Heat Storage - Pros Cons and Opinions

I'd like to start a discussion about boiler heat storage. For example, having a well insulated 40 gallon tank in series between the boiler and any heat demand with constant circulation so that it would be just an extension of the boiler reservoir. I'm wondering if there'd be any increase of combustion efficiency since the boiler would be forced to have longer but less frequent combustion calls. Currently, a typical burn call lasts around 8 minutes unless of course one of the kids is taking a long shower. It seems that the Axe doesn't achieve full burn potential very often.

Looking for pros, cons and opinions on this subject

Need to measure CO2 in flue gas to determine most efficient firing rate. I suspect it ramps up pretty quickly with forced combustion air, so is it more efficient at 8 minutes over one minute?

I think it was Berlin who linked a full report on that boiler from the 1940s by the federal government. Good if you can find and read it.

franco b wrote: ↑

Tue. Dec. 14, 2021 2:21 pm

Need to measure CO2 in flue gas to determine most efficient firing rate. I suspect it ramps up pretty quickly with forced combustion air, so is it more efficient at 8 minutes over one minute?

I think it was Berlin who linked a full report on that boiler from the 1940s by the federal government. Good if you can find and read it.

Bureau of Mines Report 4936 Axeman-Andersen Anthratube Boiler

It has been a long time since I read it but I am pretty sure it has information on combustion efficiency.

That should keep Lee out of trouble for awhile.

It seems like increases in combustion efficiency would be offset to at least some extent by increases in standby losses, not to mention the cost of any pump used to produce circulation between boiler and tank. If the boiler hardly ever runs more than 8 minutes, is it too large for the attached load? Or maybe the thermostat is too close to a radiator?

Mike

Pacowy wrote: ↑

Tue. Dec. 14, 2021 3:01 pm

It seems like increases in combustion efficiency would be offset to at least some extent by increases in standby losses, not to mention the cost of any pump used to produce circulation between boiler and tank. If the boiler hardly ever runs more than 8 minutes, is it too large for the attached load? Or maybe the thermostat is too close to a radiator?

Mike

All good points Mike. In addition to capital cost of the buffer tank, extra circulator, and piping work, the electrical demand of every heat call will be higher due to the second circulator.

To answer Lee's question - yes, the addition of a buffer tank will increase the combustion efficiency. The tough question is if there is any $$ to be saved by doing so.

Don has shown some impressive results by slowing the fan down and getting longer burn cycles. That seems like it would have much shorter ROI.

franco b wrote: ↑

Tue. Dec. 14, 2021 2:48 pm

That should keep Lee out of trouble for awhile.

Yes it will!

Pacowy wrote: ↑

Tue. Dec. 14, 2021 3:01 pm

It seems like increases in combustion efficiency would be offset to at least some extent by increases in standby losses, not to mention the cost of any pump used to produce circulation between boiler and tank. If the boiler hardly ever runs more than 8 minutes, is it too large for the attached load? Or maybe the thermostat is too close to a radiator?

Mike

I agree there would be added losses to some extent. The boiler runs for 8 minutes at a time (usually) but a few times per hour based on heat loading. I feel its a good fit for our needs, it heats DHW for a family of 5 and provides heat for our 2400 sq ft house and also provides heat for a 500 sq ft outbuilding. On the coldest days of winter while producing DHW it would require about 80% of its heating capacity.. I think the short run times are just because I'm not using very much water in my heat distribution system. Its plumbed to a water to air heat exchanger for a majority of the house.. Only 16 ft of baseboard for the two bedrooms upstairs and another 60 ft of baseboard in the outbuilding roughly 100 ft away. But the outbuilding is only heated occasionally to 68 degrees, otherwise its kept at 40.. So on mild days that zone doesn't run at all.

Heat storage doesn't make sense with coal. It does on wood fired systems because of the varying heat output.

I used a large tank, heavily insulated when I first went to a wood boiler. Between the boiler and the tank I had about 300 gallons of water. It was not very good. Now with the coal heater, you can probably ramp up and get water flowing faster, my boiler was just plain awful. I had a separate line from the tank to the boiler to allow gravity flow, it had a circulator for when the boiler got above a certain temp. I messed with it for several years until I found coal.....and of course, wood sucks....

If you're happy with the efficiency you achieve during periods of higher loads, rest assured that is almost certainly when most of your coal consumption occurs. Changes in off-peak efficiency aren't likely to do much to your overall coal use, and may have unintended consequences if they make your peak period operation less efficient.

Mike

Lightning wrote: ↑

Tue. Dec. 14, 2021 1:08 pm

Currently, a typical burn call lasts around 8 minutes unless of course one of the kids is taking a long shower. It seems that the Axe doesn't achieve full burn potential very often.

That seems pretty impressive that it can get the boiler back up to temperature in less than 10 minutes. I'm assuming this is disregarding when you're thermostat calls?

Retro_Origin wrote: ↑

Tue. Dec. 14, 2021 6:22 pm

That seems pretty impressive that it can get the boiler back up to temperature in less than 10 minutes. I'm assuming this is disregarding when you're thermostat calls?

I have the thermostat set to run frequently, it triggers a big blower to push air thru the 20x20 inch water to air heat exchanger. The blower only runs a couple minutes at a time but frequently. Perhaps you saw my other thread on this subject. I had to set it that way to fix the temp swing issue in the living room. So my point being that it doesn't draw down the boiler water temperature very much. And another variable about the burn cycle is the condition of the fire, if the fire had been asleep a while then yeah it takes longer because the fire takes longer to rev up again. But if the fire is hot and frisky at start up its actually closer to 6 minutes of burn time to bring it back up to temp.

I opened and closed the outbuilding zone to crush the boiler temperature so I could time a burn cycle.. with no heat load on the boiler it took 7 minutes total to bring the water temp from 145 to 170.. the first 3 minutes of that burn cycle was just to rev the fire. At the end of 3 minutes the boiler water increased only 5 degrees. The next 4 minutes increased the boiler water 20 degrees. Ten minutes after the heat cycle its overshot to 180. These are a fierce little boiler lol..

Upper left number is the boiler supply pipe temp. Red number on the top PID is the exhaust stream temp.. it gives you an idea of where you are relative to max output. Max is around 450 degrees.

Start of burn
At 3:00 minutes At 5:00 minutes At 7:00 minutes 10 minutes after burn cycle

I think I have figured out the worlds BEST heat storage tank!
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. And it is 100% perfectly insulated. None of those BTU's will ever escape!

-Don

Pacowy wrote: ↑

Tue. Dec. 14, 2021 4:51 pm

If you're happy with the efficiency you achieve during periods of higher loads, rest assured that is almost certainly when most of your coal consumption occurs. Changes in off-peak efficiency aren't likely to do much to your overall coal use, and may have unintended consequences if they make your peak period operation less efficient.

Mike

That seems very reasonable. Thank you