Please critique domestic hot water plan

[BruceM]

Hi Mike,  I read your post and looked at the manufacturer info on the proposed roof mount solar system.  It should work, given your climate, but I wonder if you could save some money and get a better, simpler system by using an active solar panel and circ pump.

Circ pumps are very reliable, in my experience.  There are some nice ones for direct solar driven use too. They use no timers, sensors, etc., they just pump when the sun shines on the small PV panel; maybe this would meet your design critieria.

I don't know what's out there for simple, failsafe drain back systems. They might be good. Or they might violate your design principles too much. 

I think I'd try to have just one hot water storage tank, and let the solar hot water panel feed that as well.  I'd even look at some of the very big tank propane water heaters that are designed with two sets of heat exchange coils, and maybe eliminate the tankless heater.  It might solve 3 problems with just one tank/exchangers/heater.  Just thinking out loud, I haven't studied them to closely.  You need to be able to turn down thermostat for the propane at various times of the day, to make it wait for the solar or fireplace heat.  But maybe if the temp is just let very low that would suffice, if the tank is large enough.

Best Wishes,
Bruce

[mbryner]

Hi Mike,

We're off-grid w/ LP on-demand water heater and a masonry stove w/ water loop.   If you search through some threads from about a year ago, I detailed my experience.   We have the HeatKit masonry stove which is similar to the TempCast.  

There are some things about your sketch I really like: the thermosiphon loop!   And there are a few things I have concerns about: hot water out from the water tank into the solar panel before going into the water heater.   You'll be losing a lot of heat through the panel in winter.   Lets suppose you take a shower in the evening or morning before the sun is up.   Then you are taking all that pre-warmed water from the wood stove into a cold dark panel, cool the water down and lose the heat to the outside air, then into the LPG water heater where it is rewarmed.   Unless you specifically bypass it manually every time.   Or you use a sensor and diverting valve, which violates your rules.  Maybe I missed something in the sketch.   I also worry that you will overheat your tank w/ the woodstove unless you use the water and constantly watch the temps.    Don't worry too much about the "cold water slug".   It's blown out of proportion.   Do you turn off the shower while bathing?   Our house has all copper pipes.   Sure it takes a little while to heat up the pipes before a shower, but once the pipes are hot if you turn off the hot water it will still be hot when you turn it back on and you won't get much of a cold sandwich.   Same in the kitchen.   Personally, I wouldn't worry about the hot water recirculator.   Seems like a lot of watt-hours to keep the water hot all the time.

It takes about a week to get hot but our masonry stove keeps a 500 gallon tank 100-110 deg constantly with at least 2 people taking daily (long) showers.   This violates your guidelines, but I use a Laing DC circulator pump and diff temp controller, because there's no way to build a 500 gal water tank above the masonry stove.  (i.e. http://www.altestore.com/store/Solar-Water-Pumps/Circulating-Pumps/Laing-D5710B-PV-Circulating-Pump-threaded/p6064/ ).   I built the tank myself out of a concrete septic tank, lined inside w/ 8" of rigid foam and sealed w/ silicone paint.   There are 3 heat exchanger loops inside the tank -- all of them are built of flexible copper.   1 for the masonry stove, 1 for a (future) drainback or antifreeze solar panel, and 1 for the domestic hot water.   Here are some pics:  http://www.docbryner.com/mossy_hollow/Thermal_Storage.html

Our water heater is a Rheem tankless LPG model.   IIRC it uses about 80 watts when running, probably mostly for the blower.   It handles the pre-heated water from the tank "OK".   If the water in the tank is too close to 120 deg, the water heater doesn't turn on, and there's no way to turn the water heater thermostat up to 140 because it's "locked out" unless you get the commercial model.  

So far, I've been quite happy w/ the setup.   Over 6 months, we only used 60 gallons of propane.   Water heater, clothes dryer, cook stove, oven, backup house heat.   But then winter came and Christmas, and I had to turn up the backup heater to keep the house warmer w/ grandparents here and not keeping the masonry heater hot enough, so we used a litte more than planned.   That's my setup.   Take from it what you want.  

Marcus

[mike90045]

All, this is so great.

