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I can not get my head around the over sizing of boilers

h12721aa Member Posts: 17
The question ?

 Suppose a 100 ft of

hoot water base board heating system gives off 50 000 BTU. If I run this with a

60 000 BTU oil boiler at 180° water temp. and this heats the house to 70°,

What would a 120 000 BTU boiler do?

  I don’t think it

would heat the house any faster or higher since only 50 000 are given off by

the base board.

This is no trick question, it is, I can not get my head

around the over sizing of boilers and its effect .

It seams going to 200 ft of 

base board should be the answer

What am I missing?



  • hot_rod
    hot_rod Member Posts: 16,347
    you're on the right track

    Thermal Equilibrium is what you are noodling. Here is a link to some good reading on that and heat transfer in general. page 51 talks about your question. It's really the heat emitters, not the boiler that drives the thermal equilibrium.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • icesailor
    icesailor Member Posts: 7,265
    Consider this:

    For those of us who were mentally challenged at birth, we devise other ways of thinking out complicated theories down to our levels.

    Consider the house as a car towing a trailer. The car has an engine (boiler) to do the work. Pulling the car around and the trailer. If you tow the trailer down a flat and level road, you need a certain amount of power, controlled by the gas pedal (or cruise control if you're brave) to drive at 60 MPH. It takes a certain amount or power setting to move the car, If the grade increases, the load goes up and you have to give it more gas to maintain the speed. If the grade goes down, you need to take your foot off the gas to maintain the 60 MPH. If you come to a steep grade, you might have to give it a LOT more gas to keep the speed up. If you can't keep the speed up, the engine is too small. The engine produces more power, the higher it revolves (RPM). If the engine slows down, it produces less power, Shifting into  a lower gear makes the engine turn faster and produce more power. But speed up the hill slower. You will find a speed and engine RPM that will allow you to not need to shift gears again and go slower. You may not have enough power to maintain 60 MPH. If you don't get the engine settings (RPM's correct, and be giving it enough power to get up the hill, and you let the engine slow down, the car may not ever get back to 60 MPH. You need to plan ahead. Give it the gas before you get to the base of the hill.

    If you put 1,000# of people in the car or the 1,000# of cargo on the trailer, you have to give it more gas to keep up the speed. If you go up the hill, you must give it even more gas, When you go down the hill, you take away the power. You use no power when you coast or use the brakes. That's the principal behind all Mod Con's. Variable power settings to equal the load. The temperature going up and down and the same inside is what determines the load. The engine/horsepower doesn't change, just the RPM in which it is produced. Railroad trains do the same thing. The more cars, the more weight, the more engines. They don't need it on the flat, just going UP the graded. They need big heavy engines with great big brakes to get back down the hill. With no power.

    Wrap your thoughts around this concept. When you figure it out, that paper posted will start to make sense. Its all relative to something else. As of lately, there have been no new concepts developed. Just different ways of looking at the same thing. Its like Gravity. We all know what it is. No one knows exactly WHAT if is. Other than falling isn't the problem. Its the sudden stopping that makes ask why. We all know that a mis-step on a ladder proves all theories of gravity. That what goes up, must come down.
  • Gordy
    Gordy Member Posts: 9,514
    edited July 2014
    In your initial post

    Your asking what the bigger boiler would do with the same amount of radiation.

    The bigger boiler would get up to setpoint in both room, and water supply temperature faster only in that the cycle is using more btus from the oversized boiler burner to heat the water more rapidly. Like putting a pan of water on the small burner of a stove, or the large burner which one will heat the same size pan, and water content faster? If it takes say 15 minutes to bring supply water to 180, and satisfy the thermostat with right size boiler it may only take 12 minutes with a boiler to large. The larger burner is making the supply water seek equilibrium faster.

    This can be a good thing, and a bad thing. Good in that it is quickly bringing the supply water, and room radiation up to temp to satisfying the thermostat. Bad in that it's burning more fuel to do the same work as the right size boiler, and it's doing the work so quickly that it never runs long enough to get a good efficient burn cycle before shutting down.

