Out back FM80 sizing and PV panel config help needed.

bigdogeatbigdogeat Posts: 26Registered Users ✭✭
I have had difficulty downloading the Outback FM80 sizing tool so I could use some help.

I am currently running 16 150W BP panels (2400W) through an Outback FM80. It runs perfectly and has never flipped the 80 amp breaker between the panels and the controller. I just upgraded to a 6000W split phase inverter and now I I want to add 4 panels I already have to the system bringing it to 3000W.

I currently run 16 individual panels @ 24V but I know I am right at the limit of the Outback FM80 controller for a 24V PV system. My initial plan was to leave it configured the same but run the 4 extra panels through a small PWM controller I already have. I was planning on setting it up to dump any extra energy, (once the batteries are full), into a 24V 600W heating element in a water preheater I built.

My real question is should I reconfigure the panels in series to create a 48V system, (10 strings of 2 @ 48V) or should I just go with my original plan? Could the controller handle the new configuration? Which is more efficient?

The small controller has a load dump feature which would make it really easy. If I ran everything through the Outback then I would have to do some more work to get the diversion load to work. 

Thanks for your help.

Outback FM80 Controller
16 BP 24V 150W Panels
AIMS 6000W 120/240V Split Phase Inverter
12 Trojan T105RE 6V batteries
Off-Grid
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Comments

  • EstragonEstragon Posts: 2,850Registered Users ✭✭✭✭✭
    What is the new inverter DC voltage? With a 6kw inverter, I'd absolutely be seriously considering 48v.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    It is a 24V inverter. I was using a 1800W inverter and it was always plenty, but I went with the went with the split phase inverter to run my 240V deep well pump which was the one thing that I couldn't run off of my PV system. It will only run a few minutes every couple days. Unfortunately my house was not originally designed with solar in mind.
  • PhotowhitPhotowhit Posts: 4,708Solar Expert ✭✭✭✭
    bigdogeat said:
    My real question is should I reconfigure the panels in series to create a 48V system, (10 strings of 2 @ 48V) or should I just go with my original plan? Could the controller handle the new configuration? Which is more efficient?

    Outback FM80 Controller
    If you are running the panels as single panels all in parallel. You are already hampering the ability of the charge controller.

    You should be running 2 in series, MPPT charge controllers need a voltage about 30% above the charging voltage, to work properly.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • Raj174Raj174 Posts: 644Solar Expert ✭✭✭✭
    edited April 19 #5
    If you are not going to add any other loads to the system, I would just add the panels into the current configuration. Yes, you will be over paneled by 600 watts, but the FM80 will not have a problem with it. Output will still be limited to 80 amps and power production in the morning, evening and during cloudy weather will be increased. Attention should also be given the inverter/battery cables and breaker to handle any increased power drawn by the 6kW inverter with added pump load.
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Thanks for the input guys. I really do appreciate it. I'm getting some mixed advice though. Yes, they are all in parallel right now. Originally I had them run into 4 smaller PWM controllers before I upgraded to the Outback. That is why they are configured that way. To be clear, the panels usually produce 34-39V so I don't think the present voltage is an issue Photowhit. 

    Raj17, if I add the 4 panels to I am worried about making the Outback shut down from too much amperage going in. If I switch the panels to sets of 2, the amps should drop in half, correct? I'm just not sure if the power output from the FM80 would be the same.

    To your point, the inverter and battery cables are short and 1/0. Kinda overkill but I wanted it to be good for any future upgrades. As expected, I did however trip the 150A breaker, (originally for the 1800W inverter), between the batteries a couple minutes after starting the well. A bigger replacement is on the way. 

    If I run all 20 panels through the Outback, will I be losing production vs using the Outback for 16 of them and 4 through the PWM controller? Like I mentioned, the diversion load to the heater element is super simple with the PWM controller. Thoughts?

    I am in the process of moving the panels to a larger more permanent base built for all 20 so I'd prefer to only wire them up once.