I'm hearing a lot about using a drain back system.  I have space to put a ground level tank, but the solar pump would have to be stout enough to lift to the top of a 1.5 story A frame style roof, nearly 35 feet.  I do have a spare 200W pv panel I could put on it.

I've not looked at any gear for drain back systems any suggestions of a great vendor to look at.

[BruceM]

Wonderful setup Marcus. I know Radiology is the most technical side of the medical field, so I'm not surprised, but I think you'd make a great engineer. Your system is impressive, and so are your hands on capabilities.

The big plus of drainback systems- no double wall exchangers are required, since it's all potable water. Just a loop of copper pipe in the tank does the job. With antifreeze, most codes require a double wall exchanger. I'm still stewing over drain back vs antifreeze for my new house.

I haven't built the solar hot water system for my new off grid place yet- I built the backup propane system so I could move in faster, and ran out of money. (My older home is active solar heated.)  Enthusiasm also waned as copper prices soared and I found that with a super insulated home the payback was getting out there quite a while. (I did pre-plumb for the system, and have the entire south wall of the shop available, along with space inside the shop for a 600 gallon storage tank.  I hope to start on the solar this summer if I do sell my old house next month as it appears I will.

I have a manually controlled Laing 12V pump for my long insulated loop to the house bath/kitchen. If the hot water is cold, you just push the button next to a loop of copper that sticks out of the wall just under a counter top.  The button is located such that it's a one hand operation. When the pipe gets warm, you let go, and now you've got hot water with no wasted water.  I do agree with Marcus- if the run to the heater is short, and the pressure higher/pipes smaller (smaller pipes less volume to throw away), it's probably not worth the bother.

Of the commercially available DC circ pumps, the Laing is the best quality overall, I feel.  I reverse engineered the Laing D5 and the El-Sid 10 & 20, and use my own custom electronics drive for the El-Sid 10 watt pump for my primary heat system.  I use a Laing D5 for a manually controlled hot water loop. The El-Sid 10 watt (only that model) is the quietest in conducted sound, as it has twice as many magnetic poles (and twice the drive circuitry) as the 20 watt. The 10 watt unit was an earlier design, and on the 20 watt models Juniper Labs found they could cut the poles in half by adding a permanent magnet glued in place to insure a known stop/start location of the impeller. With fewer poles to drive with, there is much more noticeable noise in the water stream, even after 75 feet of insulated Pex in the ground, and rubber hoses isolating the pump!  The El-Sid pumps have very simple, primative electronics that cause what is known as "shoot through" on the H-bridge drive of the coils and generate very bad EMI.  The Laing pumps have sophisticated electronics, and are the best of the commercial units for EMI.  They do have more conducted audio noise than the El-Sid 10 watt pump.  

I'm going to have a look at drainback systems myself.   The designs that just use a big pump to drive them are simpler- if the pump stops, the whole thing drains, period.  There is beauty in simplicity, but it means a bigger pump to be replaced more often, and a lot more PV to drive it.  

I might add the complexity of dual-operation; an RV type diaphragm pump to get the panels filled, and a small circ pump to keep it going. Drain back for this type of system requires a high point air inlet solenoid valve.  The system must also have an air purge valve and small pressure tank.

I'll have to look around for more info on these systems.

[mbryner]

We're sitting in the airport, dropping off our exchange student, so texting from phone, arrrghh.

Thanks Bruce!  Yes, I prob would have liked engineering.  I was a biophysics major.  Basically a bio major & physics major, w/ lots of calculus.  Still, I didn't do many calcs on this.   Mostly intuitive.  

You mention having to have a double wall exchanger by code.  Yes, my workaround is having the all the exchangers in the tank, w/ the stagnant hot water in-between.  Very easy rule if you build your own non-pressured tank and use flexible copper.  

Mike, you don't have to have a strong pump to pump 1.5 stories up.  You just need it strong enough to pump up from where the small drainback tank is located.  I.e. in the attic.  The weight of water to pump is only the difference btwn level of the drainback tank & solar panel.  Doesn't matter how long the circuit is, within reason of course.  (we're talking low flow rates & assuming low resistance)

Marcus

[mbryner]

Bruce, thanks for the info on the Laing pumps.  Should have known you would know all about that & would reverse engineer them!  That's awesome!     

