Pick up factor for steam boilers?

Timco

So you've perfectly sized the boiler. I've seen it dozens of times. Boiler heats house great, runs the entire time, never makes 10oz even.

Should a pick up factor be figured in to make that pressure? Do we want it to reach pressure and cycle during a heat call?

JUGHNE

So at 10 oz on a design day (typically coldest day) do all the rad heat all the way across?
And would they have to go 100% across to keep house heated?

Timco

JUGHNE said:

So at 10 oz on a design day (typically coldest day) do all the rad heat all the way across?
And would they have to go 100% across to keep house heated?

Even on non design days some rads get half hot. There's a venting issue I'll be fixing though. I'm just talking in general or as a rule of thumb. Do we want to see a gradual climb to 10oz or better rather than no pressure readings and running the entire time?

JStar

I usually add 10-20% to my sizing, and have had no issues keeping my systems working adequately. I've even had good success with negative pick-up factors. Cycling is innefficient. A steam syste typically works best when it simmers along for most of the day.

ChrisJ

Please keep in mind I'm a homeowner, not a professional.

I'm currently running with 10% above my installed radiation and the system works beautifully even with fast radiator vents, though these were carefully chosen with a lot of experimenting.

Personally I'm against the whole 33% thing and feel 10% is plenty and honestly, I think Dave Brunnell has the right idea.

Taking Another Look at Steam Boiler Sizing Methods
https://heatinghelp.com/systems-help-center/taking-another-look-at-steam-boiler-sizing-methods/

Mark N

Chris I agree that first boiler definitely looks oversized for 4 small rads.

Mark Eatherton

Between you me and the fence post, I think the pickup factor (seen it between 33 and 50%) is extreme overkill and is useful when the system is first started COLD. After that initial start, the oversizing contributes to short cycling, which as pointed out is wasteful.

I've done systems with ZERO pickup, and they worked just fine. If you really want the boiler to cycle, throw some non electric TRV's at it, and as a room becomes satisfied, you shed that square footage from the load.

As for shoulder season days, you should be cycling on/off. If not, you've got some other problems to track down and kill, like holes in your heat exchanger discharging into the flue gas stream.

Ideally, if a person could modulate the flame and combustion air going into making steam, similar to the way a modcon tracks the load, you'd be in an ideal situation whereby demand is perfectly matched to energy availability. Thermal equalibrium as Siegenthaler says...

I am still waiting for someone to take a neg reg gas valve and variable speed blower/burner assembly and retrofit it into a cast iron vessel steam boiler and modulate the burner based on fluid temperatures, instead of steam pressures and have the worlds first fully modulating residential steam boiler. Surprised that Steamhead, Brunell, Gill or Pajek or one of the other steam gurus have not done this yet. I guess they know the liabilities connected to playing with the firesides of these systems.

Quite frankly, I am surprised that one of the major steam boiler manufacturers hasn't done something on this scale. I suspect they have, but haven't released it yet. Again, liability issues...

BTW, Lochinvar's new Fire Tube XL boilers come with a 10:1 turn down, and there is research to take it even lower yet. We have the technology... One of these days, the appliance will fire at the beginning of the heating season, and will stay on until the spring thaw.

ME

Henry

I agree with you for small residential systems but for large apartment blocks and commercial buildings, it is a must. They usually have night setback, therefore a cold start. In industry we need 50% factor at a minimum. Start a large kettle and undersize the boiler (s), you can have runaway steam and even vacuum in the line that will steal from other process.

I design large steam heating boiler replacement with a modulating burner, even for humidification. I try to use as much as possible Heat Timer controls. If it is full modulation, we have a control made for us to our specs.

ChrisJ

Henry said:

I agree with you for small residential systems but for large apartment blocks and commercial buildings, it is a must. They usually have night setback, therefore a cold start. In industry we need 50% factor at a minimum. Start a large kettle and undersize the boiler (s), you can have runaway steam and even vacuum in the line that will steal from other process.

I design large steam heating boiler replacement with a modulating burner, even for humidification. I try to use as much as possible Heat Timer controls. If it is full modulation, we have a control made for us to our specs.

That sounds like it would be terribly hard on a boiler, no? I add 12-14 gallons a day into my house when it's cold out just to maintain 35-40% RH.

Henry

This is NOT residential, it to humidify commercial buildings, industrial plants, office towers and very large condos. The boilers start at 25HP and can go to 100HP. They only provide humidification.

Henry

ME, any retrofit into an existing boiler requires here, recertification of the installation! The burner manufacturer would also need certification of his burner onn all sorts of boilers. Both cases makes it too costly for the residential customer. BTW, CSA charges $3000 minimum to certify a gas appliance!

ChrisJ

Henry said:

This is NOT residential, it to humidify commercial buildings, industrial plants, office towers and very large condos. The boilers start at 25HP and can go to 100HP. They only provide humidification.

Ah,
I'm going to assume because it's commercial and industrial that the boilers are treated well and proper water treatment is used which is why losing so much water doesn't bother them?

Henry

Water quality needs to be checked every day. We have pretty good water at 7 grains of hardness and most areas do not have fluoride added to the water only small amounts of chlorine. Daily blowdowns are the norm. Most installations will have a steam inection system into the condensate tank to maintain at least 170F which helps in removing oxygen and other gases.

Timco

Great replies and thoughts to all. Thanks. I like these discussions and take in all the opinions.

Fred

This topic probably will never be resolved to everyone's satisfaction and I think that is because every steam system has its own eccentricities. Is a 30% pick-up factor right for every house, maybe, maybe not.
In my case, I have about 100 ft of 2.5" and 3" Mains, about 200 ft. of radiator run-outs and 700 sq. ft. of EDR.
Here in Ohio, we probably have 2 months (January and February) where the outside temps are such that the boiler runs every hour and long enough to get my 15 radiators hot. The other months, October, November, a good portion of December, virtually all of March and April almost have to be treated like sholder months where the boiler runs but not regularly enough to keep the piping and those radiators hot enough (even though the Mains are insulated) to not require some pick-up factor.
I get it that the capacity of the boiler can be spent heating the Mains and run-outs first but we need to keep in mind that that infrastructure also has to maintain that heat throughout the heating cycle as well. you can't simply say "OK the piping is hot, I don't need to factor anything in for those components once they are hot." They still require energy to maintain temp. They have an EDR value.
If one were to calculate the EDR value of the Mains, piping and cycle heating requirements to heat/ or incrementally reheat everything for each house, just like we do for the Radiator EDR, on a case by case basis, some probably can get by with less pick-up.
I think the industry locked in on 30% because it was a reasonable average factor and the small deviations probably don't justify the additional time spent to do that survey on a case by case basis.
To answer the OP's original quetion, No you don't want to build in a Pick-up factor that increases pressure. That is called an over-sized boiler. The lower the pressure, the faster steam moves. Any boiler Steam rating over and above that which matches the EDR of the radiators and whatever it takes to support the infrastructure is overkill. Calculate the EDR rating of your pipes and see how far off the 30% is for your particular house/system.