Dan Holohan’s website, www.heatinghelp.com, includes a message board called “The Wall.” Regular participants include some of the brightest and most dedicated practitioners of hydronic and radiant heating in North America. As a result, the useful information found here rivals that found on any online forum in the industry.

With Dan’s return to Plumbing & Mechanical, Supply House Times and PM Engineer (pme), we are again sharing with readers, with Dan’s permission, some of the posts (edited by PM staff for style and grammar) that pertain to technical problems in the field or other items of interest.

Replacement boiler sizing

Original inquiry: Replacing an old HB Smith 25 mills, 6-section boiler (1,600 square feet steam). Existing installed radiation only measures 503 square feet. Two stores on the ground floor have been disconnected. However, the supply piping is still there serving 14 small radiators on the upper floors. Do I trust the new boiler rating of 540 square feet or do I need to go up a size due to the large supply pipe sizing in basement? Also, just to throw something else into the mix: Is there any difference in boiler quality or steam supply if the boiler riser requirement is one at 3 inches or two at 2 inches? HeatJockey

Replies: I have run into this situation before and it can leave you scratching your head for the correct answer. First, are the disconnected stores ever going to be reconnected? That is the million-dollar question. If so, you have to calculate that load into the replacement boiler. If not, then I would size to the existing load, then add for pickup and piping loss of about 15 percent. That would leave you with needing approx. 600 square feet. Also, insulate those mains and check all vents or traps.
As far as the risers go, I always would go with a boiler with two risers, keeping the exit velocity low and avoiding the possibility of a tipped water line. GregMaxwell

Going up a size is more comfortable than cutting it close with a good customer. Reassurance from someone else stops the “Am I being an idiot?” feeling. HeatJockey

Have you done a heat loss? Even a quick heat-loss calculation will help. You don’t have to heat up the entire radiator if the radiator is larger than you need. Quite often I go smaller than the connected load if the heat loss is much smaller than the connected load. If the heat loss is larger than the connected load, you go larger and plan on adding radiators.
You do always have to take into account that the supply steam piping was sized for all of the original radiators. Paul

The boiler with one 3-inch has the lower steam velocity. You can figure the area of a circle. One 3-inch is larger than two 2-inch circles. Point odds to 6 point odds, so roughly a 15 percent increase in volume on the pipe.
The 3-inch requires big wrenches, threaders and more sweat. The 2-inch is easier for the light-duty shop and is almost large enough to do the job. Now two  2 1/2-inch risers are better than one 3-inch riser. Charlie from wmass

Do we always need to fill the radiators with steam? That will certianly provide adequate heat, but you will be oversizing the boilers more times than you think for the simple reason the steam system was oversized to begin with because of the foul-air issue. The original radiators may have been oversized by 30 percent.
Varivents will shut down the boiler and prevent overheating, but you may get short- cycling on certain jobs. On small buildings, this does not matter that much. On a bigger building it matters a lot. Right now, I’m estimating a job where the engineer specified a 6,000,000 Btu boiler when a 4,000,000 will do just fine. Can you imagine the cost difference on this job? Paul

Paul, the correct answer is no, we do not need to fill every radiator to heat the building properly on most days. We need to fill every radiator on design days. Also remember if the radiators are in rooms below 70 degrees F, they will call for more steam to heat across as the effective square footage per section increases the lower the room temp gets. Customers also will find the radiator that does not warm across and call you out on it, even if the fuel bill drops and the building is perfectly comfortable. Charlie from wmass