Why undersized steam boilers waste fuel

HeatingHelp

Why undersized steam boilers waste fuel

Read the full story here

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.