There’s an old joke about how a contractor sizes a residential boiler. He stands across the street from the house, puts up his hand and counts the number of fingers that obscure the house. However many fingers he uses is the number of sections he needs for the boiler in that house.

While it’s amusing to think of a contractor actually sizing a boiler that way, the truth is that the methods used by many are not much better.

Some estimate that approximately 85% of all boilers in the United States are the wrong size for the house, and most of those are oversized rather than undersized. Contractors often add more capacity to the boiler because they want to make sure it isn’t undersized. However, an oversized boiler doesn’t do the customer any favors.

Oversized boilers waste fuel and ultimately shorten the life of the system. With fuel costs higher and higher, it pays to run the proper calculations to ensure that customers are receiving the correct size boilers — and paying lower utility costs.

Look to the Experts

It may be surprising to know that most boiler manufacturers do not publish boiler sizing information. Instead, they use a guideline known as “H-22,” which is published by the Hydronics Institute Division of Gas Appliance Manu-facturers Association (GAMA).

The official title is simply “Heat Loss Calculation.” Available for the calculator or as a software program, this publication shows contractors exactly what they must do to determine the correct size boiler (either gas or oil).

“We recommend that contractors follow the heat loss guides from the Hydronics Institute Division of GAMA. You’re really not sizing a boiler, you’re sizing the application,” says Jeffrey Alex-ander, vice president-marketing, Peerless Heater Co., Boyertown, PA and vice chairman of the Hydron-ics Institute Division of GAMA.

“The contractor needs to size each room only if it is a new home and he will be installing radiation or other means of heat distribution (e.g., radiant tubing) in each room. If it is a replacement, the contractor need only do a whole-house heat loss.”

For a hydronic system with a water boiler, it is imperative that contractors perform the heat loss calculation. Then, count up all the connected radiation on the job and see what that comes to.

Also, look at the size of the boiler that’s currently there as a point of reference. If the boiler is gas-fired, clock the gas meter to see the actual input. If it’s oil-fired, check the oil nozzle and pump pressure to see how they’re firing.

The other issue, which is most important, is to talk to the homeowner. Ask whether or not the house receives enough heat, or if there are any problem areas.

Steam Sizing

Steam needs to be sized to the existing square feet of radiation. “The contractor must go through the house, measure the radiation, and calculate the square feet. All boiler manufacturers’ literature shows square feet of steam, as well as showing input, gross output, and net output,” says Alexander.

Something to be aware of as well is that in older homes with steam boilers, asbestos insulation may have been taken off at some point, leaving the pipes uninsulated. In this case, the contractor should oversize the boiler a little bit for that extra loss of insulation, or else the boiler could end up being undersized.

Of course, the best thing to do is to insulate the pipes, then properly size the boiler.

“If contractors go through and look at the heat loss, the connected radiation, the size of the boiler that’s there now, and talk to the homeowner, he should use whatever the smallest of those four things are,” says Roger Prevost, owner, Quality Equipment Sales, a manufacturer’s representative, Pottstown, PA. “So if the heat loss says they need 100,000 Btu but they’ve been heating it with 75,000 Btu, then go with the 75,000.”

Another benefit to checking all aspects of the application is that the homeowner will be impressed, and that contractor is probably going to get the job. Homeowners want to see contractors going the extra mile and being interested in giving them the right equipment.

Don’t Judge By the Cover

Boilers may also end up oversized due to the fact that contractors simply replace the equipment with what’s there already.

“What they don’t think about is that the homeowner has probably added storm windows, better insulation, siding on the house, things like that, which have reduced the heat loss in the house,” says Joe Coppola, boiler marketing manager, Slant/Fin, Greenvale, NY, and chairman of the Education Committee, Hydronics Institute Divis-ion of GAMA. “We find that boilers are often as much as two times too big for houses, which is ridiculous. It causes a lot of fuel to be wasted.”

Prevost notes that it just doesn’t make sense to skip doing a heat loss calculation, because it’s so easy. “I can do a heat loss calculation in 10 minutes. I did it riding down the road one time while I was on my cell phone. The contractor told me the size of the house, how much insulation was in the walls and attic, and I assumed the house had about 15% to 20% of the wall area as windows. I came up with a heat loss of about 54,000 Btu.

“When we actually went to the house and measured it, it was 51,000 Btu. He was going to put in a 140,000-Btu boiler.”

Imagine all the fuel saved on that installation alone. Imagine how much fuel (and cost) you could save your customers by taking an extra 10 minutes to determine the correct size boiler for their homes. Not only will customers save money on equipment, they will definitely save on their energy bills. And you’ll really be a hero.

Sidebar: Smaller Than You Think

You should not size a boiler based on the square footage of the house, but people do it every day.

Contractors may use an old rule of thumb, such as using 23 Btu per sq ft to size a boiler. That’s wrong, because it’s not the square footage of the house that matters, it’s the heat loss through the walls and the windows, and that doesn’t relate to the square footage of the building.

Joe Coppola, Slant/Fin, relates the following: “I have a 2,300-square-foot, split-level house on Long Island. I did a heat loss calculation on my house, and I didn’t believe it, but the heat loss was 58,000 Btu. Now, the boiler that was in my house was 200,000 Btu. Of course, I had added insulation, storm windows, and things like that.

“When I went to replace the boiler, I said, ‘Geez, it just doesn’t seem right to put a boiler in that small.’ Besides, at the time there weren’t any made that small, so I had to put in a 105,000-Btu boiler with a net rating of 76,000 Btu. Now this boiler is oversized, and it will waste fuel because it’ll short cycle.

“It’s too big. It’s like putting a V8 engine in a Volks-wagen — it may be cool, but you’re not going to use it.”

Publication date: 05/28/2001