What happens if the boiler is sized to small?


Old 08-29-13, 06:23 AM
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Join Date: Aug 2013
Location: USA
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What happens if the boiler is sized to small?

From reading on this forum as well as other places I realize that oversizing hot water boilers is a common problem. and can cause short cycling which impacts efficiency.
My fear is that even though I've done heat loss calculations 4 or 5 different ways that I might undersize the unit if I spec it myself.
What problems will slightly undersizing a boiler cause? Of course it may be hard to really undersize it too much as gas boilers smaller than 55K are not very prevalent.

My heat loss calculations have been between 43175 and 54652 with even a ROT calculation of 57888. ( 35 X 1608)
I used slant fin calculator , Crowns calculator, Weil Mclain and even PEX calculator.
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Old 08-29-13, 06:44 AM
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Frank, I think you've got enough results to be very confident. Plus, Manual J, which the various heat loss programs are based on, has a built-in safety margin of 20-25%, so it is recommended that you resist the urge to add even more.

But, since you ask, what if the boiler is undersized? Here is the theoretical answer: When the outdoor temp drops below the effective design temperature, the circulating water will drop a few degrees below design, say from 180 deg to 170 deg - while the boiler fires continuously. I believe that you have plenty of extra capacity in the radiators, right? So, what will happen is that as the circulating water temp drops, the efficiency of a conventional boiler will actually increase and deliver more heat, but at a slightly lower temp. With excess radiation, you will still be able to heat the house with that lower temp water.

Not to worry.
Old 08-29-13, 09:02 AM
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Thank you.
Now to find a contractor who really knows whats going on.
Old 08-29-13, 10:39 AM
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^^^ Hardest part of the project, good luck with that! ^^^
Old 08-29-13, 02:35 PM
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Remember too... that your OUTDOOR DESIGN temperature only occurs for 1% or less of the entire heating season. This means that if the boiler is correctly sized, for 99% of the heating season it will STILL be OVER sized.

Let's say heating season is what, from Oct 15 to Apr 15. Six months... 180 days... that means that even if the boiler is somehow UNDER-sized, it will likely only be so for what, 2 days? Grab a sweater! (or a hoodie, as your fashion sense dictates)

As an experiment, if you are interested, go into one of the heat loss programs and change the Outdoor Design to something higher... recalculate and look at how much less the heat loss is.

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