Two Steam Boilers in Parallel?

Adam_13

Can two steam boilers be piped together in a gravity return system? I don't see an issue on the supply side if each boiler is piped properly with it's own header, however I'm not sure about the returns. The system has three dry returns. If I bring each return below the water line separately and then connect them together, can I pipe this common wet return into both Hartford loop connections? I am concerned that if only one boiler is firing water will back up into the other boiler.



This is for the same system that the Reed radiators are on. It is a large Victorian house that had been converted to a two-family. One large steam system heats the whole house. The owner would like to have a new boiler installed, and he is planning on moving into the house himself and having it slowly converted back to a single family.



In sizing the boiler I have found that the 14 radiators and all the piping will require a boiler with a DOE capacity of about 230, 000 BTUH. I am exploring splitting this load equally between two smaller boilers. Using an outdoor thermostat I can set the system to fire one boiler when it is above, say, 40 out, and both when it is below 40. My thought was that with one boiler firing, though it would be very much undersized, it would still be able to heat the first few sections of the steam-only column style radiators in the house for mild weather. Then, of course, both boiler would fire in the cold weather and be able to fill the whole system with steam. Also bsmt access is not great, and we have a better shot at getting two smaller boiler down than one large one.



Another consideration in favor of two boilers is that the owner is single at the moment and is only planning on living in one of the apartments while he renovates the house. Thus for a good potion of the next several years probably about half the radiators will be shut off. Again my thought is that running one of the two boilers for this period would be more efficient then firing one big one that would in effect be over sized. As he restores more of the house and as his family grows he would then simply be able to flip a switch and bring the whole system back online.



Any thoughts and comments on this plan are greatly welcomed. Thanks, -Adam

gerry gill

yes, you can

but when we have done it we prefer to fire the full load then drop some off as pressure rises...a boiler that size i would just add a low fire valve to instead of a second boiler.

The Steam Whisperer (Formerly Boilerpro)

Yes, you can.......

As to whether you use one or two:  On hot water systems I usually figure that the improvement in overall seasonal efficiency of stage firing over a large single on/'off boiler starts at loads right around there.  However, with steam its a little harder to judge because unless the second boiler is valved off from the supply, there are some additional standby losses when only one of two boilers are running.  Probably your lowest standby losses on a stage fired boiler will occur with power burner boiler designs where air flow across the heat exchanger is minimal when the boiler is not firing.  Add a stack damper, and the lost heat will mostly end up in the boiler room.  You may also want to look at a modulating input boiler....but the burners and added controls are fairly expensive.  I have used the Slantfin Intrepid boilers with a PowerflameX4M burner for gas modulating with a Heat Timer modulating control.  R and D electronics also has a modulating control in the works for steam, and hopefully will be able to better control modulation of the burner than the Heat Timer, which appears to use a very primitive software.



Here are a few pics of installs.   Both just use a two stage thermostat as the primary staging control. One is in a regular one pipe system the other a WarrenWebster vapor system (two pipe vapor)

Adam_13

Thanks

 for your responses guys. I'll look into pricing for two boilers (with vent dampers) vs. one boiler with a modulating burner. Unless the stand by losses of one boiler not firing greatly affects the output of the other, I'm inclined to still consider two boilers. The bsmt is damp and musty so a little heat down there would be beneficial. 



From the pictures I see that you have connected both risers from the boilers into a common header with what appears to be a common equalizer. Is there just one Hartford loop connection for the two boilers at this equalizer? Is this piping only for two boilers that always fire together, or would each boiler have to have it's own header, equalizer, and Hartford loop if they fired separately



As far as the modulation firing is set up, do both boilers fire in high fire or low fire together, or is one in high fire and the other in low to supplement it, etc.?



One last question, what are the devices I see near the bottom of the equalizers?



Sorry for all the questions, but I am very interested in steam and always take every opportunity I have to learn more. By the way, I'd love to hear more about and see pictures of how you addressed the Webster modulation system



Thanks for your time, -Adam

The Steam Whisperer (Formerly Boilerpro)

Info.....

These stage fire separately as called on by a two stage thermostat.  Both use just a single harford loop(or gifford loop for better stability).

Modulation would be with only one boiler with a burner than fires at any input between its top and bottom range.  I have a small one running from 75,000 to 200,000 input and another running from 100,000 input to 320,000 input.

The devices are probably the black boxes which are the primary operating water level controls for the whole bank of boilers.

Check out Slantfin's Caravan modular boiler line for details on this piping layout.

The webster system which has one of these boiler pairs  is going to be updated with radiator supply orifices to match the later webster systems that allowed modulation.  I have this in mind for some other two pipe systems I am looking at too.  I have an article not yet published on this process, but if you look up Henry Gifford you will find many of the fundamentals for converting a standard two pipe system to modulating operation.

Adam_13

Thanks

boilerpro for all your help and information. Sorry for another late reply. Once again this is the first I was able to get back to the computer this week. I'm looking forward to researching more about Henry Gifford and the Gifford loop. All the best, -Adam