What is the best method when sizing an oil fired hot water boiler? Is it best to go by the DOE rating or the IBR rating? In the past I have always used the IBR rating but I have read differnet arguemnets on this and was looking for your input. After doing a heat loss on a building if I find the heat loss is on the high side of a 3 section boiler I may recomend a 4 section and down fire the boiler to the lowest recomended rate for better efficiancy. Is this wrong?
Terrible answer, I know.
The accuracy of the heat loss is the key factor. Regardless of whether you use Manual J based software, or I=B=R based software, the accuracy is critical.
If the construction quality actually DOES meet specs, as in insulation is figured acurately as installed around doors and windows; penetrations of "warm" interior walls through to the attic and vented crawl space get SEALED; windows and doors are the correct R-value, and infiltration is verified with a blower door test and accurately calculated into the heat loss, etc., then you can figure using the DOE number.
Here's the thing, though. Most times the boiler that you choose WON'T be between the DOE and the IBR number, so it won't be an issue. It starts to matter more on bigger size boilers, in larger buildings, where the difference is wider.
The only reason I see to oversize an oil boiler would be if it had a tankless coil in it being used for domestic hot water. Then the water heater side has a minimum to meet demand.
I use the DOE number, myself, but I'm pretty conservative on a heat loss survey.
The other huge variable is what temperature outdoor air and indoor design temperature the job is figured for. Those can make or break a job. If a guy expects all zones 78° F on the coldest night of the year at 3:00 AM, that would need to be figured in. It'll cost him to run that fat boy every other day of the year, though. It won't last as long, either.
If you figure the baseboard short, you won't be able to recover from setback quickly. Same thing if you figure stingy on design temperature difference and then use the DOE number. The boiler will recover slowly on real cold days.
Extra baseboard (designing for a lower temperature water temp) will put a nice load on the boiler when it runs, IF THE BOILER ISN'T OVERSIZED. Zoning allows for the load to be directed more into lived in zones by setting the other zones to a lower temperature. This is called diversity of load.
It's a balance of construction quality, heat loss survey accuracy, and expectations of the system performance. Hydronicsman designs for equipment longevity, long inexpensive run cycles, and equipment cost control. He can and does shave boiler size real close to what the needs are, only because he thoroughly understands how the building is constructed and will behave.
There are safety factors already built into the programs, so adding more safety factors makes no sense to me, unless you don't believe your own numbers. The I=B=R number is merely a 15% safety factor. That's all it is.
Don't use the DOE number unless the Heat Loss survey is very accurate. If it IS accurate, and you are too close for comfort with the DOE, then consider going up a size.
Only downfire an oil boiler as low as is published in the I&O manual. That's a fine way to control size.
Thanks Noel, and smarks for asking. I wondered what your take was on the subject. I would defer to your expertise any day.
Thats right, it does depend. If the home has a bunch of zones, copper bb, DOE will do ya if the numbers point that way. Big 'ol gravity converted high water content..maybe IBR looks like what to shoot for.