On demand hot water

26 Posts
Aug 26, 2009 09:37 pm
On demand hot water

I am building a off-grid home in upstate NY and need to install a propane on-demand hot water unit. Eventually I will incorporate solar hot water but given the fact I'm near broke, it'll have to wait.
The system I have is a 1K array with 16 Trojan T-105's. I see 6K per day in the summer and probably 3.5K - 4K in the winter.
I'd love to hear if anyone knows of a unit that only draws a mere trickle of electricity when not in use and only runs temp controls when in use.
I know Bosch makes one that turns on with water pressure but it's BTU rating is too small for our household needs.
175,000 BTU range would be about right and no standing pilot is a must.
Thanks with any input.
Dave C.
462 Posts
Aug 27, 2009 11:49 am
Re: On demand hot water

Dave, why do you have to install an on demand hot water heater? 175,000 BTU's? A normal hot water heater is only around 40,000 BTU's, and probably a lot cheaper to purchase and replace as well as to operate. You should consider solar first. If you are installing a PV system I would think that if you invest in a single or two solar hot water panels and run the pump off the PV system that the cost would be way more economical. If this is a vacation home and you are not there much, then there will be plenty of time to heat up a tankfull of water.
 If you are building this home from scratch, then maybe you should consult with someone who knows solar. You could incorporate a sunspace to house the tank that can heat your home off season and while you are away and incorporate the hot water tank into this space as well.
 Lots you can do besides invest lots of money into PV......
26 Posts
Aug 27, 2009 01:11 pm
Re: On demand hot water

It will be a year round home and my refrigerator (already in use) is propane as will be my hot water.
My "solar window," where my PV panels are located, is about 100' down hill from the house and is already beyond the re-design stage to accommodate for solar hot water (no mount, no roof, no-go)
Everything is up and running and I'm not buying any new PV panels (8-130 Kyocera's) to accommodate an on demand hot water unit.
I will definitely invest slowly into solar hot water in the future. One step at a time and I know propane is good back up (as is solar) to pump through radiant flooring when we are away on vacation (if ever)in our cold dark winters.
The BTU rating of 175,000 is needed given it will be our only source of hot water and the runs it will make throughout the home.
Thanks for the input.
462 Posts
Aug 29, 2009 09:21 pm
Re: On demand hot water

Dave, 100' isn't that crazy of a distance for hot water. Just think of a large house with panels on it's roof. The runs from the roof into a basement can reach this distance in some cases. Now think of the Alaska pipeline. It delivers hot oil over miles. The trick is in the insulation. If you bury it below the frost line and insulate it may be possible to have SHW. Plastic water service pipe works well in this application. An insulated elevated line can also be done. Think of a chain linkfence with the hot water line as the top rail.
 If the hill is south facing, panels can be installed upon an earth berm using the ground for insulation.
 Are you installing a wood stove? Another easy way to make hot water. You mention radiant heat. In the summer you can gain heat from the floor and pump it into the tank, heating your water and cooling your house at the same time.
 Just don't give up on SHW yet until you realize all you can do. As a plumber and mech. engineer, I am not a big fan of instantaneous HW makers. I've seen them come and go over the years. And when service time comes, you may find the manufacturer out of business. Taking a couple showers at the same time can be an issue. Also determine your water quality which may hinder materials used in the heater. 
26 Posts
Aug 31, 2009 12:51 am
Re: On demand hot water

Without going into crazy detail about my site, it's just not well suited for SHW. I'm really just looking for information for off-grid on demand hot water. I'm dialed into everything I could do for SHW and have determined its just not in the cards at this time given the logistics and the money so.....
As of now I'm building an off-grid, strawbale, timberframe home I've designed and milled all the lumber myself and know the in's and outs of my site to that end.
Any input on a model would sure be helpful.
462 Posts
Sep 2, 2009 11:37 am
Re: On demand hot water

David, if on demand is your choice, check out "Noritz". They manufacture on demand heaters. They make models that have direct ignition based on water flow. Operation power is 120 V using 115W or 250W if freeze protection is needed, so around 1 amp or 10 amp draw off of your 12 V solar PV system. They can be purchased at any local plumbing supply.
26 Posts
Sep 7, 2009 09:09 am
Re: On demand hot water

Thanks Tom. I will look into the Noritz line and see what they have to offer.
99 Posts
Sep 10, 2009 12:33 pm
Re: On demand hot water

I highly recommend the Takagi TK-Jr.  At only about $600 (+$300 for venting), it is very cost effective.  You can also set it up as a closed combustion unit, as I have with mine, so that it draws combustion air from the outside instead of pressurizing the building.  It tops out at about 140,000 BTU though.  For me this is plenty since I have both passive solar in my house as well as a solar thermal system, which really doesn't cost that much if you source the materials yourself.  I bought 3 x 30 evacuated tube arrays for $695.00 ea., plus $37.50 S&H ea., plus tax, for a total of $2,329.35.  And I built a 900 gallon hot water storage tank for just a few hundred bucks (wood frame, styrofoam insulation, EDPM liner, cement board, and tile).  You can put the evacuated tube arrays on simple ground-mounted frames.  I would highly recommend more research into this route.  Nonetheless, if you improve your insulation and infiltration and thermal mass in your house, you will probably find that 140,000 BTU per hour is plenty anyway.  That's over 3 million BTU per day!
99 Posts
Sep 10, 2009 12:42 pm
Re: On demand hot water

BTW, the Takagi uses a 120VAC electric ignition.  As far as I can tell, it draws nothing until it sparks.  Also, while the flame is actually going, it has a fan to actively vent the unit.  The draw doesn't register on my Outback inverter however, so it must be miniscule, probably 10 watts or less.

