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.


New Member
Solar DHW is about 20% of home energy cost, and in the system you have, quite feasible to implement, but a standard dual coil tank is useless for solar since both heat exchanger coils are generally far too small for effective heat transfer. (Boiler coils are typically 8’ of copper tubing, since the boiler provides 180 degree water on demand - solar heat exchange coils are designed to operate at lower temperatures, generally 130 degrees F maximum and therefore need more surface area, generally a coil of about 80-100’ of copper tubing.

The key to solar thermal design is to use solar to raise the temperature of the incoming water from a typical 50 degrees to one much closer to the 120 degrees most DHW systems produce. This difference in temperature is called the ΔΤ or delta T. The lower the ΔΤ the less energy is needed to reach desired output temperature. This energy requirement is calculated easily with the heat energy formula Q=VρCΔΤ or energy = volume of water in pounds of mass X specific heat of water (which is conveniently 1) X the change in temperature. If your cold water is 50 and you want 120 deg hot water, the delta T is 70 degrees. If you’re hearing 80 gallons, that’s 666 pounds of mass. Specific heat constant is 1. So it takes 46,648 BTU to heat the tank.

Now - if you preheat the cold water to 100 degrees with an in-line solar equipped tank so that the boiler tank only has to peak it to 120 degrees, the boiler only has to provide 13,320 BTU. In most climate zones, two 4x8 solar collectors and an 80 gallon solar tank can knock that out even in overcast days.

Space heating is doable - especially if you’re using in floor radiant heat, which uses much lower temperature water than baseboard / radiator heating. It requires a large thermal mass of storage water - a 500 gallon insulated tank with a 100’ copper solar coil on the bottom and another 100’ copper coil near the top that will preheat you boiler circulation zone return water (again lowering the ΔΤ) so the boiler uses less energy.

Solar thermal is a little trickier than PV because you have to use the heat captured and don’t want to make too much / it’s in efficient and can be dangerous. Generally you size for summer and peak in winter with another heat source - the goal is to get 50% of your annual energy from solar.

Rheem makes a really nice line of solar tanks, with built in peaking capability - the electric element peaking one is my favorite. Here’s a link:



Solar Enthusiast
I built from scratch DHW panels and heat exchanger and have been using it for 15 years. I am in Arkansas. It has 60 gal. Tank, heat exchanger with 2 pumps. I bought a controller. Pumps are 60 watts each. From May to September I have 155 deg water except for cloudy days. October through April 140 is great a lot of days 120 deg. In the winter the pumps run 7 to 8 hours.
8 hours x 120 watts = 1 kw to rise the water from 80 deg ( usually after tank mixes) to
120. If I started over I would have 1 pump 60 watts and winterize it. I use LP tankless when solar doesn’t cut it. And no you can’t run the half heated tank through it. Tankless will over heat. I did the math 1 time for solar hydronic heating of my 1000 square foot. If I could find a insulated milk tanker trailer I could make it work. It takes a lot of btus over the night to heat a house.


New Member
You’re right, tankless heaters aren’t appropriate for use with pre warmed input water. Never suggested they be used.

And yes, stainless steel dairy tanks - the rectangular 500 or 1000 gallon types - insulated on all sides with 6” SIP are typically used around here for overnight storage / transfer mass. Up here in Colorado’s mountains, 12-16 4x8 collectors to heat the mass.

These days, it’s a lot cheaper and easier to use a ground or even air sourced heat pump system with a high temp DHW option and offset the electric use with PV. Few people want to deal with thermal and fewer understand how to design, install and maintain it.