Convert Englander 28-3500 to thermal mass heater?

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Swamp_Yankee

Member
Oct 18, 2018
75
Hunterdon County, NJ
I have an old Englander 28-3500 that heated my old house for 10 years. The new house is hydronic heat and someday I'd love to put in a gasifier, but in the meantime the 28-3500 just sits in the basement as the current owner of my old house did not want it. I am considering putting up a moderately sized building on the property that I could potentially use the 28-3500 in, but it would be way overpowered for the space in its regular configuration with the blower, etc.. I was reading some articles the other day about thermal mass heat storage using sand and had a thought: The 28-3500 is essentially a plate steel stove inside a heavy gauge sheet metal box which forms the air jacket which feeds heated air into ductwork. After sealing up a few openings in the rear the entire air jacket could easily be completely filled with clean dry mason sand. The result would be a plate steel stove completely surrounded by a void free mass of sand.

The idea would be to get the stove running full tilt with a very hot and clean (for non-EPA stove anyway) burn for a period of time, heating the "sand jacket" which hopefully would then continue to release heat slowly and gently into the structure for a period of time after the fire is out. This is completely an experimental idea that I'm apt to try since I have the stove, I don't really have a use for it, and the only cost to me would be a load of sand. Also, if it turns out to be a failure I could easily use a shop vac to remove the sand and the stove would be useable in its original form.

Finally, I'm well aware that this would void all warranty (not that it's under warranty anyway), not be covered by insurance in the event of fire, etc...I'm not that worried about fire risk inside of a steel building sitting on a concrete slab. In any event, I'm curious to hear what others think of this idea.
 

EatenByLimestone

Super Moderator
Staff member
How many lbs of sand do you think that jacket will hold? Most thermal mass heaters weigh tons. You may have better luck blowing the hot air through a masonry mass built around the stove. Might want to leave room or a hatch to change the blower though.
 

jalmondale

Member
Dec 16, 2021
140
NY
Could you run metal tubing through the gap instead, and circulate water around it? That would let you have a larger thermal mass storage than just what fits in the gap (possibly you could combine this with radiant floor heating). The two things that would worry me would be cooling the firebox too much (which might be ok since this wouldn't be in the firebox itself) and overheating the water to produce steam (you should be able to get an emergency release valve, though).
 

Swamp_Yankee

Member
Oct 18, 2018
75
Hunterdon County, NJ
How many lbs of sand do you think that jacket will hold? Most thermal mass heaters weigh tons. You may have better luck blowing the hot air through a masonry mass built around the stove. Might want to leave room or a hatch to change the blower though.
Blasting hot air into the masonry is an interesting idea-kind of like a masonry heater but pulling the heat from air warmed by the stove rather than the flue gases. The trouble is it would be difficult, expensive and relatively permanent as opposed to a cheap experiment that could easily be undone if it doesn't produce the desired results.

That said, masonry heaters that weigh many tons are designed to heat a decent sized home whereas I'm looking to slowly release heat into a 350 or so square foot space.

Could you run metal tubing through the gap instead, and circulate water around it? That would let you have a larger thermal mass storage than just what fits in the gap (possibly you could combine this with radiant floor heating). The two things that would worry me would be cooling the firebox too much (which might be ok since this wouldn't be in the firebox itself) and overheating the water to produce steam (you should be able to get an emergency release valve, though).

Water would definitely be a better heat storage material but it would add a lot of complexity and expense to do it right, not to mention the safety issues. Still on the fence so I'll wait for others to weigh in before I decide to give it a shot or not.
 

qwee

Burning Hunk
Jan 17, 2013
236
Idaho
A masonry heater guy here. My concern would be that you are trapping a lot of heat in the metal firebox without escape. So it might get really hot really fast. Wood stoves aren't designed for really high firebox temps - warping or cracking could occur. In masonry heater design, designers know this is going to happen (up to 2000 F) so build expansion zones into the mass heater design. But, maybe your idea will work.

Any way you could take a safer more passive approach? Like a brick or stone structure around the stove with an air gap. This jacket of mass could absorb some of the heat. I suppose your idea could work if you kept your fires small. This would give you the 'smaller' heat you desire while not overloading the woodstove's metal.
 
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Swamp_Yankee

Member
Oct 18, 2018
75
Hunterdon County, NJ
A masonry heater guy here. My concern would be that you are trapping a lot of heat in the metal firebox without escape. So it might get really hot really fast. Wood stoves aren't designed for really high firebox temps - warping or cracking could occur. In masonry heater design, designers know this is going to happen (up to 2000 F) so build expansion zones into the mass heater design. But, maybe your idea will work.

Any way you could take a safer more passive approach? Like a brick or stone structure around the stove with an air gap. This jacket of mass could absorb some of the heat. I suppose your idea could work if you kept your fires small. This would give you the 'smaller' heat you desire while not overloading the woodstove's metal.
Good point, but part of the reason that I got on this kick is because sand seems to take up heat very readily, so I would hope that the firebox would be giving up heat to the sand at such a rate that the steel would not degrade. Of course, another interesting question would be even if it did, what would happen anyway since the firebox was completely encased in inert sand? Of course the front of the firebox/door seal would not be so that could certain crack and cause an issue. I found this which was interesting:

Sedimentary Sand​

As a sedimentary material composed of the compound silicon dioxide, sand is found on beaches and in deserts all over the world. Sand has a low heat transfer coefficient of 0.06 watts per square meter degree Celsius. This means it can retain heat for very long periods of time and explains why the sand on the beach of a hot country remains warm hours after sunset. A 1-kilogram container of sand will cool from 104 degrees F to 68 degrees F in 5 hours, 30 minutes.

 

jalmondale

Member
Dec 16, 2021
140
NY
Based on the weights listed for the kits here: https://www.soapstonesupply.com/031.php, it looks like you'd want about 1400 lbs of firebox plus something with a high specific heat capacity to heat 350 sq ft - if the stove is around 400, that would be 1000 lbs of sand, which is probably more than you can fit in the steel jacket.
 

EatenByLimestone

Super Moderator
Staff member
Thats about half a pallet or 20 bags. Thats not too much. If you dry stack concrete blocks and fill them with sand it'd be pretty easy to do. Google says a 8x8x16 block is 38lbs. 10 bags of sand and 26 concrete blocks. It should be pretty easy to sneak up on 1000lbs. Maybe $200 in materials at Home Depot? Go to a concrete yard and the bulk sand will be $30, probably $50 in block.