Pressurized glycol expansion tank sizing

Orion_134

So based on some loose calculations and guessing both collector volume (1.2 gal x3) and pipe type (3/4" copper 84' total loop), my calculations are a loop volume of 6.7 gal with 3.6 gal on the roof. So that seems to tell me I need an expansion tank with ~4 gal acceptance volume and ~8 gal total volume. Does that sound about right for a two story house with storage and expansion tanks in the basement? 

hot_rod

The coil in the solar tank. Did you include that?

Orion_134

Copper 3/4" 84' 2.1 gal

Collector 1.2?*3 gal

Tank 2.2?

I haven't been on the roof to get any identifying info on the collectors, and the internet has failed me on tank specs. I'm going off of some other specs I found. 

hot_rod

A #30 should be adequate. A # 60 would give you some wiggle room. If the coil in the tank is large diameter, it could hold another 3 gallons.

The math to size a solar expansion tank will make your head spin😉

Shop for a solar rated tank, Amtrol had them, Zilmet USA in RI has them on their site. 

hot_rod

Sizing a solar expansion tank in the appendix here

https://www.caleffi.com/sites/default/files/file/idronics_3_0920.pdf

Orion_134

I did some digging on the new water heater and it claims 143' of 5/8" for 2.6 gal. Kinda close. 

hot_rod

You will want to run that solar pump on speed 3. That 5/8 HX is going to consume some pump head.
The higher the flow rate the better with solar, scrub that heat out of the collector and put it into the tank before Mother Nature takes it😉 3-5 delta, as long as velocity is acceptable.

With solar thermal we often pressurize the tank a few psi lower then what we pump the system to. This allows a bit of glycol to go into the tank. So when the collector drops to cold winter ambient, the pressure gauge doesn’t go to 0 and you panic about a leak. Viessmann solar calls this method a “safety seal”. Another reason to size generously on the tank

Orion_134

That's a good strategy.

What do you mean by ”3-5 delta"? I'm reaching back to my heat transfer class in college and... well...aging... 😂

hot_rod

When the pump is running, sun on the collector, check the supply and return temperatures. You want a good flow rate, indicated by the temperature difference. That pump station has a Grundfos vortex shedding flow sensor, but you need a controller that can read that output.

Often people think that slow flow allows the collector to get hotter and more energy harvest. The problem is you have a single pane of glass and maybe 1/2 insulation around the collector. So a hot collector loses heat to the temperature surrounding it very quickly. ST can reach 50% efficiencies under ideal operating conditions

Swimming pool collectors can reach 90% as the ambient and collector temperatures run close together, very little is lost at the collector.

The Idronics 3 I linked shows the math on that. If the SRCC test data on those collectors exists, you can plot the efficiencies of a collector under different operating conditions.

See an example of a common size flat panel collector efficiency curve in Idronics 3

Orion_134

So it sounds like you're talking about delta_t? I would have expected a much greater delta between supply and return? 

hot_rod

The delta is what you tell it to be. You change the delta by increasing or decreasing flow, pump speed.

The control I sent you has a variable speed function built in, use it! By watching the collector temperature it increases pump speed as it sees the collector temperature rise, RIS parameter. Again, it attempts to pull that heat out of the collector as fast as possible, bank it in your tank.

That parameter is adjustable, I think factory default is 10°

That isolar control is a microprocessor built specifically for solar. It has a dozen important functions to really optimize and protect your system. Read through the manual, feel free to ask any setting questions.
If nothing else the digital temperature display is nice to have, see collector and tank temperature at any time. It has a 3rd sensor you can mount anywhere. Some installers like to mount it at the top of the tank to see what you actually have. Sensor 2 is always near the bottom of the tank to trigger on the pump.

It is pre wired and pre-configured to work out of the box, plug and play. But you should take advantage of the custom settings.
collector over-heat, tank over-heat, glycol protection, night time dump, vacation mode, etc.

Read that Idronics 3 also, it explains the concepts of solar thermal, then the control manual makes more sense as to why it has all those parameters and settings.