solar dhw with tankless backup

Larry Weingarten

...answer with a question, but is Your Navien the type with the small tank built in and how far down can it modulate?

The problem as I see it with tankless backup to solar is that when the solar is giving you 110 degree water and you want a touch more, gas tankless cannot give you the right small amount of heat.

Yours, Larry

Unknown

solar dhw with tankless backup

I have recently started installing 2 solar panels on my roof with a navien tankless as backup. So far I have the navien installed and the old oil boiler and indirect fired wh removed. So the family still has hot water and I am ready to install the final components of the solar system(drainback w/8o gal internal coil,tekmar 157,evac tube)I have a copy of Caleffi idronics 3 which has a diagram from I believe Seganthaler (fig 24)that shows a motorized 3 way valve bypassing the twh until temp needs to be increased, which at that point the valve shifts and directs water through the tankless. Question is.. Is this needed and if so wont this allow a good shot of cold water onto the unsuspecting showerer when the valve shifts?

Thank You
Tony

Unknown

No my Navien is not the unit with the little tank (A model) I didnt choose this option because I didnt want any standby lost. Future system will include 6 more panels for space heating and will rarely need backup.

Unknown

sorry, 15,000 low end
Tony

hot_rod

How will it behave

when it sees solar water supply to it at say 70, 90, 110F? Some of those heaters start shutting down when the see warm incoming water.

If you use a 3 way valve to supply solar till it drops to say 120, then switches back to a cold supplied instantanous, you really minimize the amount of solar harvert-ability.

hr

Unknown

I understand what your saying Hot Rod but the problem I have is the shift in the 3 way valve is going to really let a shot of cold water through, chances are good when im the one in the shower. If I pipe it directly through the tankless, the water temp will degrade slowly but may not turn on before the water temp has become to low to finish my shower. hmmm.
I will try it directly but leave provisions for the 3 way if needed. Fortunately this is in my house because either way would be unsuitable for a customer.
Thanks
TONY

Unknown

I thought the idea of the valve was to bypass the instantaneous when solar temp were adequate, not to switch from preheat to cold supply.

If solar pre-heat is above 110 running it thru an idle instantaneous will unnecessarily heat the instantaneous's HX and we will be loosing some of or solar heat to this uninsulated coil that is now acting as a radiator, this makes the 3 way seem like a good idea to me.

The variable speed pumping feature, energy calculation, and reasonable price on the 157 makes that a very attractive controller.

hot_rod

At what temperature would the valve switch?

for bathing perhaps 105F, but for dishwashing you want to stay closer to 120- 130 for soap to work and bacteria issues? If you switch through the TWH at 120 will it modulate properly with a low delta T? Sounds like some brands, and models do, others will not?

As for the cold slug, it seems plumbers are adding small tanks and recirc pumps in some cases to overcome this issue. In my mind that defeats much of the TWH benefit.

This article explains the pros and cons of the tank addition

www.taunton.com/finehomebuilding/how-to/articles/add-a-tank-and-pump-to-a-tankless-water-heater-to-save-energy.aspx

Unknown

Yes, the Navien unit I purchased did not have the small tank with the internal circulator. I decided against it for the same reason as you said, it tends to defeat the purpose of the tankless,but in hindsight for the extra 140 bucks I should have gone that route.(Navien makes a model with 1/2 gal tank and circulator included)I could have piped it in tight to the three way valve and could all but totally eliminate the slug of cold water.Hmmm hindsight is always so clear.
Thanks