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rocket mass heaters

why is there no fear of boomsquish in a residential boiler?

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Location: Connecticut

posted 1 year ago

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Hello all,

Just getting my second cup of coffee here so maybe my brain isn't working right yet. Butttt....what's the feature/s of a residential oil fired boiler that makes it safe enough to run inside a home? It's not a large amount of fluid per zone, there are zone valves that could fail, pumps that could fail, etc. I assume there are safety features on these pumps and valves that shut the burner off if anything fails?

I know anyone looking to heat water with a RMH is warned (rightly so) of the "boomsquish." It seems most are pointed towards the tank over the RMH design (Tim Barker or Geoff Lawton design)

Just wondering the difference between a boiler and any RMH water heater design that's been proposed?

John

Thanks in advance!

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Hi J,
I am not a hot water/plumbing/heating expert, but I wanted to respond while we wait for an expert to reply :)

I do think there is a fear of boomsquish in a residential boiler, but they have been commercialized and regulated enough that most of the safety features are already installed to mitigate explosions. Again, I don't think it is a lack of fear, simply that the fears have already been dealt with.

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We have a residential boiler. ( natural gas) We just had some servicing done since it's a recent house purchase.
As I understood what the plumber and gas guys said, there are basically a couple of ways that explosive level pressure is avoided. First, it's not designed to ever get very high,( there are gauges to check that things are in the proper ranges) and that there are points that are built to fail and allow venting without catastrophic failure. It would make a mess, but no "boom".
Ours is a cast iron boiler, but while that style is still good and legal, now all new installs are basically something closer to on-demand heaters. Our type also is required to be checked yearly as well, mainly for carbon monoxide levels but also to make sure it's running in the right ranges.

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Peter E Johnson

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Mostly the pressure relief valve will release pressure if something goes wrong with the system. They also have thermal shutdown systems if the boiler gets too hot. In my parent's gas furnace it turns the gas off after the water gets to a certain temperature, but turning the heat off from burning wood isn't as easy. Maybe you could use something to shut the air off to the fire if the water temperature gets too high? I'm sure there is a testing agency that has a number of tests that a boiler has to pass to meet code. If it's made to run at 30psi the testing agency might require it to survive 120-240psi so that as the pipes corrode there is still enough iron to hold the pressure in 50 years.

I think the point of calling it "BOOMsquish" is to communicate that it's not a system to be built haphazardly, because it can kill you. I've seen posts on this forum where people were talking about using copper coils to heat the water up. I don't know exactly what they were thinking, but having copper in direct contact with flame isn't a good idea. The small amount of sulfur in wood will eat the copper away over time. Also you need to do pressure calculations on any tank that you're trying to use as a thermal mass within the system. If you look at the pressure ratings on these pipes at McMaster https://www.mcmaster.com/products/pipe/high-polish-stainless-steel-tubing-for-food-beverage-and-dairy/?s=pipe The wall thickness stays the same from 1/2" to 3", but the pressure rating goes from 3400psi to 540psi. The bigger the pipe or tank the thicker it needs to be to hold the same pressure. I have no idea how much pressure something like a 55 gallon drum could hold (maybe 5psi), but I wouldn't feel comfortable pressurizing it at all. Mostly because steel rusts, and even if it was okay when it was installed it'll get thinner over time.

Plus they don't want Rocket Mass Heaters to get a reputation of being explody, so discouraging people that aren't qualified to make a pressurized water systems will save lives, and not tarnish the reputation of RMHs.

While looking around I did find some links about using thermal siphons with wood cook stoves that appear to be pressurized.

