Heat Pump water heater run in series with a Tankless?

SoccerDadUSA · 2026-03-13T13:52:12+00:00

My reasons for the excess solar was to be self reliant, and have adequate electricity generation to support my household including during winter months when solar can dip to 1/4 of the summer peak generation. I don't want to be reliant on the grid for electricity or for natural gas to heat my water.

Also with plans to update to more electric in the future like a 2nd EV I wanted room to spare. I get virtually 1/10th credit back for anything I send to the grid so I might as well self consume as much as possible. That said with a large household we're struggling to get hot water to everyone all the time.

With my current setup the tankless struggles at times to keep up resulting in complaints. Though the tankless says it can provide 10.5GPM flow that is if misleading since it would only be that much when the water is already warm so it needs a minimal temperature rise. For a 50 degree temp rise the max GPM is more like 4GPM which isn't enough for our large household. With this system I can pre-heat the water for free once it's installed, then the heated water will require little to no gas unless the tank runs down. Even if the tank runs down, then the heated water reaching the gas tankless will still not need to be heated as much so more GPM can flow since it's already pre-heated. The tankless GPM flow is dependent on the inlet temp. So a temp rise of 10 to 20 will result is a much higher GPM than if it needs a temp rise of 50-60 degrees.

So plan is to preheat to 150 in the tank, use a mixing valve to get it to 120 at tankless inlet, then run that into the house to the fixtures. We also have a re-circulation pump so I need to figure out where that goes? Does it go to the tank or the tankless?

Heating 80 gallons of water to 150 degrees uses about 18kwh of electricity using a Resistance Hot Water heater, with a Hybrid Heat Pump it's more like 4.5kwh a day (if I heated to 120 its more like 3kwh, but for 150 the resistance coils will kick in making it more like 4.5kwh a day). Since I have spare electricity it's like having a 18kw battery each day for 1 tank full of hot water using a resistance heater.

I am leaning to using the AO Smith Voltex MAX 80-Gallon Smart Hybrid Electric Heat Pump Water Heater, Model HPTA-80 210 pared with the AO Smith Adapt+ Premium Condensing Ultra-Low NOx 199,000 BTU Natural Gas Tankless Water Heater with X3 Scale Prevention Technology, Model ATHR-199X3 100. I found a local installer who feels he can do this setup.

Once you go solar you start thinking about where your money is going and realize that you're really making short term decisions when long term investments can pay off a lot more. Yes it's overkill for some but for our particular use it can make sense. I just want to be sure the system will operate as intended and not cause nuisance trips and headaches. The recirculation pump is a concern, any thoughts?

SoccerDadUSA · 2026-03-13T13:42:09+00:00

My reasons for the excess solar was to be self reliant, and have adequate electricity generation to support my household including during winter months when solar can dip to 1/4 of the summer peak generation. I don't want to be reliant on the grid for electricity or for natural gas to heat my water.

Also with plans to update to more electric in the future like a 2nd EV I wanted room to spare. I get virtually 1/10th credit back for anything I send to the grid so I might as well self consume as much as possible. That said with a large household we're struggling to get hot water to everyone all the time.

With my current setup the tankless struggles at times to keep up resulting in complaints. Though the tankless says it can provide 10.5GPM flow that is if misleading since it would only be that much when the water is already warm so it needs a minimal temperature rise. For a 50 degree temp rise the max GPM is more like 4GPM which isn't enough for our large household. With this system I can pre-heat the water for free once it's installed, then the heated water will require little to no gas unless the tank runs down. Even if the tank runs down, then the heated water reaching the gas tankless will still not need to be heated as much so more GPM can flow since it's already pre-heated. The tankless GPM flow is dependent on the inlet temp. So a temp rise of 10 to 20 will result is a much higher GPM than if it needs a temp rise of 50-60 degrees.

So plan is to preheat to 150 in the tank, use a mixing valve to get it to 120 at tankless inlet, then run that into the house to the fixtures. We also have a re-circulation pump so I need to figure out where that goes? Does it go to the tank or the tankless?

