Best way to plumb a hot water solar heat exchanger into a gas fired on demand radiant heating system

[lightmind]

Hey everyone,

I am installing a solar hot water collector system and will have a heat exchanger to supplement a gas-fired boiler 6 zone on-demand slab radiant system.

The current system fires the boiler as the zones call for heat. The boiler heats the water in the calling loop(s) to 160 degrees or so very quickly and then the gas turns off and the circulation pump continues until demand is satisfied.

I am unsure of the correct and most efficient way to plumb the heat exchanger from the hot water solar into the boiler loop.

I have a pump on both sides of the exchanger controlled by the solar controller and separate from the boiler system circulation pump and have the controller setup for on-demand currently. But where do I inject the supplement hot water supply and return?

 

[Dana]

There's no way you should be blasting 160F water into a slab radiant system. Even at the 99% design condition most slabs won't need more than 100F water unless its an uninsulated barn, with the door left open.

It's probably more cost effective (= more fuel savings for a lower investment) to fix the radiant system design and figure out the maximum actual temperature requirements before trying to marry it to low-temp solar thermal input.

A dumb brute-force approach would be to put a buffer tank between the radiation and the boiler/solar, and set up the controls for the boiler to only fire when the solar isn't keeping the tank hot enough.

 

[Sponsor]

 

[lightmind]

The system was set-up with the home long before I arrived and has been working the same way for 15 years without a problem. I do know that 160F is a lot, probably should be between 100 to 120F . However the boiler is already set to the minimum. I think to lower the temperature would require a new aquastat unless there is a better way. But, that is not what I am asking about.

I would like to know where in the current radiant boiler plumbing I should insert the supply and return from the solar hot water heat exchanger? My goal is to have the water the boiler sees when a zone demand is initiated already hot enough that the gas is not needed to heat the water. So the boiler circulation pump would come on but the water would be heated by the hot water solar heat exchanger. As long as the boiler sees a temperature above the minimum setpoint the gas should not fire. BTW the solar storage tank is 120 gallons and today for instance the water is at a toasty 165F .

Thanks, I appreciate your help and experience.

 

[Dana]

Thermostatic mixing valves can take any boiler output temperature and re-mix with the return water from the radiation to deliver any temperature you like.

A cast iron boiler will be damaged by entering water less than 130F. Even dropping 140F may result in excessive flue condensation. If you're cold starting the boiler an piping the return water from the slab radiant directly into the boiler with no mixing of boiler output it's abusing the boiler.

There is no generic boiler or generic plumbing associated with it. Without a model number, a schematic &/or photos of the near-boiler and zone plumbing there's no way to divine if there is ANY place that would be appropriate for inserting the heat exchanger from the solar, let alone a "right" place.

 

[lightmind]

Thanks again. As I said the boiler was system was installed with the house. I do appreciate your advice, it appears that the design was lacking. The boiler is a Weil-Mclain Gold CGA. I will be looking into it asap.
I am thinking that what I can do now is create a solar loop circulation circuit that keeps the water in the boiler hot by circulating the hot solar water heat exchanger heat through the boiler supply and return. Then, when one of the radiant loops calls for heat I can divert the solar loop supply with a three way valve from the supply of the boiler to the return of the radiant loop manifold. Sounds theoretically possible, what do you think? BTW the circulation pump for the solar loop is a variable speed that ramps down upon reaching the setpoint, so I don't use a mixing valve.

I put a Espar Diesel boiler in my RV and it is always circulating the water when it is on. I do have a 5 gallon buffer tank to keep the heater from cycling too much. The system works well for heating and hot water on demand.

 

[Dana]

Don't install the solar on the return to the boiler. Take pictures of the boiler plumbing and the zone manifold/plumbing.

Is there going to be ANY tank to store heat when there isn't much of a heating load?

There are at least a half-dozen sizes of CGa series, and it might matter, so which size is yours?