Boiler size to EDR...or maybe to venting?

ethicalpaul

Boiler size discussions are ever-present here but today I had a new thought about boiler sizing.

We know that many steam homes are over-radiated (too much radiation for the heat load) because of the Spanish Flu or whatever reason depending on the age of the home.

So in many many cases, the call for heat is satisfied before the radiators all fill with steam (sometimes way way before)

So the radiation present in the home isn't even the limiting factor. The venting is.

If you have a 40 EDR radiator that only ever fills up with steam 1/2 way it is effectively a 20 EDR (give or take). It will only be radiating (condensing) where the steam gets to in the radiator.

Has anyone ever thought of or actually done sizing per venting? I think it would only come into play with vents that were installed with "slow radiator venting" in mind. And it means someone could mess up a system with improper venting (which is already the case anyway).

Curious what you think

ChrisJ

EdTheHeaterMan

You just got to love PAUL!

Jamie Hall

Something along those lines can be done with orifices -- @The Steam Whisperer has commented on this a number of times. What you are doing is restricting the rate at which steam can enter the radiator, thus restricting the output. But that only works on two pipe systems, and further it only works if the boiler is very well matched to the now restricted output, and maintains very low pressures.

Thinking about it, though, consider this: in a one pipe system, the radiator vents -- slow or fast -- will not close until the steam gets to the vent. The only way you can restrict the fill of the radiator (for instance, the halfway mark you envision) is by turning off the steam supply at that point -- or by having a boiler which is sufficiently undersized that it simply can't supply the load placed on it by the radiation. The difficulty in either case is arranging the radiator venting so that they each fill at the rate needed for that space, and having absolutely maximum main venting -- likely including main vents on all risers or runouts also calibrated to that each radiator sees steam at very close to the same time.

Can it be done? Possibly. The balancing would be very difficult, to put it mildly.

ChrisJ

@Jamie Hall
I'm confused. Why do you feel you need to stop the radiator at the half way mark?  The slow venting makes it so everyone gets a piece of the pie, it's literally the orifice you speak of.  It's just on the outlet instead of the inlet.
What happens is as long as venting is done correctly and all of the radiators heat at the right speeds once the boiler maxes out the radiators just stop advancing and continue heating the same amount.
It works.
It really does.
Radiator vents don't see steam or even warm air in my house 90% of the time.

Jamie Hall

I don't want to stop the radiator at the halfway point -- the OP does. Don't look at me...

ChrisJ

Jamie Hall said:

I don't want to stop the radiator at the halfway point -- the OP does. Don't look at me...

The boiler size will stop it automatically and naturally and the high limit will never be needed.
I'm sorry but it's not good to constantly have a boiler "bouncing off the rev limiter".
Let the radiation limit the pressure not a control that can fail.

ethicalpaul

Jamie Hall said:

I don't want to stop the radiator at the halfway point -- the OP does. Don't look at me...

I don't want to stop it, but the fact is that they almost always do stop. What I said was:

> So in many many cases, the call for heat is satisfied before the radiators all fill with steam (sometimes way way before)

Is this wrong?

EBEBRATT-Ed

Maybe a compromise would be to size the boiler for the actual heat loss instead of the EDR but still keep the piping and pickup factor.

ethicalpaul

I could see that if the piping factor were about 5% like it should probably be

Jamie Hall

Ah. Misinterpreted. No, it is not wrong for a radiator to not be filled with steam all the way when the thermostat is satisfied. It's just the way that particular radiator works in that space. Nor is there any harm to it.

There is a fundamental difference between steam heat (and, to a lesser extent, gravity hot water) and either hot water heat or forced air. In the former case, steam, the system is entirely dependent on very small naturally created pressure differences, coupled with small controlled losses in the piping and valves, to govern where the heat transfer medium goes, and how fast it gets there, while in the latter case there is forced circulation.

Take the latter case first. In a hot water system, you can easily manipulate balancing valves (with relatively large and adjustable head loss) to direct the heating medium to where it is needed, at any given moment. Or you can vary the speed or operation of a pump to do the same thing. Further, you can vary the temperature of the heating medium at will, to match the heating demand of a particular space. You can even have different temperatures for the medium for different spaces. No problem at all. Therefore, it is very simple to match the power input -- the boiler size and modulation -- to the required power output -- the heating demand.

So -- size the boiler to the heat loss, control the circulation with valves and pumps and modulate the boiler output power according to load.

Steam, for better or worse, doesn't work that way. You cannot modulate the power output of steam -- you condense a pound of steam and you get about 1,000 BTU, whether you need it or not. More important, however, is that it is remarkably difficult to control the very small pressure differences which drive circulation -- indeed, to a certain extent steam systems are self-balancing that way. The problem here is obvious: ensuring that the various radiation gets the amount of steam it needs to heat the space.

