OK, folks -- time for some creative thinking about a 16-unit building we are trying to weatherize in SE Wisconsin.
It's two stories arond a central courtyard, with most units accessed by a door direct to outside. (That is, most units have either two or three walls exposed to ambient, with a wall that's about R-6. We won't be able to touch the walls, but the attics/secondfloor ceiling will go to R-50 or R-60 before we are done. )
The 16 one-BR apartments have two small (14kBtu) natural gass wall heaters (without circulating fans) for space heat; one in the LR and one in the bedroom. The wall heaters test at 50% to 60% SSE. (The exhaust coming off one of them was over 1,100 degrees F.!) The wall heaters are 40 years old, and the pilots light are prone to blow out. Each 500 SF unit has two wall heaters and it's own 40 gal NG water heater -- they average 500 therms of NG per year.
We can't make the numbers work to replace both wall heaters. I know I can install one 84% SSE wall heater with a built-in circulating fan in the living room, and it will have more than enough capacity to heat the whole unit.
What we need is a creative and code-acceptable method to assure that the bedroom gets enough heat from the living room into the bedroom at the design temp of -7 F. Defining "code-acceptable" -- if I can justify it in the 2009 IBC, I can probably get acceptance from local Code folks.
Wow, with R-6 walls comfort will be a challenge no matter what.
Without knowing construction details and what you can cut into ducting possibilities may or may not exist. You'd need to get the air intake as close to the heat source as possible to get enough warm air to be meaningful.
I once had a similar problem and I ended up pulling a FHW circuit off the DHW for make up heat when the outside temp dropped.
Do you have heat loss calcs? If you know what the room needs you'll be closer to getting the answer you need.
I'd install a fan that pushed air into the bedroom from the living room. Could optionally wire it to the new heater, so it would always run when the heater was on. Grainger sells some very small blowers. Could construct your own little AHU and mount it up towards the top of the wall that divides the two rooms?
Or, maybe a wall-mounted Panasonic fan (very quiet) that has a Tamarak transformer installed on it to lower the CFM, if it's too high (70 cfm)? You could just hard-wire it and let it run all year long...
I look forward to hearing where you land with this one!
Don, I was curious if you had come up with a solution yet. I like to see how other people solve thorny issues, it can broaden the mind:-)
Check out the links and discussion under the post below that addresses distribution with a point heat source. In particular, SWA did Building America research on comfort and distribution of ventilation air in homes heated with a gas space heater. Homes were better insulated, but also much larger than your apartments.
R 6 walls leave much to be desired, however, I would guess that the windows are a bigger source of discomfort. Can you add high performance storms to raise the mean radiant temperature of the bedroom?
Have you been following the work that the folks at CEE (MN) are doing on combis in WX (with SRC) for both heat and hot water? http://www.mncee.org/Innovation-Exchange/Projects/Current/Retrofitt... Achieving distribution may put you over your cost effectiveness or budget constraints, but it could be worth getting rid of three inefficient appliances for one condensing one. The CEE pilot was constrained by using the home's existing ductwork. There could be a modest distribution system for your apartments that could be installed since you are already willing to think outside the box. Once you raise the combustion efficiency of the heating system, water heating may be your largest gas load. Addressing the gas water heaters could also move you past some combustion safety backdrafting issues.
CEE's work clearly demonstrated the need to verify that you have field conditions that will result in the rated equipment performance (and ability to condense) over a variety of load conditions. This was achieved through a combination of lab testing and field commissioning of systems.
How is mechanical ventilation currently achieved?
Don, keep me posted on this. Sounds like a great project and opportunity to learn.