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Solar and Rinnai

dynamic Member Posts: 20
So I have a job that I am installing heat and a/c via an Acadia heat pump.

So today, I get called in by the owner and wants to know how the solar system works with a Rinnai. I did not install the solar system. But it did make me question a few things and now I need help with answers.

The solar company has a 16 array of Sundra tubes on the roof, (2) 40 gallon electric water heaters, a flat plate heat exchanger, and 30 feet of baseboard in the crawl space as a dump zone. The home is energy star rated, so it is extremely tight, has the concrete in the crawl space completely encapsulated with 4" of foam and spray foam in the sill plates. The crawl space floor is dirt. This system is piped through a Rinnai tankless water heater. The house will be empty more than not.It is a british rental house.


My questions are:

A) I say that the dump zone baseboard will affect my cooling loads. The solar guys say that it wont, they say the earth will absorb the energy produced from the dump zone.  That dump zone will be working all of the time as the tanks will heat up and remain unused.

B) The solar guys claim that the Rinnai will work seamless in conjunction with the solar system. I believe that there are going to be short cycling issues with the Rinnai. I believe that because if the solar can only heat the tanks to 110 and the Rinnai is set at 125 degrees, when the water flows through the Rinnai and turns it on, it will heat up to 125 too quick and shut down. This will cause water temp fluctuations. I say that there needs to be a buffer on the outlet of the heater.

So please help me. I will be talking with the solar guys next week.



  • hot_rod
    hot_rod Member Posts: 22,505
    What is the output

    of that array. Look it up on the SRCC site. I would guess 20- 30,000 BTU per DAY. So you are not talking about a lot of energy to disapate. Some will go to DHW even in an empty home.

    Is the crawl space ventilated to allow some of that energy to escape? What about insulation between the crawl and living space, the floor joists insulated?

    Not all brands and models of tankless get along with solar pre-heat, for the reason you mentioned. it seems like more and more tankless are being installed with a small buffer to address some of the issues like cycling and the cold sandwich.

    Fire it up and see how it cycles and run the concept by the tankless manufacture for their piping suggestion.

    That is one of the challenges with SDHW when there is little or no load for extended periods, with solar thermal you have to use it or lose it.

    I think the flat plate collectors work better in a small loaded system like that. You can use the re-cool or system cooling function to cool the tank down at night. But it does take energy to "shed" the temperature.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Fortunat
    Fortunat Member Posts: 103
    if noone is home

    If no one is home to use hot water, why would the A/C be turned on?

    A 16 tube Sunda system is pretty small and if it is controlled properly and heating both 40 Gallon tanks, the use of the dump zone will be very modest, even in the absence of any DHW demand. At 180 or 190 degree tank temperatures, the collectors will be over 200 degrees and even evacuated tubes are starting to roll down the efficiency curve at that point.

    Your other question is a good one. As someone else said, not all on demand water heater's play well with solar. For example the ones that don't actually measure water temperature but just use a proportional gas valve can result in substantial scaling risk when fed pre heated water.

    In general though, the Rinnai is pretty decent as a complement to solar. It modulates pretty low so the problem you describe is rare (though not impossible). We generally install the mixing valve between the solar tank and the rinnai and set it to 125 degrees and then set the Rinnai setpoint to 116. That way, most of the time the rinnai stays off.

    In the case when the solar preheat temp is just below the Rinnai setpoint AND the hot water draw is small so that the demand is below the Rinnai's minimum firing rate (10,000 BTUh input), you will in fact have short cycling of the water heater.

    I'd encourage you to install the system with the option to add a buffer tank in the future, but leave it out for now and ask the homeowners for feedback about how it works.

    Good luck


  • atomic
    atomic Member Posts: 9
    issues with on demand

    The on demand heaters can only turn down so far, and if the incoming water temp is 90'F or higher the things shut off the gas as output temps get too high. This is a glitch in these designs that render them unpredictable. We now prefer to install electric back-up tanks to complement solar because of this issue. And with on demand efficiency around 80% for the affordable units, and electric tanks at 92% eff., energy savings result. Make sure to add some insulation when using tanks to reduce standby losses.
  • Jack
    Jack Member Posts: 1,047
    It can work well...

    but a couple things. I represent Rinnai and back in the day was in the solar business. My preferred way to pipe a solar/Rinnai is to install a 3-way valve between the solar storage tank and the Rinnai. My preferred 3-way is a Bonomi. The way Bonomi supports the ball in their 3-way gives them the lowest turning torque of any 3-way I've seen. This allows a small hp actuator to be used to drive the valve. You could do the same with solenoids but I just hate solenoid valves. to unreliable in my experience. The 3-way valve is controlled by your A-stat or solar controller based upon available temp at the top of the tank. When hot water is available, through the 3-way by-pass the Rinnai and go direct to the house (tempering valve). When the tank temp drops divert to the Rinnai and use it to peak the water temp.

    It is true that you have to kinda play with this diverting temp a bit and you must understand how the system will operate. Rinnai has recently improved their flow/fire operation. In the past the minimum flow and fire to initiate operation was 15kbtu and .6gpm. The new units require .4gpm and 10kbtu to operate. This is a substantial improvement all the way around but really improves operation with solar as it reduces the "balance point" hassle. You need to test the operation of the system to properly advise the owner as to how their system will work. It is possible at LOW flow rates in the 100-110 tank temp range that the Rinnai may not be able to fire. I haven't seen this with the new units, but given the way tankless work and the fact that I haven't seen it all (yet) that we could end up with and issue until the tank temp cools to let the Rinnai do it thing. This is an issue at low flow rates. These systems work well. Make sure you are clear on these issues though before you "put it out there" to your customer, especially on a job you didn't design or build.  Btw, one thing that impacts the on-demand with solar is the tempering valve.

    I've also had dealers put a tempering valve ahead of the Rinnai and reduce the temp so the Rinnai always fires. I don't care for that as the purpose of putting in a solar system is to maximize the solar contribution. To my mind that means when you have hot water in the solar tank, use it. The goal is to be pumping as cool a water temp as possible to the collectors to maximize their efficiency. Let us know how you make out.
  • hot_rod
    hot_rod Member Posts: 22,505
    a valve like this

    we build thermostatic and motorized versions of this mixer. We have not brought them to the US market yet. Is there a need?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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