Adding solar hot water. DHW only or hydronic heating as well?


New Member
I am looking at adding solar hot water to our place and was hoping to get some help with the design theory.

The current hot water system is roughly set up like this:
Existing System - Simplified.jpg
There is an oil boiler that heats water for 7 zones. The first 6 zones are radiant heating in the house. The 7th "zone" is a 40gal DHW storage tank. When one of the thermostats in the house, or the DHW storage tank, requests heat then the pump for that zone and boiler kick on. It appears to be a closed loop with a cold water inlet (not pictured) to keep the system topped off.

My first thought is to just replace the existing DHW storage tank with a larger tank that has 2 heating elements. Then connect the boiler to one element and the solar hot water to the other. Something like this:
Option 1 - DHW Only.png
I've run across a few designs like this so I don't think I'm too far off. The idea is that the solar collectors will keep the DHW tank heated most of the time but the boiler can kick in to supplement our DHW needs if it's cloudy or whatnot.

But what if I wanted to also heat/pre-heat the water for the radiant heating to stop burning as much oil? (Assuming the solar collectors are large enough.)

I've had a few ideas like adding a small tank to the boiler's system and using excess heat after the DHW solar heating element to heat that 2nd tank. Or maybe leave the existing small DHW tank in place and add a large tank to the boiler's system and just pre-heat all of the water via the solar collectors?

Anyone have a similar setup or thoughts on such a system?

Thanks in advance :)


Solar Enthusiast
JesseC: A few comments to get you started: using pumps on each circuit is old school, and highly wasteful. I see that is your existing system. My first thought is rip that out and replace. I've done them, as large as 30 plus pumps. These days we use Caleffi or Uponor distribution manifolds, and put a Grundfos Alpha 2 pump (or other brand like Taco, B+G, etc) on the manifold. Instead of typical small circulators that use about 45-90 watts each, we see the Grundfos running at 8-28 watts and replacing numerous individual pumps. Plus of course less restriction, less hardware, etc.

80g is a pretty big DHW tank. Most boilers can make a couple gallons of hot water per many people do you have taking showers at the same time? I use an 80g tank for the buffer tank, and a 45 gallon DHW. Have never been short of hot water.

If you hook up solar hot water panels, the big decision is pressurized with a heat exchanger, and generally speaking a glycol solution. Or, do a drain back system. I prefer drainback due to our very cold winter temps. You are likely to use a water to water heat exchanger on that solar collector. B&G makes nice ones that are reasonably priced. Get one with at least 1" fittings.

Take a look at TACO zone control panels. Very nice, very simple.

Good luck.