Please critique domestic hot water plan

[mike90045]

All,
Asking for some advice on what I want to be "self managing" low energy, domestic hot water system.  Off grid, so electrical consumption reduction is very important.

Let me explain the layout of the attached sketch (.jpg)

Cold water would first enter a heat exchanger loop, in the flue, above the secondary combustion chamber, of a site built masonry heater. http://www.tempcast.com/   The loop is 3/4" SS high pressure pipe, installed a few degrees off horizontal (to initiate thermosiphon) it would connect to insulated hard copper pipe, and travel to a 2nd floor, 85gal  storage tank, located nearly directly above the heater.
The proposed tank looks like it may be a RHEEM Marathon Thermal Storage Tank #MTS852000. It is a plastic (polybutene & filament wound fiberglass, 170F limit) tank with a layer of insulation (R value not stated) and an outer plastic shell, it also has several ports in different areas. Manual at:
http://globalimageserver.com/fetchDocument.aspx?id=7db02d15-1283-4cdf-a3fe-7ab73aac67e3

In the winter, I've been told, each firing of the masonry heater, can be expected to transfer 30,000 BTU to the water loop.  The heater can only be fired 2x a day, 12 hours apart. I calculated that 30K BTU into 85 gallons would be a 44F rise of the tank temperature, per firing. Right ?

I believe, with 45 bends, and not 90 elbows, this would self-thermosiphon. Anyone know how to verify this, before it's built, and found not to? I don't want to have a circulation pump if I don't need one.

Then, this tank-o-warm water, would feed the inlet of a rooftop solar water heater, which in cloudy winters, not provide much gain, but I'll take what I can get.  It could be bypassed if it looses too much heat, or hopefully, a bit of gain is to be had.  The Rheem Solaraid rooftop unit is supposed to have some sort of high temp valve (according to the plumber) that "burps" a slug of too hot water, to keep it's temperature within limits.

Output of this, finally, feeds the high efficiency, tank-less water (which has a 2 gallon tank in it) and a 2nd generation controller, supposed to deal well with any inlet water temps, and prevent the "cold-slug-o-water" in the middle of your shower.

Then the faucet recirculating hot water scheme, with a return line run from the most remote faucet.  Some brand of pump.  Taco is mentioned a lot. Grundfos and Wilo also supposed to be OK.  Taco has a "D-Mand" system with timer, thermostat, remote.  Who makes a good pump, that's easy to rebuild ?  Taco, Grundfos, Wilo ?   Magnetic coupling - pro/cons?  Bronze vs. SS ?
Taco has a $30 "aquastat" for cycling the pump off/on after the timer kicks it on, so once the pipe heats up, it can save a few watts. Any idea if these work reliably, or if they are always failing.

Open system, no inlet check valve, about 70psi water pressure, fed from elevated tank.

So, there is a bunch of questions, and I hope to get some critique and/or advice, before I go down the wrong path too far.

Thanks,  Mike

[LowGear]

Hi Mike,

I'm curious why your just not using a few / many loops of soft copper for the collection and thermosiphon part of the system.  No joints required even with 1" material.

Casey

[mike90045]

Hi Mike,

I'm curious why your just not using a few / many loops of soft copper for the collection and thermosiphon part of the system.  No joints required even with 1" material.

Casey

I'd understood from the bldg department, that the heavy wall, underground rated (I forget if it's L or M) was needed to "protect the room occupants" so a blowout would happen someplace else !

[dieselgman]

I wonder about your pumped "recirc loop" and how that would work with a tankless "on-demand" type of water heater.

Also, the variable temps feeding that tankless heater would require some serious engineering to achieve a stable output temperature - I guess 'second generation' means it has a very smart controller?

Off-grid hot water systems I have seen generally have not been too great with electric hot water heating, what is the heat source in your tankless unit?

Very nice schematic by the way! 

dieselgman

[BruceM]

Mike-

My understanding of the masonry heaters is that you get a very hot, short time fire; this makes me worry about the thermosiphon being able to handle enough flow to do the heat transfer to the tank.  You might be better off with a small DC circ pump since the run time is limited, and you could then save bundle on copper.  I am a fan of simplicity too, so if enough data could be had to support the use of the thermosiphon, I'd do it too. In either case a pressure and temperature relief valve on that loop would be wise, in case it turns into a steam generator. That would also eliminate the issue brought up by the inspector, and I think his input may have been baloney.

