gas tankless for solar water heating backup

Royboy

anyone care to comment on their best practice for using a gas tankless backup for solar hot water? mixing valve used as a bypass/diverter? something else?

hot_rod

It depends on the brand and model you are working with. Some work fine with pre-heated solar feed to them, others not at all. Some brands offered a solar version back a few years, I think they tweaked the computer programing.

If you have a solar unfriendly version,a few mixing valves can correct, but you are adding additional maintenance components. In hard water conditions, the mixers could require yearly disassembly and cleaning. I suggest softening the water if you start adding thermostatic valve assemblies.

If you use mixers, pipe them so that the same cleaner acid that you run through the tankless, also run through the mixers. Clean the entire assembly in one shot by pumping a tankless cleaner solution.

We offer special solar/ tankless mixer stations in Europe for this application, but you can easily build your own, with two off the shelf valves.

Here are a few choices for thermostatics, or with one motorized.

With a three valve setup, the valve determines which source to flow, and a final 3rd valve assures you are always mixed to a safe temperature, should the solar tank unto 160 or so.

Or be sure to buy a tankless that can handle warm input, and ignore the above..

SWEI

That 262 looks interesting -- I'll have to read up on it.

We typically install a TMV at the solar tank outlet and feed the tankless from that, set for 4-6°F below the TMV setpoint. As Bob pointed out, you have to make sure the tankless will fire on a small ΔT. If the solar system is sized right (in our climate) we find that electric tankless really shine in this application.

hot_rod

SWEI said:

That 262 looks interesting -- I'll have to read up on it.

We typically install a TMV at the solar tank outlet and feed the tankless from that, set for 4-6°F below the TMV setpoint. As Bob pointed out, you have to make sure the tankless will fire on a small ΔT. If the solar system is sized right (in our climate) we find that electric tankless really shine in this application.

The 263 is another option.
Two concerns for the US market, low CV and it would need to be low lead construction and certification.


http://www.caleffi.com/sites/default/files/file/01164_09en.pdf

Jack

That is a pretty cool set-up HR. What I have done in the past is specify a Bonomi 3 way, electric actuated valve run off the tank temp. I like the Bomomi's as they have a low turning torque so you can use smaller actuators. Getting the hot water to the building supply mixer directly maximizes the solar contribution and makes the most sense...which Caleffi apparently knows. I have seen solar tankless systems where the water is simply run through the tankless. I can't say that it has hurt the tankless but I don't see where it makes sense triggering the limits on the tankless all the time.

hot_rod

Yep on all points. Knowing the solar DHW pre-heat can run in excess of 160°F on a good day, either on purpose or with temperature protection functions enabled.

I'm not sure how all the different tankless like that? Is there ever a hard lockout condition established from over temp conditions? maybe a push button thermal disc, for example.

That would be a drag to manually reset all the time.

We have that same motorized solution, but it needs a control to drive it also. I like the non electric thermostatic approach best.

With any valved solution keep water quality in mind. At elevated temperatures, minerals really precipitate out at a higher rate.

Lastly, remember any device used in potable water systems need to be low lead certified, not all the valves out there carry that listing.