Water as mass

A little backstory first. I found the girl of my dreams to share my dreams with, but she likes her baths. Living on a mountaintop with rainwater as our only source of water, there had to be some give and take involved. I promised her a bathtub if she would limit her water usage. The deal was struck. Tossing around some ideas for the perfect solution, maybe I could kill two birds with one stone. So I make my inquiry here to get some feedback on my latest idea.

Instead of using cob for the mass in a RMH, could a bathtub full of water be the mass instead?

I was thinking of a brick enclosure around the tub large enough to satisfy the exhaust requirements of the RMH, and a lid over the tub that would line up with the enclosure to double as a bed for company and also as a seating arrangement for ourselves during the day. And an adjustable outlet on the enclosure lid to regulate the humidity level in the house.

I know there's more involved than this simple description, but before I set out designing it I thought I'd get some feedback first.

Thoughts?
As always, thanks in advance.
Bill

In a word, no.

In two words: Legionnaires Disease

This is a air borne, lethal disease that stems from heated water that causes a certain type of bacteria.

There are ways of treating water sir. Think Hot Tub. And the water would be changed weekly I should think. More often during rainy weather when there is an ample water supply.

Hi William;
Congratulations on finding the girl of your dreams!
And then ... convincing her to  move to the top of the mountain! Good Job!

About your idea.
I think you would be happier with an outside rmh in your new bathing room or your new outdoors hot tub :)
Makes installing drain lines much easier, and any issues with open water / humidity indoors are eliminated.
I think  building a traditional rmh / couch indoors would keep your gal happy as well!

It might be worth looking into Japanese baths.  They keep the bath full for days at a time and reuse the water for many baths and usually keep it quite warm.

I have no idea how they keep the water safe.

Another idea is something like these new saltwater hot tubs.  No chemicals, just a lot of saline water or something.  Be like bathing but like a spa at the same time?  

Yes William, I do congratulate you on getting a great girl.

I had a couple of false starts before I met my wife Katie. I often feel saddened this time of years because I see so many people looking for their next love. It was great to hear that you have found yours.

Hot Tub or not, I do wish you the best.

I would heat a large reservoir of water much hotter then the bath and pour myself a bath when I wanted one with water that absorbs heat from it but does not come in contact with it. Heat exchange instead of heating the tub itself. Drain the water to water a garden or flush a toilet... Hold on tight to a mountain loving lady they are hard to come by.
Cheers,  David

i remember a few years ago permies member "f-styles" was running his rocket mass heater exhaust through a residential hot water tank somehow.  i don't know if he still uses it or not, but the tank was basically his mass "bench".  he said it stayed warm rather than hot but im sure you could improve the temps if you wanted to.  then you could fill the tub from the residential plumbing as usual.   otherwise i like outdoor hot-tub idea.  would definitely want a good drain and thus a faucet or way to fill it

Yep. Rocket stove exhaust through a residential/commercial gas water heater (or fabricate your own heat exchanger and pipe it through a storage system of some sort--old water heater tank) seems to be the best alternative. If you set it up correctly the tank heating can be done with thermosiphon -- no pumps -- and let gravity work in your favor.

Following the simple no answer.

There is a simple reason behind not using water as mass, besides legionella.

Heat transfer rate. Water heats up really well, but also does it cool.

And it can't be heated more than 100C°.

This means that the 350C° gases out of the barrel, will boil the water. if there isn't enough. And if there is too much, convection will cool the rest of the water, and you'll have a mass that is not optimum, and cools too fast.

i'm not sure if he's still using it, but permies member here F-styles did it. and used it.  maybe he's still using it today.  i've heard it can't be done but also i've seen it been done.  he has pictures and an article all about it on here.  i think the article is called "rocket mag stove" or "using water as a mass" or something of that order.  you can also search for "f-styles" in the forums and i'm sure you'll find it.  you might have to build one of those primitive technology hot tubs outdoors :P

Thank you all for the insight to my query. I had forgotten just how hot the exhaust gasses can get, so thank you Satamax for the reminder. And because I'm no engineer and also don't fancy the idea of going up in a violent water tank explosion from my lack of knowledge in that field, I guess I'll scrap the idea and resort to the more traditional use of an electric water heater, to my Stacy's delight.

It'll be two years yet before we move up to North Central Tennessee to our mountain retreat, but Stacy visited the property with me not long ago and fell in love with it and the remoteness of the mountain. I had to make some concessions for my city gal to live so primitively on my mountain, but not so many as to give up what I've strived so hard to achieve building my dream. I am one lucky guy! So thank you all for the kind words and encouragement. I've waited twelve long years for her to walk into my life. All seems right with my world. I am blessed indeed.

i have a newer 40 gallon propane hot water tank.  it has a violent fire underneath, and it literally has a chimney heat riser going right thru the center of the residential hot water tank.  they are designed to hold residential water pressure an also designed to be fired.  i've always dreamed about running a rocket exhaust through that residential water tank stack, after the bell, the exhaust gasses would only be 200 degrees and thats far lower and safer than a an intense propane fire which is normally diretly under the tank and the hot gasses travel directly up the tank center.  if you're referring to flashbang that occurrs in a small coil or copper coil, but if you use the normal residential propane hot watwr tank method, which ive been dreaming about for years, you wont have the flashbang issues.  thats why there is no coils in a residential propane hot water tank, it heats a large mass slowly instead of heating a tiny amount in a tube coil.  

this is how they work, the "TP Valve" is a pressure relief valve, they are found on every hot water tank , they will release pressure before any emergency arises, and they are threaded, so if you are concerned for any reason you can thread a garden hose onto the relief and run it to a drain.  that likely wont be necessary because this is proven technology.  that's what the relief valve is for, but i've never seen one actually need to relief, but it's there just incase, just specifically for that reason :)  

Where there is a will there is a way  I also live up on top of a rather large Hill not sure I would call it a mountain but I have a well that goes down almost 300 feet and a girlfriend crazy enough to put up with me and our off grid living so I feel your pain and love to at least some affect that said I would suggest a addition type room off the side of the house something like enclosed gazebo to be used as a Jacuzzi house I'm not sure what your climate is like where you're at though homemade solar water heaters are fairly easy to make as well if you have the climate for it  I personally would suggest using a secondary miniature rocket stove just for heating the bath or some kind of dual exhaust where you can swap the exhaust of your rocket stove on the fly to move between your conventional bench and you're jacuzzi or some combination thereof that would allow you a little more control over the water temperature but I definitely suggest a separate room for a Jacuzzi as moisture issues can become a problem very quickly     also if you have the space for a big awesome shower you could do a small one or two person jacuzzi inside the shower with a retractable granite lid allowing the jacuzzi to double as a shower bench

the idea i had was this

Plenty of people proposing this type of water heater. But, so far, i have only seen one of this type in use. And no feedback on it's performance.

Well designed water heaters are a few and far between.

Geoff Lawton's water heater. I'm aware off.

https://www.youtube.com/watch?v=1oDpmmsqHwQ

Crude design, but IIRC, it functions.

This one, i don't know much about.

http://batchrocket.eu/en/applications#chb



And there is this thing, which i don't know much about.

http://batchrocket.eu/en/applications#pizza