How far can we down fire?

Phil_17

Hi all, long time lurker here with a question.



We have a nicely working single pipe steam system; fall 2007 MegaSteam 513, about 460 sq ft of radiation by my calculations using the charts from TLAOSH, house heats evenly, boiler never makes any real pressure until the radiators are about full (which often means that it never makes any measurable pressure at the boiler itself), vaporstat cuts out at 8 oz. and in at 2 oz. and the system almost never cycles on pressure.



This leaves me thinking that we could be a bit low on the range of firing rate with the boiler, but I like that it works this way: most radiators never get hot all the way across until the depths of winter, and some of the vents never need to close. I've spent a lot of time (and a little money) learning all about steam (at least single pipe), and have spent a fair amount of time tweaking things to get this system working nicely the way it is. Things are super quiet, it doesn't use hardly any water (only need to add maybe two or three times a year) and the system has been basically trouble free. Happy, happy!



So, to the question: We are seriously considering a renovation in part of the house (this is a big old New England farm house and the back Ell is in need of some serious help). The current plans would call for removing about 150 sq ft of radiation (planning to install radiant floors throughout the renovation, probably with a new secondary boiler to handle that load, and maybe relocate the indirect DHW from the steam boiler to the new one), and my question is how badly is this going to hurt the system that we currently have? I know that the existing boiler will be seriously oversized for the new load, and I'm wondering about options to down-fire it (how far can this reasonably go?) or whether we're just going to have problems (lot's of cycling on pressure, spitting vents, perhaps creating hammer problems where there are now none, etc.)



Nothing is definite right now, but we're likely to get started on the renovations over the winter, so this may come into play during this heating season.



Thanks!

-Phil

jpf321

you can experiment a little...

by turning off the rads slated for removal and seeing what effect that has..



steamhead is really the best to answer a downfire question.



you won't have any of the ugly effects (sound, banging, spitting) you mentioned above (assuming pipe pitch is maintained and boiler piping is reasonably correct), worst case you cycle more often on pressure.



why move the indirect from the original now-over-capacious unit to a new one that you should be buying just the right size.

Phil_17

moving indirect?

Good point on just turning off the rads in question. I'll make a note to run that experiment and see what happens.



As for the indirect, there are a couple of thoughts. 1) Running this off the steam boiler has been hard on the circulator, I've had it fail twice in the past 3 years (under warrantee, but still it's something I'd rather not worry about), 2) the new boiler should be super efficient, probably condensing (radiant floors using warmboard typically don't need more than 110-120 water) so why not put the DHW on the most efficient appliance we have? 3) with only the steam on the steam boiler, and radiant running half of the house, we might be able to keep the steam system idle for much longer in the shoulder seasons (thus saving more fuel)



Still early in the design/spec phase of the project, so these are great questions that we will need to put some energy into answering.



Thanks!

Steamhead

Or you might

see if the steam system will handle the additional load. Remember, the rooms where the addition connects to the original house won't have "outside walls" once the addition is in place. So they won't need quite as much radiation.