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# formula for sizing boiler

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Member Posts: 2,124
especially if the Domestic Hot Water comes of the boiler as a seperate zone. The 75K boiler is way over sized...unless there are huge hot water demands and the DHW comes off the boiler as a zone....kpc

• Member Posts: 19
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how-to size a boiler for a H.W. system?

i'd like to figure out if the BTU output for this boiler is sufficient.

it's a relatively small (1200 sq.ft.) 2-bed, 2-zone, single family home. i've calculated the cubic area as 9,728.

The current boiler for this house is 44,000 BTU, and another identical house has a 75,000 BTU boiler. My problem is that I can't get the heat calcs from the plumber who installed the 44,000 BTU boiler.

Would someone be able to help me determine the correct size boiler necessary for this two story, hot water system, or at least figure out if 44,000 BTU is insufficient?

the homeowner has just closed on the house, but when i have been at the house the boiler is firing pretty frequently.

With the correct heat calcs, i could have the plumber change the boiler if necessary.
thanks for any help.
• Member Posts: 19
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calculating heat loss

also, i'm not clear on how to calculate the heat loss for this house. It is new construction.

I'm really just looking for a rough estimate to figure if this boiler is under-sized.
• Member Posts: 498
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Chances are that a 44000 Btu/Hr boiler is oversized for a 1200 Sq Ft house of modern construction.

To calculate heat loss... go to the top of the page on the "button bar" and click "Heat Loss." That will open a Slant Fin web page - where you can fill out a form to get a free CD with a program to calculate heat loss on your house. NOte that the numbers are very conservative. If it says that you have a heat loss of 40,000 - you probably only have a real heat loss of 30,000. Many people have found that boilers sized by this (and all the other programs) are oversized.

Best of luck with this.

Perry
• Member Posts: 19
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big help, i appreciate it

thanks!
• Member Posts: 1,718
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Reality

If the heat loss is that low, then the boiler is fine since you won't find a conventional cast iron boiler much smaller than that anyway. It also depends on the location of this house. Where is it?

Another question. How is the domestic hot water made?

Also, from my experience, new construction is not necessarily tight construction. There may be some areas of abnormal infiltration.

Massachusetts

• Member Posts: 19
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house is in Providence

it is constructed on a slab, no basement. It has a direct vent water heater, the boiler is a Burnham 44,000BTU.

So, maybe it's just that the other identical house is way oversized with 75,000BTU?

i'm just hoping the HO doesn't start calling to complain her boiler is constantly firing and that her gas bills are too high and stuff like that.

thanks for reassuring me that the plumber probably sized the boiler correctly, even if he doesn't have the heat calcs to show me.
• Member Posts: 1,718
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The straight

I'm not saying its right. There are many factors involved here. Do a proper heat loss and go from there.

Massachusetts

• Member Posts: 223
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Duty cycle not frequency determines adequacy

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"the homeowner has just closed on the house, but when i have been at the house the boiler is firing pretty frequently".
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Firing frequently is not the issue. The "on"/"off" ratio determines the adequacy under the existing outdoor temperatures.

For example, if the outdoor temp is 20 degrees and the "on" time is 5 minutes and the off time is say 10 minutes, then the duty cycle is 5/(5+10)=1/3, it says that you are using 1/3 of the available 40,000 BTUH , If the indoor temp is 70, then you are using 13000BTUH for a temp diff of (70-20)=50. Therefore, in this case if you doubled the differential, you would need an average of 26000 BTUH to handle an outside temp of -30 degrees.(70--30=100)

Note that the frequencty of these cycles has NOTHING to do with the adequacy. They come about from too small a boiler or thermostat setpoint differentials .Also, a note of caution, the test should be done on a cloudy day or at night over a period of at least a few hours, since solar radiation would impact the numbers.

The formula as I had done previosly on another thread, which will give a estimate of the outddor temp that the heating system can sustain i.e. the design outdoor temp is obtained from

Ti-Td=(Ti-Totest)/duty cycle

from which

Td=Ti-(Ti-Totest)/duty cycle

For the example I gave

Td=70-50/1/3=-80 degrees

where

Ti, Totest= inside temp, outside temp during test

Td= design outside temp (outside temp capability w boiler full on, i.e. duty cycle=1

which shows a hugely oversized boiler but is just an example.

By the way, what is the make of that 44,000 BTU boiler?. I've been looking for an oil fired one of that size and would appeciate any input.
• Member Posts: 315
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Ted Nailed It....

"New construction is not necessarily tight construction..".

If you do the math with the Heat Loss Calculation, it will put you in the ballpark; good luck.

• Member Posts: 1,860
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44Kbtu figure is the firing rate yes?

If so, your actual net output --which is usually what you compare your heat loss to--is more like 30+Kbtu. Or in a small house like yours some pros may say you can use the DOE Gross output, which is higher than the net but lower than than the firing rate (gross input). (HO here.)
• Member Posts: 19
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you guys are rocket scientists

to figure out all these crazy equations i've read on the wall.

to answer the one thing i understood in zeke's post; it's a gas fired Burnham boiler.

i'm messing around with the slant fin heat calc software now, and i'm leaning more toward the assumption that 44,000 BTU is sufficient for this house and the identical house w/ 75,000 BTU is more than sufficient.

i appreciate everyone's knowledge, for sure.
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