tankless with solar pre-heat

hot_rod

any experience with different models that work with warm incoming water? I like the new Eternal GU100,000 with the small tank and all stainless construction. It seems a nice size for residential the has pre-heated source and could be supplied with a 1/2 gas line, nice for retro-fits..



The marketing folks tell me it would void the warranty to use it with solar, although no reason for that? Seem like a perfect product for this application.



Anybody have experience with this new model?



hr

Paul Rohrs_14

Minimum Delta T?

I am curious as to why they would void the warranty as well.  I know that one tankless manufacturer (gained notoriety from Paul Harvey) needs a minimum of a 40 degree Delta T to sustain the minimum firing rate.



My opinion, the more sophisticated the control for tankless, the more opportunity for unit to not fire when solar preheated water enters the unit.



Best,



Paul

Jack

No experience with the Eternal

But, Paul, the deal with any tankless, Rinnai or otherwise is the GPMxDeltaTx500=BTU. Push those numbers around based upon the min flow and min btu to initiate operation.



I do not like to supply the water from a solar tank to a Rinnai or any other tankless, as all you arei doing is giving superfluous operation and cycles to the limits, when the solar waters very hot. I think a limit is a safety device and as such should not operate except in a safety control situation.



I have seen people put a tampering valve ahead of the tankless. That works, but does not maximize the solar contribution. It is simple, but kinda crude.



My preferred application with solar is to put a 3way between the solar tank and the tankless, controlled by an aqua stat in the top of the solar tank. When solar hot is available you divert the water directly to the house thru the tempering valve. When the water in the tank drops to X* (you have to figure that one out) the 3way diverts to the Rinnai. This is where you have to "play around" on the set-up as the above formula comes into play. You want to pick the the tankless that will give you the lowest flow and btu input. That way your approach temp is lowest.



Hot Rod, I think it was you who first told me that Caleffi makes a 3way suitable for this application. For another option I like the Bonomi 3way. They have the lowest turning torque in the business (that I'm aware of) and therefore you can use the small elec valve actuator..

VictoriaEnergy

solar + tankless & Warranties

Most on demand hot water heaters won't work with solar.



The issue is this:  Most on demand hwhs have sensors that measure flow and outlet temp only.  They have no inlet temp sensor.  So when preheated water enters the unit, it still fires the burner up to a rate close to where it should be for the expected outlet water temp.  When it sees the outlet water temp is way higher than it should be for that rate of water flow and BTU firing rate, the control shuts the unit down thinking it has a fault with the gas valve over firing the unit. 



Most warranties cover the item only when it is installed and operated the way it was originally intended.  Anything outside of what they allow in their instructions is almost always excluded.  It does not nessalry mean the item will fail if used differently, it just means they can't (and don't want to) guarantee anything that was outside the scope of the original design and testing process.

hot_rod

via a valve

Hot Rod, I think it was you who first told me that Caleffi makes a 3way suitable for this application. For another option I like the Bonomi 3way. They have the lowest turning torque in the business (that I'm aware of) and therefore you can use the small elec valve actuator..



Thanks Jack, yes Caleffi has a valve solution but I don't believe it is available in a no lead version yet, with the correct threads, and also required US listings. It does offer a safe, simple solution.

TonyS

Just installed on in the shop this morning

was given a dealer price so I could test the unit. Fired right up, but I had to go ,tomorrow I will take some pics and run it through its paces.

As far as the solar unit goes Hot Rod, I have been running mine directly through the Navien but first going through a tempering valve. It has been working fine for 4 years now.



Was at Bosch solar training a while ago and they had an idea they said was very common in Europe, using a mixing valve in reverse to act as a bypass valve.

I haven't tried it yet but I have a job starting soon that I am going to try it.

Look for yourself...page 62 of this manual

http://www.bosch-climate.us/files/201110041959120.Bosch_Solar_Design_Guide_102009.pdf

hot_rod

these valves

we build for the European market. There is also a motorized version. I'll get a sample or build one up and try it.



www.caleffi.com/en_IT/Technical_brochures/01164/01164.pdf

michael_34

Navien backup

I have used Navien successfully in the past, but I did put in a Caleffi three way motorized valve to bypass the tankless if the solar was hot enough. The system is controlled by a Resol E controller.



Can you use it without the bypass and just pass through the Navien? I'll check the manual when I get home.

HDE

Improvements are coming

I have spoken to a few tankless manufacturers on this subject.



First, quality electronically controlled tankless units do have cold water inlet thermistors so they are monitoring both inlet and outlet temps in conjunction with flow rate to control firing rates accordingly.



However being innovative, efforts were made in design to provide rapid hot water delivery as quick as possible to assist in minimizing hot water delivery time complaints. With that, most units fire as fast as they can to provide hot water, before realizing that heat is sometimes not needed. You see, tankless manufacturers designed units to make hot water, they weren't expecting hot water to be fed to the tankless heaters.



With that comes some newer innovations, at least one manufacturer Navien states that their next premium unit due out later this year will have an optional DIP switch to select a solar mode. This will delay firing, but will prevent temp overshoot by feeding hot water to the inlet.

ColoradoDave

Rinnai w/ Solar

Rinnai specifically told us to put a mixing valve coming off the solar to temper the incoming water down to a maximum of 130 degrees.

We did have some issues with minimum flow rate however.  We adjusted the mixing valve down so that the flow rate (.6gpm required by Rinnai) was at least 1 gpm on minimum fire.  I forget the actual temperature we adjusted the incomming mixing valve to, but I believe it was about 90 degrees.

The system has worked flawlessly since.  We're getting at least a 40 degree pre-heat (ground water in CO is usually in the 40's to 50's depending on where you live.

HDE

But such a waste

Minimum flow rates in conjunction to minimum fire rates of tankless can cause these near 40 degree rise results.

But such a waste, firing the tankless when an abundance of solar heated storage is available.

Jack

I agree

that the tempering valve ahead of the tankless is a waste of both gas and solar energy. As well, pick the Rinnai with the .4gpm and 10kbtu minimums. That is why you put the 3 way between the tank and the tankless. Max solar contribution of high temp water and only call the tankless when tank temp drops below the threshold...of comfort. You do have to work at the break point temp and Rinnai set-point temp to arrive at the best compromise.

NRT_Rob

if you have significant solar

use an electric on demand. infinitely modulatable.