Oh,  here's another trick I did:  the diff temp controller has only AC output & my batt bank is 48 v anyway, so I bought a cheap LCD monitor DC power supply off eBay & spliced it into the pump.  Don't know how efficient it is tho'.

[BruceM]

I just started looking at drainback systems.  Markus, I didn't think about a small drain back tank just below the panels, inside the heated house envelope.  Glad you thought of that for Mike's situation. That means Mike could just pick one of the more powerful Laing DC circ. pumps to do his job, and he could have a very simple system, with little extra expense on the PV panel to drive the pump.  

Here's a good selection of Laing DC pumps with head and GPM info (two of the pumps can deliver over 16 ft of head):

http://www.houseneeds.com/shop/laing/laing-pumps-dc-solar-vario-d5-38-090-b-lmb15107977.asp

Mike, if you need to put the drainback tank (10 gallons typically) or pipe to it in a non-heated space in the attic, that can be handled too, you just use antifreeze in the distilled water for the "closed loop drainback system".  That's what they are called here, and they have a nice color system diagram of one:

http://homepower.com/basics/hotwater/

one more nice article on drainback system details:
http://www.plumbingengineer.com/march_11/solar_feature.php


I'm not a fan of water on the roof, so I have the south wall of my shop clear except for windows along the top foot of the 10 foot wall.   My drain back would have to be a buried plastic drum.  I have some unique problems with EMI/magnetic fields in my shop so would still opt. for operation by the EL-SID 10 watt pump with my own electronics, and have a 120VDC RV diaphragm pump that could be used for start up and air purge. My battery bank is 120VDC, though I also have 12V at the shop.

What I did find while looking for good drain back info was info on how to reduce the area of copper absorber of a flat plate collector, by surrounding it with a cheap air collector.  Given the price of copper these days, this could be a real boon for me.  I plan to do my winter space heating (in floor pex) via the collectors, so I need a lot more area than for a domestic hot water system.  I previously did some calculations which showed that a 60 degree panel angle should give me enough heating for domestic hot water only in the summer.

http://www.builditsolar.com/Experimental/ShurcliffPart3/khanhms.htm

After some more reading on drainback vs closed loop (antifreeze) systems I found that drainback systems aren't recommended for hard water. Bummer- my water has a TDS of 186. I sure don't get much sediment in my main water storage tank though...so maybe this is a red herring?  I know it takes very little calcium build up to reduce thermal conductivity. (Edit- no problem, I just have to make it into a closed loop drainback system using distilled water. See the home power link above.)

Marcus- What do you use in you fireplace loop- just water or do you add corrosion inhibitors and such?  What about biological warfare in your 500 gallon tank (algae, bacteria, etc.)?

[David Baillie]

Hi Mike,
When we first set things up we were using a tankless takagi jr for hot water and in floor radiant.  The problem I ran into was yes it was extremely efficient on propane use but it had a 14-18 watt ac load to run the main board and then a 70-80 watt ac load when there was call for heat plus the taco circulator at about 35-40watts.  At the time our power supply was extremely limited so it was too much.  That unit's other problem was that it anti freeze option used an electric element!  We switched it out for a tanked direct vent water heater.  Less electric probably more propane.  The woodstove has a hot water loop in it with the plan to preheat a seperate tank but we never seem to get around to it...  All that to say that efficiency in one area can come with tradeoffs in others...
David

[mbryner]

Bruce, somehow I missed your previous post until David just posted.   David, you make me want to double check how many watts our Rheem tankless is using at idle.  15-20 watts seems like an awful lot.

I would think using distilled water (or antifreeze) rather than hard water would be important in a drainback system of solar panels because the calcification would affect the small channels in the panels.   I'm definitely planning on an antifreeze system in mine if it gets installed this summer.   Drainback is a little harder because of always needing to run downhill, elbows, etc.    Simple closed loop, like our stove loop, is much easier (IMHO) because you can fill it from the highest point w/ antifreeze and seal it.   If the pump fails you may have to replace the antifreeze that got cooked out in the sun, but that's not a big deal.