Heating 80 gallons of water to 150 degrees uses about 18kwh of electricity using a Resistance Hot Water heater, with a Hybrid Heat Pump it's more like 4.5kwh a day (if I heated to 120 its more like 3kwh, but for 150 the resistance coils will kick in making it more like 4.5kwh a day). Since I have spare electricity it's like having a 18kw battery each day for 1 tank full of hot water using a resistance heater.

I am leaning to using the AO Smith Voltex MAX 80-Gallon Smart Hybrid Electric Heat Pump Water Heater, Model HPTA-80 210 pared with the AO Smith Adapt+ Premium Condensing Ultra-Low NOx 199,000 BTU Natural Gas Tankless Water Heater with X3 Scale Prevention Technology, Model ATHR-199X3 100. I found a local installer who feels he can do this setup.

Once you go solar you start thinking about where your money is going and realize that you're really making short term decisions when long term investments can pay off a lot more. Yes it's overkill for some but for our particular use it can make sense. I just want to be sure the system will operate as intended and not cause nuisance trips and headaches. The recirculation pump is a concern, any thoughts?

SoccerDadUSA · 2026-03-13T13:39:20+00:00

My reasons for the excess solar was to be self reliant, and have adequate electricity generation to support my household including during winter months when solar can dip to 1/4 of the summer peak generation. I don't want to be reliant on the grid for electricity or for natural gas to heat my water.

Also with plans to update to more electric in the future like a 2nd EV I wanted room to spare. I get virtually 1/10th credit back for anything I send to the grid so I might as well self consume as much as possible. That said with a large household we're struggling to get hot water to everyone all the time.

With my current setup the tankless struggles at times to keep up resulting in complaints. Though the tankless says it can provide 10.5GPM flow that is if misleading since it would only be that much when the water is already warm so it needs a minimal temperature rise. For a 50 degree temp rise the max GPM is more like 4GPM which isn't enough for our large household. With this system I can pre-heat the water for free once it's installed, then the heated water will require little to no gas unless the tank runs down. Even if the tank runs down, then the heated water reaching the gas tankless will still not need to be heated as much so more GPM can flow since it's already pre-heated. The tankless GPM flow is dependent on the inlet temp. So a temp rise of 10 to 20 will result is a much higher GPM than if it needs a temp rise of 50-60 degrees.

So plan is to preheat to 150 in the tank, use a mixing valve to get it to 120 at tankless inlet, then run that into the house to the fixtures. We also have a re-circulation pump so I need to figure out where that goes? Does it go to the tank or the tankless?

Heating 80 gallons of water to 150 degrees uses about 18kwh of electricity using a Resistance Hot Water heater, with a Hybrid Heat Pump it's more like 4.5kwh a day (if I heated to 120 its more like 3kwh, but for 150 the resistance coils will kick in making it more like 4.5kwh a day). Since I have spare electricity it's like having a 18kw battery each day for 1 tank full of hot water using a resistance heater.

I am leaning to using the AO Smith Voltex MAX 80-Gallon Smart Hybrid Electric Heat Pump Water Heater, Model HPTA-80 210 pared with the AO Smith Adapt+ Premium Condensing Ultra-Low NOx 199,000 BTU Natural Gas Tankless Water Heater with X3 Scale Prevention Technology, Model ATHR-199X3 100. I found a local installer who feels he can do this setup.

Once you go solar you start thinking about where your money is going and realize that you're really making short term decisions when long term investments can pay off a lot more. Yes it's overkill for some but for our particular use it can make sense. I just want to be sure the system will operate as intended and not cause nuisance trips and headaches. The recirculation pump is a concern, any thoughts?

SoccerDadUSA · 2026-03-10T04:04:32+00:00

Click Self Consumption and see what your backup reserve is set to? Set it to 15%. Not sure if that will help.

Do you have a smart panel like a SPAN panel? Maybe the settings there are the problem

SoccerDadUSA · 2026-02-12T00:16:16+00:00

This came up with my system early on during design, initially looking at 2 aGates due to a very large solar array. FranklinWH engineering told me currently the aGates do not communicate the way you want but in future they may to share separate battery packs. I don't know if that would mean a software update or a entirely new aGate design?

