Vacuum tube system balancing

Zxcv1984

Good day,

I have a gravity draining, vacuum tube solar collector system used for heating my house.
For the sake of keeping it short, please refer to : https://forum.heatinghelp.com/discussion/170794/evacuated-tube-copper-pipe-seized-in-collector#latest for additional detail on the system.

Last summer I observed a few days a week of stagnation during the summer, on sunny, cloudless days. Without any comparative and limited experience with these systems, I assumed it was caused by the system sizing, designed for the winter season.

I'm living around 47 deg latitude.

Now, working from home thanks to COVID, I've had the opportunity to monitor the system more closely. From about December 21st up to now, the circuit temperature raises above 100 deg C from 10h30 to 14h30 approximately, again on sunny days exclusively.
The exact duration of the event varies a bit with each days, and as the days grew longer.

What I've observed is that even with the pump running at 80% of max speed, the circuit temperature slowly creeps up, and by the time it kicks at 100% speed it's too late to curve the tendency, and the system shuts down.

Today I've tried something: circuit temperature was about 90 deg C, pump speed at 80%. I forced the pump at 100% and, lo and behold, the circuit temperature fell to 50 deg C within 5 to 10 minutes.

A part of me think I should adjust the pump speed incremental increase to allow it to reach 100% faster, but then again I'm wondering if there may be something misadjusted that prevents, for example, the circuit to prime properly an stay primed, or if the two collector stacks may receive unbalanced flows.

I'll put a hand drawing so you can understand my circuit.
FYI: I've checked the sender in October, it's accurate.

Any input would be appreciated.

mattmia2

Sounds like the circulator might not develop enough head at 80% to lift the water up to the roof. May need to start a 100% and ramp down

Zxcv1984

@mattmia2 thanks for the input. I didn't mention the controller on my initial post: the controller (Deltasol BS Plus) start the routine with 2 minutes "Fill" at 100%.

Bouncing on what you say and the observation that when forcing 100% pump speed I could lower the temperature, me think it may be less a question of having enough resistance at the end of the circuit to allow priming and more about balancing both branches. I'll try that first chance I get. Crossing my finger I can try it tomorrow

mattmia2

Or air is getting in somewhere and the down side of the loop isn't able to counterbalance the up side of the loop.

hot_rod

What model circulator are you using?
I’ve found that the Grundfos 15-100 is great for DB, high head low gpm. It’s almost impossible to over pump solar, especially those tube systems. You want to scrub the heat as quickly as possible, get it in the bucket.

Do you know the lift?

However it does pull a bit over 1 amp and the relay in the control is a 1 amp. The relay does have some wiggle room, the traces on the circuit board become the weak link. We sold dozens of DB pump stations with that control and 15-100s

vesmandaric

@hot_rod
Thank you Hot Rod (wroooomm, wrooomm... ) for Caleffi brochure, very informative.
A quick question, before I go and understand all schematics:
My system:
Vacuum tubes, closed loop, heating double coil 200L tank.
Tank supply hot water for the house and upper coil goes to hydronic floor heating system.
There is auxiliary instant gas water heater on the hot line from the tank towards In Line of floor heating.
What I want to achieve:
If water from solar tank is under 40C then return (cold) water from floor heating should not go to the tank (long loop) but directly to gas heater, thus shortening the travel (40ft each way) and avoiding heat loss.
Is there any way to achieve such piping with two thermostatic mixing valves?

hot_rod

we build a valve like that, SolarCal 262 or 263. Build your own with a couple off the shelf 3 way thermostatic