Revisiting dual steam boilers

John_173

This is a continuation of sorts of a thread from early November (if you’re interested it’s <A href=http://forums.invision.net/Thread.cfm?CFApp=2&Thread_ID=51252&mc=12>here</A>
or if that doesn't work, cut & paste this:

http://forums.invision.net/Thread.cfm?CFApp=2&Thread_ID=51252&mc=12

Thanks to Boilerpro, Ebebratt, Gerry Gill, Noel, & Terry Tekusan for your earlier thoughts.

The tandem boilers should make it easier to get the new equipment in the basement and provide more efficient fuel use. My problem is that my both first & second choice contractors think I need a receiver and two boiler feed pumps. They both are concerned about the time lag that may occur getting the condensate back to the boilers. I would like to keep things as simple as possible (gravity return). I think I need at most a passive equalizing tank and no pumps. The Slantfin modular boiler document (Noel’s link) in November’s post is great, but includes single boiler feed pump and receiver returning condensate to the bank of boilers.

While no one can positively answer this question long distance, what do you think?

Second question – I’m leaning to Smith G-8 (wet-base, LPG-fired) boilers. First choice contractor says he can get whatever I specify. Nearest distributors are out of state (I’m in SC). Am I foolish to select a brand not locally obtainable? Remember, this is SC – NOBODY has residential steam!

A simplified description of the system:

Two-pipe steam, approx 825 EDR, circa 1930.

Old Bryant LPG boiler is dead, terrible near-boiler piping. All that will be ripped out.

All returns are dry, until right at the boiler (well, that’s good, anyway).

Motorized zones, receiver, condensate return pump (all to be ripped out).

Poor-to-moderately sealed envelope, 18” stone walls with plaster/wood lathe interior.

Three zones:

First zone is 606 EDR with approx 83’ supply, 12’ dry return and 95’ dripped return, plus risers to all three floors.

Second is 203 EDR, about 68’ supply and 48’ dry return, plus risers to 1st & 2nd floor.

The last is about 16 EDR, 36’ supply, 40’ dry return, plus riser to 1st floor.

Well, this is already too long. Thanks for your time reading this.

gerry gill

with all overhead returns

there will be a return lag..your suggestion of a passive equalized tank is the best option in my opinion..a feed tank would work but now involves added stuff..more stuff, more problems..a condensate tank is dumb..if water can make it back to the condensate tank, it can make it five more feet to the boiler..

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John_173

Thanks, Gerry

I'm thinking the volume of the passive tank needn't be huge - that is, no more than the volume of one boiler.

That, of course, raises the question of the volume of the Smith boilers. Not much info seems available through the website.

I suppose I could use a cast iron receiver, plumbed inline on the return as the equalizer. (I'm assuming CI is the best material.)

A question comes to mind, why are overhead returns slower? Just the reduced slope?

More generally, is there more printed reference to tandem boiler installation? Or have you (& others) inherited the technique from the Old Masters & refined it?

Again, thanks.

John_173

More questions

1.) It would seem to me the equalizing tank should be between the two boilers & equidistant from each. Or doen't it matter?

2.) I’m guessing the tank is the place to plumb fresh water feed.

3.) Is the feed manual or automatic? (Automatic would need to be able to sense low water condition from either boiler.)

4.) Supply & return shut off valves are not recommended (page 10). I can understand the need to pipe each boiler identically to equalize & minimize pressure drop. However, part of the attraction (in my dim mind) would be the possibility to take one boiler off line for service & still produce steam with the other. Is this just another example of my bad thinking?

5.) The Slatfin modular boiler document shows steam system piping with an isolation valve between the header & the riser to the emitters (page 8, top drawing). Why?

Hope I’m not too demanding of info.

Steamhead (in transit)

You can use gravity return

no need for a pump or tank. Here is a piping diagram showing various ways to pipe multiple boilers as well as how it relates to a single-boiler installation. This diagram is for the Utica PEG/Columbia CEG series, which I had saved as a PDF so it was convenient to post here. You'd simply use the water line height, riser height etc. dimensions for the G8. It's very important that the Hartford loops be exactly the same height.

The same info is on pages 66-67 of "The Lost Art of Steam Heating".

If possible, split your boilers with one larger, maybe 2/3 to 3/5 of the load, and a smaller one to make up the difference. Then a Vaporstat mounted on the main header can drop the smaller boiler out when several ounces of pressure builds, and the system can then run on just the larger one. Terry did this and it worked real well, and we plan to do the same on an upcoming project.

Each boiler would have its own automatic water feed valve, LWCO and Vaporstat. As long as each boiler has its own Vaporstat, over-pressurizing should not be a problem, so I can't see any reason to not use isolation valves. Of course the water feed connection should be on the boiler side of the isolation valve in the return.

In that Slant/Fin diagram, the valve between the main header and the system is called a "king valve" and would be used to isolate the boilers from the system. This dates back to the early days of steam heating- not sure if that would be needed today, but they show it on the diagram so I'd include it.


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John_173

Thank you. sir!

And thanks (ultimately) to Dan.

What an amazing resource! Gerry & Steamhead addressed my questions in minutes. Two (of quite a number) of the real pros, taking time out of their days off (at least I hope you guys take some time off).

And what I REALLY like is the new stuff I learn (or at least learn I don't understand) almost every time I check out the Wall. For the same reason, I've read Scientific American for the past forty or so years. You guys prove my ignorance every day & sometimes you even manage to reduce it.

Thanks & peace to you.

gerry gill

don't cut yourself short on the water volume

in the reservoir tank..remember the tank is sitting sideways splitting the waterline..so only the bottom half of the tank is below the waterline of the boiler..also, if you only want to go down say two inches in the boilers sight glass, then you will also only want to calculate the reservoir tanks USABLE volume to be what water is in the two inches that it to will drop..see what i mean? you don't actually get to use all the water that you started with in the tank..so don't undersize it..it can be placed close to the boilers, or in an ''A'' dimension anywhere in a wet return...which brings me to your question of why the lag time between wet and dry returns..picture a water level in your minds eye..if a drop of water gets added to one end, it also raises the other end..right away..now picture that drop of water going out as steam, having to condense, go into a dry return, and meander its way back to the boiler..see with a wet return, the drop your following would have dripped into an ''a'' or ''b'' dimension and you would immediately get a drop of water popping back into the boiler..but not the same drop you started with..did that make sense? thats why dry returns are slower..your actually waiting for the original drop of water to get back.

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John_173

Got it!

Or at least I think I do. Gives me something to play with while I cook supper.

I knew I was addicted to this place when I started dreaming steam....

Steamhead (in transit)

Glad to help

and the offer I made earlier still stands, if needed.

If I'm wrong about the tank, it can be added, but I've never had to do this.

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John_173

Again, thanks.

Give me some time to let things settle out. I haven't bugged you because I've been stalled on this job for a while. It now looks like it may be March before we have heat. That will make it about a year since I started searching for contractors able to do this project. A measure of the scarcity of residential steam down South.

I'm meeting a second time (first time on site) with a contractor who may well be able (& willing) to do the job. I'll see what he says & will again mention to him the possiblity of consultation with you.