These systems look sketchy to me, because you need some air in the system to compress as the temperature rises. The pipes coming off the tops of the hot water tanks will bleed all the air out of the system. I would put an expansion tank in any system that I was going to build. https://www.homedepot.com/p/The-Plumber-s-Choice-6-3-Gal-Thermal-for-Potable-Water-Heater-Expansion-Tank-FTET63-N/322406176 I have no idea how big you'd want it.

http://inthewilderness.net/2017/01/27/homestead-hot-water-thermosiphon-loop/

https://riversonghousewright.wordpress.com/about/27-thermosiphon-wood-fired-hot-water-system/

This one appears to use a single use pressure relief valve, so that if the stove is getting the system too hot the pressure only dumps off once. Instead of having the valve reset, and letting the system build pressure again.
https://www.susprep.com/off-grid-water/hot-water-free-wood-cook-stove/

https://www.iamcountryside.com/self-reliance/wood-stove-hot-water-heater-heats-water-free/

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Peter E Johnson

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Peter E Johnson wrote:
This one appears to use a single use pressure relief valve, so that if the stove is getting the system too hot the pressure only dumps off once. Instead of having the valve reset, and letting the system build pressure again.
https://www.susprep.com/off-grid-water/hot-water-free-wood-cook-stove/

Looks like I was wrong. The white thing on the pressure relief valve probably isn't a single smash pipe that keeps the valve open. It's more likely a thermostat that opens the valve if the water temperature gets too high. So the valve is probably 75-150psi instead of the 30psi that normal boiler systems run.

I couldn't find anything about a pressure rating for the tank on their website. https://www.vaughncorp.com/residential/storage-tanks/range-boiler-30-119-gallon/

pressureReliefValve.jpg

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I would also like to bring up the difference between hot water boilers and steam boilers. A hot water boilers operates at a much lower pressure and has less catastrophic explosion potential. I have a hot water baseboard heat boiler. The control of the heat by firing and turning off in intervals is something different than a high heat startup that trickles off with time. I'm sure there are ways to relieve pressure but they are not 'industrial standards' when it comes to RMH because it is so 'new'. A little bit of bubblewrap safety but the history of exploding boilers/hot water contraptions give a lot of merit for the hesitation in anything new.

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It is a series of protection devices built in. Things like T&P valves, relief valves, thermal shut offs etc. Most boilers also include an inspection service and pressure testing in many cases. You also have some sort of sheet metal containment around it that will as least slow the debris from the explosion while venting aiming the steam blast away from people.

A gas or oil fired boiler one of the first thing built in is a fuel shut off or a fuel idling system based on temperature. How do you turn the fire off quickly in a rocket stove for a short period and do it safely? Can't defuel so shutting the air off is the only answer. But now you are running unburned hydrocarbons on out. Condense enough of them on the walls of your horizontal chimney and suddenly you have a major chimney fire risk. So first major safety system negated by design and danger actually made worse. About the only other answer would be some sort of flooding system to put the fire out. But it would be a major CO risk as well as as you quench the coal they can release hydrogen and potentially generate explosion risk this way too.

Most boilers are hydrostatic pressure tested. At the very least the design is pressure tested and certain percentage off the assembly line are tested. Big thing this is done in an industrial test facility with various safety functions. Testing individual systems with varying designs, in the home would have very few safety systems and a real chance of making a big mess and doing damage.

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j sigs

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Location: Connecticut

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again, haven't had enough coffee yet, but.....

is it logical to think that if I had a water jacket from a boiler, I could mount it somewhere on/in either a RMH or a waste oil burner, in a way that would reduce the boomsquish factor?

The ultimate goal is to spend less money on oil in the winter.

Here's what I've been thinking about in case anyone is wondering where I'm going with all this

I have a pretty regular source of waste oil.

I was thinking of either 1. a waste oil fired RMH or 2. The blue flame style waste oil burners.

Burning manually outdoors about 15' from the house

Batch burning with manual firing of the boiler and monitoring the burn in person i.e. no automation. I would burn for a few hours at night after work, say 6-9pm loosely monitoring temps, etc

collecting the heat via water and some type of heat exchanger on/in the burner

pumping the water via a Taco pump

building a 300gal tank indoors to hold hot water

heating at least one zone (master bed/bath, 2 smaller bedrooms) up to about 67 deg F 24/7 throughout the winter

Heating the zone via a separate Taco pump and baseboard hydronic heaters

Probably a lot of little details I haven't mentioned or forgotten because of lack of coffee!

