My masonry stove heating hot water to 500 gal tank under basement slab -- pics

[mbryner]

Hey guys,

So after living in the new house for about 1 week, I can report some data and post a few pics of my weird hot water system.

We have a large masonry stove (thermal mass heater) from www.heatkit.com
The firebrick/refractory core heats a lot of exterior rock facing sourced from our land.
The firings are only a few hours long and it takes about 4-6 hours for the heat from the last fire to penetrate to the rock facing.
In the back of the firebox, there is a stainless steel hot water loop.  



You may remember I placed a 1000 gal concrete septic tank under the garage slab.   I've lined it with 6 inches of styrofoam and then 1.5 inches of R-max polyisocyanurate foam on the inside.  Then I painted the inside with 2 coats of Red Guard shower-pan elastomeric membrane and 2 coats of liquid silicone.    All the foam decreases the tank volume to around 500 gals.   Putting the foam on the outside of the tank would invite thermal bridging if the ground got wet (even when drained by gravel?) and 1000 gal + thermal mass of the concrete would be little too much BTU's heating load.

Here's a pic of the tank before the garage/basement slab was poured.   It has a 24" manhole on one end and 2" vertical PVC pipes for in/outs x 8.   You can also see the earth tubes made of 4" white PVC.



This is the view from the manhole into the newly sealed tank w/ silicone:



The water in the tank just sits there as a heat sink.   It doesn't go anywhere.   I added 1 gal of bleach to keep it clean.   Is that way too much bleach?  500:1 dilution ratio.   Didn't think about that until later.   Hope it doesn't eat at the silicone.   Arrggh.

Then, there are 3 loops entering the tank.  
1.   The masonry stove closed loop:  down from woodstove, into 50' of 3/8" copper coils in tank then to expansion tank, check valves, flow valve, and circulator pump.   I used a 20 watt Laing 12 v DC circulator.   The Eagle 2 differential temp controller is set to 12 degree differential and the pump is just plugged into it via a 12 V DC 60 watt LCD screen power supply.   (The fill valve is located behind the masonry stove, hidden behind the refrigerator on the other side of the wall.)  

2.  The second closed loop will be to flat plate solar panels on the porch roof, to be installed hopefully this coming summer.   They will have a similar loop setup as the woodstove.

3.  Domestic hot water enters the tank and splits at a T into 2 x 50' 3/8" flexible copper coils before coming back together at another T.   Therefore 100' of copper heating coils.   I also installed a bypass valve in case the tank is cold.

These pics show the inside of the tank with coils.  The woodstove and solar panel coils are on the bottom of the tank, and the domestic water coils are about 18" higher to take advantage of heat stratification.   The vertical white CPVC pipe just holds the thermister about at mid-tank level.







Here's how I formed the coils:



Pic of overview of the system from the mech room:   The manhole is on the other side of the wall in the garage.   Diff temp controller is blue box on the wall.  Grey box is the propane fired on-demand water heater (Rheem #RTG-84DVP Tankless Water Heater).   There are 4 pairs of PVC pips entering the tank through the slab looking left to right: 1st set for future solar, 2nd set for masonry stove, 3rd set sensors (one pipe used), 4th set for domestic water preheat.



Yellow valve is incoming cold water for domestic hot water.   You can see pipe going down into tank and then back up, as well as bypass valve:



So, here's prelim data.   Nothing too fancy.

I started making consistent fires in the stove on Monday Dec 27.  The tank temp rose by approx 10*/day from 50* initial filling temp, until it reached around 98* on Friday Dec 31.   Since that time it is plateauing.   Today before making the evening fire the tank was at 111*.   The circ pump is running constantly because the firebox interior is always much hotter than the tank water.  Even after being at work all day the incoming water from the woodstove was 120*.     If I take about a 30 minute shower to bathe the kids (3 y/o and 2 y/o like to take shower w/ daddy), the tank temp will drop by about 4-5* while preheating the water.   I have no idea how much I'm saving in propane yet because if the woodstove is being fired the tank is being heated!

Marcus

[Tom Reed]

Awesome you're in the house. Isn't it nice to have the job done.  We've been in ours 3 years now next month and some doors and trim still need to be painted. And IIRC you're still married too. May you and your family enjoy the home for many years to come!

