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Short version:

I'm currently designing my water heating setup for a house I'm in the middle of building. Can you lend me your eyes to see if there's any fatal flaws you can see in the diagram I made?

Back story:

My system is going to be fully off-grid, running on solar (~10kw system with lots of 12v AGM batteries), so I can't do gas or blow through too much power. I've picked a heat pump water heater to save on electricity use (Rheem ProTerra professional model, 65 gal), but a big problem here is: it won't be enough to heat my in-slab radiant flooring by itself. To get enough heat for radiant flooring, I'll also be putting in a solar water heater and wood boiler which will also feed into the tank.

Here's where things get even more complex. I won't be running all of the systems at the same time, so I'll need a bunch of recirculating pumps, timers, switches, sensors, etc. Basic running plan would be: run boiler/solar water heater when I'm going to also be running radiant floor. Just use heat pump in "heat-pump-only mode" and solar when all I need is shower/kitchen.

If I'm getting plenty of hot water (sun is pretty reliable in California), then I can turn on the recirculating line going into the storage tank during the day, doubling my hot water storage. Idea there is that it should give me a giant battery of hot water, which I can use to heat my radiant floor (which also acts like a heat battery). Hopefully that will help keep my house comfy after the sun goes down. It gets slightly below freezing a few times per year, so I'm hoping my BTU needs won't be extreme.

The first tank in the picture wouldn't be actively heating -- it's just for storage. As a bonus, when the recirculating line going to it is turned off, it acts as a "tempering tank," gradually raising the temp of the water to the temp of my semi-conditioned garage space (free water heating!).

I've read that running a recirculating pump through domestic hot water return with a heat pump can either produce too much heating needs (water cools as it recirculates) or fails to trigger the heat pump to turn on (it detects incoming warm water, so doesn't bother heating it further). I'm hoping to minimize this by using timers, sensors, etc. to only use that part of the system when I need it.

I know it's a crazy system, but would it work? Is there anything I should re-configure or add? Personally, I like having redundancies and options built into it.

Other info:

  • 2 people to live in house
  • ~1,400 sq ft, single story
  • Crazy-go-nuts insulation (17" walls filled with spray foam)
  • Probably going to choose Grundfos circulating pumps (whichever size works with system and has low watt use)
  • These are only heat sources for house (except for wood burning stoves)

I based the diagram off of what I've learned so far and copied some parts of somewhat similar schematics I've found.

Diagram of hot water lines

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    Have you ever heard of the KISS principle. For two people and 1400ft house it looks like work and headaches.
    – crip659
    Jan 9 at 23:51
  • Yeah, this definitely isn't a simple setup. My wife and I are a bit of crazy preppers in this regard though, so multiple options is probably where we'd end up anyhow. The main issue I had with a simple system is that I can't figure out how to heat the house with limited solar power in the winter. There's already radiant flooring tubing in the slab, so I'd like to use that. Going with just a boiler would be an option, but that's more of a hassle to continually operate. So I'd like to have a heat pump as our DHW, then back it up with something that can handle radiant floor. Jan 10 at 0:35
  • With that much insulation, a wood stove should be plenty to heat the house. I heat a much bigger house with much poorer insulation in a much colder(-20) climate with a small wood stove. Every joint in a water system is a possible leak. Every pump will use power most of the time and a point of failure.
    – crip659
    Jan 10 at 0:47

2 Answers 2

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If you want a good single source system, then look at ground source heating. Having said that I had an almost complex system in the previous house.

Where I had a custom boiler designed with the flue that went through it to extract as much heat as possible from the fire. I had two heat exchangers in it as well. Then I had

  • wood fire in the same fire box under the tank.
  • heat pump heating with its own thermostat and circulating pump
  • and an electric element inside it with its own thermostat

The thermostats were in different heights in the tank. And each system was independent. It was designed so that

  • It would primarily work with wood
  • If I was too lazy to stoke it with wood, the heat pump would kick in
  • If that was not heating it fast enough, the electric element would kick in.

Primary output from the tank was for our use and the second heat exchanger was for under floor heating.

My recommendation would be to make sure each system is independent. So that others will work if there is a fault with any system. However, if I was doing it again, I would go for ground source heating.

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Seems far overwrought, and yesterday's tech. If someone proposed this in 2006 I'd say sure on things like the lead-acid batteries and solar thermal, but today, forget it. As far as complexity, it reminds me of Wintergatan's marble machine, the issues aptly discussed in this video.

First, I doubt the manufacturer intended the heat pump water heater for space heating applications. Ask them. But they make air sourced heat pumps for water heating, also I wonder if you’re insulating like you say, will your floors even need it?

Second, adding the complexity of solar heating. This is a total lost cause. The complexity is just "more stuff to break". Thermodynamics says it is a net loss vs using PV to run a heat pump water heater. It's only about twice as efficient as PV driving a resistive heater, and of course heat pump heaters are more efficient than that. So wherever you planned to put the solar thermal, add PV instead. You really cannot have enough PV.

I'm concerned about what you're going to do for A/C. Running cold water through a hydronic system is not a win, because it will cause condensation in places not designed to catch it. I'd feel better to see you use commodity mini-split heat pumps, honestly. Not least, because they are commodities, and you won't lose heat due to a single point of failure.

On the batteries, AGMs are yesterday's news. The world has gone lithium, and if you're DIYing this, the obvious source of battery capacity is EV wrecks. It's down under $100 a kWH, and is a battery that will last 20 years in home power conditions. AGMs, you get 5-8 years out of provided you sharply limit how much of their capacity you use.

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