I live south of Tokyo, Japan where we are building a slab foundation house. Not that cold really. It may snow once or twice in the winter and it melts right away. This shows you our average temps. It is very very humid here in summer as well.
http://www.climate-zone.com/climate/japan/fahrenheit/tokyo.htm

The reason I want radiant is because central A/C and heating doesn't exist and all builders are fighting against my request for central (they all want to use wall hanging AC/heating units). I know it's a strange country. Radiant floor is more favorable here.

Here are my questions:

1. Since it is not that cold here, will I be better off with slab or floor panel. This is what concerns me the most as I peruse this forum and scour the internet. Not so concerned about long term running costs because house will probably be a rental unit some day.

Is floor panel (warmboard, or DIY) more favorable to slab heating for mitigating hardwood floor damage?

2. We don't have that much space for an extra boiler...I'm thinking below the stairs. Because of this and #1, have consider electric.

3. Do I need heating for attic or will 1st and 2nd keep 3rd warm?

4. Would like to do solid wood floor, but will strongly consider veneered wood floor.

5. Do the folk on this forum know of dealers for radiant equipment in China. We're importing a few things from China. Costs for Japanese building materials for DIYers is very expensive.

6. I read that radiant is not a good choice for hardwood floors. Do folks here agree?

House will be square with 1st, 2nd, and 3rd attic. 1st and 2nd are 770 sq each and 3rd is like 450 sq ft.

Regards,

Marcus

Hardwood floors are not a problem with radiant particularly at the relatively low heat loads you'll be seeing in Tokyo.  But if you're intending to use the radiant for both heating and cooling, radiant ceilings will be more effective on the cooling end, yet stil decent on the heating side.

Is the attic living space, or is it dead-storage or...??

Each floor of living space will need it's own heating & cooling, but the heating & cooling loads are likely to be different on different floors. (It's more like the 1st & 3rd will keep the 2nd warmer. The 3rd floor has the most surface area exposed to the outside since it's coupled to the attic & roof. The first floor has unheated earth below it, but that earth isn't very cold in Tokyo.)  If the attic is dead-space, as long as the roof is insulated and you don't have insulation in the attic floor it is protected from freezing, but may run seasonally hotter/colder than the fully-conditioned living space.

If you're using the radiant for both heating and cooling, an air-source heat pump that has both heating & cooling modes seems like the right solution here. Radiant cooling will not remove humidity, and on extremely humid days you have to guard against condensation on the radiant panel, but Tokyo isn't nearly as humid as the tropics- you can get significant sensible cooling performance out of it.  Standalone room dehumidifiers are probably sufficient for handling the latent loads on the muggiest days.

As for equipment suppliers in Asia I'll step out here- beats me.  (Does Daikin make hydronic-output heat pumps? If yes, do handle both heating & cooling?)

How are you heating hot water?  The relatively small heating loads should be very amenable to combined space-heating & hot-water setups, and you should be able to get the radiant to work at temps under typical Japanese extreme-boiling-bath temps.

Thank you so much for your help. Our "attic" is all living space except for behind the knee walls obviously (high pitch gable roof). We don't use trusses but old fashion rafters so lots of living space. It's a square house all built straight up.

I would love to use radiant for cooling, but skeptical it's feasible for the heat here in the summer (that website above is not accurate for summer temps...but winters temps are). Are the pex hosing systems separate for hot/cold? The hottest muggiest day for around 3 weeks in August is around 33-36 degrees celcius (96.8 F). Very humid here. I'd like to get that down to 22 C (71 F). Is this possible? The rest of the air condition season is about getting 30 C (86 F) down to 22 C (71 F) for another 3-4 weeks after and 4-6 weeks before August. I would love to explore this option if these temperature drops are possible but I need to talk to a professional over the phone about this. I'd love to speak to professionals with equipment to sell for this. Please let me know how to contact you.

My biggest concern with importing a heat exchanger regardless the fuel type is the power difference. My area of Japan is 100 volt (or 200 volt) at 50 Hz. America is 120 v at 60 Hz, while Europe is 240v at 50 Hz. I've talked to one supplier and he thinks he can change out his power source. Gas is better than electric for these concerns as the "heat" itself will not be electrically dependent.

Is a 70" by 40" utility room big enough (under the stairs)?

All hot water in 98% houses in Japan comes from real time natural gas element heaters. I've rarely seen a boiler or water heater. My shower hot water is gas and heated real time after the water starts to flow. Same for the kitchen hot water...same gas heater and wall controller as shower. The shower temperature buttons override the buttons in the kitchen if water is flowing in the shower (which activates the wall controller). Japan is sophisticated in this regard I believe. I have thought to import the hosing/manifold/etc and install myself (builder is agreeable) and buy a Japanese natural gas heating system like described above and combine the two. But I think that will take more ingenuity than I'm prepared to do alone.

I have another question about installing in crawl space, but I'll make a new thread.

Thanks.

Radiant cooling will not de-humidify the house like you are going to want. The wall hanging or Mini-Split systems are very good for de humidification because they run a cold coil with low air flow, that would be my choice for cooling the just heat with radiant. In fact radiant cooling in your climate might cause floors to sweat.

The ductless mini-split is what we were planning and what we have now. It's what all of Japan uses in probably 98% residential homes. Since this country is quite advance with these, it's what we'll do and drop the radiant cooling idea. They cost here about $800-$1200 USD each to buy and install. This link describes them. http://www.mrslim.com/Products/Category.asp?ProductCategoryID=24 The problem I don't like is every room in the house gets it's own dedicated outside air condensing unit. The good side is you only cool the rooms you need, we rarely run all 4 at the same time. I've never seen ones where a single outside condenser unit supplies multiple indoor air handling units. I will have to look into that. We have 4 in our current house, will need 6-7 in our new house. I've always thought multiple condensers was highly inefficient but now I'm not as sure. I've read ducting systems have way more heat loss compared to ductless. I'm guessing the cost of running multiple condensers balances out the costs of heat loss, and the over production of cold air for rooms not needing it, with ducting systems. Thanks.

You should have a look at some of the LG and Mitsubishi units, there are several out there that have one compressor and 3 indoor units.
Dan

The mini-split airconditioning units here sell over the counter for around $400 to start and go up from there.  Any installer (unlicensed electrician) can install these because they run on 100V and do not require any additional wiring.  Typically, they are installed in every room of a Japanese house--because they are usually not insulated very well.  These are heat-cool-dry units.
On the other hand, split airconditioning units using a common compressor are readily available here as well.  The only difference is in the installation and planning.  A centrally located compressor unit would require longer runs for the pair tubing.  And the larger units require larger branch circuits--thus demanding the services of a professional electrician specializing in air-conditioning. 

There are pro-s and cons to both.  Both are available, prices vary drastically between units--a lot to do with product options.  I am sure your builder will help you to determine the appropriate unit for your needs.  My biggest objection to individual units is they clutter the exterior appearance of the home.  Even a very well thought out house design looks "Rube Goldgerg"-chuto humpa when surrounded with split-ac outdoor units.