[mbryner]

Thank you Tom.  Yep, still married.      It's been quite the journey.   Moving sucks, but we're getting there slowly.

We've been in ours 3 years now next month and some doors and trim still need to be painted.

If they're wood doors and trim, what does it matter!?     Haha.   Yes, we have some of that to do still, and the kitchen pantry and dining nook are still being worked on, but it was complete enough for county and bank inspectors.

Sorry, I thought I posted here that we moved in, but with all the moving I haven't been on this forum much in the past week.   Maybe I'll post a few finished house pics also when the weather gets sunny.

[WGB]

Great job Marcus!
Wish I was as far as you are.
You might like this, little late.

[mobile_bob]

Bill:

thank you for the pdf file!

can we put a copy down in the white paper section?

this is something that ought to prove very useful to many folks

bob g

[WGB]

Bill:

thank you for the pdf file!

can we put a copy down in the white paper section?

this is something that ought to prove very useful to many folks

bob g

Have fun, white papers would be fine.

[veggie]

Marcus,

That's really a wonderful system you put together there.
You obviously put a ton of thought and hard work into bringing all those ideas together.
Congratulations on that.
Keep us posted of the system's performance now that you are up and running.

veggie

[sailawayrb]

Hi Marcus,

Great looking setup 

Are you doing 12 or 24 hour fire cycles, what is burn time per fire cycle, and how many lbs of wood per fire cycle?  From everything I have read, using a masonry heater to heat domestic water is a non-starter.  If one's firing cycle is more than every 12 hours or one uses more than say 20/30 lbs per fire (approximately 1-1.5 hour burn time), supposedly one can expect to replace the masonry core on an annual basis because of excessive core thermal stress.  I don't have any first-hand experience and I would be very excited/gratified to hear that this is not the case.

Bob B.

PS -  BTW, do you recognize where this photo was taken  

[mobile_bob]

i got a question for you  fire box fellows

in some of my older books there are designs for admittedly large scale fireboxes, that are designed with castiron
interior walls that have grooves cast in where water pipes are then affixed and a layer of  fireclay is sort of grouted over

the system is called a "water wall" and from what i have read the lifespan is quite good, but then again they are meant
for a continuous fire application?

whatever the case, what if the fire box had removable sides and top sections that were made of cast iron, then one could remove
and replace them more easily that ripping out a bunch of brick and fireclay?

any thoughts on this sort of thing?

bob g

[mbryner]

Thanks everyone.    It's only been functioning for a week so time will tell.    

Bob B.:  Hey, that's easy -- it's the new Taprock restaurant down by the Rogue here in Grants Pass.     Was it good?  Is that your wife?  Hope you had a good time.   I've only eaten there once, for breakfast and it was pretty good.   There was a restaurant there before, but it had a kitchen fire about 2 years ago and they tore it down.   That place had a lot of character and you could sit closer to the water.  

I'm firing the stove 2 to 3 times per day at 30-50 lbs each time.   The wood is quite wet.   It's seasoned madrone and construction debris (i.e. cut 2x6's, etc.) but it was out in the rain and I don't have anything else.   Goodness, I hope I don't have to replace the core yearly, but the innermost core of firebricks are not mortared to the next layer so it is relineable (if that's a word).   The outer facing is getting up to 150+ degrees and the heated bench is up to 110-120 degrees (it all depends on how accurate those infrared thermometers are).   You burn it with the damper and flue wide open to get complete combustion.   It burns with hot flames for a few hours.   Then I damper it down when there are red coals, otherwise I'll just suck cold air in the air intake in the basement and lose the hot air out the chimney slowly.    Shouldn't get much if any creosote buildup that way.

The water loop is a 1" stainless steel coil in the back of the firebox.   The coil is U-shaped horizontally, cold coming in the bottom portion and hot in the top.  It exits through the back wall.   I'm getting measurements of around 140 deg at the hot side now that the tank is up to around 110 deg.   (Maybe the tank is hotter today -- I'll check when I get home from work later; still at the hospital, just taking a break.)    In this picture during construction you can see the double wall firebrick in the firebox and the water coil in the back.