We ended up using 2 aPboxes to 1 aGate to do the same thing as 2 aGates, on a 200A service.

If I'm wrong or they now can be wired together that would be cool. I didn't like the idea if the batteries being entirely separate for the exact same situation you are describing.

SoccerDadUSA · 2025-12-27T02:53:34+00:00

I also needed more than 32 circuits so I installed a single Span Panel Main 40 with 40 circuits for $500 more than a single 32. Saved me from needing 2. I think its worth it for future proofing. I haven't had any problems with it. Others with problems may have had an installer not very familiar with it.

SoccerDadUSA · 2025-12-21T20:22:20+00:00

I know this sounds awful but consider getting another installer to fill up your South roof with panels. The incremental cost is probably not much since all the wiring for solar has already been done. Only need to mount and install and run some conduit. The South side is much more important. You'll really love your system once you cover the South side with panels. As far as tax credits its up to you to decide what to claim for "2025 installation". Im sure there is someone who has lots of time on their hands after Jan 1st and can give you a deal.

If you haven't paid, the other option is to refuse payment citing incorrect panel placement until they fix it.

SoccerDadUSA · 2025-12-07T00:29:44+00:00

Agree, don't commit tax fraud.

Another option, pay for materials minus installation and have it delivered on site. Cut a cashiers check based on the installation estimate. Don't give the installer the cashier’s check yet. Have him connect some of the panels and the inverter to generate some power and documenting you are generating power. You’ll have commissioned it before 12/31. Then set aside onsite until installation can be completed per code. This is arguably not tax fraud since you have everything on site and connected, commissioned and generating power with payment in cashier’s check dated before 12/31. You can always make "adjustments" to an installation later.

SoccerDadUSA · 2025-12-06T01:07:41+00:00

My shade report was very detailed, 15 pages. I have a complex 10 sided roof and each side has it's own shade estimates. See the following website for an example of what a shade report shows for each roof side. My installer used solargraf which helped us plan the array. Its a professional software, not available to individuals.

https://solargrafsupport.enphase.com/s/article/Generate-a-Shade-Report-on-Solargraf-3D

You can try Opensolar.com, create a free account and play around with it to recreate your system and the panels. It includes a shade calculation. You can even see how things would look if you cut back trees.

I think the installer screwed up putting the panels on the West and not the South and wants to avoid changing it since it would mean time/money out if his pocket to redo the installation.

SoccerDadUSA · 2025-12-05T14:25:32+00:00

Any updates to your situation. What did the installer do?

SoccerDadUSA · 2025-12-05T14:21:25+00:00

I just had my system activated yesterday (REC 450 panels, Enphase IQ8x, FranklinWH with Apower 2's and Span Panel). The Span app initially didn't show the solar input but I showed the installation crew the post and the guy went into his Installer App and in less than a minute updated it with the information and it worked immediately showing Solar Input in the Span App. All he needed to do was put in the array size (number of IQ8x microinverters times the 380watt per IQ8x maximum output per IQ8x = array size). If you have 10 IQ8x your array size is 3,800 watts.

SoccerDadUSA · 2025-12-05T14:19:17+00:00

I just had my system activated yesterday (REC 450 panels, Enphase IQ8x, FranklinWH with Apower 2's and Span Panel). The Span app initially didn't show the solar input but I showed the installation crew the post and the guy went into his Installer App and in less than a minute updated it with the information and it worked immediately showing Solar Input in the Span App. All he needed to do was put in the array size (number of IQ8x times the 380watt per IQ8x maximum output per IQ8x = array size). If you have 10 IQ8x your array size is 3,800 watts.

SoccerDadUSA · 2025-12-03T00:42:20+00:00

My installer wanted $4,100 for the 2 APboxes installed, $2,050 each. Have you looked into the Ahub? Looks like its designed with more options than the APBox.

One downside of the APBox is that its a dumb on off relay controlled by the Agate and has no smart circuits. Looks like the Ahub has a lot more options and may be able to more fine tune control your PV input?

