Boiler size - Burner, DOE, or Net BTU measures?

TomTA

My apologies if this is addressed somewhere else; I haven't been able to find the information.

I'm in the market for a new boiler. My existing on is 140k BTU, and of course the first person I spoke with said "well, that means you need a 140k BTU boiler, plus 50k for the indirect HW heater". I can screw things up as well as the next guy, and do it for free, so I'll do it myself.

I used the SlantFin application to estimate BTU needs, and it came in around 70k. I've been looking at various boilers, and they have lots of different BTU measures; burner, DOE, and "Net AHRI"n. Which one should I focus on? If I look at the burner measure, I could go with the smallest Burnham at 84k; if I go with the net, I need the next size up that has 115k burner capacity and 87k net.

STEVEusaPA

Please don't oversize your boiler and DON'T add anything for the indirect (and don't use guy #1).
If the input is 84K, and its the MPO @87% efficient, that's right where you want to be.
If you're nervous about it, get a full heat loss performed. If it's right there and you're still nervous there are 3 things to consider:
-You can put the indirect on priority.
-The MPO 84 & MPO 115 are the same boiler block. You can change the baffle configuration, firing rate and burner settings (with an analyzer and pump pressure gauges) to fire it either way.
-That full output is only needed for maybe 5-10 days the entire winter, if that.

If your heat loss is correct, and you like the MPO, put in the 84. If on your design (coldest) day, the boiler can't keep up, you could change it to the 115 output. Keep in mind, especially on the coldest days, don't try a deep setback (or any setback).

TomTA

Thanks. Is there any concern with the 3-pass boilers that flue gas temperatures will be too low?

Jamie Hall

TomTA said:

Thanks. Is there any concern with the 3-pass boilers that flue gas temperatures will be too low?

Not if the person who performs the combustion testing knows what they are doing.

STEVEusaPA

They do have lower net stack temps especially with the low draft and low excess air (clean burning).
You'd either have to sacrifice some efficiency to get the net stack temperature up (higher draft), install a chimney liner, or go with a balanced flue (direct vent) set up (with a Riello if that's an option) and you can vent thru the wall.

If you're going from 149k to 84k, you probably should have the chimney lined anyway.

OP-What's your city, state zip, and how many gallons of oil did you burn last year?

TomTA

Lancaster, PA 17601. Not sure how many gallons. We get DHW from a coil in the boiler, so that's a constant drain.

STEVEusaPA

If you knew the amount of gallons you used for last season I could give you a rough guess based on usage.

EdTheHeaterMan

The supply house where the contractor purchases the boiler from, may offer a Load Calculation service for the installing contractor. That said, the existing boiler you are replacing is most likely way oversized.

So here is the scoop on BTU ratings:

BTU Input on a gas heater is how you calculate your gas pipe size
BTU Input on oil heat is how you select your nozzle size.

DOE rating is the output of a boiler after taking into account the combustion efficiency and off cycle loss thru the exhaust vent. That is where they get the A F U E rating This rating was added to boilers 20+ years ago. This way the AFUE ratings for boilers and furnaces had a common efficiency factor since furnaces don't have a set duct loss number like boiler manufacturers did.

NET AHRI or I=B=R NET rating is what you use to match the load calculation. It includes 15% piping and pickup allowance assuming that the heater and the connecting pipes are not in the conditioned space.

SO @TomTA , YOU SHOULD USE: NET AHRI AND MATCH IT TO THE LOAD CALC.