Suddenly I Feel More Comfortable With My Boilers Efficiency

I've been saying on this forum for some time now that by comparing my homes coal boiler heating demand (coal usage in lbs.) to its past resistance boiler electricity requirements (electricity usage measured as KWH) I must be running somewhere in the lowly range of only about 60% to at best 65% efficient with coal (let's call it 62.5% on average). Others have told me this seems low. But now I have come across figures from the EFM website for their 520 boiler that make me feel better.

The EFM provided figures show that for the maximum 25#/hr. feed rate the net heat output of their 520 boiler is only 186,690 BTU's.

Assuming that on an "as received" basis, anthracite has typically about 12,150 BTU's (per a quick look at the Jeddo website data), and extending this out to 25 lbs., I calculate net input BTU's at 303,750 for the 520 boiler at its maximum feed rate.

186,690 nominal net output BTU's divided by 303,750 nominal net input BTU's = 61.5% nominal efficiency

I feel right at home now efficiency-wise.

That seems really low. If Keystoker is to be believed, my KA6 is 84.7% efficient.

Feeding at the maximum is bound to lower efficiency with high stack temperatures. At a lower firing rate efficiency would be much higher.

Since so many variables in a stoker or any stove are fixed, it follows that only one firing rate will produce maximum efficiency for that design. That point where volatiles and CO are best consumed and heat exchange is most effective. Going above or below that point efficiency will be lower even if more heat is produced or less coal is burned at a lower setting.

franco b wrote:Feeding at the maximum is bound to lower efficiency with high stack temperatures. At a lower firing rate efficiency would be much higher.

Since so many variables in a stoker or any stove are fixed, it follows that only one firing rate will produce maximum efficiency for that design. That point where volatiles and CO are best consumed and heat exchange is most effective. Going above or below that point efficiency will be lower even if more heat is produced or less coal is burned at a lower setting.

The EFM figures for the midrange feed rate of 12.5 lbs per hour come out to an identical 61.5% efficiency. Ditto for only 5 lbs per hour feed rate.

I thought you had a Coalgun as per your Avatar???

Carbon12 wrote:I thought you had a Coalgun as per your Avatar???

I do! I'm merely comparing my accumulated "real world" figure for the Coal Gun (62.5% efficiency) with the EFM 520 figures from their website (61.5% efficiency).

Now I'm following!

1949 Bureau of Mines independent test data for the similar AA-130 boiler show it at a measured roughly 81.5% efficiency when firing, and down to a lowly 27% efficiency (give or take) when long term idling. Add up idling and firing over time and (for me at least, and apparently also for EFM) the overall answer is about 62%.

lsayre wrote:The EFM figures for the midrange feed rate of 12.5 lbs per hour come out to an identical 61.5% efficiency. Ditto for only 5 lbs per hour feed rate.

With the same heat exchange surface how can that possibly be? Combustion efficiency would have to be dramatically lower which I doubt. Feed rates and combustion air can be changed but fire pot and heat exchange remain fixed.

The net ratings are at steady state operation, not some average of running and not running.

Their "net" figure uses a conservative number for BTU content, and the radiation ratings include a pickup factor. This rating system assures that someone with a known load within the 520 rating will actually be able to do the job with a 520.

There was a recent thread about Gentleman Janitor boilers with a similar discussion.

lsayre wrote:1949 Bureau of Mines independent test data for the similar AA-130 boiler show it at a measured roughly 81.5% efficiency when firing, and down to a lowly 27% efficiency (give or take) when long term idling. Add up idling and firing over time and (for me at least, and apparently also for EFM) the overall answer is about 62%.

Yes the average is much lower when departing from the ideal firing rate. That was my point.

So how's the efficiency during the months you heat the house?

Lightning wrote:So how's the efficiency during the months you heat the house?

Those are the months for which I compute 62.5% efficiency. The DHW heating only months have disastrously low efficiency compared to ellectricity.

Lightning wrote:So how's the efficiency during the months you heat the house?

It all depends. Is that with or without a Baro??? Just kidding. Lets not start that again.

I believe the abysmal 27% efficiency figure is from the Bureau of Mines data is for the months with DHW only usage. Far more coal is consumed merely sustaining the fire than is used in actually heating the homes water. Loads of idle time.