Sizing boilers - very confusing

ChrisJ

It's basic math, but many seem to be confused by it.

Jamie Hall

You're so right, @ChrisJ ! Including, it would seem, a dismaying number of installers... as you say, it's basic math -- but the problem is you have to start with the right numbers for the system type and building, and that seems to be difficult. Oh well...

Long Beach Ed

So many are simply too lazy to try, or never read three words about their trade.

ethicalpaul

Better bump it up one more just to be safe

EdTheHeaterMan

Boilers have 3 ratings measured in BTUh.

Gross input. That is how much fuel you put into the flame. That is how you size the gas pipe or what oil burner nozzle you need.

DOE Output. That number was added to boiler rating in the 1970s by the newly formed Department of Energy in order to compare the efficiency of boilers and furnaces. (Furnaces do not have Piping and Pickup loss included in their rating because there are no pipes, Just ducts.

AHRI Net is the number you match the heat loss to. Old boilers have I=B=R Net output. That is the number you use for matching the Heat Loss from an I=B=R load calculation Both those numbers are the same. The organization for naming alphabet entities just likes to swap letters around to keep us on out toes.

The funny thing about Net rating is that the same boiler has a different rating if it is a steamer or a water boiler. That is because steam operates at a higher water temperature. hot enough to make steam so they have a slightly lower combustion efficiency. The other reason is that a water boiler Piping and pickup is figures at 15% while the steamers are figures closer to 30%.

When sizing any water boiler to the heat loss of the building Use the NET rating.

When sizing any steamer use the Square Foot rating. This is all the math already done for you. Since steamers need to heat all the cold metal attached to them, you need to size the boiler based on all that metal. If you put something in that is big enough to heat the 100 year old house that has been completely insulated with energy efficient windows and lots of caulk, but kept the beautiful radiators, you boiler selection may not be large enough to heat all the metal pipes and radiators. So use the SQ FT of the EDR and match it to the SQ FT rating of the boiler. The BTU Math is already done for you, if yo do it that way.

I hope this short novel of a post is helpful
Almost forgot the illustrations

EBEBRATT-Ed

It's not that hard at all. But as we see on a daily basis the installers can't be bothered to read the piping or skimming instructions. So how can we expect them to size the boiler right?


Reading is important. "You can lead a horse to water, but you can't make him drink."

Same thing here many are "know it all's" and those people usually know nothing.

Even the best installers on this site don't know everything or have seen everything and they know that.


Because in this business if you think you know everything, watch out the next job will surely bite you in the ****.

KC_Jones

ron said:

and the house has a heat loss of 50,000 BTU/hr

i still would like to know how the "heat loss" of 50k BTU/hr is derived. And where/why is "300sqft roughly 72,000 BTH/hr" ? I'll give you credit for meming, but i'm not seeing any connection here mathematically.

300x240=72,000

Heatloss is derived from a Manual J.

The Steam Whisperer

EdTheHeaterMan said:

Boilers have 3 ratings measured in BTUh.

Gross input. That is how much fuel you put into the flame. That is how you size the gas pipe or what oil burner nozzle you need.

DOE Output. That number was added to boiler rating in the 1970s by the newly formed Department of Energy in order to compare the efficiency of boilers and furnaces. (Furnaces do not have Piping and Pickup loss included in their rating because there are no pipes, Just ducts.

AHRI Net is the number you match the heat loss to. Old boilers have I=B=R Net output. That is the number you use for matching the Heat Loss from an I=B=R load calculation Both those numbers are the same. The organization for naming alphabet entities just likes to swap letters around to keep us on out toes.

The funny thing about Net rating is that the same boiler has a different rating if it is a steamer or a water boiler. That is because steam operates at a higher water temperature. hot enough to make steam so they have a slightly lower combustion efficiency. The other reason is that a water boiler Piping and pickup is figures at 15% while the steamers are figures closer to 30%.

When sizing any water boiler to the heat loss of the building Use the NET rating.

When sizing any steamer use the Square Foot rating. This is all the math already done for you. Since steamers need to heat all the cold metal attached to them, you need to size the boiler based on all that metal. If you put something in that is big enough to heat the 100 year old house that has been completely insulated with energy efficient windows and lots of caulk, but kept the beautiful radiators, you boiler selection may not be large enough to heat all the metal pipes and radiators. So use the SQ FT of the EDR and match it to the SQ FT rating of the boiler. The BTU Math is already done for you, if yo do it that way.

I hope this short novel of a post is helpful
Almost forgot the illustrations

To make it more confusing all of the above goes out the window if you have an orificed system and probably many vacuum systems. With radiator orifices that are up to date with the current heat loss, you are back to sizing to the heat loss of the building for a steam system with some pick up ( I usually add about 10 to 15%. Nearly all systems we've worked on from about 1905 to WW
II, have 60% excess radiation, so we use orifices that will only allow 60% of the radiator to heat. We have numerous systems that the boilers are about 1/2 the capacity based on conventional radiation EDR x 240 x 1.33.
On vacuum systems, the btu's are stretched out for a given volume of steam, so I would expect the boiler size would go down too.

HVACNUT

Unless of course Patrick has a huuuge underwater hot tub under his rock and a 119 gallon indirect. Then 150K is too small.

ChrisJ

ron said:

ChrisJ said:

and the house has a heat loss of 50,000 BTU/hr

i still would like to know how the "heat loss" of 50k BTU/hr is derived. And where/why is "300sqft roughly 72,000 BTH/hr" ?

I'll give you credit for meming, but i'm not seeing any connection here mathematically.

Ron,
You've never done a manual J or sized a steam boiler?