Pickup factor. Help me understand

ChrisJ

I'm a bit confused by the use of a 33% pickup factor.



The way I see it, with your piping insulated as it should be the piping should dissipate a fairly small amount of heat. Now in my system my pickup factor is something like 31,000 btus. Once the piping is hot I could see it dissipating maybe 5000btus through the insulation, but 31,000?



So my question is, why do we need a 33% pickup factor if all of your piping is insulated? Wouldn't something like 5% do just fine?



Is the 33% there to makeup for missing insulation and poor venting? Is it simply a fudge factor and not actually necessary if everything is perfect?

KC_Jones

I am following this thread

I have been wondering this same thing.  As a homeowner couldn't I theoretically measure all the pipe in my system that is exposed and calculate the exact sq ft?  I am still reading Dan's books so my understanding of all this is still in it's infancy.  As far as measuring all the pipe as a homeowner I could take that time, but the rest of you as professionals don't really have that time to spend.  I work for a manufacturing company and fully appreciate the value of time!

icesailor

Pick-up factors:

When I was a youth (long ago), the Pick-up factor was how successful someone was at picking up girls.

Later, I came to understand that it was a shortcut to save people a lot of work.

If this is what you are asking, this is how I understand it.

In a heating system, there is a certain amount of measurable restriction. You can count how many feet of pipe and fittings plus anything that will cause you a restriction or loss in the system, add then all up and subtract it from the gross output of your system, which will give you a net estimate of your system.

IBR rates boilers and systems to a standard they developed. They picked some magic number after reading smiling faces and signs of the devil on some page full of computer machine code. If you look at the ratings of boilers, there are three ratings. Gross output, or what is supposed to come out of the boiler. There is a DOE rating, the boiler must meet some DOE standards. Lastly, there is the IBR NET RATING which is lower than the other two. Because IBR has rated the boiler and allowed for "normal" piping and pick-up. It was my experience that as long as a boiler I chose fell below the IBR rating, and there was no off the wall piping, I was OK. I didn't have to get involved with complicated calculations that most of my competition was clueless about.

If that makes sense to you and is what you were asking, fine. If not, its the best I can do under the circumstances.

Jamie Hall

It really isn't a fudge factor...

nor is it compensation for poor insulation or poor venting.  Indeed, if your insulation is poor, you either should use 50% -- or add all the piping into your EDR calculations.  If your venting is poor, that will throw everything off.



No, what it really is is a way to compensate for the energy -- heat -- it takes to warm all that metal up.  Just as it takes more throttle to accelerate a car from a stop to, say, 50 mph than it does to drive at a steady 50, it takes some additional energy to get the metal up to temperature in the heating system over what is needed strictly for the radiation.  Is 33% too much?  Perhaps, depending on the system.  You will see this in the system cycling on pressure towards the end of a very long run.  Is it perhaps too little?  Probably not -- but a system with an undersized boiler is a pretty sad affair, so one wants to be a bit conservative.  Is it a decent rule of thumb for sizing a system?  Yes.  It's worked and worked well, for a bit over a century now, and there isn't much reason to change it.

ChrisJ

Jamie

That is the part I do not understand.



When my system fires up, the pipes are heated first, not at the same time as the radiators.



My mains are heated literally with every ounce of steam the boiler can produce. Once they are heated the steam goes to the runouts and then the radiators. By the time the radiators get any steam all of the piping is already heated minus the loss through the insulation. I can hear this taking place as my Gorton's breath at the beginning of each cycle. Once the radiators start getting steam all vents become completely silent because the radiators condense steam far faster than the piping ever could.



Maybe I'm just not getting it. I didn't understand the friction losses in piping which result in a pressure drop for a while either but I got it eventually.

EBEBRATT-Ed

pick up

Steam cycles up and down (at least in a house job)

 Every time the boiler shuts down the steam is condensed back into water very quickly. The radiation will stay hot for a while. The piping with not as much mass doesn't hold the heat as well.

