Plumber suggesting the “Big Iron” boiler... do I need it?
I’ve had an engineer run heating calculations for my 4 story brick building split into two units. The total calculations came back at 30MBH per gas boiler with new cast iron water radiators. The plumber balked saying it should be double or triple the MBH based on the sq ft of the building. Another engineer came back with 35MBH and 30MBH, again the plumber is recommending the “Big Iron” Burnham boilers based on the size of the building. of-course the plumber won’t share his opinion with the engineer directly and vice-verse. I’m getting the sense that the plumber, who I believe has good intentions, is basing his suggestions from his gut feeling of “bigger is better” and “This is what I always do and it’s worked so far”. Since the plumber is the one doing the install I want him to be crystal clear on why a selection was made. I read some detailed calculation methods on the forum from master plumber but that was for steam. Can anybody tell me the correct way to size a gas boiler with water radiators and pex piping? I have a high tolerance for details, my plumber may disregard math but I don’t. I suspect the plumber leaves the sizing up to the plumbing supplier where he buys his equipment, again based on the total sq ft of the building. thank you
fortGreene
in Heating and Cooling 11 years and 1 month ago
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8 replies
zeroenergybuilder | 10 years and 11 months ago
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ellenlourie | 11 years and 1 month ago
There is a difference between heat loss of the house and boiler size. Boiler size is the boiler input in btuhr. To arrive to boiler size you have to add to heat loss amount of heat necessary to heat piping between boiler and radiators and account for boiler combustion inefficiency, losses trough the jacket and other losses.
fortGreene | 11 years and 1 month ago
thanks for the reply, I’m leery because the Engineer’s number seems low, and the plumber who is doing the installation is tossing out numbers that seems way too high. I ended up with 2 engineers because the 1st engineer had over committed himself and couldn’t meet the agreed deliverable. I’ve forwarded comments from this threat to the plumber and engineer. The engineer doubled down on his numbers also giving a min-max range of 35,000-45,000 BTU. The plumber is going to come up with numbers to back up his super-sized boiler suggestions, likely provided by the place where he purchases his heating equipment. This is a 1920’s building with plaster, wood, and brick non-insulated walls. A far cry from a passive house. Most of the windows (front and back windows only) are new. There isn’t an attic. The building has two separate entrances, it’s common for this area of Brooklyn. The 2nd floor has a large landing with stairs that lead to the top unit (3rd and 4th floors), this area is not factored in as heated space so that is likely the difference in sq footage you mentioned. When engineers are done they move on to the next job, I have to live with the results. I also don’t want the plumber cutting corners or performing the install incorrectly out of spite. It’s a lot of handwringing and this isn’t the Taj Mahal but as it was noted by master plvmber, I don’t want to be stuck with the wrong decision for such a key component of creating a habitable building.
cmu | 11 years and 1 month ago
My first reaction is that if TWO engineers gave you matching figures, why would you be leery? Engineers are usually conservative and probably have no reason to underestimate. That *is* quite a low figure (mine, 3200, semi-attached, has a 150kbtu boiler.) Assuming your building is attached, most heat loss is via the roof and via air leakage (a house built to almost-no-leakage “Passive House” standards, for instance, could do with half your heat.) So as a reality check, is your house well insulated & sealed? do you have new(ish), airtight windows? Weathersealed doors? Attic insulation (suggested by the low btu loss on floor iv.) Also how is there 300sqft of unheated space on floor 2?
Master Plvmber | 11 years and 1 month ago
Gennady’s right, but the reality is that there is a lot of repetition in heat loss and BTU output requirements in townhouses/brownstones in any given area and climate. The rule-of-thumb in NYC for homes of “typical” construction is 35 BTUs per square foot or 4 BTUs per cubic foot. Ive seen many energy analyses in, again, typical area homes that put that number much closer to 19 BTUs per square foot or 3 BTUs per cubic foot. So, those rule-of-thumb are very generous. That said, 3,656 x 35(btu) = 127,960 BTUs And assuming you’ve got an average ceiling height of, say, 10 feet, then: 3,656 x 10′ x 4(btu) = 146,240 BTUs. Granted, I’m not there and I have no idea what’s going on. Maybe your building is in Rome and it’s got a giant oculus in the roof. Maybe you’re really Santa and the building is in the North Pole. But if yours is a somewhat typical attached brick building somewhere in the NYC Metro Area, then I’d say you’ve got 3 people on board that could use some schooling on sound boiler sizing methods. Get this done correctly now or you’ll pay for it for the next 20 years in lost efficiency and maintenance on an oversized boiler.
fortGreene | 11 years and 1 month ago
Thank you for the replies, here is the square footage of the heated living space Basement, split between units with a mech room 960 sf – 12 MBH unit 1 1st floor: 850 sf – 17 MBH 2nd floor: 565 sf – 08 MBH unit 2 3rd floor: 815 sf – 13 MBH 4th floor: 685 sf – 09 MBH Total: 3656 sf
ellenlourie | 11 years and 1 month ago
There is no such thing as calculating boiler size based on square footage of the house. This is a sign of incompetent installer. Hot water systems are sized on heat losses trough exposed walls, windows, doors etc plus infiltrations and less internal heat gains at design outdoor temperature for cold start.
Master Plvmber | 11 years and 1 month ago
Just out of curiosity, what is the square footage of the heated leaving space of the building?