Pickup Factor in older buildings

dabrakeman · April 7

I know there have been many discussions on this forum relative to pickup factors and understand that many in their situations may find that that something less than the default 33% works. The reason for this post though is to discuss how to make the appropriate decision for older buildings or systems with that as discussed in LAOSH page 124-125 as "unusual piping".

Dan in LAOSH suggests that many of these older unusual systems need 50% or more pickup and this assumes insulated piping (page 376). However, there is no clear direction on how to determine what is unusual and properly assess or calculate a necessary pickup factor for a system deemed to be unusual.

Interested on peoples input as to whether it makes sense or is something anyone has done on an old large job (maybe over 2000sqft connected radiation) to go through and total up all the mains, takeoffs, risers, takeoffs off risers and some allowance for fittings? The equivalent sqft radiation for bare pipe based on size and length is known but then the question I would have is how much to reduce this based on the presence of insulation. LAOSH states that the insulation on supply pipes can reduce the heat loss from that pipe by up to 5x. So, would it make sense to take the calculated uninsulated EDR and assign 20% or maybe 25% of the calculated value?

I know this would be a significant labor cost so I probably know the answer whether anyone has done it on a job but maybe some have looked at it on their own personal systems?? I am trying to make sure that in a case we have pickup factor is not being underestimated at 33%. I also know that some of the original headers are still in place that served a system double the current system size but nothing is known to be removed from the portion of the system remaining so those mains I would expect to be per original design pressure drop. Being a public building I also expect a desire to do setbacks and recoveries which could increase the pickup demand.

(Yes this is related to the previous discussion UP Michigan Gem needs Help...) if more background information needed.

Jamie Hall · April 7

I personally think that even rather unusual piping -- a lot of long mains, for instance, or a number of tall risers, that sort of thing -- doesn't need a pickup more than the nominal 33%, and I'm not even convinced that it needs that much.

The setbacks and recoveries don't affect that at all. You are still sizing based on radiation, and it is available radiation which will determine how fast recoveries can take place -- not the size of the boiler.

LRCCBJ · April 8

Is the equipment gas fired or oil fired?

If you need to heat 100,000 lb. of cast iron, you can absolutely understand that the larger boiler has the distinct advantage during a recovery, independent of the available radiation.

pedmec · April 8

My experience with the older buildings has been that the asbestos insulation has been removed but not replaced. This might explain why Dan recommends a higher pick up factor in large buildings.

ChrisJ · April 8

LRCCBJ said:

Is the equipment gas fired or oil fired? If you need to heat 100,000 lb. of cast iron, you can absolutely understand that the larger boiler has the distinct advantage during a recovery, independent of the available radiation.

If recovering is a big concern like in a church or school id say multiple boilers is a better option.

Jamie Hall · April 8

no matter what you do... you can't deliver more heat than the radiation is capable of, so there isn't much point in going too much oversize... and it's what the radiation can provide which determines your recovery.

EBEBRATT-Ed · April 8

I don't see any need for more than 33% PU unless you have a lot of long pipe runs uninsulated and located in cold basements or crawl spaces.

Every job is different . Cover the radiation load and walk the building and get a general feel for the construction. Tight building or loose and drafty? Then apply a PU that makes sense.

It's not one size fits all.

dabrakeman · April 8

It is gas fired and everything sees pretty well insulated except the immediate near boiler piping (which will be done on new boiler). Much of the piping is in the understage drafty crawl space area. Being a theater area the setback and recoveries would be expected but I understand your point @Jamie that as long as the boiler is big enough to initially get steam to the radiators oversizing won't help in any way other than likely causing more short cycling during the recovery.

Anyway, what I have recommended based on totaling the connected radiation comes out a bit below a LGB-7 with default PU leaving effectively a 47% PU. Issue I have is the theater company wants a good relationship going forward with a local contractor and that contractor insists on an LGB-10 or 11 (one to two sizes bigger than is in their now and was spitting water from radiator vents) which makes absolutely no sense to me. The contractor doesn't seem to want to talk to me to convey his reasoning so I guess I have done all I can do...

