Why undersized steam boilers waste fuel

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Why undersized steam boilers waste fuel

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Patentsolver

Dan, I can't make any sense out of your explanation in terms of specific heat. It's like saying you need to know the volume of a trash can to determine the size of hose you need to fill it. It takes a certain amount of energy to raise the temperature of a pile of metal from 70 degrees to 215 degrees. But any size furnace can put out that much energy - the only difference is how long you have to wait. Don't you agree?

Jamie Hall

Patentsolver said:

Dan, I can't make any sense out of your explanation in terms of specific heat. It's like saying you need to know the volume of a trash can to determine the size of hose you need to fill it. It takes a certain amount of energy to raise the temperature of a pile of metal from 70 degrees to 215 degrees. But any size furnace can put out that much energy - the only difference is how long you have to wait. Don't you agree?

Ah. Well, you do have to heat all that iron up. However, at the same time as you are heating the iron up in some distant main, the radiation closer to the boiler is cheerfully condensing steam and radiating the resulting heat to the space around it. If it were purely a matter of heating the iron up, then yes -- any size boiler could, in principle, heat the whole thing up. Given long enough. But it isn't. As soon as the iron is hotter than the surroundings, it is going to be losing heat to the surroundings (which is, of course, the objective of the exercise!).

Steam in most heating systems condenses at just one temperature: 215 F. So the radiation in which it is condensing radiates at just one rate: 240 BTUh per square foot. The idea is to match the rate at which the boiler is creating steam to the rate -- fixed -- at which the system is condensing it.

As Dan says, the pickup factor can be likened to starting torque -- it allows the system to supply extra steam to get itself up to temperature at a reasonable rate. It is the subject of considerable debate... in an ideal world, the boiler would put out lots of extra heat when coming up from a cold start, to get the radiation and piping up to temperature, and then throttle back to the rate at which the system was condensing steam. The world is not idea...

EBEBRATT-Ed

It makes perfect sense. That is the same method use to size the end of main drip traps in a two pipe system. Find the weight of the pipe and fittings and find the warm up load with the specific heat.

I see people all the time putting 1 1/4, 1 1/2 and 2" traps on mains. They seldom require anything larger than a 3/4 trap

DanHolohan

Well said, gents. Thanks.

Ironman

It's very similar to understanding the Refrigeration Cycle: if you realize that one component (the evaporator) is adsorbing heat into the refrigerant which is causing it to boil off into a vapor; and the opposite component (the condenser) is removing that heat and causing the refrigerant to condense, then you realize that both components have to match in size to be able to transfer heat at the same rate.

jumper

Does this depend on strategy? In the seventies single apartment building boilers were replaced with multiples. Superintendent would fire them according to time of day and outdoor temperature. Wanted building extra warm in AM when occupants were getting out of bed. Pretty warm late afternoon when folks come home from work. Rest of the time not so warm.

Jamie Hall

Sure -- multiple boilers let you really fire things up and get all the radiators toasty in the morning, then you shut all but one or two off for the rest of the time until the commuters start coming home just to keep things warm, then fire them up again.

At least a good super would -- but they are getting hard to find.

EBEBRATT-Ed

@Ironman said" then you realize that both components have to match in size to be able to transfer heat at the same rate.

Don't really match in size as the condenser has to reject all the heat the evaporator absorbs + the heat of compression

Ironman

EBEBRATT-Ed said:

@Ironman said" then you realize that both components have to match in size to be able to transfer heat at the same rate.

Don't really match in size as the condenser has to reject all the heat the evaporator absorbs + the heat of compression

True enough. But I was using that as an illustration, not as the particular subject.

Steve Ebels_3

In a perfect world...all the rads would get hot and the boiler would never shut off on pressure because the radiator load or demand was a 100% match to the output of the boiler.

nicholas bonham-carter

If the main venting were of such capacity, and low back pressure, as to fill the mains first, then with Hoffman 40's on each radiator, there might be an even enough distribution to make the undersized boiler work.--NBC

ChrisJ

nicholas bonham-carter said:

If the main venting were of such capacity, and low back pressure, as to fill the mains first, then with Hoffman 40's on each radiator, there might be an even enough distribution to make the undersized boiler work.--NBC

This should always be the case.
If any radiator starts heating before the main vents are hot there's a problem right off the bat. This should never be tolerated, undersized boiler or not.