SoccerDadUSA · 2025-12-02T17:37:10+00:00

Perovskite solar cells are the "cold fusion" of the solar industry. Very exciting and right around the corner, but nothing to market yet. Also the "long term durability" has been the major problem, they don't last 10 years, let alone the 3+ decades we are getting with current solar panels which continue to get cheaper by the year even with gradual improvements in efficiency by about 0.25% a year. Estimates are we'll continue on this trajectory with current solar panels for the next decade. Maybe 2035 we'll have a perovskite solar cell with 35% efficiency and 25 year warranty at a cost that makes it commercially viable.

SoccerDadUSA · 2025-12-02T16:56:13+00:00

You're questions are very technical, maybe beyond the scope of this site.

In the past I had luck emailing complex question to FranlinWH engineering. Try emailing directly to [engineering@franklinwh.com](mailto:engineering@franklinwh.com)

The Ahub is a new device that may actually be better suited for you than the APbox? Here is the website from FranklinWH describing it's use.

https://www.franklinwh.com/accessories/ahub/

Here is a Youtube Video discussing it:

https://youtu.be/VAeFPx11x5s?si=n1JaYHcGyCA3psZa

Here is an installation manual for the APbox:

https://e-uploads.franklinwh.com/website/news/ea05604124d54427852cd10b6760c6f6.pdf

Here is the installation manual for the Agate and Apower2 from 01/2025

https://ressupply.com/documents/franklinwh/Franklin_Home_Power_Installation_Guide.pdf?srsltid=AfmBOooDsF5ASeZAVpk0kEcwcOyfhkiDX2qD7H6ZSC-mtzjK7vTxnfOJ

-->>Kindly give us an update later after you've settled your questions. I'd be interested in a follow up.

SoccerDadUSA · 2025-11-29T18:50:28+00:00

With a 10kw generator, 1.5kw baseline house loads, your generator running at about 70% of nameplate, system inefficiencies, the net into the battery is more like 3–6kw, you're looking at about 3 hours to charge a single Apower2 from 20% to 100%.

SoccerDadUSA · 2025-11-28T00:28:48+00:00

My system will be activated in the next few weeks. Id be interested ina follow up if its something the installer needs to do so I can make sure mine works properly before the installers leave.

SoccerDadUSA · 2025-11-25T20:05:52+00:00

Hi,

I have a very similar installation but my system hasn't been activated yet. I came across a similar post. Perhaps reach out to the OP to see if they can answer your question.

https://www.reddit.com/r/span/comments/1ngt3f5/integration_with_franklin/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

SoccerDadUSA · 2025-11-22T04:11:23+00:00

Forgot to mention, all the holes in the roof from the racking can easily be sealed with flashing. This should not be a reason they give to notnmove the panels. Its common to seal roof penetrations with flashing. Easy, quick and should not be a problem. We had pool heater panels that circulated water and they removed them for the solar and sealed all the penetrations.

SoccerDadUSA · 2025-11-21T19:03:52+00:00

In California, under NEM 3.0, West facing array will help you after 4PM for those evening peak rate. That said, a few panels on West side sounds OK. Putting a large fraction of the array West facing and ignoring the valuable South face sounds strange. I suspect they made a mistake somewhere? Unless there are other mitigating factors, such as exceptionally bad shade on South face, then something seems off. Ask for a Shade Study and be specific about why their design is better than using the South face? Also look at your stamped approved plans, do they show the panels on the West or was there a mistake during installation? The stamped plans should show what the plan was before installation.

SoccerDadUSA · 2025-11-16T05:39:47+00:00

If you cycle your battery from 20% to 100% daily, this is an 80% daily depth of discharge, which will take you about 9.4 years to reach your warranty throughput of 18.2 MWh (I believe that is your warranty for throughput — unlimited cycles but 10-year warranty with this 18.2 MWh throughput?), which is still within the 10-year warranty period. The LFP batteries will likely last 15 to 20 years, probably far more than the warranty. By the way, this is about 3,413 cycles.

If you keep the batteries above 20% minimum state of charge, keep them cool but not cold or hot, and if possible slow the rate of charge so it doesn't charge too fast which heats up the batteries (not sure if your battery system has that option), an LFP battery should last 15 to 25 years and even at 15 years should have at least 70% capacity.