Jamie Hall · April 8

Yeah, @dabrakeman , there are times... all too many of them sometimes! -- when the only thing you can do is give your best advice and then walk away. Sounds like this is one of them...

Best thing to do is to write a short, coherent report with your recommendations and send it to the client. Then when things go wrong later, you have that to confirm what you recommended.

ARobertson13 · April 8

With small buildings that have simple steam distribution systems pick up factors are not a difficult determination because you examine the mains and riser layout and insulation conditions. With larger prewar units with more complex and less surveyed distribution systems, the wiser course of action if the existing heating plant is operational, is to use the "Test Fire" method. This involves turning the steam system on and gradually increasing the steam output from 1/4 maximum after each cycle until the steam pressure peaks at 2psi. As for Pre war buildings in which the existing plant is non functional, use the ratings of the existing plant. If the new unit is oversized, there are things that you can do to reduce the output. Such as using a smaller nozzle on an oil burner or adjusting the modulating pressuretrol to a lower setting. You do not want to risk an undersized install. Your clients will not like it and it will result in an undersized checking account.

dabrakeman · April 8

Existing boiler is not operational. Guess what I am looking for is a way to "survey the distribution system". To just use the existing size and then find ways to deal with it seems counter to everything I thought I had learned here. There has got to be a way to properly size an entire system including distribution piping.

At one time the original system here served a firestation, village offices and ballroom and the theater. Now it only serves the theater. Seems more than likely a similar one for one was done in the past...

LRCCBJ · April 8

dabrakeman said:
It is gas fired and everything sees pretty well insulated except the immediate near boiler piping (which will be done on new boiler).

The contractor doesn't seem to want to talk to me to convey his reasoning so I guess I have done all I can do...

Since it is gas fired, you can partially remedy your size problem if you can convince the contractor to utilize a two stage gas valve. If he understands the concept, you can use a boiler that is 40% larger than necessary and then avail yourself of a reduction to 60% of rating. This is the best of both worlds. Plenty of energy for a cold start and no short cycling when the rads are full and the pressure rises.

Of course, if he is completely obstinate, there isn't much you can do. You can't fix stupid.

EBEBRATT-Ed · April 8

Unfortunately, most contractors don't like math or calculations they would rather guess because they have been doing it for 40 years and know everything.

If you wanted to make thing more accurate measure all the mains & risers and figure the amount of steam condensed from the pipe and add that to the radiation load and go with that and no PU or a small 10%pu.

ARobertson13 · April 8

There is no formula for a steam distribution system that is complex. You have to know the following:
1] The size of the mains and number
2] The branches and their location
3] The number and location of the radiators
4] The degree of insulation
I know many steam contractors use pickup calculations but I have a rule: Don't experiment on your clients and Do no harm. If there are too many unknowns. The only solution that I would apply in your situation is to attach a temporary mobile boiler determine the required output. [These units are expensive $5,000 plus a week]. Other than that you would have to get a contractor / consultant who has a lot of experience with this type of situation.

dabrakeman · April 8

"If you wanted to make thing more accurate measure all the mains & risers and figure the amount of steam condensed from the pipe and add that to the radiation load and go with that and no PU or a small 10%pu."

@Ebebratt-Ed If indeed this was done would there not be effectively less radiation in those pipes when insulated than bare?

@LRCCBJ - Yes I had seen the Low-High-Low fire option in the brochure. Thank you for your input as to the degree by which this can be implemented. This made sense to me initially if erring on the big side. This might also be what the contractor is thinking if theoretically you can bring a LGB-10 down to a LGB-7 which it doesn't quite do.

dabrakeman · April 8

@LRCCBJ I was wondering if you or anyone else could clarify for me how the L_H_L fire gas valve is implemented.

First question, is the 60% low fire adjustable in any way? I.e. if the low fire took the boiler well undersized particularly after all supply piping is heated up.

On initial startup is the boiler always on high fire? Am I correct to assume if controlled off pressure it would always start on high until pressure reaches a predetermined level i.e. 1 psi (or lower... my system sounds like crap if it gets much over 1psi)?

If the above is true wouldn't you want the low fire capacity to be ideally matched to the connected radiator load without PU and maybe a little margin of 10-15% on top?

