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Boiler sizing

Haven't checked that. The two main zones are on zone valves which will slow things a bit, and those zones are split front and rear of the house. I may go with high flow valves.


  • Paul Fredricks_6
    Paul Fredricks_6 Member Posts: 88

    I am looking at a boiler replacement for a friend. The existing boiler (gas) is rated at 197K output. I measured the baseboard radiation in the house and found 229' of it. At 600 BTU's per ft that's 91,600 btu's. They currently have a separate gas water heater, but if they replace the boiler they will go with an indirect.

    Do I need to size the boiler to the radiation load? Or to the heat loss? If the heat loss is less (probably) isn't there a possibility of running out of heat at the end of the loops? There are 3 zones, does that negate that problem?

    Any info helps. Thanks guys.
  • Weezbo
    Weezbo Member Posts: 6,232
    lot of variables...

    just off the top of my head

    i would say dial down the lengths of base board (add a couple more zones)

    think along the lines of Water usage for each bathroom / bedroom occ level

    take a look at the places to tighten the home up.. sealers insulation insulation :)

    make certain you have combustion air available for the boiler and gas heater..

    seriously consider Triangle Tubes offerings

    i am thinking, though scant the information is at the moment,..half the boiler will be plenty ..however once the insulation ,sealing windows and adding extra insulation :)

    and proper ventilation for the appliances and a change in the lengths of the emitters by re zoning...

    you could get a blower door test done and check all the numbers one more time

    see what that suggests against a boiler with half the out put of the current boiler...you could shave it down perhaps even more..

    your fuel bills would likely change drastically...

    p.s. Tell the gas company that you went with a new High efficiency, modulating condensing boiler and system zoning change ,.... or they just might think you have done something to the gas meter.

  • Paul Fredricks_6
    Paul Fredricks_6 Member Posts: 88

    It's a zoned as it can be. The first is a split loop and a fairly open floor plan so zoning wouldn't be worth the trouble.Second floor is also a split loop, split front to back. Not sure zoning that would do much either. The third zone is a family room over a garage. They are currently comfortable with how things are set up.

    This is a good sized house (don't know square footage yet) and it's pretty well sealed. Should I not worry about the amount of baseboard in relation to the new boiler size? I won't be removing any.

    I was thinking about a Knight boiler. I have to use the existing chimney as a chase way for the vent/fresh air.
  • Weezbo
    Weezbo Member Posts: 6,232
    consider this..

    the comfort that you are selling is multi faceted..

    physical comfort is only one aspect.

  • John Ketterman
    John Ketterman Member Posts: 187

    To answer your questions:

    If the heat loss were greater than the radiation output, the home would not have heated properly on below-design days in the past. If everything worked OK in the past, then the actual (as opposed to calculated) heat loss must be smaller, and so the boiler should be matched to the heat loss and should not exceed the radiation. Remember that heat loss calculations are notorious for being padded, so your calculated loss might well come out to be larger than the radiation.

    Boiler output of twice the radiation is a classic case of oversizing. But the nice thing about oversizing is that the heat comes on more quickly at the beginning of a cycle or after setback.
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,538

    The radiation is meaningless with a hot water system.Size to the heat loss.The good thing about being over-radiated with a mod/con is you'll be condensing more of the time!The # of zones is also meaningless,you can cut a pie into 4,6,8,10,12 or as many slices as you like,it doesn't make the pie any bigger or smaller

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  • Paul Fredricks_6
    Paul Fredricks_6 Member Posts: 88

    Thanks for the info everyone. When I get the heat loss done it will tell the story.
  • Weezbo
    Weezbo Member Posts: 6,232

    if i had 250 feet of base board on one loop....

    and 6 40 foot zones of baseboard ..are you suggesting that the out put would be of no significant difference ?

    not being a betting man i would druther have the 6 40 foot zones..

    the flow in gpm's and the head, the flow in gas to the boiler and the flow of $ to both the electrical and fuel supplier

    would be higher in the larger loop theory...


    my thought was downsizing the lengths of the zones (using the same emitters) would simply increase the system wide performance of the new system...

    it is like the parable of the loaves and the fishes where less is more.
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,538
    The question was heat loss

    And the # of zones would have no effect on the heat loss of the structure or the boiler necessary to heat it.The output of the emitters has no bearing on boiler sizing.

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  • Terry Trommeshauser
    Terry Trommeshauser Member Posts: 34

    right off the bat I see an average 76.33ft per zone.
    seems a little high . no?

    the output off the last half of each zone may have very limited output for lack of water temp.

    what kind of delta-t are you seeing across each zone as it is?? I suspect well over 40F (sb 20)

    my 0.02 worth.

This discussion has been closed.