Thermal store charged by Solar in the heating return circuit of a gas boiler

[wye001]

Hi All,

I've inherited an old solar heating system and I need views/advice on how to improve it.

There is a thermal store that is charged from the solar collectors. This thermal store has a thermostat that controls a pump and valve that feeds directly into the underfloor heating. The valve selects between hot water from the thermal store and hot water from a combi gas boiler. The gas boiler is controlled from a thermostat in the house.

The gas boiler is new (An Ariston Genus One).

Question 1: Would it make sense and is it a good idea to connect the thermal store to the boiler via the heating return of the boiler?

My reasoning:

1. At the moment the two systems are not integrated. It's an either/or situation depending on a valve.
2. The solar part starts pumping hot water to the house whether it is needed or not and then the valve blocks the flow from the boiler. The boiler could be pumping water into the heating circuit due to the thermostat in the house and this flow is then blocked by the valve. This seems wrong.
3. I would rather feed the hot water from the thermal store into the boiler, which will need it if demanded from the house thermostats.

Question 2: So can new boilers handle hot water in the heating return circuit? If my flow temperature is say 40C and 35C water comes out of the thermal store, vs 10C water in the cold circuit.

Question 3: Should I have a mixing valve to mix the thermal store water down to something "safe" like 30C if the thermal store is at high temperature?

 

[Zwy]

How big is the thermal storage/buffer tank?

This is a condensing boiler, the return water needs to have a higher delta T to the output water from the boiler. If return temp is too high, the boiler will not condense exhaust gas very well and heat will escape out the flue.

I would probably integrate both systems but run a large thermal storage tank after the boiler. All water would circulate from the thermal storage and a temp sensor on the thermal storage would engage the boiler if solar is not maintaining temps. This complete setup will require mixing valves so the thermal storage/buffer tank can be heated to a higher temp. The best tanks are vertical and the water will stratify in the tank. Hot water is pulled off the tank and run thru a mixing valve. As you have infloor radiant, the thermostatic mixing valve can be run at around 32°C which extends cycle time in between boiler firings. I would look into some type of mixing on the return side so boiler incoming water allows for efficient condensing of flue gas. A large enough buffer tank should have cool water at the bottom so size will matter.

You did not state if the solar system is sealed or open. Does the solar system have an expansion tank? This can make a difference. If you use a buffer tank with an internal heating coil, then the open solar system can be run thru the coil to heat the water in the tank and the boiler will use the tank water for circulation. This might be the best option even if the system is sealed. This example is a buffer tank used to provide domestic hot water but instead of using the coil for domestic hot water, the coil is used to circulate hot water from the solar system and heat the tank.



In any case, the return water to the boiler needs to be low enough for condensing flue gases. This is where the efficiency comes from. I can't stress this enough. Tanks like the one above run in $2000 USD range and is 300L which might not be large enough. I would suggest probably contacting a professional to determine what size buffer tank would work best and installation. This is something that can be done DIY but it will require a fairly large learning curve.

This may require a fairly large tank depending on the btu's (kw) the solar system produces and keep the boiler return water in the right temp range. If space is a concern or doorways, it might take multiple tanks and these need to be plumbed correctly. You might be better served keeping the systems separate and using the solar system to heat domestic hot water.

 

[wye001]

HI there, the system has two of these cordivari thermal stores connected in parallel (The two white ones at the back):



> You did not state if the solar system is sealed or open. Does the solar system have an expansion tank?

I think it is open, but I don't know how to tell. It has several expansion tanks in the circuits.

The cylinder in the front is the main DHW buffer heated by both solar and the boiler. This is another thing I need to look into. At the moment the boiler is maintaining the temperature at 50C, but the solar also feeds into that. Essentially the system is split in two. It's controlled by a Resol DeltaSol BX controller set up like this (with no direct connection to the boiler). The first tank is the one in the front of the photo, and the second tank in the diagram is the parallel set of white ones in the background.



This setup connects to the heating like this:

1. There is a thermostat on the white stores, set at 30C +-
2. It drives a pump + valve that interrupts the CH flow from the boiler and pumps hot water through the floor circuits.

It is this nasty mechanism I'm trying to fix, as it currently blocks the output from the boiler completely with this valve, when that thermostat turns on and it seems wrong to me.





I have been thinking of replacing that red pump and valve and feed into the return flow of the boiler:

 

[Zwy]

HI there, the system has two of these cordivari thermal stores connected in parallel (The two white ones at the back):

View attachment 256092

Probably about 300L each.

> You did not state if the solar system is sealed or open. Does the solar system have an expansion tank?

I think it is open, but I don't know how to tell. It has several expansion tanks in the circuits.

If there are expansion tanks then it is a closed system.

The cylinder in the front is the main DHW buffer heated by both solar and the boiler. This is another thing I need to look into. At the moment the boiler is maintaining the temperature at 50C, but the solar also feeds into that. Essentially the system is split in two. It's controlled by a Resol DeltaSol BX controller set up like this (with no direct connection to the boiler). The first tank is the one in the front of the photo, and the second tank in the diagram is the parallel set of white ones in the background.

View attachment 256093

T's between the tanks would be better before the pumps. This helps with stratifying both tanks and keep temps balanced.

View attachment 256099

This really should be plumbed as primary/secondary loop where the buffer tanks could be the hydraulic separator. Here is a diagram with primary/secondary loop but the same principles apply.

What is Primary-Secondary Pumping?

What is primary-secondary pumping?

www.pumpsandsystems.com

It should be a heating loop and a distribution loop, each loop has a pump for circulation. The boiler pump is pumping all the water and this is not correct. Flow rates will vary depending on what zones are on in a multiple loop system and the boiler may be starved for water in the current configuration. You will need to add another pump and rework the plumbing. I was looking for a diagram to explain how this should be done but couldn't readily find one. You might be better off using a low loss header for hydraulic separation. I suggest if you want to learn, this forum can help you.