Jamie Hall · April 8

All quite reasonable thoughts, @dabrakeman . In steam systems, the high/low transition is usually controlled by pressure, with the objective being to keep the system pressure within a rather narrow band consistent with best operation for that system (this will vary with the characteristics of the system, but is usually on the order of a few ounces to a pound or so for "normal" systems). Unfortunately, the burner characteristics are such that the two firing ranges do not truly modulate continuously, but switch between a high rate and a low rate. Depending on the specific burner designs, these two rates may be adjustable.

At the present time I am not aware of any gas fired smaller steam boilers which truly modulate, which would require a different pressure control sensor. However, in large power boilers -- both oil fired and gas -- modulation in various forms is used, although it may be stepped by means of turning specific burners on or off in a multiple burner array.

dabrakeman · April 9

If the connected radiation is pretty well known and the pickup piping is a bit more speculative how might we use this information to guide boiler size choice if a 2 stage gas valve is used? For now assume low fire is 60% of high fire as that seems to match most of what I have found. Would it not be logical to:

1) keep the low fire boiler capacity less than the known connected radiation sqft with 33% pickup factor? If it was greater than that then in all likelihood even after the boiler switches to low fire it would continue to short cycle on pressuretrol cutout since pressure wouldn't fall to the gas valve vaporstat cutin during low fire because there is till just too much steam. There is little to no pickup when everything is hot anyway.
2) assume you would not want the low fire boiler capacity to be much less than a boiler sized directly to the connected radiation with no PU. At least not much less. Maybe no more than 5% less or so. IF you hit it perfectly once low fire is triggered it would go into a virtual steady state. The fact that many seem to cycle quite frequently from low to high to low tells me that maybe letting this go a little lower is common??

Those would define the upper and lower boundaries of unacceptability "red" but then there is probably a more ideal green zone within bounded by yellow zones...

Using this logic in my example where measured radiation is 1702sqft (and I had originally suggested a LGB-7 rated at 1936sqft with a very ample PU) if a 2 stage gas valve is to be used then boiler size could be allowed to range up to ratings between 2135sqft and 2560sqft. So a LGB-8, maybe a LGB-9?

LRCCBJ · April 9

dabrakeman said:
If the connected radiation is pretty well known and the pickup piping is a bit more speculative how might we use this information to guide boiler size choice if a 2 stage gas valve is used? For now assume low fire is 60% of high fire as that seems to match most of what I have found. Would it not be logical to:

1) keep the low fire boiler capacity less than the known connected radiation sqft with 33% pickup factor? If it was greater than that then in all likelihood even after the boiler switches to low fire it would continue to short cycle on pressuretrol cutout since pressure wouldn't fall to the gas valve vaporstat cutin during low fire because there is till just too much steam. There is little to no pickup when everything is hot anyway.
2) assume you would not want the low fire boiler capacity to be much less than a boiler sized directly to the connected radiation with no PU. At least not much less. Maybe no more than 5% less or so. IF you hit it perfectly once low fire is triggered it would go into a virtual steady state. The fact that many seem to cycle quite frequently from low to high to low tells me that maybe letting this go a little lower is common??

Those would define the upper and lower boundaries of unacceptability "red" but then there is probably a more ideal green zone within bounded by yellow zones...

Using this logic in my example where measured radiation is 1702sqft (and I had originally suggested a LGB-7 rated at 1936sqft with a very ample PU) if a 2 stage gas valve is to be used then boiler size could be allowed to range up to ratings between 2135sqft and 2560sqft. So a LGB-8, maybe a LGB-9?

1) Keep the low fire at the exact level of the radiation without any pickup factor. In fact, a good argument could be made to keep the low fire even less than the connected radiation presuming the connected radiation is oversized to the heatloss. The problem with this approach is that the pressure would drop to a level that the boiler would likely call for high fire to lift it back up a bit. You really cannot go wrong with low fire being as close to the calculated radiation value (without pickup factor).

2) It certainly can be less. However, the control of low fire via pressure becomes an issue. It won't stay at low fire for very long if it is much less.

