How big a rocket heater for 250,000 BTU

[blackhall]

I am thinking about building a new house of about 3,000 square feet. I'd like to heat it with circulating hot water in the concrete first floor (40x40 foot slab) and heat exchangers on the second floor. In addition I can add an unconstrained amount of additional thermal mass in water storage. I'd like to heat that water with a rocket heater because I have pretty much unlimited firewood that it would be good to have some way to get rid of. 

So, the rule of thumb for sizing a furnace in my area would say that I need a 150,000 BTU forced air to service this size house. Theoretically I could get by with a slightly smaller hotwater system. Normally it would be important not to over-size the heating plant but since I have all that storage capacity I don't think this is a problem-- Run the thing flat out until you want to quit, coast until you exhaust your storage capacity. 

Problem is I cannot find data that would let you calculate the size of the rocket based on a quantifiable output.

Any suggestions?

[satamax]

What kind of volume is the house.

[blackhall]

The first floor is 40 feet by 50 feet with 10 foot ceilings. The second floor is the same size except there is a 25 by 30 foot atrium in the center. 

Mark

[Sidney]

Peter has a section on this subject in his document.

batchrocket.eu/en/building#size

But it uses kW instead of BTU's

Here in Canada we also use BTU's. My oil furnace is a 140k BTU unit. so I'm curious to see the answer.

[satamax]

Well, 12/15 inch batch i would say.

[Deleted]

125 mm — 3.5 kg — 12.95 kWh — 44187.23 BTU
140 mm — 4.9 kg — 18.13 kWh — 61862.13 BTU
150 mm — 6.0 kg — 22.20 kWh — 75749.54 BTU
175 mm — 9.5 kg — 35.15 kWh — 119936.78 BTU
200 mm — 14.2 kg — 52.54 kWh — 179273.92 BTU
230 mm — 21.6 kg — 79.92 kW — 272698.36 BTU
250 mm — 27.8 kg — 102.86 kW — 350972.89 BTU

[peterberg]

Easy to recalculate BTU's from kWh's and the other way around.
www.cleavebooks.co.uk/scol/ccpower.htm

One remark though: while using 250,000 BTU/hr you are able to warm up the house in say, an hour or two. Don't expect a mass heater will do that because that one is giving off its power 24/7. Most cooled-off brick dwellings I know of need two days or more to drive the cold away using a mass heater. What I want to say is this: you need to look at the heat losses of the house which should be compensated for by the heater, not so much at the power of a forced-air furness which is heating an existing house.

On the plus side: the house won't be completely cold in a weekend while you are away. The more mass in the house, the longer it will take to warm up and cool down. One extreme example: I built a mass heater during the fall of 1989 in the French Alps at an elevation of 1900 meter (6230 ft). The house was built against a slope so the backwall happened to be the mountain itself. It took the owner an awful lot of fuel (about a quarter of his winter stockpile) and a fortnight to warm up that pile of rock, after that he ran the heater just once a day to compensate for the losses.

He complained by telephone about the shortage of power every day of that two weeks, once that was over he happened to be very satisfied. His neighbours were wondering why they couldn't see any smoke, was he running the heater or what? The owner invited them to see for themselves: no fire and the house warm all the same!

[blackhall]

Sidney--

Thank you. That is precisely the kind of information I was looking for-- Also my research to date had not found Peter's Van der Bergs site and that is extremely helpful. 

With the tables everyone else has thoughtfully provided it would appear that the section of the burn chamber and riser should be about 8.6 inches diameter.

I particularly appreciated Peter's story of using an Alpine mountain as a heat sink. 

Mark



 

[dendub]

Hello,

One of the contributors (who seems to have deleted his/her profile) had posted this:

--- start of quote ---

125 mm — 3.5 kg — 12.95 kWh — 44187.23 BTU
140 mm — 4.9 kg — 18.13 kWh — 61862.13 BTU
150 mm — 6.0 kg — 22.20 kWh — 75749.54 BTU
175 mm — 9.5 kg — 35.15 kWh — 119936.78 BTU
200 mm — 14.2 kg — 52.54 kWh — 179273.92 BTU
230 mm — 21.6 kg — 79.92 kW — 272698.36 BTU
250 mm — 27.8 kg — 102.86 kW — 350972.89 BTU

 --- end of quote ---

3 questions:

1) Do I get this right as to the information provided (using the first line)?
a burning chamber of 125mm (square) that is fed 3.5kgs of wood per hour produces 12.95 kWh i.e. 44187 BTU in 1hr

2) Does the community believe these to be correct?

3) I am trying to think about how to improve autoclave's energy consumption to sterilise mushroom substrate (say 121 celcius or 250 F).
autoclave is advertised at 24kWh power.
I am thinking that if I feed water to the autoclave that has already been heated by a rocketstove, the electric consumption will be lower.
Hence why I need the table above to be (understood by me) and validated so that I can plan for what I need, namely to warm 250 liters of water to say 90 celcius by a rocketstove (I think I would need the 200mm chamber or larger).
I then will need to think on how efficient the design to transmit as much of the energy from the burnt wood to the water. 
If someone has data on how efficient one can get to transmit heat from the stove to the water, I'd love to get it. Thanks.
Does this make sense?

Thanks,
Denis

[masonryrocketstove]

Dec 16, 2023 0:12:50 GMT -8 dendub said:

autoclave is advertised at 24kWh power.
I am thinking that if I feed water to the autoclave that has already been heated by a rocketstove, the electric consumption will be lower.
Hence why I need the table above to be (understood by me) and validated so that I can plan for what I need, namely to warm 250 liters of water to say 90 celcius by a rocketstove (I think I would need the 200mm chamber or larger).
I then will need to think on how efficient the design to transmit as much of the energy from the burnt wood to the water. 
If someone has data on how efficient one can get to transmit heat from the stove to the water, I'd love to get it. Thanks.
Does this make sense?

There was a pretty comprehensive study done on fuel efficiency for water pasteurization (bringing H2O to 71°C to kill bacteria and viruses in drinking water) using an institutional rocket stove with internal brazed copper plate heat exchanger.. They found that the on-demand water heating method (through the heat exchanger submerged in the pot of boiling water) was far more efficient in fuel use than heating a batch of water to a target temp. It also allowed for the construction of a smaller (60 liter) boil pot and smaller rocket stove than a batch burner would require. And the smaller fire under a smaller boil pot is much easier / less damaging on the equipment than a huge fire under a huge boil kettle for heating the whole volume of water as a huge batch.

The full report on the method and real-world implementation of the project is available here, with all the calculations:
www.mdpi.com/1996-1073/13/4/936

Pretty sure you could use this for bringing the water up to around 90°C by slowing the flow-rate and increasing the residence time in the heat exchanger. The “water pasteurizer” output could then be plumbed directly into the autoclave’s water intake.

..Best of all is that you could seal the whole pot skirt on the institutional stove to eliminate smoke seepage common to typical InStove designs.. which have to fit their cooking pot loosely enough for it to be removable for cleaning afterward. An internal heat exchange water pasteurization system like this would not have a removable pot requirement, so that pot skirt gap could be sealed gas-tight with something like ceramic fiber.