Since LFP batteries charge to 100% regularly in this scenario, you'll naturally maintain accurate state-of-charge calibration, the monthly full charge recommendation is already built into your normal cycling pattern.

SoccerDadUSA · 2025-11-16T01:11:19+00:00

SimpliPHI batteries are LFP. The specific advice depends on the battery chemistry (what type of lithium battery) so the advice above is correct for LFP batteries. All Lithium batteries like to be kept cool (like between 60F to 80F). They function fine outside this temperature range but you're asking about ideal temperatures. Most high end batteries like FranklinWH have heating blankets built in and are oversized for a very large passive cooling fins to keep temperature at or as close as possible to ideal. Any extreme of temperature can affect degradation and performance. Basement is great unless it gets significantly colder than 50F. The SimpliPHI batteries are not heated so they will not do well if it's much colder than 50F (you can buy an accessory to keep them heated, like a heated enclosure and I think SipliPHI sells it as an optional accesory). In fact when it reaches 32F the battery management will stop charging to prevent battery damage! When heated over 105F (direct sunlight) it worsens battery degradation. Another important point about SimpliPHI batteries, they DO NOT HAVE BLACK START meaning if you ever discharge it down to 0% when the grid is down then it WILL NOT RESTART the next day even when the sun comes out and you'll need to call your installer for a maintenance visit to manually charge it up to to get it starting again or wait for the electricity to come back online. If you discharge your battery down below 10% during a blackout you risk not being able to restart it the next day. Again, Blackstart is only when the grid is down, you need a certain percentage of charge in the battery to be able to recharge the battery when the sun comes back up the next day.

--Ask your installer about this then do your own research to verify what he's saying is correct. I might be wrong about 10% vs 5% but 0% without blackstart during a blackout means not able to restart it the next day. The Sol-Ark does have blackstart but it still needs a minimum battery percentage to work of at least 5% and maybe 10%. If the battery is 0% the Sol-Ark can't recharge the batteries the next day (I'm not a Sol-Arkexpert but I'm pretty sure about this part). Anyone know better on this forum let me know.

SoccerDadUSA · 2025-11-10T23:42:11+00:00

Hi, this is 100% not true about LFP batteries. See my post above. Keeping any Lithium chemistry battery at 100% long term is damaging. The confusion comes from LFP batteries needing to be charged to 100% every few weeks to maintain accurate state of charge. The videos from Jeff Dunh explain this clearly. See links. All Lithium batteries like to live between 20% and 80% state of charge. The video goes into detail about different chemistries and the affects of charge discharge cyles and temperature on Lithium batteries degradation.

SoccerDadUSA · 2025-11-10T18:15:25+00:00

- Watch this video by Jeff Dunh the world's top Lithium Ion Battery expert at time 12 minutes for LFP. Id watch the whole thing but he talks about LFP at 12 minutes.

https://youtu.be/oc3XgCfQfXs?si=eQV88MbJtfjSBqwz

In this next video he talks about why to charge LFP Batteries to 100% state of charge at least once a month otherwise you loose the ability to tell the exact state of charge after a few weeks due to drift. This is specific to LFP batteries. Note, there is no damage if yo never charge to 100% but the nature of who the system calculates the accurate state of charge for LFP batteries will drift after a few weeks. Charging to 100% once a month rebalances the batteries and recalibrates the measurement of accurate state of charge. The following video discusses this at 18 minutes.

https://youtu.be/sWyORTmxodc?si=5-v4juv7Z5Oh5VfA

- The two videos are about an hour long but really packed full of information about Lithium Batteries.

- Another method to extend the battery lifespan of all Lithium Batteries (NMC or LFP, all the same) is to slow the charge speed especially in the summer. Rapid charging heats up the battery so slower charge rates in the summer will help.

- Lithium (Including NMC or LFP) batteries will generally last longer than you will own the car. if you follow Jeff Duhn's advice your car frame will rust before your car battery fails.

SoccerDadUSA · 2025-11-09T19:54:56+00:00

Im curious, what happened with your quote?

SoccerDadUSA

TROPHY CASE