Thanks in advance.

For reference, I'm in CT Zone 6a in a residential neighborhood. House was built in '86. Currently have a Burnham V8 Oil fired Boiler w a beckett burner. I average about ~800 gal of oil usage per winter.

Peter E Johnson

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A shell and tube heat exchanger would probably be the best way to go. You wouldn't need a "high" pressure 300 gallon tank for your thermal mass, and since you're planning on putting the boiler outdoors it would require a lot less antifreeze on the outdoor side of the system. It would require two pumps, one for the higher pressure boiler side of the system, and one for the low/no pressure thermal mass side.

Shell and Tube Heat Exchanger Wiki

I quickly looked on ebay, and high pressure high temperature shell and tubes aren't cheap. This one says 8BAR (116psi), and 50C (122F). I'd design a large factor of safety(FOS) for the pressure, and a decent FOS for the temperature. A FOS of 4 on a 30psi system would be 120 psi, and 116psi is close enough for that.
Ebay shell and tube

At 30psi water boils at about 340F. I honestly don't know how much of an FOS you'd want for the temperature, maybe 1.3 to 1.5. You'd probably want a temperature rating around 442-510F. So that ebay heat exchanger isn't even close to what you'd want.
Water P-T graph

If you could find the datasheet for a particular heat exchanger it's possible the manufacturer has a built in FOS, and the "max" on the label is an acceptable temperature to run at all the time. You'd need to dig through the documentation to find that, so it's just safer to stack another FOS on top of the manufacturers potential FOS.

j sigs

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Location: Connecticut

posted 1 year ago

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Peter E Johnson wrote:A shell and tube heat exchanger would probably be the best way to go. You wouldn't need a "high" pressure 300 gallon tank for your thermal mass, and since you're planning on putting the boiler outdoors it would require a lot less antifreeze on the outdoor side of the system. It would require two pumps, one for the higher pressure boiler side of the system, and one for the low/no pressure thermal mass side.

Peter, thank you! I'm not sure Im following you 100%. What would you suggest for a thermal mass?

Also, if the boiler is outdoors, and only burned for a few hours a day, wouldn't I need MORE antifreeze on the outdoor side?

Again, thank you for your input! I'm not an engineer so sometimes the concepts don't jump right out at me as obvious. Regardless, I thank you!

There's no way I could buy a shell and tube heat exchanger, cant afford it. If I could scavenge one from the junkyard, possibly, but even then I don't really know enough about them to know what I'm looking for. I've seen some built out of old electric water heaters. I could possibly pull that off.

Peter E Johnson

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j sigs wrote:
Peter, thank you! I'm not sure Im following you 100%. What would you suggest for a thermal mass?

Also, if the boiler is outdoors, and only burned for a few hours a day, wouldn't I need MORE antifreeze on the outdoor side?

Again, thank you for your input! I'm not an engineer so sometimes the concepts don't jump right out at me as obvious. Regardless, I thank you!

There's no way I could buy a shell and tube heat exchanger, cant afford it. If I could scavenge one from the junkyard, possibly, but even then I don't really know enough about them to know what I'm looking for. I've seen some built out of old electric water heaters. I could possibly pull that off.

The thermal mass would still be the 300 gallon tank.

It sounded to me like you wanted to pump water out of the 300 gallon tank to the rocket mass heater which would require a lot of antifreeze to stop it from freezing.

Doing this project on the cheap is a good way to get the BOOMsquish. The only place I've ever seen a used shell and tube heat exchanger for sale was at an auction at a shut down oil refinery. They're not very common outside of heavy industry.

Edit: The MSPaint picture came out really small.

RMBsimplified.png

Here's a simplified version of what I was trying to describe with an expansion tank or safety valves.

j sigs

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Location: Connecticut

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Peter E Johnson wrote:

j sigs wrote:
Peter, thank you! I'm not sure Im following you 100%. What would you suggest for a thermal mass?

Also, if the boiler is outdoors, and only burned for a few hours a day, wouldn't I need MORE antifreeze on the outdoor side?

