Series Piped Water Heaters - Opportunity Load and Propane

MarkP

You know, I never tried it.  The manual says it won't work using pre-heated water.  I do notice when the water in the pressure tank is used up and the pump comes on.  The water from the tank outside is usually colder and I have to adjust the shower valve slightly.  Maybe it would work OK but the mfgr just got tired of complaints from people who thought they shouldn't have to do any adjusting.  Anyway I am optimistic that this system will provide for my daily needs.  A 119 gallon tank ought to last me several days from fully hot to unusably cold even without heating in between and I expect that the solar system to be able to keep up with my use.  I got the heater for $250, a real steal, so after the initial investment there really is no downside for me as long as the batteries get charged.

Dave Angelini

BB. said:

For many (most?) solar hot water systems, you need a tempering valve on the output of the solar hot water tank. The system may heat the water to ~180F, but you do not want much more than 120F or so at the shower/bath tube fixture.

The tempering valve can limit the maximum temperature to the Rinnai--But you will always be using LP. Another option would be to have a solinoid switch--When the solar tank drops below 110F, it switches the input to the LP Rinnai.

Looking around on the web, I see Rinnai US has a "hybrid" booster. Connect to your water heater (gas/electric, possibly solar?):

https://www.rinnai.us/hybrid-tank-tankless-water-heater

And Rinnai in Australia and New Zealand seem to offer solar solutions (solar collector+storage tank+Rinnai):

http://www.rinnai.com.au/hot-water/solar-hot-water-systems/

And a few years ago, they had a RH360 system that appeared to be designed for solar hot water support, but I do not find any current information about the product:

http://www.ecobuildingpulse.com/products/solar-tankless-water-heating-backup-from-rinnai_o

-Bill

If one is letting the controller get the tank water get to 180F, they will be needing a new tank long before it's time.

Estragon

If using pv, I would control with thermostat. If using direct solar, the temp from the collector tank could be controlled with a mixing valve to avoid scalding and maybe issues with the tankless downstream.

I'm not sure how to control the temp in the collector tank with direct solar though. The circ pump could be set to turn off at higher temps I guess. It should probably have an expansion tank somewhere too, so the PRV on the tank stays closed? I would likely use a simple collector with just a coil of black poly water pipe in the sun. Maybe that wouldn't get hot enough to be an issue. It expands some in the heat, and as a water line it seems fine sitting in the sun all day.

Estragon

Interesting Rennai links. It sort of implies their heaters don't mind hot water input.

I could probably rig up a "hybrid" heater with my unit. It looks like it's just a tank (no second heat source) that the heater circulates to. The advantage to that would be the ability to supply multiple fixtures at once, and to eliminate a problem with all on-demand heaters - it takes the heater a couple of seconds of flow to fire up. If hot water was used recently, this ends up sending warm water already in pipe, followed by a couple seconds of cold water, followed by hot water. Okay once you're used to it, but the warm-cold-hot sequence is a bit wierd if you're already in the shower and think you had the right setting based on the warm part.

Odd they don't do the solar thing in the US. I can see Australia being a good market for this, but you'd think areas like AZ would be good too.

mike95490

I have 3 heat sources in series.   My final is a Eternal Tankess, and it's fed via a tempering valve to limit it's input to 120F.
Things have been running just fine.  First is a 100gal storage tank heated by a thermosiphon loop in the Masonry Heater.  That 90F water goes to the roof top Rheem heater, and it's output goes to the propane tankless.  only in the summer, does the mixing valve start to temper the water.

solarvic

I have been experimenting with a 24 volt  300 watt dc element connected to my battery and it keeps a 50 gallo0n waterheater supplied with hot water. It useses about 13.25 amp continuiously. I did it that way so I can use the inverter for supplying  house. I have been using grid power to keep the battery charged . I tried to ac couple with a grid tie inverter to my off grid inverter but it wouldn,t work as I didn,t have the right off grid inverter. So I was left with the choice of selling my off grid inverter and getting the right one. I advertised it here but there wasn,t any interest so I decided to use charge controlers since I already had them and the wire. I can,t get approval to use the gridtie inverter as It overloads my service panel.

Dave Angelini

Estragon said:

Odd they don't do the solar thing in the US. I can see Australia being a good market for this, but you'd think areas like AZ would be good too.

Why would you say that? Just because the store here does not do much with solar hot water from the sun does not mean that many offgrid don't use the sun. It works much better in winter than PV does if you are offgrid and need hot water daily.

Estragon

Dave, that was my point - solar hot water would work well in many parts of the US, so wondered why Rennai apparently chose not to sell solar gear.

Dave Angelini

Solar domestic hot water does and has worked well. When it started here and other southwest places in the 70's long before PV they had not perfected the drain back system and were plagued with bad design in freezes. 

I don't know anything about Renai but if I needed an on demand I would do what Mike did in his schematic. We do similar but really do not need the on demand as when it is cold enough, the wood stove is on.  Many of my clients just do the standard for radiant heating Offgrid and tap it for domestic hot water. Just a tank, collectors, a few pumps, and a drainback valve. The controller can be elaborate or just a low and high hot water snap switch and a timer. many just use an open loop and drain it in cold weather. Some just throw a blanket on in the southwest. Other places it has to be a drainback. On really overcast days it easily gets up to 130F.

Using PV just seems clunky and there is very little chance anyone I know would want to depend on PV for daily, reliable, hot water for cleanliness and health. In winter it is just another thing to break the camels back offgrid if it tries to heat water on low battery. It is one more thing I have to manage for (some) them and I like smooth sailing.

For an enthusiast of using PV for hot water go for it.

 I always wonder how they keep it hot enough in winter that a woman would approve