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# boiler sizing

#1
09-10-12, 07:26 AM
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Join Date: May 2006
Location: usa
Posts: 60
boiler sizing

When i size for boiler replacement I measured total ft x 600 . and the 3 zone house requires a 5 section boiler. The home owner asks that if we not calculate the basement 50ft and size for just the 1st and 2nd floor giving us a 4 section boiler. Im thinking that the saving are minor and We should be concerned about the what ifs. What if the basement all zones call and we have a undersized boiler. We will get low water temp return. This can shock the boiler and cause damage or couse it to condense? Does this sound right. and to not go with a smaller boiler.. What do you think?
Thanks,

#2
09-10-12, 11:00 AM
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Join Date: Oct 2006
Location: USA
Posts: 23,539
Cool, I'm not sure what you mean by this:

size for boiler replacement I measured total ft x 600
What do you mean "total ft" ?

Are you talking about feet of installed baseboard?

If so, that is wrong. You don't size the boiler for the btu of the heat emitters installed in the home, you size the boiler for the HEAT LOSS of the home.

A 5 section boiler is most certainly way too big, as the 4 section may be also.

Just how big is this home? How many floors ? How many SQUARE FEET of living space?

#3
09-10-12, 01:09 PM
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Join Date: May 2006
Location: usa
Posts: 60
I understand manual j is the most precise way and I do them all most every other day . I was told tho that with boilers having the baseboard all ready sized for the house to just match the output load of the baseboard . 2nd floor 50' and 1st has 70 ' and the basement has 52' . Total 172 ' base board.
what if the manual j says I need 140' ? I'd remove some Finns?
172* 600 = 103,200 but output. I was lookiNg at Weil McClain cga4

#4
09-10-12, 03:23 PM
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Join Date: Oct 2006
Location: USA
Posts: 23,539
In theory your idea would work, but let me ask you this:

How do you know for a fact that the baseboards are "already sized for the house" ?

Please tell us how many square feet the house is.

From that number we can arrive at a 'sanity check' number.

This sounds like around a 2000 square foot home, and if you need 100K BTU to heat that, there must be outside walls missing!

No, you don't need to remove any fins. The only thing that happens when you have MORE heat emitter than you need is that now you can heat the home with COOLER water, and that saves money.

#5
09-10-12, 04:21 PM
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Join Date: May 2006
Location: usa
Posts: 60
Shouldn't I be concerned with low water return??
The homeowner is concerned with over sizing and I just emailed him that. I will perform a manual j load calc.

#6
09-10-12, 08:42 PM
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Join Date: May 2006
Location: usa
Posts: 60
Manaul J was performed and 64,000 btu is needed. 104 ft base board required.
So my weil mcclain cga-4 net IBR is 77,000 and looks to be the proper size. then the cga 5 that is over sized.
With this 3 zone set up. would i have to worry about return temps getting below 130* with all operating on a very cold night?? how one figure this out?
Thanks,

Last edited by coolmen; 09-10-12 at 09:05 PM.
#7
09-12-12, 06:25 PM
Member
Join Date: Jul 2007
Posts: 2,465
I might even consider the CGA-3. Since the heat loss is 64k and the DOE is 59k and you have 3 zones. A heat loss is about 15% to 20% heavy and the boiler size can be reduced when zoned.
The return water temp is somewhat of a concern but not as much as everyone thinks. It is more the volume of cold water than the temperature of the return. A simple boiler bypass cool return to hot supply will alleviate that problem.
Bypass_Piping_Explaination
Another option is look at a cast iron boiler that can return 110f return water temp.

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