    Again, thank you so much.
  • VicVic Posts: 2,907Solar Expert ✭✭✭✭

    Hello bigdog..,

    Technically,   the OutBack  FM series of CCs have a maximum input current spec,   of 80% of the TOTAL Isc of all of the PV strings.

    Presently,  with 16 "strings"  of single PVs  that spec is exceeded (76 Amps).   And adding more will further exceed that 64 A Isc input Limit.

    Photowhit is correct,   that even 72 cell PVs,   like your BPs  will not have quite enough input voltage (hot PVs,  Flooded batteries,   or needing to EQ,  etc).   SO,  you can switch to the recommended strings of two PVs.  This will be more than enough increase in Vin to the CC,   AND  halve the  string Isc totals.   A win-win.

    Just being a bit TOO TEDIOUS,    Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Raj174Raj174 Posts: 644Solar Expert ✭✭✭✭
    edited April 19 #8
    @bigdogeat
    @Photowhit is correct, the most efficient and problem free panel configuration for your MPPT controller would be 2 in series. I would consider using 2 controllers on a single battery bank an unnecessary complication. Do you need the extra charging amps from the PWM controller at the top of the day? The PWM controller will bring an extra 16 amps when the FM80 is at max output. That's a little over a 14% charge rate for your battery bank. Doable, but a little high.  I would go with 20 panels, 2 in series by 10 in parallel on the FM80, but your call.  
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
  • PhotowhitPhotowhit Posts: 4,708Solar Expert ✭✭✭✭
    edited April 19 #9
    Okay, I'm not sure what the situation is, Outback lists that the FM80 is most efficient at;
     


    I would never run a 60 VDC input to a 48 volt battery bank as equalizing voltage would be higher than input voltage.

    I use the 30% and have repeated it in rote, from Interaction with  @boB (for some reason the linking feature doesn't appear to be working). boB is an engineer at Midnite Solar now and designed/helped design the Midnite Classic and helped design the MX series when he was with Outback.

    I would be interested in his ideas on this, low overhead efficiency. Perhaps you could run all in parallel. Still would not recommend it, lots of wires coming in if you are extending from 4 previous arrays, now into one. Should have a breaker/fuse on each panel or string of panels... just to busy.

    I suspect When you say you are seeing voltages;
    bigdogeat said:
     To be clear, the panels usually produce 34-39V 
    What you are seeing is a voltage when the panels are in a null state, and not charging for the most part. Since the Charge controller is working turning the current on and off rapidly, you are seeing a voltage between the VMP of about 34.5 volts and the VOC 43 volts on the charge controller or you are reading the VOC of the panels with the charge controller out of the loop. If current is flowing in any serious manner, it would be hard to exceed the VMP.

    I think there were several names, but most of the 24 volt nominal panels made by BP shared similar specs, I looked one up;

    http://abcsolar.com/pdf/bpsx150.pdf




    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Wow! Awesome feedback guys! It's the perfect time to switch the panels to 2 panels in series per string. Seems like the safest way to go. That is the way I will wire it. 
    Raj174 said:
    @bigdogeat
    @Photowhit is correct, the most efficient and problem free panel configuration for your MPPT controller would be 2 in series. I would consider using 2 controllers on a single battery bank an unnecessary complication. Do you need the extra charging amps from the PWM controller at the top of the day? The PWM controller will bring an extra 16 amps when the FM80 is at max output. That's a little over a 14% charge rate for your battery bank. Doable, but a little high.  I would go with 20 panels, 2 in series by 10 in parallel on the FM80, but your call.  
    The only reason I was even considering the PWM controller for the extra 4 panels  is because it is super easy to run the excess power produced into the water preheater that I put together. It is installed prior to my regular on-demand propane water heater, It would help charge the batteries when needed and then dump excess into the heating element. I figured preheating the water would save a fair amount of gas especially if it is just excess energy produced anyway. And I already have all the components for that. And the element is 600W exactly the same as the 4 extra panels I would run it from. Seems easy in my head. lol  I think I would have to do some more research and buy some more pieces in order to get the Outback to do it. 

    But since we are on the subject, is there an easy way to have the Outback send a certain amount of excess energy into a dump load? I can't send too much or it will fry my element.

