Small elec water heater as pre-heat for gas tankless?

cdrecdre Solar Expert Posts: 78 ✭✭
Sounds complicated, but wondering if I can get the best of both worlds...

I'm planning my off-grid system - likely to run 4kw of PV with an Outback 2.

I expect I'll have a good amount of excess generated energy, and I'm thinking it may make sense to discharge that energy into a small 40 gallon electric heater as a pre-heater for a tank-less propane heater. I feel like I'd rather invest in more PV than putting money into a solar water heater.

Might make sense to go with just the tank-less at first and see how much excess energy I'll have, but I thought I'd run the idea by folks here first.

Also, any recommendations on a tank-less heater which works particularly well off-grid?

Comments

  • rollandelliottrollandelliott Solar Expert Posts: 834 ✭✭
    Re: Small elec water heater as pre-heat for gas tankless?

    where do you live?
    seasonally or 24/7?
    solar hot water seems to make more economical sense?
  • BB.BB. Super Moderators, Administrators Posts: 29,497 admin
    Re: Small elec water heater as pre-heat for gas tankless?

    Depending on how much "extra power" you have--A heat pump based water heater will be 2-3x as efficient as resistive heating (at least in moderate to hot climates).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Small elec water heater as pre-heat for gas tankless?
    cdre wrote: »
    Sounds complicated, but wondering if I can get the best of both worlds...

    I'm planning my off-grid system - likely to run 4kw of PV with an Outback 2.

    I expect I'll have a good amount of excess generated energy, and I'm thinking it may make sense to discharge that energy into a small 40 gallon electric heater as a pre-heater for a tank-less propane heater. I feel like I'd rather invest in more PV than putting money into a solar water heater.

    Might make sense to go with just the tank-less at first and see how much excess energy I'll have, but I thought I'd run the idea by folks here first.

    Also, any recommendations on a tank-less heater which works particularly well off-grid?

    Wow, a whole bunch of implied questions....

    1. If your were on grid, the cheapest way to heat water would actually be a heat pump rather than propane! Being off-grid, propane is cheaper than anything you do with PV.

    2. Since you are off grid, the propane tankless is cheaper, although a preheat with your surplus PV (after the batteries are charged) may be a help. Just watch out that the preheat temperature is not so high that the tankless has a problem with not overshooting its set temperature at its lowest burner output and your lowest flow rate.

    3. As far as the electric requirements of an off-grid propane tankless, I have no useful information.
    SMA SB 3000, old BP panels.
  • PhotowhitPhotowhit Solar Expert Posts: 4,994 ✭✭✭✭
    Re: Small elec water heater as pre-heat for gas tankless?

    I have a similar situation, I have an excess of PV for winter months(even for my Battery bank) and after speaking with someone who has a small 12 gallon 'point of use' water heater in their cabin, I'm gong to use a 10 gallon 'point of use' for my water heater over the winter and maybe as the primary water heater in the future depending on how the winter goes. My friend and his wife lived fine on a 12 gallon version, though on the grid. I was given a small one(6 gallon) but I found a 10 gallon version at Ace with 2" R16 insulation and might just install it and hope it will serve most of my needs.

    I had hoped to use my current 40 gallon as a 'prewarmer" to bring water up to room temp in the winter time, lots of difficulties there, it's insulated, likely want to filter the water before it goes into the 'new' water heater, or so my plumber friend says, something about lime from the 12 year old water heater. Still haven't done anything toward installing so watching threads as well, but I might have some hard info for you by spring. I still don't have the last tree down, before I put up the array,...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Small elec water heater as pre-heat for gas tankless?
    Photowhit wrote: »
    I have a similar situation, I have an excess of PV for winter months(even for my Battery bank) and after speaking with someone who has a small 12 gallon 'point of use' water heater in their cabin, I'm gong to use a 10 gallon 'point of use' for my water heater over the winter and maybe as the primary water heater in the future depending on how the winter goes. My friend and his wife lived fine on a 12 gallon version, though on the grid. I was given a small one(6 gallon) but I found a 10 gallon version at Ace with 2" R16 insulation and might just install it and hope it will serve most of my needs.