Though my experience is slow vents aren't always necessary either. I have a few radiators that are close to the boiler that need to have slow vents on them, but ones further away get away with Gorton 6's and Gorton Cs and yet will never steal all of the steam.

I've found, at least in my situation, it's directly related to the total length of pipe between the radiator and the boiler.

If radiators tend to steal steam, throw Hoffman 40s or Gorton 4s on them and the ones that are slow to heat throw faster vents on such as a Gorton 6 or C.

jumper

Problem is load on radiators varies. Ideally TRVs on two pipe systems results in less demand most of the time. Then you want undersized boiler or burner running all the time. But that means pipes stay warm and consume fuel. So maybe to save fuel you do want burner off and cold pipes. Unless those pipes are uninsulated and right against outside wall my guess that it's better to have warm pipes all the time. Because cold pipes condense more steam. It's the old exergy thing.

Dave in QCA

By nature, I guess I am a sceptic. I tend to question a LOT of things I am told. And so, when I started reading this article, by the time I got to the end of the first paragraph, my reaction was, 'Whoa.... Wait just a minute!" But in the end, the answer that Dan presents as to why oversizing is important is related ONLY to the waste that occurs because of uneven distribution of the steam. Half of a building over heated to keep the cold half at 70F is waste.

1.33 factor for pickup and piping loss is an accepted and proven method of preventing uneven distribution of steam. Is it the only method that will work? No. The other comments attest to that. But, in the world of the installing contractor who cannot afford to assign a tech to a job to fiddle and tune, it is a reasonable method for sure.

Other methods include vent adjusting and balancing on a 1-pipe systems. Inlet orifices, or regulating plates on 2-pipe systems. It is also interesting to note that Dunham's description of the benefits of vacuum operation as in the Differential Vacuum System and other various versions that Dunham perfected, always included the more even distribution of steam.

mikeg2015

I think there's an opportunity to use stem boilers sized closer to the load on one pipe systems. But it requires I think :

1) well insulated headers,
2) very slow and controlled radiator venting and benefits from TRV's in common spaces to manage differences between temps on different floors at different outdoor temperatures. Small vents available include (Gorton or MoM #4 and ventrites) and well vented main. 3) You may need to go a step farther and vent the main into the flue for increased draft (assuming you have a very tall chimney) to ensure the main is lower pressure than the radiators.
4) A 2 stage gas valve would be ideal for a soft shutdown to prevent rapid vacuum formation on vapor systems with large headers.

Question becomes, how small is too small. Can you "tame" the steam to partially heat up headers.

Maybe it's good enough just to downsize so that DOE matches EDR.

nicholas bonham-carter

I think the steam whisperer has sized some two-pipe systems close to the heat loss, with good result.
We know that hot water systems can be sized to the heat loss of the building, so why not steam, if you you can put up with a slower response time?—NBC

ChrisJ

nicholas bonham-carter said:

I think the steam whisperer has sized some two-pipe systems close to the heat loss, with good result.
We know that hot water systems can be sized to the heat loss of the building, so why not steam, if you you can put up with a slower response time?—NBC

Old habits die hard..................

The pickup used to be much larger with hot water too. Wonder how many are still following the old rules for those systems.

JoeEngineer

It takes a minute to wrap your mind around the whole thing; this reveals an advantage of steam over hot water, since the cold, at every point in the system, condenses the steam. As pointed out, a property sized system heats every pound of cast iron, so it is reaching out to the far ends of the system with "full capacity" ready to give up 1000 btus from every pound of steam, hot water is only able to provide say 20 Btu's (20º∂t) even with a reverse return system the water is losing temperature and consequently heating ability as it goes through its sub loop. the phase change of steam to condensate allows the (properly sized) steam system to provide (close to) instantaneous respond to the heating load

EBEBRATT-Ed

@JoeEngineer

it gives and it takes away. with hot water as soon as the burner and circulator start you start moving heat towards the load. With steam you have to boil the water first. You get that latent heat back but with some increased temp going up the flue and boiler jacket loss with steam

jumper

I agree with JoeEngineer. Heating is to maintain temperature in living space as opposed to putting energy into it. Until radiators get hot enough to heat living space fuel is wasted when it is heating pipes and water and iron. Once an under sized boiler gets some of each rad hot the burnt fuel is maintaining temperature in rooms. Of course you still need more boiler capacity for design conditions.

