Sauna/Showerhouse Design Ideas

[generalstark]

I am presently designing a sauna/showerhouse to be built next summer.  The structure itself will have approximately 100 sq. ft of interior space and will be constructed with cordwood masonry on a floating concrete slab.  It will also be partially earth sheltered.  The space will be divided into two rooms with one being the sauna and the other being the shower/changing room.  The shower/bath  will be a cast iron claw foot tub.  The structure will be sited next to out 24' diameter yurt that is heated with wood and has no running water.  We presently carry water from a nearby well but are in the process of protecting a spring from which we will have a gravity fed supply.  This will not be plumbed directly into the yurt, but to an outdoor freeze proof spigot, and eventually to a cistern that will feed both the yurt and the shower.  We are located in NW Vermont where it gets quite cold in winter.  We have good solar exposure in the morning and early afternoon.

My goals for the design include:

-Simple and cheap systems that are easy to maintain.

-Radiant floor heat for the sauna/shower house.  We will also have a wood stove for heating the sauna room.

-Hot water for the shower.

-Solar hot water/propane and or wood heating for the radiant floor and shower hot water.

-Solar electricity for lighting and pressurizing water for the shower. (the spring will not provide enough pressure)

I have found a source of used solar water panels that I can get quite cheap, but I would like to be certain that these would be the best/cheapest way to heat water for heating and the shower.

I am looking for any advice for designing this system to be cheap and simple and easy to maintain.  Any help would be greatly appreciated.

[bob golding]

have a look at build it solar.com. they have a very good article on making a solar thermal panel using PEX tubing instead of copper. lots of other good stuff on thermal solar as well.

bob golding

[GaryGary]

Hi,

I'd have a look at this system:

http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm

The way that I use it, its just producing domestic hot water (showers etc.), but the collector area and tank size could be expanded to do what you want to do.

Its a very simple and inexpensive system. You could build the collectors as shown, or, if you have a good source of used collectors you could use them.

It seems like the large and non-pressurized tank would work out well for your needs.  For the shower water, you could use the large PEX coil heat exchanger.  For the radiant slab floor (which I really hope is well insulated underneath!) you could either draw hot water directly out of the storage tank and circulate it through the floor loops, or use a copper pipe coil heat exchanger.  If you want to collect heat from other heat sources (like a wood heater), you can pick up heat from a coil in the firebox and circulate this heated water through a pipe coil heat exchanger in the storage tank (just be careful to include a pres/temp relief).

While the tank construction may not look like a shiny stainless steel tank, these box style, EPDM lined tanks have a very good track record.  I have heard from people who are just getting around to replacing the first EPDM liner for a 1980's tank!

They are cheap to build in large sizes and you will want a pretty large one.  For what you want to do, you will also want a lot of collector area.

Its unlikely that unless your building and slab are very very well insulated that it will be practical to heat entirely with solar, so you will need backup heating sources, and using this large central, non-pressurized tank makes it easy to add heat from other heat sources to the common storage tank.

If you make the collectors like the two designs I show, I recommend that you use the copper tube, aluminum fin design rather than the PEX-AL-PEX, aluminum fin design.  The copper is more forgiving of high temperatures that might come from stagnated collectors, and with the price drop in copper, and the better performance of copper, there is not a lot of money to be saved using PEX.

Between the collectors and the rest of the system, there have grown to be a lot of pages, and its getting a little hard to wade through -- this page gives a roadmap of all the pages, and makes it easier to not miss something important:

http://www.builditsolar.com/Experimental/PEXColDHW/RoadMap.htm

Don't know if you have thought about this, but the project may qualify for the 30% solar tax credit.  If so, you would want to get those used collectors if they are SRCC certified -- the certification is "required" for the tax credit.

Gary

[generalstark]

Thanks for the info!

It does seem as though the insulated heat storage tank would be a good option for this design.

My questions are regarding space.  One design requirement is to keep the interior square footage of the sauna/showerhouse at about 100 sq. ft.  I plan on locating all the systems for the shower hot water and radiant slab (which will be well insulated) inside the structure, most likely under and behind the benches in the sauna.  Clearly, space is limited.  

How should I calculate the size of the tank?

Initially, I would probably include a coil heat exchanger for the shower water, and the radiant floor.  If the water in the tank is the water that circulates in the solar collectors, how could I easily add additional heat to the tank water from a wood stove or propane on demand water heater?  A heat exchanger or directly?

Matt

[juddley]

Hi Matt,

I did a remodel on a 10 year old wood fired sauna. Problems were>

1. Original structure had no stub (stem) walls on top of Concrete slab floor to keep wood above floor level. Everything had rot because a Sauna has huge amounts of condensate inside.... even more with with a wood fired.

   
   

2. The stove access door was located inside of sauna.... thus ashes, wood, had to be carried through the entry door and made a huge mess. We converted to a unit with firebox access on outside of building.