When the boiler starts again it has to reheat the piping again. Some of the steam is condensed back into water without getting to the radiation so it does no useful work other than reheat the pipe (which is the pick-up)

Jamie's example of a car accelerating is spot on.



You are correct in that the pick up factor is seen as somewhat antiquated by some.

No doubt we need to have a pick-up factor. 1.33 may be overkill for some jobs but needed on other jobs.

 It's a judgment call. Will the boiler ever have to pick up the piping and a cold house at the same time??

SWEI

This is part of why

modulation and two-stage firing are so wonderful.

Mark N

Pick Up Factor

Chris the actual pickup will vary depending upon the temp of water in the boiler and the temp of the piping. The hotter everything is the less pick up needed, the colder more. But whatever the actual pick up is it must be met every cycle. The boiler and pipes must be brought up to steam temp before any steam gets to the rads. For example lets say it took 10 minutes to get steam to the rads and the boiler ran a total of 20 minutes. In that example the pickup was 50%. In actual btu's it would be whatever the output of your boiler is for 10 minutes. The boiler rating on the boiler are based upon 1 hours time. Rarely does your boiler run for 1 hours time.

JStar

Pick-up

I use 33% for uninsulated pipes and 15% for insulated. No complaints yet. Agreeing with SWEI, a two-stage valve will handle 100% capacity on start-up and drop down to the (Output minus Pick-Up) in Low Fire.

nicholas bonham-carter

an interesting question

Chris, I think you are on to something here. The pickup loss may not be necessary, IF YOU CAN WAIT A LONGER TIME FOR RECOVERY [from setback]

As you have pointed out, the piping has been heated by the time steam arrives at the radiators, so why include the added 1/3 over the EDR?

This question could be answered by those who have adjustable burners, which can be more easily dialed down to match the heat loss of the radiation to the output from the burner.--NBC

Dave in QCA

Piping Loss and Pickup Factor

This is a somewhat difficult subject to understand and its an even more difficult subject to explain. I have to bow to Jamie for his analogy to the accelerating car, it is excellent!!! I had never thought of it in that way before, but it's spot on!



I'm going to jump in and give my 2 cents too and see if I can help to make it easier to understand too.



The "pickup" factor is actually two separate factors that have separate purposes. First, the piping loss factor takes into consideration that piping will give off heat and in doing so will condense steam, even after it is fully heated up. So, in order for a boiler to be able to maintain a fully heated system of radiators and piping at full temperature and and maximum output, the heat losses or emission of the radiators and the heat losses of the piping must be matched by the output of the boiler. The amount of heat loss from an average piping system that is insulated is estimated at 10% of the capacity of the radiators. So, if the radiators on a system add up to 200 Sq Ft, the piping losses can be estimated to be equal to an additional 20 Sq Ft of radiation.



But, the piping loss factor only takes into consideration the piping losses of a fully heated system. Steam systems by their nature are difficult to heat up in an even manner. By comparison, a hot water system uses a 15% piping loss and pickup factor, or only 5% pickup. The result is that old gravity hot water systems with huge cast iron radiators are very slow to heat up, but they do eventually heat up. In the heating process, all of the radiator will heat up gradually and all at the same time. Starting from stone cold, to luke-warm to warmer, and eventually hot, if the boiler has been firing continuously. The key point is that the radiators throughout a well designed system will heat uniformly.



This is not the case with a typical steam system. On a steam system, an additional pickup factor of 24% on insulated piping is needed. On a typical steam system what would happen with a boiler sized too small to give adequate pick-up, some radiators will heat first, and others later, sometimes not getting any steam at all until other radiators are blazing hot. The negative effects of unbalanced steam distribution are especially undesirable in mild weather when steaming cycles are short and only partially heat the radiators. Undersized boilers in these situation will often leave some of the radiators with no steam at all.



Fast vents on steam mains will improve the likelihood that steam will arrive at the radiators at the same time. Slow, make that VERY slow radiator vents will further improve even distribution. Actually, I have seen some cases where marginally sized boiler worked just fine, but the size of the mains were small, reducing the amount of heat loss and the vents were all Hoffman #40s, which are pretty slow.