Next I'm going to attach a thermister on the preheated water pipe coming out of the tank to see how efficiently the dual 50' coils are transferring heat to domestic hot water.   That will  give me an idea of how much propane I'm saving, or at least the # of BTU's saved by calc'ing the temp differentials.

The 50' coil seems to be very effective.   While I haven't measured it specifically yet, after the masonry stove loop gives it's heat to the tank, the pipe coming out of the tank feels about what the controller reports the tank temp is, and is definitely much cooler than the hot pipe going into the tank.

What concerns me somewhat is that the masonry stove still hasn't got the house up to more than 70* on the main floor yet.   Outside temps are in the upper 20's and low 30's lately.  Yes, I understand the concept of radiant heat and how you will feel warmer even though the room temp is cooler.   And it is trying to heat a whole 2600 sq ft house.   Upstairs is quite comfortable: in the lower 70's.   Downstairs the backup thermostat reports average of 64 deg but the walls are 68 deg when I measure w/ the IR thermometer.   We have a lot of windows, too, and no curtains or blinds yet (even if they are argon filled high effieciency windows it still feels cold next to a window.    We're still experimenting and I'm trying to keep the backup propane boiler turned off.

Marcus

[mbryner]

Hi mobile_bob,

You last post came in while I was writing so I didn't have a chance to address it above.   That type of firebox sounds very interesting.  I'll do some research on it, but it's a little late for me.   I didn't design the water coil in my stove.    It came with the package, albeit an add-on from Heatkit.   I don't think Norbert Senf would offer it if it were prone to early failure.   He is one of the leaders in masonry stove building in North America.      You'll notice though, in my stove you can remove the inner-most layer of firebrick fairly easily when necessary.

Marcus

[sailawayrb]

i got a question for you  fire box fellows

in some of my older books there are designs for admittedly large scale fireboxes, that are designed with castiron
interior walls that have grooves cast in where water pipes are then affixed and a layer of  fireclay is sort of grouted over

the system is called a "water wall" and from what i have read the lifespan is quite good, but then again they are meant
for a continuous fire application?

whatever the case, what if the fire box had removable sides and top sections that were made of cast iron, then one could remove
and replace them more easily that ripping out a bunch of brick and fireclay?

any thoughts on this sort of thing?

bob g

Here's a guy who did more or less what you are thinking.  Basically a standard wood stove that is surrounded by several tons of brick. 

http://www.builditsolar.com/Projects/BioFuel/DougMasonryStove.htm

He has other things on his site you might find intetresting too.

Bob B.

[sailawayrb]

Bob B.:  Hey, that's easy -- it's the new Taprock restaurant down by the Rogue here in Grants Pass.     Was it good?  Is that your wife?  Hope you had a good time.   I've only eaten there once, for breakfast and it was pretty good.   There was a restaurant there before, but it had a kitchen fire about 2 years ago and they tore it down.   That place had a lot of character and you could sit closer to the water.  

Marcus

Yeah, didn't think you would struggle with this one.  The food was OK, but I found the cabin to be more interesting.  Yup, the fine looking woman in the photo is my wife Gayle.  We happened by the Taprock for breakfast after fueling up before the long drive back to Seattle on the 28th.  Spent six days at our Evans Creek hide-a-way.

I'll comment more on what little I know about masonry heaters tomorrow.  I had a busy day at work today and will see worse tomorrow so need by bedtime 

Bob B.

[mbryner]

The builditsolar.com website is a great resource.   The admin, Gary, has even emailed me and our house is posted on that site as of a few weeks ago.

Here's his page on masonry stoves and woodstove water heating in general:

http://www.builditsolar.com/Projects/BioFuel/biofuels.htm#Wood

[Crofter]

Cast iron is a problem if it gets run dry or freezes. Stainless pipe much more foregiving, no rust, can be acid cleaned, resistant to flue gas acids, easily custom fabricated. Pipe can be run at much higher pressures compared to the flat cast "water wall system"

Depending on your system design and your water mineral content you may have a scaling problem. A closed loop running glycol may be necessary. In limestone country using continuous makeup water you have to be prepared to do acid descaling or your heat transfer drops away off. Whatever your system be prepared to dump heat and pressure in case you lose circulation or you have steam to deal with.