. If overshoot occurs, reduce anticipation slightly for a shorter on cycle. Experiment. Make sure that all rads heat some on each cycle.