The above can be difficult to explain. Even when multiple boilers were bid at same price many decision makers opted to replace single over sized boiler with another one.

ChrisJ

@jumper

Heating a living space is literally putting energy into it.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1802894#Comment_1802894

"Until radiators get hot enough to heat living space fuel is wasted when it is heating pipes and water and iron."

Unless that pipes and water and iron is in space you don't want heated, every BTU you put in you get back when that equipment cools down. Nothing is wasted.

DCContrarian

An experienced man once said to me: "The purpose of a heating system isn't to provide heat, it's to provide comfort." A lot of wisdom in that. This thread has me thinking of him.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1802962#Comment_1802962

What does that mean?

A heating system doesn't provide comfort by providing cooling so how exactly does it provide comfort?

My steam system provides heat. I guess there could be others out there that provide avocados but mine just does heat.

jumper

» every BTU you put in you get back when that equipment cools down. Nothing is wasted.«

If that is the case why bother with A/B dimensions; feed pumps; and so on? Steam powered injectors are relatively inexpensive.

ethicalpaul

https://forum.heatinghelp.com/discussion/comment/1802923#Comment_1802923

A lot of the heat of the pipes is in the semi-conditioned basement and yes some of that heat makes it to the living space, or provides a sort of buffer, but I think it's incorrect to say nothing is wasted. A lot of it heats the ground around my house or gets blown away by the relatively drafty basement

My upstairs radiators are supplied by pipes in the exterior walls. Some of that heat is also wasted.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1802963#Comment_1802963

I'm talking only about in heating system. You could expand it to say the purpose of an HVAC system isn't to provide heating and cooling, it's to provide comfort.

Comfort is providing the right amount of heat (or cooling) when and where it's needed. Just look at the example in the story, half the building is overheated and half is underheated. That's not comfort.

I usually use this saying when people ask me if it's more efficient to keep their house cooler in the winter. Well, you'll save money, but you'll be less comfortable. The purpose of the heating system is to keep you comfortable. Is it worth it to spend less and be less comfortable? Only you can answer that.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1802973#Comment_1802973

That heat is lost continuously whenever the system is running. The original claim was "until radiators get hot enough to heat living space fuel is wasted when it is heating pipes and water and iron." There's nothing special about the heat that goes into warming the system up.

ChrisJ

@DCContrarian that I can agree with.

It's cheaper if you just turn the heat off but you pay for comfort and if I'm paying $200 a month already what's the point making it so I'm chilly to save $20.

Dave in QCA

It seems like the discussion has drifted from the science and thermodynamics of building and its heating system to the science of human psychology and how people sense comfort.

ttekushan_3

The way I look at it, the pick up factor in large part insures that heat can be delivered evenly throughout the structure, all other things being equal. If you have a lot of leeway in manipulating venting and the piping layout is fairly simple, it will take less pickup factor to achieve this. If you have a building complex with convoluted piping you might need more. I’ve saved fuel by upsizing in one instance and downsizing in another. The difficulty with being a contractor walking into a situation is that it’s difficult to find out just how well it was actually working beforehand. A landlord might say “it’s been fine.” A tenant might say, “yeah it’s fine once we plug in the space heaters in the back bedrooms with the cold radiators!” The landlord might also say “it’s eating me alive in fuel bills!” If I have the freedom to attack the whole of the system, it’s usually possible to improve all those situations. I like to do everything I can to get the heat distributed quickly and evenly by every means available and let the cast iron act as the flywheel between cycles. One pickup factor doesn’t fit all, but the luxury of a boiler that modulates itself into and out of the pickup factor is wonderful.
In any case I’ve found that well distributed heat, comfort, and economical operation go hand in hand.

jumper

If vents stay closed longer then system breathes less. Steam works to expel air and that is not best use of steam. There is a distinction between heat energy and heat that increases temperature. Back in the seventies I heard smart guys discussing benefit of keeping the boiler going full time somehow. Some pointed out that a cold boiler extracts more energy from flame. Eventually the consensus was that intermittently cold boiler wastes fuel.