   
   

3. Had to add external combustion air port through the masonry wall because whomever build it forgot to do so.

   
   

4. Interior surface has to be water proof on an outside free standing installation in order to keep the walls from absorbing way too much condensate..... unless you plan to keep it fired all winter long. The mold that grows deep in the walls is really hard to get rid of.

   
   
   

Most Saunas are wood lined and if I were going to do one new from scratch I would make sure there is stringers behind the wood with vents at top and bottom of each stud cavity.

We later added a vent in the ceiling for fair weather times to "Air Out" the inside in good weather.

And finally.... the Electrics/Controls.... Make sure to buy the absolute best damper, thermostat and air control you can get. Make sure it has a safety "fail safe" for "Over-Temp". Wood burning is very hard to regulate and the inside temp can be deadly if not closely regulated. It is no longer legal in my state to use wood fuel for a sauna or steam bath. Also..... though likely not relevant, they are not allowed on many Homeowner Policy's.

As an extra safety device I added an audible alarm for 130 degrees F (Wet Sauna) just in case all else failed and the owners fell asleep....

Good luck with your project.

Judd

[scottsAI]

generalstark,

Northern climates the slab edges must have a foundation extending below frost line.

http://home.howstuffworks.com/home-improvement/repair/house4.htm

Insulate outside of this foundation, cost less by using less... maybe. (depends how deep)

Since you must dig a foundation, why not put the tank below the changing room?

Save space. Tank must be insulated, triple use the foundation/tank/insulation into one!

Benefits: Tank warms area above, more floor space above.

Make the solar heat collector as GaryGary suggests into the roof, not separate saving money.

Sister wanted something like yours, called it a bath house.

The Sana was convertible into a jacuzzi plus a windowed view (special door).

The Sana floor is the jacuzzi cover.

Housing market crashed before they sold, did not get to build it.

Have fun,

Scott.

[GaryGary]

Hi,

Collector and tank size are not that easy to determine precisely, but a rough way:

Estimate the heat loss using this calculator:

http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

Get your average temperatures from weather.com, and plug these in as the Design Outdoor Temperature -- then the in the output, the "Design Heat Loss" will be the loss at that temp in BTU/hr.  Do this for a typical day for several months of the winter to get an idea what your heat losses will be like through the winter.  Don't use extreme record temperature -- just the historic averages.

For water storage, each gallon of water holds about 8 BTU per degree of temperature change.  So, if you have a 300 gallon tank, and you can take it up to (say) 150F, and extract heat down to 90F, then it is storing (300 gal)(8 BTU/gal)(150F - 90F) = 144,000 BTU -- about equivalent to 2 gallons of propane burned in an 80% efficient furnace.  

On a sunny day, you can roughly figure that each sqft of solar collector will add about 800 BTU to the storage tank.  About 1.5 gallons of water going from 90F to 150F will store this amount of heat -- so, you need about 1.5 gallons of water storage for each sqft of solar collector to store one full days worth of sun from the collector.  Most people use 1.5 to 2.0 gallons per sqft of collector.

Your slab will also store a bit of heat, but you can't vary the temperature very much.

For the water heating for showers, most of the rules of thumb say about 20 sqft per person per day -- that's showers plus other hot water needs.  

Or, just looking at showers, a 10 minute shower at 2 gpm at 110F with 50F ground water requires about

(2 gal/min)(10 min)(8.3 lb/gal)(110F - 50F)(1 BTU/lb-F) = 10,000 BTU per shower.

So, for 4 showers a day, you would want to add enough collector to produce 40K BTU per day over the space heating -- plus any other hot water needs.

You might be able to figure out a way to extract some of the heat remaining in the shower water before it goes down the drain.

so, as a very rough cut, with a 10X10 building, R40 ceiling, R30 walls, some windows and floor losses, and 0.5 air changes per hour, with an outside temp of 30F, your heat loss might be about 4000 BTU/hr, or 100K BTU/day -- add another 30K BTU for hot water needs, and this is 130K BTU per day.  

On a sunny day, (130K BTU/800 BTU/sqft of collector) = 160 sqft of collector.

A tank size of (1.5 gal/sqft)(160 sqft) = 240 gallons.  

This (very very roughly) would take care of needs on a sunny day and through that evening and into the next morning a bit.  If you want to account for some cloudy weather before going to backup heat, you need to add more collector and more storage.

I guess you would have to think about whether a tank sized for solar heating (as above) would also work for your backup heat.  Seems like it might?

For adding heat to the tank from the wood stove, I think another copper coil heat exchanger would work.  Be very careful about using and maintaining a pres/temp relief valve -- steam is like dynamite.

You might be able to just circulate water from the tank through the floor loop -- thats what I do:

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

These heat exchanger coils tend to be bulky, so you will need a fair bit of tank size just to get the HX coils in.

The demand water heaters are good for boosting tank water up to some needed temp.  They can be set to only add heat when the inlet temp falls below some level you set.

One thing you will find is that more insulation and better sealing will make your solar go further, and will also reduce backup heat load when you need it.

Gary