I suggest using manual control (w/ timer relay?) for the recirc loop; then you can just push the button at the end use point, wait for the indicator light to go out, and have hot water. Automatic systems tend to eat a lot of heating energy.  You will have to use a substantial pump, as you must meet the flow requirements of the chosen tankless heater.  They will cause a substantial head loss.  Look carefully at the AC or DC power requirements of the tankless (I assume it's propane fired.)  I haven't looked at them in 7 years, but the ones that could do what you wanted then could be a significant phantom load on your inverter system, all night long. 

Its very nice to see someone doing something smart about saving energy.

Best Wishes,
Bruce

[Tom Reed]

Hi Mike,

A couple of things in the design that concern me. Since is sounds like you'll be getting some fairly hot water out of the masonry heater there is a possibility to over heat the storage tank. Also sending heated water to the roof mount solar will drastically reduce its efficiency and in some instances heat may be lost. And the 40w load for the "tankless" unit is a large demand especially if it's 24/7. 

On my system I have an electric 50 gal hot water heater feeding into a LP fired heater. When the charge controller goes into float the electric heater is run on 120v to preheat the water into the LP heater. The LP water heater uses no electricity so it is not a load during no sun periods.

In your situation I'd plumb the masonry stove into directly into a 100 gal solar storage tank with electric backup with a thermosiphon loop. Connect a thermal solar collector into the heat exchanger in the solar storage tank. I'd go drain back, but you can go glycol this way too. When the sun warms the panel, you'll also have power for the pump. Feed the hot water from the 100 gal tank into the cold inlet of the LP fired one to add any additional heat needed. Optionally add a mixer valve out of the LP heater to prevent scalding.

[LowGear]

Hi Mike,

Take it from a high classed slum lord and one of the cheaper guys (as has been testified to here on this blog) on the earth that using the thin-wall or "Save two bucks a stick" copper is not the long term choice of wise people.  If you plan on living in the place for more than five years buy "L"!!!

http://www.plumbingsupplyhouse.com/bathroom-fixtures/different-grades-of-copper-pipe.html

And so the building department requires that you do it right.  My question is still how come you're not using soft L grade copper for your heat exchanger loop?

Tom has a point.  40 watts for 24 hours is almost one KW per day.



Casey

[BruceM]

I didn't notice the 40 watts on the tankless. Hopefully that's a peak power for fans or   Better check to see what the "standby" current draw is, 'cause 40 watts is too much for a 24/7 load.   A PIC microcontroller needs about 0.006 watts, even a power hog controller would only be 20x that. 

[mike90045]

Thanks all. 

The fellow from the masonry heater company claims that the thermosiphon to a 2nd floor tank works well, and has never had troubles with it.  If my calculation of 30K BTU = 44F rise per burn, is right, then even if I had 2 firings , and never used any hot water, the 85 gal plastic tank might get to 130F (40F feed water in winter, +44 + 44 per firing = 128f)   So i think i still have some margin, since the side port feed for the thermosiphon loop, is only at the top 3rd of the tank, it hopefully will mix a bit and not cause a superheated slug of water at the top.  I guess I could plumb in a 150 deg thermostat at the hot inlet, and have that trigger a recirc pump.
I'd be feeding plain domestic water into the tank/thermal loop, and then that preheated water to the roof tank, which has a commercial heat exchanger fluid that does not mix with the domestic water.    The plumber has been using the Rheem rooftop heater with the Eternal LPG fired tankless, and it's smart enough to only burn the fuel it needs to.  It has a blower for forced draft, so I'm hoping the idle draw is much less than the 40W, which includes the blower.  It's also  rare hybrid, with a 2 gallon internal tank, which is suitable with hydronic heating and instant hot at the tap circulation schemes.  "Easy integration with recirculation and no
startup lag eliminates cold water sandwich"  (oh specs:  Electrical Consumption Standby 8w, Max 45w   - so that's "only"  192 watt hours idle, per day)
http://eternalwaterheater.com/   http://eternalwaterheater.com/pdf/downloads/2011%20Eternal%20Spec%20Sheet%204.20.11.pdf        Taco has a "D-Mand" system with timer, thermostat, remote, which looks pretty interesting, but I was hoping to locate some feedback - didn't seem to see any on line.   

