Replacing Cast Iron Burnham

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swcooper

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Owing to a unique winter inspired set of consequences, I find my self about to replace my 100kBTU Burnham Holiday boiler in my 2000sf 2 storey home.

I see a couple of threads suggesting 100k BTU is way over the actual thermal load on there, although we assessed a quick napkin calculation on the baseboard, etc hanging off it and came to 85kBTU worth.

Here's the numbers though, in zipcode 08528:
1st Dec-Jan3rd 193.3cf/197.4therms
Jan3rd-Feb1st 210.5cf/215.1therms
Feb1st-Mar3rd 193.3cf/198.1therms - gas for hot water and heating only, cooker/dryer is electric.

From loose recollection we have about 80 ft of baseboard upstairs and 70ft downstairs, with a small kickspace too. There's a small 6ft baseboard in the basement that may end up disconnected after this.

We were just going to get a similar replacement Burnham ES2-5 but we've run into some possible snags with chimney lining (is there any reason this would have requirements for anything more stringent than the previous one used?) so figured I would look further into the idea this would be oversized for the task at hand. What does my actual thermal load look like? Should we be looking for a smaller sized unit instead for better efficiency? Installer says the exterior air temp sensor will keep the efficiency up if it is a bit too much.
 

Dana

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If that house was actually being heated during that period even the very smallest ES2 (the 3-plate ES2-3 ,with 59000 BTU/hr of output) would have plenty of margin for your heating loads.

The Somerset Airport logged about 3200 heating degree days between 1 December and 3 March, during which time you used about 600 therms. That's 60,000,000 BTU/ 3200= 18,750BTU/HDD of input BTU, or 781 BTU per degree-hour. Assuming 80% combustion efficiency that's 625 BTU/degree-hour that went into the house (the rest went up the flue.)

Assuming a 99% outside design temp of +10F and a heating/cooling balance point of 65F (the HDD base), that's 55F heating degrees...

...times 625 BTU/degree hour comes to 34,735 BTU/hr for an implied heat load, which is a pretty typical number for a 1800-2500' house with clear-glass storm windows and 2x4 walls.

The ES2-3 delivers nearly 2x that much- you'd be good do about -40F with that boiler. With the ES2-5 you'd be good to well under -100F. Expecting a cold snap? :)

If you assumed 600 BTU/ft for the baseboard that means you have about 140' of baseboard then? If you take the 59,000 BTU/hr of output of the ES2-3 and dividing by 140' of baseboard you're looking at ~420 BTU/ft, which balances at an average water temp of ~155F. Since the ES2 series tolerates return water temps as low as 110F you won't have to do anything special in the hydronic design to protect the boiler when you downsize.

The higher efficiency of the boiler means it delivers cooler exhaust, and if the chimney liner is terra-cotta you'd have corrosive condensation happening in the flue, especially if it's oversized for the BTU output of the boiler (likely). The direct vented ESC3 side-vented is comparable to the ES2 (slightly more efficient) and would allow you to just brick-up the boiler port to the old flue.
 
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swcooper

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Thanks Dana. I actually discovered it's an ES2-4 that's made it's way down to the basement, so a milder case of overkill. It's not really in a position conducive to side venting, so we'll figure it out with the chimney.
 

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Thanks Dana. I actually discovered it's an ES2-4 that's made it's way down to the basement, so a milder case of overkill. It's not really in a position conducive to side venting, so we'll figure it out with the chimney.

The ES2-4 has 88,000 BTU/hr of output, which is about 3x oversized for your ~30K heating load, which means it won't really hit it's AFUE numbers. If it's still in the crate and can be swapped for the 3-plater it's probably going to be worth it in the long run.

I expect you're on the hook for the proper sized stainless steel flue liner if going atmospheric-drafted, which often makes it more expensive than the direct-vented types. If it's on an exterior chimney there is elevated backdraft potential, which is another reason why going direct vented is better when you can.
 

Dana

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IIRC if it's 83% efficiency or lower it's still legal to vent gas appliances into properly sized terra-cotta lined flues.

But since 82% is the minimum legal efficiency for boilers & furnaces sold in the US that doesn't leave very many options, eh?
 
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