Replies

1. 
   Riversong | Dec 27, 2009 01:42pm | #1

   Graham,

   If it's the aesthetics of a masonry hearth, you can't do better than a masonry heater. But, depending on what other sources of heat are contemplated, it's very possible that such a heater could overheat your space. (I hope you're not planning an actual fireplace, since even with glass doors they are terribly inefficient and increase the exfiltration of air from the house with the typically poor-fitting dampers.)

   If, for instance, you intend to incorporate some passive solar strategies, then that could contra-indicate a masonry (high mass) heater. The same arguments used against combining radiant slab mass floors with passive solar (which I argue are somewhat wrongheaded) are more powerful with a masonry heater which can maintain a higher suface temperature than a radiant slab and is much more difficult to turn off.

   Both a high-mass radiant floor and a high-mass radiant masonry heater will continue to emit heat when the sun starts to shine. The slab heat source will be turned off by the thermostat, while the fire in the heater might continue to burn.

   Which isn't to say that a masonry heater won't work, but it will require more attention than a low-mass woodstove which can offer quicker heat when the sun's not shining and be damped down when it is. Even with a woodstove, I advocate a thermal mass surround to prevent local overheating in a house with a very low heat load. But such a secondary storage mass, because it's decoupled from the stove, won't hold either as many Btus as the masonry in a heater or have as high a surface temperature (so radiant return will be slower).

   What envelope system are you considering? If you're interested in the modified Larsen Truss cellulose system that I've developed, I'm also in north central VT and am available for consultation.

2. 
   John Silvernale | Dec 28, 2009 08:49am | #2

   I have been in my superinsulated home with radiant floor heat for 17 years in northern MT.It is also passive solar. Things can get warm if the temp falls well below zero (-40) during the night and the sun shines the next morning. my air to air exchanger takes up a little of the excess heat, I open door to the garage(insulated) and dump the excess heat there for heat storage thru the cars.

3. 
   homedesign | Dec 28, 2009 02:11pm | #3

   John S.
   Your idea of storing and using heat from the garage has merit.
   I question the way you are doing it.
   Your garage should be disconnected from the pressure boundary of the living space.
   Do you really want to eat and sleep with your car?
   Perhaps you could run a "Garage Tube" thru the garage (similar to an earth tube concept).

4. 
   hpnWsbX6gm | Dec 28, 2009 11:51pm | #4

   Thanks for your responses.

   Robert, I am considering either a double stud wall with dense pack cellulose or a REMOTE wall system 2x6 OVE, with dense pack cellulose in the wall cavity and 4" rigid board on the exterior. I have read about your modified Larsen Truss before and was concerned it might be too difficult. I would like to explore this further. Is there an email address that I can reach you at?

5. 
   John Silvernale | Dec 30, 2009 11:25am | #5

   John Brooks
   I understand what you are getting at, not having to heat garage or plug in vehicles is worth fighting with wife about door being open. thou I have a screen door buffering the site of the cars and stuff. I also have a earth tube suppling my whole house ventalation( vanee air 2 air exchanger)

6. 
   Riversong | Dec 30, 2009 11:10pm | #6

   Graham,

   You can reach me at HouseWright (at) Ponds-Edge (dot) net.

7. 
   Pete Powell | Jan 03, 2010 09:11pm | #7

   Graham, I designed, built and lived for quite few years in a house in central MD with R30/R40 construction, a Tulakivi fireplace and a cored 8" concrete floor slab for thermal storage of passive solar gain and excess heat from the fire, and never experienced any overheating problems. The system worked with a thermostatically controlled fan in the cathedral ceiling forcing warm air into the slab on a temperature rise over 75 degrees. With careful use of the fire after some experimentation we were able to maintain fairly consistant comfort levels and the backup propane forced air furnace was almost never needed unless we were away. Since we were putting relatively low temperature air into the slab there was not an overtemp problem from the radiant slab. On sunny days, accumulated solar heat would also be blown into the slab with the same blower and no fire would be needed that night . I am an architect and have designed quite a few houses with similar setups using radiant forced air slabs with Airfloor, Flexicore precast or double 4" pours with 2" conduit used to form air passages, and all have been successful in this climate. A Tulakivi is not the cheapest wood burning stove you can buy, but we feel they are the best by far for many reasons, having heated with several kinds of wood backup heaters in our passive solar homes for over 35 years. I highly recommend that you consider it further. The key to maintaining comfort with these stoves is learning to adjust the burn to suit outdoor conditions, solar gain and slab temperature. If you overheat the house, you are simply burning too much wood . It's not rocket science! Good luck