Drainback solar?

Steve M_2

I'd at least give him a price to upgrade to 3/4. The only concern I would have is that drainbacks require high head pumps and 1/2 may not be large enough. It depends on the high required, do the math. You might ask the customer how many pumps he's been though and go from there.

Robert O'Brien

Drainback solar?

I have a customer with a 20 year old drainback system w/ a leaking storage tank.They want change basically everyting except the panels.I'd prefer to leave it drainback but it has 1/2" supply/return from panels.I've read/been told a drainback system needs a min of 3/4" to facilitate draining.Any thoughts?

Larry Weingarten

If...

... the panels were never damaged by cold in twenty years, the 1/2 inch pipe is proven and probably fine. I'd want to make sure it had good slope back to the drain-back tank, no traps and was big enough to handle flow for however many collectors are on the roof. I'd also check for a working vacuum breaker just above the pump instead of any air vent on the roof. Hope that helps!

Yours, Larry

hot_rod

size the pipe to the flow

how many collectors and what is the flow rate they require? If they have an SRCC sticker on them it may indicate flow rate.

Assume 3 collectors t 1 gpm each = 3 gpm flow. 1/2" can handle 3 gpm. The return pipe needs to be sized to provide at least 2 fps velocity. This allows the 'drain back '' water to carry the air along with it, back to the tank, when the collector fills.

I have seen one system that included a venturi in the return pipe to increase velocity and "speed up" the drain back"

Not all drainbacks require high head pumps. The pump sizes to the lift head. The lift required is calculated from the top of the drainback tank fluid level, to the highest point at the collector. See attachment below.

It may be fine to series a couple 15-58 together and drop one off after the siphon is established. No need to use a high head circ once the circulation is established. That high head current draw could consume more energy than you collect, some days.

One high head circ on variable speed is another option. Just be sure not to lower the speed too much and lose the siphon.

Pressurized drainback is the way to go. If the components are all suitable for pressure use. The pressure assures you never flash to vapor in the collector and maintains some pressure NPSH at the circulator.

No need for vacumn breakers on drainbacks, those were required on drain down systems. Different animal, and a good one to avoid ;0

hr

Robert O'Brien

Thanks,

I'm strongly considering changing it to a glycol system and be done with it

Larry Weingarten

OK,HR...

... I'm showing my age :~) Do you have a schematic of pressurized drainback? (the picture you attached with a heat exchanger in the tank?) Low maintenance is essential with solar and getting rid of a vacuum breaker is a move in the right direction.

Yours, Larry

ps. Remember the "Sunspool valve"? That was my idea of draindown, where the water was wasted nightly. To me drainback uses the same working fluid/water which drains back to a small tank.

hot_rod

dp

hot_rod

closed loop glycol

solar does bring some "issues"' also. Cost, creep factor ( seeps around fittings) pumping and hx derate, messy spills, etc.

But the biggest factor with glycol is stagnation. All system stagnate, some much more than others.

System stagnate when power is lost, unless PV powered, when components fail, but mostly when the tank or load is satisfied. Depending on the systems usage, stagnation is possible every sunny day.

While solar fluids are designed to accomodate temperatures in that 325- 350F, much higher with evac tube systems, it does take atoll on the glycol inhibitors.

Yearly glycol checks are a must. This will give you an idea on how often the glycol has been stressed.

Good news... most solar systems contain 10 gallons or less in the piping, collecter, and hx. So glycol replacement is not as big a deal as a snowmelt or hydronic boiler system.

If the drainback has served them well, keep it going.

hr

hot_rod

a sneak preview

of a page from I-dronics 6, due online any day now.

This issue deals entirely with combi solar/ hydronic systems. Plenty about drainbacks and formulas and tables for calculating DB system pumps, tube size, and siphon head calcs.

Remember pressurizing a drainback system it increases the max siphon head. By pressurizing you raise the boiling temperature, and prevent flashing in the collector.

Some experts claim DBs produce 10-15% more btus than glycoled systems, all things considered.

Yes I remember the water guzzling Sunspool valves. I still get parts requests.

hr

Robert O'Brien

HR

I'd much prefer to keep it drainback,it's impossible to repipe and the 1/2" has been doing the job for 20 years,I'd be on the hook if it didn't in the future,though

Larry Weingarten

Going back...

...in time again, do you remember the "Trendsetter" system, (I understand it's back)? That was where the backup heater sat on top of a box that was your solar tank. Could heat from the drainback system you pictured be convected up into a DHW tank? Or, could a large coil be put in the tank to act as a preheater only when water is used?

I think I'm looking forward to I-dronics 6!

Yours, Larry

hot_rod

DHW from DB

there are a number of ways to heat Dhw from a drainback. Internal or external HX are two of the most common. You will see 4 or 5 different methods in I-6.

We have a unique approch to simplify the components and lower the cost of pressurized DB DHW systems in the works.

I wonder how the Trendsetter system drains back the evac tube collector header?

I still worry about the life expectancy of evac tubes. A collector should last 20 years or more without replacement. Most flat panel collectors do and still perform. I'm not convinced evac tubes will run, with the vacumn intact, both in the glass and in the copper heat pipe, for 20 years plus.

I notice they have a 5 year warranty on the tube collectors.

A number of solar contractors weighing in here indicate about a 30% tube failure rate in the first 5- 6 years. Best read the fine print, the manufacturers may warranty the tubes, shipping and replacement labor may be the owners responsibility?

hr