    Adding more radiation to solve the problem would be counter productive IF the present radiation is of proper size. All you would be doing is bringing the room to its set point faster, and the boiler would still short cycle.

    What over radiating Is good for is allowing one to use lower AWT to heat the space. With a conventional boiler you can only go so low with out proper boiler protection. With a modulating condensing boiler it's the cats meow to get that return water temp low to condense.

    This is all thermal equilibrium as Hot Rod said. Mother Nature is always trying to achieve equilibrium.
  • h12721aa
    h12721aa Member Posts: 17
    where will the temperatur go

    Thank You gentleman for the advice and explanation.

    May be I should have post this in the boiler section? 

    Hot Road, I have been reading that page and still wondering after

    the water gets to 184 degree and the burner is still heating the water where do

    things go, some thing has to go some where??

    And now for the main problem!

     In need of advice.

      I am going to

    install a new oil fired Boiler for my hot water heating system.

    The house, build in 1985, 60 by 25 feet 1500 sq/ft. it has

    112 lin. Feet of slant Fin type of radiators. 

    The boiler right now is a Burnham RS 111, DOE/HTG 120 BTU, Net

    104 BTU. With a hot water coil build in. The Burner is a Beckam RWB

    I like to replace the boiler and burner with a Buderus (maybe)??  And a 41 Gal. Amtrol indirect fired Water

    Heater already purchased.

    Can the Beckam Burner RWB be used with the next boiler?


    I did a Heat Loss with the Hydronic Explorer and it shows me

    48,893 BTU/HR.

    If I use the Pex- Supply House.com Calculator I get 41000 BTU.

    If I use 112 Feet* 500 it becomes 55000 BTU (I was told I

    could use that formula).

     I don’t know if I

    should go by these numbers.

      Does this sound OK

    to you or is there something out of line? All advice would be welcome.

  • Steamhead
    Steamhead Member Posts: 15,350
    edited July 2014
    Assuming those numbers are accurate

    even the smallest Buderus would be oversized. It has a Net rating of 74,000 BTU per hour:


    I found two better choices. These are 3-pass boilers like the Buderus:

    Burnham MPO-IQ84, with a Net rating of 69,000 BTUH:


    Slant/Fin Eutectic EC-13P, also with a 69,000 BTUH net rating:


    You can't go wrong with either unit. Note that you don't need extra boiler capacity to operate the indirect water heater tank.

    Oh, and don't even think about using the Beckett burner from the old RS-111 boiler in a new boiler. The Beckett AF burner that is specified for the RS-111 is not capable of moving air thru a 3-pass boiler. If the new boiler comes with a Beckett, it will be an AFG or NX which is matched to the new boiler. Don't try to save a few bucks this way- it will cause no end of trouble.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • h12721aa
    h12721aa Member Posts: 17
    MPO-IQ 84

    Thank you Steamhead for the advice .I am looking for the MPO-IQ 84 now but there is only 1 Year guarantee compared to the Buderus. How is the Quality?

  • Jason_13
    Jason_13 Member Posts: 299
    Burnham warranty

    The 1 year is on parts and lifetime on boiler. A very inexpensive 5 or 10 year warranty for parts and labor is available.
  • h12721aa
    h12721aa Member Posts: 17
    Ball valve and circulator

    Thank You for the information.

    Now I have one more question, on my old set up the circulator

    is on the return side of the boiler, on the print of the new boiler they show

    the circulator on the supply side after the

    Air scoop. Old and new are both Burnham Boilers. Does it

    matter? I will be going with three circulators for 2 zone baseboard and 1 indirect

     domestic hot water.

    Also they show a cut off and supply valve as a screw

    (turning ) type and then also ball valves in a other lines. Could I use ball

    valves for everything?

    Thanks    Hilmar