    Thanks again everyone. You are all great!

  • PhotowhitPhotowhit Posts: 4,708Solar Expert ✭✭✭✭
    Will you have a thermostat for the water heater? I think you stated it was a DC water heater?

    Water heating elements are pure resistance loads, so they don't care if it's ac or DC, but the thermostats will care as the make and break a connection, DC can maintain an arc much easier than AC, since AC passes through '0', or turns off, 60 times a second.

    Elements can be use from an AC system for DC loads, or for lower AC loads. I'm using a 240v - 3600 watt element on 120v system and it uses about 900 watts. at 60 volts it would quarter again to about 225 watts.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • EstragonEstragon Posts: 2,850Registered Users ✭✭✭✭✭
    A bit of a conundrum IMHO. If you're relocating and rewiring panels, and only want to do it once, the right way is probably to wire (eg) strings of 3 in series to a 48v bank. This would make more sense for running the pump, keep wire size reasonably on both the pv and bank, and be more efficient for the controller.

    Unfortunately, you already have the 24v inverter, so there would be hassle and possibly costs to switch to 48v at this point.

    With 12 x 6v batteries, you have 3 strings of 24v (not ideal in itself), or 1.5 x 48v strings. If newish and in good shape, you could add the other 1/2 48v (4 batteries) for two strings. If not, you could make one string, and swap the extras in for spares as they fail.

    If it was me, I'd probably bite the bullet and go to 48v.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • VicVic Posts: 2,907Solar Expert ✭✭✭✭

    Photowhit,

    boB's general recommendation for minimum MPPT CC Vin vs battery voltage has been 30% in excess of the highest battery voltage,   during charge  (with the hottest PVs).

    The following is stolen from Note 2 in the MidNite String Sizing Tool's result,  after submitting PV array data:


       ...   "Most all MPPT controllers will want to see a minimum of 130% of the actual high battery voltage. So if we have a 48v battery and it has an Equalize voltage if 62.3 volts than we would multiply that by 130% and we would need a minimum of 81 volts on the input on the hottest day of the year in order to have enough headroom for the MPPT to work   ...   "

    FWIW,    Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Photowhit said:
    Will you have a thermostat for the water heater? I think you stated it was a DC water heater?

    Water heating elements are pure resistance loads, so they don't care if it's ac or DC, but the thermostats will care as the make and break a connection...
    I am using a 30 gallon AC water heater that had gotten upgraded. I just replaced the lower element with a 24VDC 600W element and left the top element disconnected. I thought the current thermostat on it would work. Perhaps not?

    I haven't supplied any power to it yet awaiting y'alls advice on wiring the system.
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Estragon said:
    A bit of a conundrum IMHO. If you're relocating and rewiring panels, and only want to do it once, the right way is probably to wire (eg) strings of 3 in series to a 48v bank. This would make more sense for running the pump, keep wire size reasonably on both the pv and bank, and be more efficient for the controller.

    Unfortunately, you already have the 24v inverter, so there would be hassle and possibly costs to switch to 48v at this point.

    With 12 x 6v batteries, you have 3 strings of 24v (not ideal in itself), or 1.5 x 48v strings. If newish and in good shape, you could add the other 1/2 48v (4 batteries) for two strings. If not, you could make one string, and swap the extras in for spares as they fail.

    If it was me, I'd probably bite the bullet and go to 48v.
    Estragon, you are right, a conundrum. Mostly financial. This system has been several years of upgrades and is a work in progress.

    I just spent about everything I had earmarked for the solar project on the new inverter. The 24V was at least $1,200 cheaper than the 48V. And it got my well to run which was my next big goal. I know my battery bank is pretty light, but I manage it pretty well. And with the extra panels I think it will be fine even with the extra load.