If you're still at a stage to do so, I would absolutely recommend you install at least one wood-burning stove (I love my BIS Ultima, but any high-efficiency unit would do) as a back-up and top-up heat source.  I wouldn't want to depend solely on solar or propane, just in case.  I'm assuming that you have woods on your property (or nearby) and could thus collect firewood if you had to.
26 Posts
Sep 11, 2009 08:08 pm
Re: On demand hot water

Thank you for the info.
I'm looking in to the Takagi now and it seems to be well suited for my needs. I'm going to call the manufacturer and find out what the draw is when idle. I'm sure it's not much.
I'm looking at a larger model than the JR. and it draws less amps with a higher BTU range.
We'll be digging a well on Monday or Tuesday and depending on how much gallons per minute we get at around 150' (all we can afford) will determine if we need to build a cistern in the basement. If we do, I'd love to see photos of your design.
Thanks again,
1 Posts
Sep 16, 2009 02:59 am
Re: On demand hot water

Since the Takagi uses a 120VAC electric ignition how does this work if your inverter is in sleep mode? I am off the grid with an old Trace inverter and I have been trying to find a water heater that is eligible for tax credit and can still work with an inverter.
26 Posts
Sep 16, 2009 07:42 pm
Re: On demand hot water

Not quite sure how that works. You'd need to have the inverter running all the time I guess.
Maybe it would be worth running a very small dedicated inverter for only the on-demand and let your main inverter rest.
I'm sure it would only need a modified auto parts store inverter...maybe 400 watts depending on the unit.
If the on-demand unit had a pilot light it may not require 110 AC unit its called for, thus waking up your inverter.
Also the the micro-hydro igniters in some Bosch models may work the same.
Let me know if you figure anything out.
99 Posts
Sep 25, 2009 06:45 pm
Re: On demand hot water

It's really not feasible to live without some phantom loads.  E.g. my smoke and carbon monoxide detectors are drawing from AC power (w/battery backup).  My septic system overflow alarm is AC.  I have home automation switches which draw a few micro-amps each.  I have a home security system which obviously draws some power when armed (though I'm looking into powering that with DC through its emergency battery terminal).  In order to remember my favorite stations, my stereo remains in standby.  And the big one -- in order to record television shows automatically, your DVR and satellite receiver must be on all the time.  Thus, it doesn't concern me that my water heater may be consuming a few micro-amps to monitor if there's any water flow so that it can turn on.  Even if all of these phantom loads together consumed 20 watts, that's only 480 watt-hours per day.  And since my batteries are full for most of the day, this doesn't concern me much.   

The batteries are the weak point of the system -- it's much cheaper to add a few extra solar panels to your system so that your batteries are only depleted at all during the night than to add more batteries to carry a really sunny day over a few extra days.  Actually, in my system, my batteries often are fully charged at night because of my wind turbine -- the wind usually picks up in the evening just as the sun goes down.  But if you're finding that you must cut out phantom loads because your system just can't swing the extra power, a single 80-100 watt panel (costing under $500) could more than cover all of your phantom loads.  To me, that is cheap peace of mind.  I don't want to have to worry that leaving my DVR on might shut down my whole system. 

I'm not really sure how "sleep mode" works, but I can run small loads while my Outback inverter is in sleep mode (they call it "search mode").  It constantly checks for loads and I think it can handle small loads on a separate circuit than when it's fully awake.  Maybe something like a little inverter inside the big one?  I'm not sure about that though.  Maybe my inverter is just on all the time because the phantom loads draw enough to prevent it from sleeping.  In any event, my batteries don't get drawn down excessively so I don't worry about it.

If you're really concerned, perhaps, as Dave suggested, you could put the water heater on its own dedicated inverter.  You could use one of those cheap car inverters meant to run a laptop since the load is small.  Sort of the reverse of a power brick.  But first ask yourself if this is truly the only phantom load you'll need to consider.  If, like me, you'll have multiple, then just forget about sleep mode.  There's a point where you can take efficiency too far.  Remember, the reason we have electricity in our homes is to provide comfort and convenience.  To worry about your inverter and to eliminate all phantom loads defeats the entire purpose.  Then it becomes a chore instead of an asset.

Also, I wouldn't recommend a pilot light water heater, as that's a waste of propane.  The amount of electricity drawn to spark and run the fan are negligible (and free from the sun!).  Don't waste propane, which must be trucked in.

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