Sq ft of steam:sq ft of EDR

NO!

The square feet of steam rating on the boiler is reduced by 33.3% to account for piping and pickup losses. The value needs to be increased by 1.33 to match the total radiator EDR.

That being said, most buildings need a bit of a pickup and piping factor (although this is a frequent "discussion" on HH).

I have installed a steam boiler with a 10% pickup factor and it works perfectly fine. Some more difficult installations with giant uninsulated pipes in the basement might need a 20% pickup factor.

No steam boiler needs 33.3% pickup factor unless the NBP is completely in error.

Yes EDR and square feet of steam are the same thing when talking about radiation. 1 EDR or 1 square feet of steam is 240 BTU/hr.

A boiler name plate shows how many square feet of steam the boiler can supply. The square feet of steam rating on the boiler includes how many square feet of radiation the boiler can support and includes a 1.33 pick up factor for warm up load.

Yes it is.

Yes it certainly is a loaded question.

If you found a boiler with an net output of 230,400, the manufacturer would have a value of 173,200 on the rating plate for steam. This takes into account the 33% piping and pickup factor.

So, looking at this way…………..Chris is correct in that the actual capability of the boiler is 33% greater than what is on the nameplate. This would precisely match the radiation WITHOUT a piping and pickup factor (usually necessary in some percentage).

It's not so much that it is a loaded question, @AndythePlumber , as it is one where there are two aspects which are important — which get left out of the discussion.

The first is that no responsible contractor should build something for a real world application which is just exactly capable of matching the required performance or loads to be placed on it. Nor would any responsible engineer specify such a thing. Without getting too philosophical about it, unhappily this is a way of working which is getting less and less common, and we see the results in everything from major projects to consumer goods, driven by a desire to reduce prices, save energy, or otherwise, quite bluntly, skimp.

The second for this commentary is specific to the heating and ventilating business, and it is that there are too many variables involved which are either poorly known, unmeasurable, or rest on questionable data or assumptions. To rely on them for design without incorporating some allowance for error is — and I'll not mince words here — just plain stupid.

Before someone shrieks, I'm all for working with a system, such as it might be a heating system, and refining it once it is in place and working to make it work better — and ideally the equipment used will allow for that (all too often in today's equipment it won't, but that's another topic). But that does not mean that one can design for the most optimistic case and install something with no margin.

I'm not a champion of that value, but I'm a champion of the simplicity of comparing an easy value against an easy value so that there's no chance of one person talking about gross btu while another is talking past them in net btu.

And too, too often, homeowners are just told "this is what I'm installing" even though the person saying that has invested no time or energy to determine if they are even close.

So against that, I would much rather a customer had some knowledge of their radiation and could see clearly how good a match they are getting to a proposed boiler.

We agree except I'm not too worried about shutdowns on pressure for a boiler that has a 1:1 size match of sq ft of steam to EDR because at least for atmospheric gas boilers, I can see no significant harm. At that size match it will be a pretty long call for heat before it shuts down on pressure, and there is no significant loss to efficiency or harm that I have ever seen presented for a steam boiler.

Properly measuring radiation, then selecting a boiler that is appropriately sized to that radiation is hardly "bean counting". And 33% pickup factor is plenty more than "barely good enough".

Can we all agree Jamie, just to set some common ground that a 2x boiler is too big and that a .5x boiler is too small? 😅

If you are referring to the McDonnell Douglas F-4 Phantom, that is an intriguing comment that merits fuller explanation than can be given in a thread dedicated to the proper sizing of steam boilers…

I think we probably can — but with an important caveat: the more closely one wants to design — pretty much anything — to be exactly right and fit for the purpose, the more skilled and knowledgeable you have to be, and the more exactly you have to know what the purpose is and what your design can actually do. And that is not handbook stuff. Handbooks — like that danged pickup factor —exist so that the person who may not be as skilled and knowledgeable, or who is not in a position to study and refine the demands on the project, can do something which will work, and which they can guarantee will work.

This remains quite interesting.

The general assumption since the beginning of time is that you MUST size the boiler to the radiation. What if you sized it to the heatloss (with a suitable safety factor). Is it a certainty that such a setup will not function properly? It's still oversized for 99% of the heating season. What is a certainty is that Dave has come close to this with two pipe systems and he has not experienced any serious problems with it.

So, why do we still cling to the mantra of sizing to the radiation that has been passed down for decades?

That's my beef.

My question is, if @Jamie Hall didn't know any of the steam rules, and had to start from scratch how would he proceed?

No, I mean no rules.

He's starting from scratch engineering a steam heat system, no knowledge of anything heating related.

I'm betting he wouldn't want the boiler producing more steam than the condensers can handle.

You know, that's an interesting question you have there… I suppose, starting from scratch, I would certainly try to size the boiler output as closely as I could to the total worst condition load on the system. Which would be a first class pain! As I say, rules of thumb and handbooks. There are a lot of variables… fortunately, most of them are pretty well understood, however. Some of them are not — or at least I haven't found good sources for them — such as the variation on the output of a radiator for differences in space temperature.

And so on…

I'll think about it!

If I were starting from scratch, I'd do a heat loss for ,say, 5-10 degrees below design day (Fudge Factor 1) and add radiation to match that; then size the boiler for a 15% pickup factor (Fudge Factor 2).

Assumptions :pipes are insulated

So, on the occasional day that's below design, I'd still have adequate radiation AND if all else fails I still have some headroom with the smaller pickup.

Still makes for a smaller boiler than what's usually seen.

Exact matches to design day and internal losses leaves no room for error when external variables intervene and, at the end of the day, no one wants to be cold.

"Steam systems seem like they've been left behind." Probably because (almost) no one is installing them new.

Question for another thread, but I do wonder why that is.

And of course that is the least predictable variable of the whole lot! What are the expectations of the end user?

Two-pipe vacuum systems don't have to be skimmed?

I started to write a longish commentary on this — that is, some of the considerations for sub atmospheric steam systems — but rapidly realised that it was getting long — even for me!

While there are indeed some advantages to operating a steam system at such pressures, there are also some major problems associated with it. Most of them revolve around establishing the initial pressures required and then maintaining them while at the same time preventing loss of refrigerant (water) and preventing the entrance of non-condensibles (air). This is no mean feat even for conventional heat pump systems (heat pumps, air conditioners, refrigerators, what have you) which operate well above atmospheric pressure.

On balance, the complexities which are added would not outweigh the advantages to be gained.

However, that is not to say that a small efficiency gain could be made in one of two ways is systems were designed to operate part of the time at subatmospheric pressures. One would be to have the boiler operating at condensing temperatures for some or all of the time after the initial firing in a cycle. I haven't worked out whether the cycle would need to be long enough to entirely fill the system with steam, however. If that were true, there would be no real advantage. The other is that forcing the system to drop subatmospheric at the end of a cycle would enable transferring the residual heat in the boiler to the target space. This second can be done in any two pipe steam system which is piped using crossover traps and a single main vent by the simple expedient of making the main vent (or vents) Hoffman #76 vents — which are still made. Some folks have also suggested accomplishing the same objective by placing a low cracking pressure check valve before a regular vent (the cracking pressure, however, has to be very low). Either of these methods will work. There is a question of is it worth it, monetarily? Both #76 vents and low cracking pressure checks do not come cheap, so one needs to look at how much energy can be recovered this way. In the days of large high mass boilers, and particularly in the days of coal fires, which don't just turn off, the answer was —-a significant amount, with the result that such systems, while not common, weren't that unusual. Today, however, the amount of heat stored in a modern boiler is, in relation to the overall system, almost trivial.