    The choice was buying the inverter that fixed my well problem or upgrade the battery bank slightly. But then a 48V battery bank isn't much good with an 1800W 24V inverter and my well still wouldn't work...
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    To add to that last comment, I would love to bite the bullet and go 48V, but that would be about a $3,500 bullet...
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Did I mention that limited budgets suck?...lol
  • PhotowhitPhotowhit Posts: 4,708Solar Expert ✭✭✭✭
    Vic said:

    Photowhit,

    boB's general recommendation for minimum MPPT CC Vin vs battery voltage has been 30% in excess of the highest battery voltage,   during charge  (with the hottest PVs).

    The following is stolen from Note 2 in the MidNite String Sizing Tool's result,  after submitting PV array data:


       ...   "Most all MPPT controllers will want to see a minimum of 130% of the actual high battery voltage. So if we have a 48v battery and it has an Equalize voltage if 62.3 volts than we would multiply that by 130% and we would need a minimum of 81 volts on the input on the hottest day of the year in order to have enough headroom for the MPPT to work   ...   "

    FWIW,    Vic


    Did you miss my post about when Outback's FM80's most efficient?

    It claims to be most efficient at "60VDC input w/48V battery at 53.1VDC (98.44%)" that is from their FlexMax spec sheet which I copied. I did NOT make that up, I would very much love to hear bob's input on this! A 48 volt battery bank charging at 53.1 volts from an FM80 that has an input of 60 VDC is not getting the 30% head space that I and obviously you have been touting.

    PS this software is really getting on my nerves, not I can't flag people in posts...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • VicVic Posts: 2,907Solar Expert ✭✭✭✭

    Hi Photowhit,

    I did NOT miss that,   and,  am certain that the CC would be quite a bit "more efficient" at a Vin of 60 V on a 48 V system,   but complete charging/EQing Flooded batteries simply would not be possible,  unless the batteries were already HOT (probably would not work,  even then).

    Have a system here  that is 48 V FLAs,   using 72-cell PVs (35.4 Vmp,  per).   Avoided trying to run strings of two,  'cause,  the venerable MX-60 would not have had enough headroom,   to find a good Vmp verses required battery voltage.    IMO,   there is more to efficiency,   than CC heating.

    FWIW,   Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Photowit, that did seem pretty strange to me but I'm no expert. BTW, I just noticed that you run a Prosine 1800. That's exactly what I ran for years and just swapped out. Super reliable and no issues ever. I have 4 now. Two backup that I never needed and the one I just took out plus one that is 220 but not split phase. I just don't need them any more. (Although I will probably keep one as a backup)
  • PhotowhitPhotowhit Posts: 4,708Solar Expert ✭✭✭✭
    FWIW - 53.1/60= 113% 
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Ok,  earlier I almost said no such thing as too technical... lol   Thoughts on my previous post before you guys got even nerdier than me? 

    Raj174 said:
    @bigdogeat 
    @Photowhit is correct, the most efficient and problem free panel configuration for your MPPT controller would be 2 in series. I would consider using 2 controllers on a single battery bank an unnecessary complication. Do you need the extra charging amps from the PWM controller at the top of the day? The PWM controller will bring an extra 16 amps when the FM80 is at max output. That's a little over a 14% charge rate for your battery bank. Doable, but a little high.  I would go with 20 panels, 2 in series by 10 in parallel on the FM80, but your call.  
    The only reason I was even considering the PWM controller for the extra 4 panels  is because it is super easy to run the excess power produced into the water preheater that I put together. It is installed prior to my regular on-demand propane water heater, It would help charge the batteries when needed and then dump excess into the heating element. I figured preheating the water would save a fair amount of gas especially if it is just excess energy produced anyway. And I already have all the components for that. And the element is 600W exactly the same as the 4 extra panels I would run it from. Seems easy in my head. lol  I think I would have to do some more research and buy some more pieces in order to get the Outback to do it. 

    But since we are on the subject, is there an easy way to have the Outback send a certain amount of excess energy into a dump load? I can't send too much or it will fry my element.

    and
    Photowhit said:
    Will you have a thermostat for the water heater? I think you stated it was a DC water heater?

    Water heating elements are pure resistance loads, so they don't care if it's ac or DC, but the thermostats will care as the make and break a connection...
    I am using a 30 gallon AC water heater that had gotten upgraded. I just replaced the lower element with a 24VDC 600W element and left the top element disconnected. I thought the current thermostat on it would work. Perhaps not? 