In theory, you could size the boiler to the heat loss, but only if you could, at the same time, control the flow of steam to the various radiators so that they all got the steam at the rate they needed it. I won't say it can't be done -- indeed, it can, with careful control of source pressure in a very narrow range (remembering that head loss from friction and orifices and such things is related to the approximately square of the flow rate) and some form of control over the head loss going to the various radiators. @The Steam Whisperer has had good success with this approach using orifices -- and patient work to balance the system. Several different vapour steam systems use either calibrated valves or orifices to accomplish much the same puprose. In principle, in fact, if you get the system perfectly balanced, you can run the system with a very small boiler -- just enough to meet the load requirement plus any losses in pipes and the like, just like a hot water system. But what do you do when the load changes? If you manage to alter the pressure differential driving the system from, say 4.0 ounces per square inch to say 2.8 ounces per square inch, you will get half the flow, all else being equal -- but all else isn't. The flow through an orifice (or metering valve -- and different valve types differ) doesn't change in the same way the flow through a pipe does. What was perfect balance at one pressure may be remarkably far off at another. You can't change the heat output per unit flow. Perhaps most relevant is that there is no way to actually modulate the power output of the system evenly. You can change the output of the boiler -- but you can't control, except at one output, where the power will go.

What you can do is make sure that the power source -- the boiler -- is big enough to actually power the entire system, size the system to actually meet the worst case demand on it -- and then turn it on and off to meet lower demands.

Remember that a heating system is a system, not a jumble of parts.

ethicalpaul

I feel like the difference between a #4 and a #5 (for example) is exactly the kind of precise metering you are talking about.

ChrisJ

Jamie Hall said:

More important, however, is that it is remarkably difficult to control the very small pressure differences which drive circulation.

Remember that a heating system is a system, not a jumble of parts.

I'll take this as a compliment. Thank you.

TonKa

After 1 1/2 seconds of thought, my gut tells me you'll need Igor Zhadanovsky's ideas on vacuum assist to do this.

dabrakeman

It might be easier to modify ones boiler size way below connected edr based upon personal experience with the the system and that home but in the same case more difficult for even an expert to come in an size it in a similar manner without any of that on site experience.

AdmiralYoda

Wouldn't it be more productive and possibly less expensive to re-size the radiators to the needed heat loss? Maybe by removing a few sections you can get the EDR you need. Depends on the construction of the radiator too I guess.

ethicalpaul

I wouldn't consider re-sizing radiators to be less expensive than very much

Gsmith

Very interesting discussion. It may be possible to somewhat fine tune the boiler sizing using some other criteria than connected radiation, and for those interested, go for it. (Venting rate, Igor Z’s vacuum system, etc), but as an owner of a nearly 100 year old single pipe system, with 44 years in the house I think the original design criteria was not too bad, mostly from reading this forum the basic design criteria for steam systems (EDR plus pickup) works pretty well but is frustrated by numerous knucklehead moves: wildly over sizing the boiler, incorrect near boiler piping, uninsulated mains, setting pressuretrol too high, etc. if the dead mens long time basics were followed most steam owners would have a pretty reliable and comfortable system.

Is there room for some improvement? Sure, but get the basics right first. Then maybe jut slightly undersize the boiler as compared to the connected radiation.

I’m amazed when I think about it how well my old system works. It was after all designed to heat my house adequately on the 2% or so coldest day of the year, which it has never failed to do. And on all the other days the entire system is wildly oversized, boiler, piping, radiation, yet it modulates remarkably well, because once balanced via venting, the radiators fill and radiate just enough heat to satisfy the thermostat. Yes on the very coldest days, just a few a year, I have some uneven heating, the second floor gets on hot because the hot air rises through my open stairwell, but on those days I can simply compensate by turning down three or four adjustable air vents on second floor radiators if the cold spell lasts any appreciable time. Takes less than 2 minutes with the Ventrite #1’s I have on the second floor rads.

I don’t have any other mechanical systems in my life that are nearly 100 years old that work so simply and reliably. Just have to eyeball the sight glass once a week or so to check water level, not really difficult.

As I said it seems the bulk of steam problems reported here are seem to be caused by not following the few basic principles needed rather than not carefully optimizing the steam system. And my system was a coal fired snowman, converted to oil, then replaced about 44 years ago with a gas fired boiler that is about 35% Oversize for the radiation. It now runs on a vaporstat set at 1 psi cutout, 1/2 psi cut in, and it almost never shuts off on pressure but shuts off on the t stat reaching the set temp. And that’s with men my earlier stupidity, before I found this site and Dan’s books, having removed several radiators, replaced one with fin tube baseboard and one with cast iron baseboard. Still, I’m quite satisfied with the simplicity, reliability and comfort itprivides. If I tinker with it a bit more I could probably wrangle a little more efficiency, but it’s works well now, so is it worth it?

Jamie Hall

Excellent, @Gsmith ! And you are so right. The basic system is very simple and utterly reliable. Most of the problems encountered are errors of one kind or another -- sometimes original, but more often later, from misunderstandings. Or are patches to fix patches...