Yes, you can use the larger boiler and setup the two stage gas valve to hit as close to 1702 square feet as you can achieve. But, be careful of the pickup factor. DO NOT include it in the calculation for low fire. High fire takes care of that.

dabrakeman · April 9

@LRCCBJ Thank you for your comments. I think we are mostly on the same page. I may be rationalizing a little higher limit on the low fire upper limit given that it would still be better than a standard sized boiler with a single stage burner once the system is filled with steam.

I guess another and perhaps simpler way to approach it would be to say how far off could I possibly be on the PU estimate. "In some cases" @DanHolohan suggested even 50% should be used. If this were worst case then my totaled radiation EDR plus 17% would account for that. Boiler rated size ~1992sqft rather than 1702sqft. The low fire on this hypothetical boiler without PU would be 1195sqft which is only ~1% smaller than the exact level of the standing radiation without any pickup factor.

So, either way shouldn't go over an LGB-8 (2264sqft) for my 1702sqft standing radiation even with L-H-L gas valve.

(note if you read this and it changed it is because I reread the LAOSH and Dan never actually said there would be cases in which 60% would be appropriate thus redid the calculations with 50%)

mattmia2 · April 9

Maybe Dan will answer at some point, but I suspect he is thinking of huge mansions and schools with thousands of feet of piping and long runouts to make sure everything is hidden.

The fact that the contractor wants to oversize the boiler without finding some unusual amount of EDR for the piping is a very bad sign for getting the boiler installed such that is operates properly.

LRCCBJ · April 9

dabrakeman said:

So, either way shouldn't go over an LGB-8 (2264sqft) for my 1702sqft standing radiation even with L-H-L gas valve.

The LGB-8 has an output of 724 MBTU. Without the pickup factor, the boiler delivers 3016 square feet of steam. With a .60 valve you have 1809 square feet.

The LGB-7 delivers 2579 square feet without the pickup factor. With a .60 valve you have 1547 square feet.

So, you have to make a decision. If your standing radiation includes some of the piping that will also deliver energy, the LGB-8 will be fine.

You should definitely not consider anything above an LGB-8.

dabrakeman · April 10

The contractor is assigning 8sqft per section to this 3 column 37" radiator because he measured it near 12" deep. So he figured it is more like a 4 column than a 3 column. In retrospect he admits he may have just measured it across the top. Still trying to get someone up there to measure depth at multiple heights because the unusual (and attractive) contours take a bit more scrutiny, particularly since nobody has been able to identify the radiator to find a rating. I believe it is much narrower at the bottom than the top and also is probably closer to 9" in some of the middle sections. Therefore, I was using the standard 5sqft knowing I probably have a 10% or so safety factor because of the hoods blocking some chimney effect on all these radiators. These 3 column radiators make up 60% of the total standing radiation on the system so it is significant. I haven't been able to get up there all winter but will probably get there end of June when to reexamine them myself. Below are refresher pictures of the representative radiators of question.

With 8sqft per section assumption instead of 5 the total EDR would be 2327sqft which if employing a 2 stage gas valve on the boiler could lead one to a LGB-10 based upon the previously discussed low fire logic.

I find it interesting that not one radiator in Every Darn Radiator shares a similar non-uniform depth top to bottom (except the occassional flute at the very top). The radiators that do cast just a shadow of doubt include:
1) the Star Radiator on page 25 that is made "wide" at 11 1/2" depth top to bottom and whether it is 2 column or 3 (hard to tell from the picture) is rated at 7sqft per section. Made to substitute for some reason for a 4 column.
2) The Imperial on page 76 that as a 3 column 9" depth radiator boast 6.5sqft/section

LRCCBJ · April 10

dabrakeman said:

2) The Imperial on page 76 that as a 3 column 9" depth radiator boast 6.5sqft/section

I am inclined to believe that the 6.5 is very close to the right number. They are definitely larger than 5 but I highly doubt they are 8.