    I had hoped to use my current 40 gallon as a 'prewarmer" to bring water up to room temp in the winter time, lots of difficulties there, it's insulated, likely want to filter the water before it goes into the 'new' water heater, or so my plumber friend says, something about lime from the 12 year old water heater. Still haven't done anything toward installing so watching threads as well, but I might have some hard info for you by spring. I still don't have the last tree down, before I put up the array,...

    You raise a good point: There is a definite difference between a point-of-use heater with a small tank and a slow recovery period and a tankless, which heats the water on the way through and requires enough power coming in to do that heating in real time.
    SMA SB 3000, old BP panels.
  • cdrecdre Solar Expert Posts: 78 ✭✭
    Re: Small elec water heater as pre-heat for gas tankless?

    This will be in Belize, full time (and not seasonally).

    Doing a little more google searching, I read a study which listed many of the same concerns. http://www.buildingscience.com/documents/reports/rr-0806-tankless-solar-hot-water-heating-integration

    I guess until a tankless heater comes out which is specifically made to handle inconsistent input temperatures, a patched together solution probably won't be a good fit.
  • erneerne Solar Expert Posts: 41
    Re: Small elec water heater as pre-heat for gas tankless?

    If you are going to preheat water for a gas demand water heater you need one with a censer in it that leaves it off until the temperature gets low enough to need more heat. the Aqua Star 125-S has this feature. be sure it has a censer.
  • cdrecdre Solar Expert Posts: 78 ✭✭
    Re: Small elec water heater as pre-heat for gas tankless?

    I'm guessing an open loop gas fired system will probably be the most efficient choice then.... (no freezing where I am).

    Any recommendations on open loop gas fired systems? Seems most are electric.
  • PhotowhitPhotowhit Solar Expert Posts: 4,994 ✭✭✭✭
    Re: Small elec water heater as pre-heat for gas tankless?
    cdre wrote: »
    This will be in Belize, full time (and not seasonally).

    How much 'season' do you have in Belize?

    When I was in Puerto Rico, I noticed lots of people have cisterns? or water tanks of some sort on their roofs, my guess is that is all the hot water they need, I think there is some elevation in Belize, so perhaps that wouldn't work.

    I do wonder if the water is chloinated and if the influx is enough to keep the tank clean... I guess you would use more water as it would mostly be just tepid...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • cdrecdre Solar Expert Posts: 78 ✭✭
    Re: Small elec water heater as pre-heat for gas tankless?

    Had a new idea today on this... I have a couple of 10 gallon gas water heaters installed in a few old RVs. They run great on the locally available butane. Thinking maybe I'll start by pulling one of these heaters, properly venting it, and using it on the house. Then, if it looks like I have enough power for a diversion load, I may get fancy with the MX 80 and a solid state relay. Thinking I would rig up another 10 gallon tank for pre-heating and rig it with either an AC or DC heating element. If I start maxing that out, I can upgrade to a 30 gal or so tank...

    Some really good info on setting up an MX 80 for a diversion load here: http://outbackpower.com/forum/viewtopic.php?f=29&t=2865&start=0

    Any thoughts?
  • BB.BB. Super Moderators, Administrators Posts: 29,497 admin
    Re: Small elec water heater as pre-heat for gas tankless?

    Watch that you get the correct solid state relay that can switch DC... The typical ones are AC only (need the zero crossing voltage to turn off of AC).

    And make sure you have a hefty heat sink--I think at least one person here had problems with hot SS relays and short life.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cdrecdre Solar Expert Posts: 78 ✭✭
    Re: Small elec water heater as pre-heat for gas tankless?