My question is still how come you're not using soft L grade copper for your heat exchanger loop?

not sure I understand this, as copper inside a firebox would not last long.  The heater companies all use stainless  http://www.hilkoil.com/ for the firebox loop.  The loop is domestic water, at about 60 psi, and thermosiphons to the 2nd floor storage tank over the heater. There is no "exchanger, it feeds into the tank, either side ports, or Tee's at the hot outlet, and drain, if I used a conventional tank.

Good feedback.  I need this BEFORE I buy anything.

[LowGear]

Hi Mike,

The heater companies all use stainless

Good enough for me.

Casey

[akghound]

From my experance I would bet the top of you tank where the hot water resides (from the thermosiphen) could reach over 200*(f). Keep in mind that calculations and real world reality can be far far apart. Just saying ....... things could get HOT. I wonder what it would be like if a tank on the second story would fail.
Ken Gardner

[mike90045]

From my experance I would bet the top of you tank where the hot water resides (from the thermosiphen) could reach over 200*(f). Keep in mind that calculations and real world reality can be far far apart. Just saying ....... things could get HOT. I wonder what it would be like if a tank on the second story would fail. Ken Gardner

Yes, that's a concern.  I've not been able to find out from Rheem, what temp their safety valve blows off at.   And they do recommend the drip tray, and I'd not have it any other way.  So, maybe it's a generic cheapo electric water heater with blanket, pull the elements out, and I have side taps.  Change it out when ever it starts leaking.....

[BruceM]

That's good news on the thermosiphon, Mike.  There's so much less design risk when you can get performance info on similar systems.  You can probably save some watt hours on the almost tankless heater by shutting it off at night, but your PV/battery/etc. system may be big enough to laugh that off.   

What's the freeze-proofing scheme on the passive solar water heater/tank on the roof?  I had a Sola-hart system in Gilbert, AZ.  Loved it.  It had electric heating for freeze proofing, but it was rare to need that in Gilbert.  Where I am now it's quite cold in the winter, so all the systems are active drain down or antifreeze and heat exchanger systems.

Best Wishes,
Bruce

[mike90045]

....  What's the freeze-proofing scheme on the passive solar water heater/tank on the roof?  I had a Sola-hart system in Gilbert, AZ.  Loved it.  It had electric heating for freeze proofing, but it was rare to need that in Gilbert.  Where I am now it's quite cold in the winter, so all the systems are active drain down or antifreeze and heat exchanger systems. 

It's made for city folk, and has an internal 1500 watt electric heater ... so you never run out....  Ha!  I only have a 3Kw PV array, which will suffer in the cloudy winters, location and climate a little bit wetter and cloudier than Tom in Hopland.
It's got a exchange fluid rated to -something.  Location 100 year lows are only about +18F, so that leaves the inlet and outlet susceptible, any usage of hot water, brings tempered water up thru the pipe from the warm storage tank, which may actually thermosiphon a bit in the single feed line. 
Currently, I'm counting on the average " low " of +25, coupled with it's 45gallons of warm water, and occasional usage, bringing another slug of tempered water up to the tank, and keeping the tank & lines from freezing.  I'll have to install a couple thermocouples at the inlet and outlet, to see whats happening, and maybe have to drain and bypass it for the winter.    I'm really trying to work all this out, so it's not dependent on timers, sensors, drainbacks, pumps and such, just the electric and LPG supply for the tankless.   Maintaining the PV, 'roid and batteries is going to be enough, without wondering if a bunch of other stuff is going to need daily or weekly fussing with.  I guess if push came to shove, I could plumb in a recirc pump on a timer or sensor, so if the inlet got down to 34F, it'd switch on and pump a minute or two from the 2nd floor tank to keep it frost free. (violates my guideline about relying on sensors and pumps)
  More thinking must do !

[Tom Reed]

I like the fail safe design of drain back systems. You also won't have 45 gal of water on the roof getting cold and never need the heater element. Yes it does take a bit more energy to raise the water to the collector, but that is small and can be provided by a solar panel that gets sun when the collectors do.

If you keep up with the anode a good glass lined steel tank should last a lifetime, depending on how your water is. Glad you're making progress on the place. Perhaps on one of your trips through we can get together again. There is another board member here in town you'd probably enjoy meeting too.