    I haven't supplied any power to it yet awaiting y'alls advice on wiring the system.

    I just want to get this system wired up forget about it so I can start getting the garden in now that the frost is hopefully past.

    Thanks fellow nerds! lol




  • PhotowhitPhotowhit Posts: 4,708Solar Expert ✭✭✭✭
    With 24 volts the thermostat might work fine, and might only work for a while until the contacts become carbonized... I've done more asking and pondering a DC heater than I ever thought possible.  I only heat with AC and mostly I've been fine 90% of the time, with a few luke warm showers kicked in. 

    I've seen a DC element with it's own thermostat, but it's sold by a company that I won't do business with...

    Guess is it's best to use it as an opportunity load with the charge controller running a relay and running AC to the water heater.  Setting it to come on once you reach float. But you have a pretty small array, so it might keep kicking on and off...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Thanks for the input Photowhit. You've been really helpful. Think I'll experiment with it for a while since I already have all the components.
  • SolarMusherSolarMusher Posts: 172Solar Expert ✭✭✭
    Vic said:

    Hi Photowhit,

    I did NOT miss that,   and,  am certain that the CC would be quite a bit "more efficient" at a Vin of 60 V on a 48 V system,   but complete charging/EQing Flooded batteries simply would not be possible,  unless the batteries were already HOT (probably would not work,  even then).

    Have a system here  that is 48 V FLAs,   using 72-cell PVs (35.4 Vmp,  per).   Avoided trying to run strings of two,  'cause,  the venerable MX-60 would not have had enough headroom,   to find a good Vmp verses required battery voltage.    IMO,   there is more to efficiency,   than CC heating.

    FWIW,   Vic


    Hi Vic,
    You are right, I wired the same panels (Solarworld/BP 175 mono 72 cells) by two in serie 11 years ago on a MX60 and it was a nightmare in summer. Vmp was just too low to fully charge a 48V bank.
    FWIW, Erik

  • VicVic Posts: 2,907Solar Expert ✭✭✭✭

    Hi Erik,   Thanks for that info.

    Hope that all is well with you.     Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • EstragonEstragon Posts: 2,850Registered Users ✭✭✭✭✭
    Preheating water ahead of the on-demand propane heater is probably okay, but you may want to check the manual/specs. Some have a wide enough range of output modulation (eg 20000 to 180000 BTU) and be okay, but some may not play well with (eg) 110°f input water temp.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    My on demand water heater does have a fairly wide range of output. 10,300 to 150,000 so hopefully that will work.

    Wired the system up tonight just like you guys recommended  10 strings with 2 panels each. All running through the Outback. Not going to mess with the other controller per y'alls advice. Fingers crossed it fires up!

    I did see a strange thing when I looked at the Outback tonight. Dead of night and it was showing 18.7V even though it was dark out and the circuit breaker from the panels to the controller was flipped off. Should be any other sources coming in... When I switched the circuit breaker on the voltage slowly started dropping down to about 3V or so. Is that normal? Hopefully there is a decent explanation. Thanks!
  • Raj174Raj174 Posts: 644Solar Expert ✭✭✭✭
    It's normal to have  a low voltage on the PV input when it's dark outside. My Classic 150 reads about 14 volts a night.
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
  • bigdogeatbigdogeat Posts: 26Registered Users ✭✭
    Cool. I thought maybe the aliens were messing with me again...
  • EstragonEstragon Posts: 2,850Registered Users ✭✭✭✭✭
    10-150000 btu range sounds good. I think mine's about the same, and I use it via heat exchanger to heat glycol loops for infloor heating. The loop flows are set such that there's ~20°f temp drop from supply to return. The heater is set to 120°f in summer (for visitor scald protection) and 140°f otherwise (better heat output). It mostly modulates output to maintain the output temp, but does cycle on/off a bit.

    As long as the preheat is set a bit (eg 20-30°f) lower than on-demand setting, it should work well.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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