    Thanks. Looking at a relay from here: http://www.power-io.com/products/hdd.htm

    They actually sell them mounted on a heat sink.
  • SurfpathSurfpath Solar Expert Posts: 393 ✭✭✭
    Re: Small elec water heater as pre-heat for gas tankless?
    cdre wrote: »
    Any recommendations on a tank-less heater which works particularly well off-grid?

    We installed an Eccotemp L5 tankless propane about a month ago. Connected it directly to the house hot water line (mounted to the back of the house, on a concrete block wall, very close to where the shower area is located so that the hot water would not have far to run). Built a little 'house' around it to keep out excessive winds and rain - kept big openings at the top front (where the heated air runs out) and at the bottom 1/3 where the controlls and connections are. Water is strained by the whole house culligan filter.

    It seems to run great (check out the Amazon reviews, lot of them there):

    My experience:

    Very hot, plenty heat for one shower. If pressure drops below 30psi, may even be too hot. Havent tried 2 showers at the same time, but I'd guess this would max the unit.

    A few caveats:

    The shower head and hose it comes with is cheap and I do not recommend relying upon them.
    It seems to not like "mixing" in water from the 'cold' line. My Moen plumbed shower head has just one handle and I just turn it all the way to hot. But that's fine as you can set the temp on the unit. Just 1 or 2 tweaks and we seem to have it right.
    It produces a lot of heat at the unit and I feel I need to keep an eye on it (make sure to turn it off every time soon after using it).
    My instinct says I may need to clean it out every 4 or 6 months (run white vinegar through the heat chamber), in order to increase product life.

    Overall, not bad for a $110 purchase.
    Spouse happy = priceless

    -SP
    Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, Honda 2000 watt genny, 100% off grid.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Small elec water heater as pre-heat for gas tankless?
    Surfpath wrote: »
    Very hot, plenty heat for one shower. If pressure drops below 30psi, may even be too hot. Havent tried 2 showers at the same time, but I'd guess this would max the unit.

    A few caveats:

    The shower head and hose it comes with is cheap and I do not recommend relying upon them.
    It seems to not like "mixing" in water from the 'cold' line. My Moen plumbed shower head has just one handle and I just turn it all the way to hot. But that's fine as you can set the temp on the unit. Just 1 or 2 tweaks and we seem to have it right.
    It produces a lot of heat at the unit and I feel I need to keep an eye on it (make sure to turn it off every time soon after using it).
    My instinct says I may need to clean it out every 4 or 6 months (run white vinegar through the heat chamber), in order to increase product life.

    Overall, not bad for a $110 purchase.
    Spouse happy = priceless

    -SP

    Most likely the problem is that you are using a low flow shower head, and if you start to mix cold, the flow rate of the hot water drops below the point at which the tankless heater can regulate the temperature.
    You are correct in your solution, since you do not want to just use more water. You have to lower the set temperature of the unit so that you mix little or no cold water.

    You will also find that trying to use a shut-off valve in the shower head to conserve water will cause slugs of too hot or too cold water or both.

    Finally, you need to realize that the ability of the tankless to regulate temperature at a given flow rate will be reduced as the temperature of the incoming water increases. There is a lower limit to how much the burner can cut back.

    Different tankless units will have different minimum flow rates in relation to the amount they are heating the water. At the other end of the spectrum, some will actually throttle down their flow rate to the point that they can still maintain the set output temperature. That will cause both showers to be lower in flow if you use two at once.
    SMA SB 3000, old BP panels.
  • Eric LEric L Solar Expert Posts: 262 ✭✭
    Re: Small elec water heater as pre-heat for gas tankless?

    I tried the SSR idea proposed in this thread over on Fieldlines and it's working very well for me so far.

    I post some of the details of my experience in the linked thread, but the short version is that for about $40 in parts (wire and an SSR), I'm using my surplus pv to power the electric water heater. So far we've had more than enough hot water from the surplus power; in fact our water is now much hotter than when we used the grid. This method may require a Midnite Classic CC; don't know if others have an adjustable PWM output signal mode with something like the waste not hi function on the Classic.
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