EdTheHeaterMan · April 10

Depending on the age of the system, some pick up lines are better than others
https://www.youtube.com/shorts/KnzUO6ZnTQ8

mattmia2 · April 10

Is this 2 pipe or 1 pipe? With 2 pipe there are a lot of things you can do to make a boiler smaller than the attached radiation work. With one pipe it is a little dicey. What you are talking about so far is over sizing by a lot but i'm trying to figure out if there is an out if you do under size it.

dabrakeman · April 10

1 pipe.

LRCCBJ · April 11

mattmia2 said:
Is this 2 pipe or 1 pipe? With 2 pipe there are a lot of things you can do to make a boiler smaller than the attached radiation work. With one pipe it is a little dicey. What you are talking about so far is over sizing by a lot but i'm trying to figure out if there is an out if you do under size it.

To me, those rads are absolutely tremendous for a typical room size. So, undersizing the boiler to them will likely not be an issue provided proper vents are installed that effectively prevent the radiator from filling on a typical 30 minute cycle (at worst).

If the rads provide 2X the heatloss...........why do you ever want to fill them with a properly sized boiler?

Seriously, what room can accept 24K BTUH..........which one of those rads might produce when full?

Jamie Hall · April 11

I like the comment "With one pipe it is a little dicey. ". Oh yeah. Balamcing a system which is seriously undersized is bad enogh on two pipe, where at least you can resort to starving some radiators with the inlet valve (or if you have the patience, orificees). But on one pipe? I wouldn't want to try.

There is another downside: if the boiler is undersized, it will not shut off on pressure. This is often regarded as desirable -- but again on one pipe, if you have any thought of using temperature regulating VENTS for temperature control in a room -- an otherwise useful option at times -- they simply won't work. For a TRV to work, the boiler must cycle off periodically for the vent to control. A ten minute on followed by perhaps one minute off for the duration of a heating call is reasonable (there's something to be said for Cyclegards... an unintended side benefit).

Jamie Hall · April 11

And furthermore...

In my humble opinion, far too much anxiety is wasted on a steam boiler cycling off on pressure (provided the pressure is reasonable -- no more than a pound or so for most residential systems, and some much less). Yes, there will be an efficiency loss -- but it is small. In my view it is better engineering to size the boiler correctly to the rest of the system and then, if better or local control is needed, work on that. Note: I am not advocating for significant oversizing. I'm advocating for right sizing. Which takes a good deal more thought... and attention to the system as a whole, rather than regarding it as a random collection of isolated bits.

bburd · April 11

This building is a theater, not a residence. Large radiators in the lobby area are usual in such buildings due to the amount of cold air that comes through the doors as hundreds of
people enter and leave.

Jamie Hall · April 11

bburd said:
This building is a theater, not a residence. Large radiators in the lobby area are usual in such buildings due to the amount of cold air that comes through the doors as hundreds of
people enter and leave.

All the more reasoning for right sizing. There are times when you are going to want every single BTU the radiation is capable of providing. And probably more...

dabrakeman · April 11

A quiet performance is also paramount (excuse the pun) to success of this system in a theater. I expect it would need a full cast (whoops) of the radiation that is there since there is no major thermal modifications to the original stone building and windows and no evidence of radiators being cut (ouch) in the theater itself. To help avoid bad acting (oops) pressure needs to stay under ~14oz or so and at the same time not have prima donna (...) rads that have to be vented too quickly or slowly to accommodate the high volume of steam. In my experience that is when you start warming up the symphony (sorry again). This is why running a limited production (I give up) with low fire makes a lot of sense.

I really appreciate all the good input and advise provided here. Going to have to make a call on those 3 column radiators once I am up there and get good measurements this summer. Still going to be a slight guessing game on the edr unless someone has further insight or any epiphanies on them. At least it seems we have held off the boiler purchase now until then so that is a success. Just needs to be ready to go before the coming winter.

Jamie Hall · April 11

That was a superb performance, @dabrakeman ! You conducted it well...

dabrakeman · April 11

Still an understudy!

ariccio · April 24

https://forum.heatinghelp.com/discussion/comment/1798477#Comment_1798477

Jesse Smith recently put to words what we all know too well:

https://asteriskmag.com/issues/05/lies-damned-lies-and-manometer-readings

Pickup Factor in older buildings

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