The negative aspect of using the 34% piping and pickup factor for insulated mains or the 50% piping and pickup factor for uninsulated mains, is that on long cycles, when the system becomes fully heated, the boiler will start cycling off and on because of pressure. It is just a part of the beast. Of course, as has been mentioned, two stage firing is a great benefit in this situation.



It should be noted that in two pipe systems, when inlet orifices are installed on the radiator, since they separate the radiator from the boiler and main pressure, the pickup factor can be omitted. Only the piping loss needs to be factored. Also, according to writing by Dave Bunnell, aka, the Steam Whisperer, if very slow vents are used on one pipe systems, the pickup factor can sometimes be omitted as well.



Hope this is helpful. on some level.

gerry gill

the long and short of it is

you don't need the extra third 'all the time'', but since much steam history has gone to the grave with dead men things get confusing.



Its piping pick up factor ----today



wasn't in the old days..it was Piping and Pick Up Factor..two separate entities..notice the word that got lost over time. AND...changes the whole meaning when that word gets dropped..you needed some extra to heat up and fill the pipes, AND the pickup factor was for coming out of setback..the extra steam would allow the system to build up some pressure and compress the steam into a tighter area cubic foot wise, which delivered more btu per square foot..if the building ran 24-7 like maybe a police station or factory, the boiler only needed a little extra for the pipes but no pick up factor at all..



see how the loss of a single word over time changed the whole meaning.

ChrisJ

Ah!

I get it now.



Thank you all for responding!

gerry gill

here is some reading Chris

that should clear up everything.



https://www.ideals.illinois.edu/handle/2142/4349

SWEI

What a treasure!

Clearly written, evidence-based, and entirely useful.



thanks~ 

KC_Jones

Excellent Information

Thank you all for the information quite informative as usual!  My curiousity on this subject is because I am trying to size my new boiler.  I currently have a WM EG-45 which I now know is oversized so I am definitely downsizing it.  I have 265 sq ft of connected radiation which gives me 63600 BTU load.  A new WM EG-35 is listed at 62000 net IBR which in theory should work.  That's why I wanted to understand the pick up factor so I could better decide.  I was just wondering if that pick up could be cut back slightly for certain systems based on pipe mass, insulation, venting etc.  What I am hearing is that could be possible?  I will have to continue reading Dan's books and educate myself further before I decide.  Oh the other reason I am thinking I can go as small as a WM EG-35 is because I am actually only firing 4 of the 6 burners in my WM EG-45 which puts it about the same input as the EG-35.  Again thank you all for the explanations!

ChrisJ

Interesting

Thank you for posting that Gerry!



Right now, my thoughts are, at least on a system like mine the piping and pickup factor should be kept as small as possible.



Reasons are the radiation is already 30% oversized for the house. That alone should be plenty to bring the temperature up during a recovery without oversizing the boiler ontop of it.



Maybe it will cause run times to be longer, but something tells me the run time on a modcon sized to design temp won't exactly be impressive either.

Mark N

Steam Study

That study was a very interesting read. With the boiler a 130 degrees it took 5 minutes to make steam and then on average an additional 12 to 17 minutes to get steam to all the radiators at all 3 firing rates. Very slow steam distribution by modern standards. The main venting most likely needed to be increased. What would be interesting to see would be some data on the effects on a steam system that has a hot water loop sized near pickup factor. Start the boiler at 130 degrees and run only the steam zone. Time how long to start steaming and how long till steam is at all the radiators. Next start the boiler at 130 degrees but run the steam zone and the hot water zone and time everything again. Run the test a few times and take an average. Would be nice to run the test at night with about a 30 degree outside temp. With the hot water loop removing the built in pickup factor the steam would be operating at the net rating or close to it.

jumper

just guessing

Maybe boiler has to be over sized because it has to remove air from heating system. On multi-residential building where boiler was replaced with a train of small boilers, and with mechanized venting, heating season began with only one small boiler fired up.