Should a properly sized steam boiler run flat out at design temp?

ariccio · January 3

Essentially what I'm asking, are steam boilers in high rise multifamily buildings like other heating systems, where we'd expect other systems to run nonstop or perhaps >80-90% of the time on the coldest day of the year, should a properly sized steam boiler be firing continuously to maintain 1-2psi on the coldest day?

My gut says "maybe no", since when a boiler shuts off, there's considerable heat still produced as the pressure declines and then as the radiators cool off long after steam delivery ends.

I'm going to pull the last few weeks datalogger recordings tonight, and I do suspect I'll see steam reaching this radiator for maybe only 50% of the time on those days when it got down to 15°F here in NYC. My gut feeling is that we're *way* way oversized, since the boiler the coop board installed was even larger than the boiler the building was built with in 1960!

LRCCBJ · January 3

@ariccio
Essentially what I'm asking, are steam boilers in high rise multifamily buildings like other heating systems, where we'd expect other systems to run nonstop or perhaps >80-90% of the time on the coldest day of the year, should a properly sized steam boiler be firing continuously to maintain 1-2psi on the coldest day?

Yes.

But it would be a veritable miracle if you could fine one properly sized for he design day. The only way this is possible is if the installer ignored the amount of steam that that the boiler can generate (safety factor of 33%) and installed a smaller boiler because he knew you wanted to run 90% on the design day. NOT HAPPENING.

ariccio · January 3

Are you saying that it would be *correct* to design it to run 90% of the time but the installers don't do that as a matter of covering their butts so they don't end up with an accidentally undersized boiler, or something else?

What are the issues - other than wasted energy - that a boiler that's more than 50% oversized would have? From what I've learned here, I think surging would be one of the main issues, followed by shortened life caused by short cycling?

LRCCBJ · January 3

Are you saying that it would be *correct* to design it to run 90% of the time but the installers don't do that as a matter of covering their butts so they don't end up with an accidentally undersized boiler, or something else?

Not quite.

The boiler manufacturers specify what each boiler can do in terms of steam output. This number has a 33% safety factor by design to manage the pickup and piping factor when the boiler starts cold. No installer would dare to go below this value under the assumption that he doesn't need 33% pickup factor. Why take the risk……….??

Only someone extremely versed in the specific building and the piping in the building would have sufficient knowledge of whether to go BELOW the manufacturer's specification. Of course, he takes the risk if he does that.

If you are a contractor, it is "correct" to follow the recommendation of the manufacturer. Anything else puts the contractor at risk of miscalculating and ending up with a boiler that is a bit too small on the design day.

I will state that I have NEVER seen this happen on HH. The 33% is way more than sufficient for the safety factor.

Long Beach Ed · January 3

A properly sized boiler would not likely be 50% oversized. As LRCCBJ states, the design goal is to have a boiler that operates 100% of the time at design temperature. The engineer (or master plumber) doing the design has great latitude in his selection, however. A pick-up factor must be considered that would start a cold system on the coldest day of the year. How about wind chill? That should be considered too in the design. And there's a safety factor too.

In practice, the engineer will usually oversize the boiler for these reasons. He's not going to be sued if the boiler is too large, but he will hear about it if it's too small.

If 50% oversized too large? Maybe yes, maybe no. Industry standards specify a 33% pick-up factor, and most manufacturers note that this should be adjusted for "special conditions".

In short, only the design engineer can answer your question. He knows why he specified that size boiler, and should be able to defend his calculations..

109A_5 · January 3

I wonder if there is an actual industry standard for heating equipment duty cycle running at design day temperatures.

With my small residential system, at more than 5 degrees below design day temperature, my boiler runs at about 65 to 70 % duty cycle. My boiler (300 Sq. Ft.) is undersized for my 347 EDR. I bet the house did not need 347 EDR, but that's how they did things almost 100 years ago. 100 years ago, did they even use design day temperatures as a reference ? BTW I have no idea what the size of the original boiler was.

Jamie Hall · January 3

Oh boy. Here we go again. Sizing a steam boiler for a residence or other small occupancy is easy enough — they are sized, or should be sized, to power the connected radiation. As noted, the EDR rating includes a 33% sizing factor (whatever you want to call it), so it is best practice not to go over that — and usually the system will work well undersized.

Design Day temperatures will affect the amount of installed radiation. Once that is set, the Design Day heat loss has nothing to do with it.

Now — for a larger building, particularly multiple occupancy, things get a good deal more difficult, and an engineer and architect will get involved and try to get an estimate as to what percentage of the various heated spaces will be calling under various conditions. They will then size the boiler to power that much radiation — usually not the full radiation. This is best regarded as an informed guess… Older practice often took a conservative route, and designed close to the installed radiation. However, in newer practice with the use of thermostatically controlled valves or vents, it gets trickier.

There are two considerations of cycling on pressure: it's a way to match the boiler output to the load. If the boiler size is reasonably close to the load, the hit on efficiency, while present, is small. If the boiler is way oversize, it may be significant. However, if you are dealing again with multiple occupancy, and occupants are controlling temperature with thermostatic vents, the boiler must cycle off to 0 psig gauge from time to time so that the vents can gain control.

The residual heat in the boiler or radiation should not be a consideration, if the controls are correctly used. However, in larger buildings that residual heat can be taken advantage of in smoothing temperature variation when the boiler is off, and a variation in control strategy can be to adjust the boiler cycle timing (minutes on per hour) in accordance with the outside air temperature (with, of course, a pressure control to prevent overpressure). This is sometimes used.

Long Beach Ed · January 3

https://forum.heatinghelp.com/discussion/comment/1831611#Comment_1831611

The "engineering standard" is sizing a boiler to run at 100% on the design day. That's why it's the design day. But the engineer must also use his discretion to design the system for other factors besides temperature, like wind chill, building anomalies, boiler and control design and unusual piping design.

The 33% "pick up" is a quick and acceptable shortcut to choosing a boiler in smaller jobs.

Jamie gives a great explanation regarding boiler/system selection.

dopey27177 · January 4

In the old days when NYC was a lot colder than it is today the design temperature for a heating system was zero outdoor temperature. The piping pick up factor increased the size of the boiler a little bit.

If the boiler was sized properly the boiler at zero outdoors would run about 90% of the time especially if the steam piping system was one pipe.

In the 1960s the federal gov. changed the design temperatures base line to 10 degrees as the lowest out door temperature. This change caused problems in some newer buildings.

Many buildings in NYC had new double glazed windows installed this caused boilers in existing buildings to be oversized by as much as 15%.

Boiler replacement in buildings where double glazed windows were installed can be very tricky and boiler sizing needs to be carefully, preferably by an architect and an engineer.

A steam boiler can be oversized by as much as 20% and not cause problems.

There is not a thing called a good guess, there a thing called care and caution because once installed the discussion comes to late.

Jake

Should a properly sized steam boiler run flat out at design temp?

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