Stoker Coal Boiler that would best meet my needs

A bit of re-thinking here:

If the three homes are seeing water that (on average) is 4 degrees lower than what it was when it left the boiler, and the homes baseboards are then achieving the typical 20 degrees of drop from 340 feet of HW baseboards on top of this, then the 4 degree heat loss from the underground travel alone is roughly the equivalent of 4/20th's of your 340 feet of HW baseboards. That comes to 4/20 x 340 = 68 feet of "baseboard equivalent" for the underground passage. Call it 70 feet.

70 feet of "baseboard equivalent" + 340 feet of "actual baseboards" = 410 feet of "effective baseboards" that the boiler is actually seeing.

410 ft. x 550 BTU's/ft. = 225,500 BTU's of boiler output per hour required to max out the system from a home heating only perspective.

Add to this that each home requires about 32 gallons of hot water per day, for 96 total gallons of DHW per day (call it 100 gallons). Due to the brief nature of DHW demand lets say that on an average day this quantity of DHW is used over a total of only 2 hours. Thats two hours each day where 50 gallons of water must be heated from 50 degrees to 130 degrees. 50 gal. x 8.33lbs/gal. x (130-50) = ~33,000 BTU's of "peak" DHW demand.

Summing it all up I get 225,500 BTU's + 33,320 BTU's = 258,820 BTU's of required boiler output.

At 80% efficiency when firing this would require 322,900 BTU's of input for the boiler. This need for roughly 320,000 BTU's of input would put you squarely into the EFM 700 arena, and would be out of the nominal rated reach of an AA-260 or AHS 260.

This points once again to the EFM 700 as your choice without having a professional heat loss calculation done.

Larry, that would require the 700 to be burning 30 lbs an hour in a typical COLD winter day. That might be theoretically possible, but that would be like running wide f***ing open, and no one on this forum would do that to a piece of equipment. Your domestic requirements are taking too much btu away from the boiler, hence a 'superstor' or equivalent would be in order to smooth the transition during high use periods. Don't over think this please.

This BTU requirement would be for only the absolutely worst case scenario of consistent sub zero temperatures outside. As I had previously said, on an average winter day the hourly BTU requirement is only about 15% to 20% of the worst case scenario, so on an average winter day their burn rate would be only about 4.5 to at most 6 lbs. per hour.

For most days the AA-260 would clearly be plenty, but if and when it really gets and stays cold outside, in my opinion the EFM 700 would be nice to have on hand.

Let's just round it up to a 900 and call it a day.

Mike

Used boilers in the EFM 520 to 700 size, an AA260, AHS 260, Keystoker K8-K12, are going to be a bargain at $2000, and an OK deal at $4000. This for a used unit, removed from service, but NOT rebuilt.
A rebuilt one can be between $5000 to $9000, depends on what unit, what condition etc.

New boilers are from $6000 up. I think a LL220 is available around $6000, not sure, give 'em a call
or check the manufacturers forum and email them.

As Mike mentioned these boiler are made to industrial standards, they are TOUGH. But they do get
worn, and corroded when not serviced when needed.. Often a boiler is run untill it breaks down then
is just 'band-aid' repaired untill the next break-down..
A boiler that has been run hard and not maintained can be in need of a LOT of work and parts. So make sure what you might be buying if you buy used.

There are gentlemen on the forum who rebuild used units, and offer some warrantee on the units they
sell.. something to think about.

Greg L.

Northern Maine wrote:Rob & Larry,

according to measurements of baseboards in all three homes we have just about enough. Heat loss from the outdoor unit to each home varies from 3 to 5 degrees.

What I meant to say here and for clarification the 3 to 5 degrees is total for all three homes combined! Sorry!

That would place you at about 365 ft of "baseboard equivalent". You are now down to roughly 201,000 BTU's per hour required for home heat, plus whatever is required for the DHW. You can pretty much spread (or smooth out) the daily DHW related BTU demand from 2 hours of peak to 24 hours of overall DHW demand by using indirect hot water tanks in each home. Allowing for some hourly "stand by" loss of BTU's due to the indirect tanks efficiency ratings, that would bring the DHW demand down to about 3,000 BTU's per average hour.

Summing it all up you are at about 204,000 BTU's of (maximum, extreme outside temp conditions) required hourly boiler output capacity, and the AA-260 is back in the game.

When you say indirect DHW what do you mean? The hot water from my outdoor boiler circulates to my oil fired boiler in my home. I have a 50 gallon hot water tank that has a coil running through it that upon demand a circulator on the oil fired unit kicks on and runs hot water through the coil and heats the water. All three homes have this set up. However, one of the homes DHW is never in use during the heating season.

Northern Maine wrote:When you say indirect DHW what do you mean? The hot water from my outdoor boiler circulates to my oil fired boiler in my home. I have a 50 gallon hot water tank that has a coil running through it that upon demand a circulator on the oil fired unit kicks on and runs hot water through the coil and heats the water. All three homes have this set up. However, one of the homes DHW is never in use during the heating season.

An indirect hot water tank (HWT) has heating coils within it that take their heat from boiler flow, just as for a baseboard heater loop. An indirect HWT has no heat source of its own. It acts exactly like an additional zone, only instead of calling for boiler water flow due to a thermostat heat call, the boiler water flows through the indirect hot water tanks internal coils due to an aquastat heat call. The aquastat probe would reside within a well that is immersed into the indirect HW tank. Boiler water and DHW never come into direct contact with each other. Boiler water is boiler water and hot water tank water is sourcing from your well or city water main. Thus the term indirect.

An additional advantage of the indirect hot water tanks is that they do away with the need for an internal DHW coil within the boiler. DHW coils are eventual sources of leaks and trouble, and you will not have any problems with one if you don't have one.

Larry...I have a "TOP PERFORMER" this is the name on the tank, that has a 30 sq ft heat exchanger in it...when the water temp in it drops....it calls for the circulator on the oil unit to kick on an push hot water thru the heat exchanger...is this good or not good?

Larry...I have a "TOP PERFORMER" this is the name on the tank, that has a 30 sq ft heat exchanger in it...when the water temp in it drops....it calls for the circulator on the oil unit to kick on an push hot water thru the heat exchanger...is this good or not good?

Northern Maine wrote:Larry...I have a "TOP PERFORMER" this is the name on the tank, that has a 30 sq ft heat exchanger in it...when the water temp in it drops....it calls for the circulator on the oil unit to kick on an push hot water thru the heat exchanger...is this good or not good?

This sounds to me like it might be an indirect hot water tank, so I'd say it would be a good thing. You would want to convert it over to the coal boiler to get away from the need to heat it via an oil boiler though. That or if the oil driven heat unit is integral to it, replace it with a dedicated indirect HW tank. Ditto for the other two homes.

The other two homes are set up exactly the same way...but, the oil burners on each unit are not even hooked up to run in any of the homes....basically the oil units are acting as a hot water holding tank and thats it....does this make sense?

Northern Maine wrote:The other two homes are set up exactly the same way...but, the oil units are not even hooked up to run in any of the homes....basically the oil units are acting as a hot water holding tank and thats it....does this make sense?

Not at all to me, but my understanding here matters little if the units are working as expected.

In other words our Outdoor boiler (in the outbuilding) constantly pumps water to each homes oil unit