Yes, I did fill the woodstove loop w/ distilled water initially.   Then, when we were leaving home for vacation I turned off all the breakers and watched the TPR valve promptly blow because of course the pump was tied to the breakers.   Duh.   Steam everywhere.   Totally forgot that the woodstove wasn't cool yet.  Anyway, we had to leave right away on the vacation so I just filled the loop w/ water from the hose and turned that one breaker back on.   That was 1 year ago.  There's probably some calc buildup now, but it's a closed loop; how much calc buildup can you get in a couple gallons of closed loop.   Also, I built in 2 hose bibs in the circuit so I can hook up either a hose to refill/flush, or attach a vinegar circulating flush.   Now when we leave for a few days in winter, I always make sure the woodstove circ pump and fridge have power!

Biological warfare in the 500 gal tank?  Well, I poured about 1/2 gal of bleach into the tank when I first filled it and there's no growth medium insde.   Silicone walls and copper pipe are not the greatest petri dishes.   Haven't noticed any smells.    Probably should check every so often, though.

Marcus

[BruceM]

Thanks, Marcus.  Looks like I may have plenty of time to plan my solar setup.    

[mike90045]

Well, with a bunch of suggestions from here, and other sites, I've revised the PLAN more, and sure hope this will be a successful one.  Now I'm looking for a reliable, low power, Differential Controller (brain) for the solar loop.  The rooftop tank is insulated, and freezing should not be an issue for the insulated inlet and outlet.
The  tankless heater uses 40w when the vent stack fan is running, but only a couple watts on standby. It prevents freeze by propane use, not electric heat.
I'm not above more improvements if you notice something else. If there are check valves that seal lightly, that would be preferred, or else I can try to do without, or may have to put in an expansion tank. All lines are at house pressure, about 50-60 psi, from a tank on a hill. Except for heat ex loop (copper & stainless) lines are planned to be PEX unless folks think CPVC is better to use for the potentially hot hot water from the summertime roof collector.

[vdubnut62]

Marcus, have you looked at Bacquacil (?spelling?) shock? It's like 15 or17% h2o2. Prob easier on the copper.
Ron

[mike90045]

Marcus, have you looked at Bacquacil (?spelling?) shock? It's like 15 or17% h2o2. Prob easier on the copper.
Ron

It was good enough to find in google Is chlorine or baquacil better  http://rob-dr-duck-coxworth.wrytestuff.com/swa32658.htm

Looks to be some weird stuff, and very hard to remove trace residues from the system.  

For my water, I'm using a slow sand filter, and then polishing it with ozone, bubbled up from an ozoneator, about 50W to run the UV light and air pump.  If water is clean enough, I think I can set ozoneator on a timer, and just run it a couple hours a day.

and I'm just running plain domestic water thru all the loops, except for the factory exchanger fluid in the rooftop collector. It's tank is above the collector, so I should not re-radiate my stored heat at night.

[sailawayrb]

Chlorine is not all that harsh on copper or likely anything else that one would have in their system.  After all, all city water is treated with chlorine and if it destroyed copper or plastic plumbing we would hear/know about it.  Cheap standard household bleach is all that is needed.  The trick is to only introduce enough chlorine into your system such as to only have a low residual amount of free chlorine left in the system after the chlorine has fully reacted with the elements in the water.  A fully mixed solution takes about 8 minutes to fully react at 60 deg F.  

The best way to accomplish this is to put a small known amount of chlorine into a small known amount of sample water and then use a chlorine test kit to determine the residual free chlorine.  Keep adding known amounts of chlorine and keep testing until the desired residual free chlorine level is reached.  Once the right total amount of chlorine for the sample is determined, one can easily scale up and determine the total chlorine need for the system by knowing the total water quantity in one's system relative to the sample quantity.  

For a closed system, one should be good for a long time because you have effectively sterilized the system and nothing should get back in.  

For an open system, one might want to consider getting a Stenner chlorine pump, pump control module, and an injector valve to maintain this ideal low level of residual chlorine.  These injector valves can inject chlorine into systems that are operating as high as 150 PSI.  

And BTW, this Stenner approach is commonly used for treating problematic drinking water too.  Associated PDF file attached.

Bob B.

[mbryner]

Like Sailawayrb says, if it's a closed loop system and you sterilize it, then there's not much to worry about.   And bleach is ubiquitous & cheap. 

mike90045,

Your new system design is better.   I'm still a bit confused how the solar panel won't thermosiphon all night. 

Marcus