Just had an inquiry about rocket mass heaters
for apartments, and other situations where a low-mass heater might be necessary for structural reasons.
(Boats, RV's, and manufactured homes might also fall into this category.)
There are several reasons why a rocket mass heater
might not be the best choice for such a situation, and ways to use the same design principles to improvise something else that might be suitable.
We've definitely run into problems with rental spaces in general, especially when the landlord is incommunicado about permanent alterations. Even if the landlord is in favor, there are other hassles to get through.
- Apartment buildings are often taller than single-family residences, meaning the building itself can act more like a chimney and have more severe negative-pressure or wind gust problems.
- If you are trying out your idea in someone else's space, you need it to work from the get-go. The most reliable exhaust for good draft is a conventional chimney, exiting above the roof and any nearby obstacles.
In some buildings that can mean going through someone else's space or tacking a lot of insulated stovepipe (spendy) onto the outside of the building.
- For a horizontal vent, there are some detailing issues with water
but the most important thing is the wind. If you are on the upwind side of the building, or near a corner that creates gusts and eddies, you may not be able to do a horizontal exhaust without the stove backdrafting in certain wind conditions.
- An outside air supply could be one way to neutralize many of these issues, but it too must be protected. Ideally, it would be in the same place with regard to wind and building pressure (maintaining a neutral balance) but would not be close enough
to intake the exhaust.
- Mass heaters store heat using the dense thermal mass, usually earthen masonry or conventional masonry (brick, stone, etc). Heat storage seems to correlate with density: heavier materials store more heat. It is possible to make lighter masonry heaters, but they are still heavy compared with iron stoves and can't store as much heat as full-mass masonry heaters.
- The mass of a typical rocket mass heater
is usually 120-200 lbs/sf. That's comparable to light industrial use, library stacks, or a large aquarium or waterbed. Not impossible that an apartment floor could handle it, or can be reinforced to handle it, but tricky.
- Shoring up or creating a masonry foundation may require work underneath the floor. If there is another apartment below, there is more inconvenience, cost, and risk/liability compared with a ground-floor heater that can't fall on anyone.
- Adding thermal mass of any kind may reveal or create structural issues, especially in older apartments. Old or hidden problems like a leaky shower
can lead to rot, and sooner or later the entire bathroom lands in the downstairs neighbor's lap. You don't want to take the risk or blame for precipitating a last-straw situation. Find out the design limits, check that the building is still in good condition to handle these loads, and then keep track of the extra mass you are adding. Try to stay below 1/2 of the load limit on any given square foot. You can also measure the floor level and distance from a nearby fixed point, such as the garage floor below a loft apartment. If you notice any deflection (bending or tilting) of ANYTHING, stop adding mass, remove some, and address the structural issues.
- Without the mass, there is not heat storage.
- Exposed horizontal stovepipe is widely recognized as a firetrap. Even if you are confident you can work around the dangers, there is little chance of official approval.
- Pebble-filled mass heaters do not offer a secondary seal like full-masonry heaters, so hidden leaks are possible. They also may not store or transmit heat as efficiently (a number of people
are trying this out and we're looking forward to more data).
- Good insulation seems to be the most popular and effective low-mass option. If it's possible to improve the insulation, the heating loads get much smaller, and smaller heaters can handle these loads. Portable dwellings particularly suggest insulation, as mass and heater fuel are both costly to haul around.
Legal / social issues:
- Rental properties and their owners are often held more strictly to local
building codes and insurance requirements, where owner-builders have more freedom
to proceed at their own risk.
- Permitting and approval of mass heaters can be expensive, difficult, and time-consuming; local officials may not know how to apply existing code to an EPA-exempt, site-built, hybrid mass heater.
- Putting in a heater without attracting notice from the landlord and/or building officials may be difficult given the need for an outdoor vent (and possibly an outside air source).
- If permitted under the Masonry Heater ASTM standard, one of the requirements is a 12" thick, reinforced, non-combustible foundation starting below frost level or 12" below grade.
- If they are willing to consider an installation over existing suspended (wood
) floors, the alternative appeals process may be slow and/or spendy.
- Renters have a reputation among landlords for mistreating property and appliances. The landlord may be wary of letting one renter build an unusual mass heater, as it might not appeal to future tenants (or they might not be responsible enough to run it safely). Unusual things can just be more of a hassle to manage in a rental situation. A long-term lease agreement might alleviate some of these concerns.
- A 'portable' mass heater could resolve some of these issues, but the landlord's permission is likely still required for both installation and removal. (Some rental agreements make any installations the property of the landlord.)
Benefits of small / interior-walled spaces:
- On the plus side, apartments usually have a much smaller heating load, so a more compact masonry stove might not need the same mass to produce a similar balance of warmth.
- Smaller, more sheltered spaces are easier to heat. This means apartments in general need less heat than a comparable exposed dwelling. A small, super-insulated space can be heated with body heat, cooking, or a candle. Cupboard-beds, four-posters, blanket forts, and Paul's article about saving 87% on his heating bill are examples of what can be done by reducing the space to be heated. Many steps can be taken to increase comfort even if you don't own the walls.
- Portable / Pebble style
rocket mass heater: You could consider a lower-mass heater such as Paul Wheaton's pebble-style / box-and-fill construction (also removable should
the need arise).
- Smaller masonry heaters: European apartment heaters, and a few American manufacturers, include moderate-mass masonry heaters. You could look into a Tulikivi heater, or a soapstone woodstove, then use it as a springboard to discuss less-expensive "hybrid options" (rocket mass heater
). Kiko Denzer's 'Heater Hat
' is a clever example of a tiny masonry heater built onto a woodstove.
- Improved radiant heat: If the apartment already has a fireplace or wood-fired heat source, you could look at improving that heater (e.g. with a Rumford retrofit) and/or creating more storage to sustain the warmth after the fire is out (a brick hearth or backsplash designed to catch radiant heat from the stove, dense 'art
' or brick 'end-tables' in front of the fireplace, a heater hat as above, or just pile bricks on the stove....
- Passive-solar techniques: You could also look at improving your thermal mass heat storage for sunlight. Jars of colored water or oil in the sunny windowsills, stone-slab or tile (even over carpet), stone or glass coffee-tables and curios, and other thermal mass additions can store a lot of heat while spreading it out at acceptable loads for an apartment floor. Insulating drapes or wall-hangings can help you trap that heat at night. Use shady / exterior walls for storage, e.g. wardrobes or enclosed book-cases, with insulation between furniture and wall.
I'd love to hear / see pictures from folks who have done some experimentation in this space. Especially if you've built a "lower-mass" mass heater. Brick cookstoves, ovens, and fireplace retrofits count too.