Again, thank you for your input! I'm not an engineer so sometimes the concepts don't jump right out at me as obvious. Regardless, I thank you!

There's no way I could buy a shell and tube heat exchanger, cant afford it. If I could scavenge one from the junkyard, possibly, but even then I don't really know enough about them to know what I'm looking for. I've seen some built out of old electric water heaters. I could possibly pull that off.

The thermal mass would still be the 300 gallon tank.

It sounded to me like you wanted to pump water out of the 300 gallon tank to the rocket mass heater which would require a lot of antifreeze to stop it from freezing.

Doing this project on the cheap is a good way to get the BOOMsquish. The only place I've ever seen a used shell and tube heat exchanger for sale was at an auction at a shut down oil refinery. They're not very common outside of heavy industry.

Edit: The MSPaint picture came out really small.

initially i just wanted to wrap a copper coil around a heat riser or something and plumb it via pex al pex (because that's what I have on hand) and go right into the 300 gallon tank. but it's the copper coils around the heat riser that sounds like everyone is saying is a bad idea for the BOOMsquish factor. still thinking about the most effective heat exchanger...

how about this, i followed your lead with MSPaint lol, and I left a bunch of things out butttt...with some strategically placed valves and a pump with both inlet and outlet manifolds AND a lets say 10-ish gallon "glycol bypass tank" (just made that up lol) you could eliminate the need for 150 gallons of antifreeze and also ditch the tube and shell heat exchanger? just switch over to glycol mix when finished with my batch burn. that way if overnight temps drop a 50/50 mix gets you protection down to -34 deg F which is well beyond my needs here in CT

what do you think?

glycol-bypass-system.png

Peter E Johnson

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I'd avoid the use of copper, because the sulfur in the wood will corrode it fairly quickly. Stainless steel would be a much better option.

With that setup you'll have a volume displacement problem between the two tanks if you want the RMB to have pure 50/50 antifreeze in it. If it takes 40ft of 3/4inch pex to go both ways you'll have a volume of V = pi * (.75/12)^2 * 40 = 0.49 cubic feet of fluid. 0.49 ft^3 * 7.5 gallons/ft^3 = 3.7 gallons per antifreeze purge. Which would only allow you to purge the system ~3 times before you're out of antifreeze. You could set the antifreeze system up to pump about three gallons out at shutdown and then three gallons in to the 10 gallon tank at startup to reuse the antifreeze, but you're always going to get some mix diluting the antifreeze fairly quickly. Your main water system would get a slowly increasing concentration of antifreeze, and I don't think you're suppose to flush glycol down the drain, and that would be a lot of antifreeze water to dispose of correctly. I don't know if your current oil heater is hooked up to city water, but pumping glycol into a boiler water system that is attached to city water would undoubtedly be illegal. Even though boilers typically have pressure regulators from the city water to the boiler that shouldn't allow backflow they still won't like it. I wouldn't do it that way.

L Cho

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j sigs wrote:again, haven't had enough coffee yet, but.....

I have a pretty regular source of waste oil.

I was thinking of either 1. a waste oil fired RMH or 2. The blue flame style waste oil burners.

If you have a supply of waste oil, then a burner would be a more controllable option. A proper control system can turn it on and off, to achieve the desired water temperature.

A waste oil fired RMH doesn't really make sense. A RMH is specifically designed to burn full on, until the wood is gone. There is no way to modulate the burn, so any attached water heating would require a vented system, preventing vapor pressure build up, and the subsequent boom.

L Cho

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Here's a simplified version of what I was trying to describe without an expansion tank or safety valves.

As the temperature of the RMH cannot be modulated, any loop of water, or coolant, flowing through the core (your green loop) would need to be vented, to allow steam to escape. If you are running the coolant through a heat exchanger, then it would be possible to keep the coolant temperature below boiling, just like your automobile radiator keeps your engine coolant from boiling. If you can cool it enough with the heat exchanger, then it won't boil. But a fixed size tank will eventually run out of thermal capacity, leading to your coolant boiling, and a possible boomsquish.

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