Sizing a boiler

Bewildered homeowner

I know this is old territory. I tried to do my homework, but I'm coming up with numbers that are making me doubt myself.

I have an older house (c. 1920) with a Columbia boiler (oil) that supplies hot water to the radiators and to a hot water storage tank. There's no separate water heater.

I looked at my bills for 2003 and got the degree days for my area (Philadelphia) for 2003 and matched them up. The coldest month was January with 1,200 DD. In that time frame I used approximately 255 gal. of oil. The conversion chart says that a gal. of oil produces 140,000 BTU, so I do the math and get 1 million BTUs per day. Since daytime temps are higher and the thermostat is set back at night, I'm fudging things and dividing by 24 to get the BTUs required per hour.

It comes out to 43,000 BTU/Hour (255 gal. * 140000)/35 days/24 hours.)

And that's if I squeezed every BTU out of a gallon of oil. With an older furnace, I'm probably getting 50%, so last winter in the coldest month, I only needed 22,000 BTU/hour.
That's what has me worried. Both numbers seem too low.

Unfortunately, the Columbia is on its last legs and I have to replace it. I've read here and elsewhere that most boilers are sized too large and in fact one quote I got was based solely on the square footage of the house (about 1,200 including basement). It was for a 145 DOE Heating Capacity MBH. That's about 7 times what I need by my calculations.

I'd be grateful for some pointers on how I went wrong or confirmation that I really do need a very small boiler.

Wayco Wayne_2

I almost always

do a heat load on the houses I am replacing boilers in. I am still amazed at how oversized we as contractors choose our systems. In the old days they didn't have computers to quicken the process so they used rules of thumb that had plenty of buffer in them. One old timer said they used to pace off a certain amount of steps from the house and hold up their hands in front of their face with arms extended. How ever many fingers it took to cover the house with one eye closed was how many sections of boiler they would install. I have taken boilers out that were oversized by 300% Some engineering manuals say to over size by 25% on purpose to have "recoverabilty" on real cold days. I've also heard from some who say if temps get too low outside you should be so close that the homeowners are required to put on sweaters. Find a contractor who does heat loads and pay him for an accurate heat load and then start making decisions. WW

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Tom Meyer

Heat Loads

Doing accurate heat loads isn't as easy as it looks. Then again, the definition of "accurate" is relative.

First, with a very few exceptions, most propietary heat loss software has a built in "fudge factor" around 10-15%. That's assuming you have the actual U-values of the structure. Imagine wondering exactly what the insulation is between the walls of a house built in 1920. Even if we did know, it has to have settled leaving an uneven distribution. Also, penetrations through those walls, (windows, doors, etc), are calculated based on the total opening, not the pane/door size. Where did the installers stop insulating around the windows? Then you have piping losses, ACHs, air leakage, on and on.

The best approach to take to is to make some assumptions. Typically, engineers like to calculate to three decimal places. In reality, we're lucky to get sizing to the nearest 1,000 BTUs.

We've decided the best thing is to get close, and recommend a modulating boiler to help make corrections to get the most accurate delivery for the actual load.

Tom Meyer
Senior Designer/Trainer
Precision Hydronics Corp
www.precisionhydronics.com

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DaveGateway

Averaging degree days doesn't work

Remember that a boiler needs to be able to support the COLDEST day of the year. You've just averaged out the temp over an entire month. You need to know the highest degree day of the total month to figure out the ratio of most oil used on the coldest day. Now calc the total BTU's used that day to see what you need per hour. Your boiler does not run 24hr/day, 35 days straight. 145K does sound high, but not if you're going to feed and indirect WH and need good recovery time. On smaller houses the domestic HW load is usally larger then the heat load! No such thing as a 22K oil boiler. 60K is the smallest I've seen. 90K+ will probably be the minimum to support a 40 gal indirect with recovery under 30 minutes with priority.

Boilerpro_3

You left out one step, I believe....

that 22,000 btu/hr is for an average winter day for that month. For the month you used that was about 25F outdoors. The low design temp for Philadelphia is 14F according to my texts.
Here's what you need to do:
22,000/ (1200 DD@65F/ 30 days in month) = 550 btu/F degree temp difference between in and out.
and then:
550btu/F x (65F -14F) = 28050 btu/hr needed on that 14F day.
One other things needs to be accounted for and this is the fact that heat loss accelarates as the weather gets colder (non-linear) because air leakage accelerates. I typically add about 15% to cover this extra heat loss.

So:

28050btu/hr x 115 % (1.15) = 32,257 btu/hr

so this would be your heat loss on that coldest day.

With a total square footage, including basement, of 1200, I'd say this is probably about right. Getting a boiler close to this size will, alone, say you a bundle on your gas bill if your curent boiler is way too big.

As a comparison, my,old, but well insulated and weatherstripped 2,800 sq ft home on two above grade floor with 700 sw ft of glass only needs 52,000 btu/hr on a -4F day here in much colder northern Illinois.

Yes, those low numbers are scary at first, but they are almost always right.

Boilerpro