thermo-siphon

[artv]

Hi all,....I notice that the hot water I store is loosing almost all of its heat over night.
I manually by-pass the collector (on the roof) in the evening via a shut-off valve.(on the hot out)
The storage tanks still have access to the collector via the cold in,but no circulation because hot out is shut off.
My question is ....will the hot stored water thermo-siphon to the cold roof top even though  it can't circulate???

[rossw]

Hi all,....I notice that the hot water I store is loosing almost all of its heat over night.
I manually by-pass the collector (on the roof) in the evening via a shut-off valve.(on the hot out)
The storage tanks still have access to the collector via the cold in,but no circulation because hot out is shut off.
My question is ....will the hot stored water thermo-siphon to the cold roof top even though  it can't circulate???

How big are the pipes?
When you "shut off" the hot side, what are you using? If the valve is (lets say) a large brass ball valve, you will get a surprising amount of heat across it if there is a decent temperature differential, and yes you'll lose heat that way.

If you don't have everything super-insulated (I know I'm guilty of that, but then chunks of my work was done by licensed and allegedly "expert" plumbers), you will be surprised, perhaps even horrified, at how much heat leaks out all over the place.

If you have access to a thermographic camera, it's a wonderful tool to see where it's all seeping out, and insulate. Any exposed, hot metal fittings or pipes will lose heat very quickly. The smaller your tank and the greater the temperature differential, the worse off you will be.

[GaryGary]

Hi,
Maybe you could describe your system in more detail? 
Tank size?  Collector size?  Is there pump circulation during the day?  When you say losing all the heat -- how much is that -- from what temp to what temp?

You can lose a lot of heat from thermosyponing at night if the collector is above the tank, but the shutoff valve should cut the heat loss way back.

As was mentioned, insulation on the tank and pipes is important.

Is it possible that you have a circulation loop going to the house plumbing?  This can happen if you have any sort of circulation system to keep the water hot -- like this one:
http://www.builditsolar.com/Projects/Conservation/Recirc/RecircEnergy.htm

Feeling all the pipes with your hands might give some clue to where heat is leaving.

Gary

[artv]

Hi Ross,..I'm using all 3/4" copper on solar side ,then changes to 1/2" to house.I haven't insulated pipes or tanks yet,but will once I get everything working properly. I've already had to make 3 changes so far.

Hi Gary,..Here is a schematic of what I've got so far
The picture in the first post is of the system minus the tanks and pump.The horizontal pipe is the hot out of the solar stored water.
The white line (one of the changes) was put in to go directly to home h/w tank ,replacing the cold feed on that tank.
The lower vertical tap is the valve I use to shut-off at night. I've been looking for some form of electric valve that I can put on a timer, to replace that valve, if you know of such a valve would appreciate the info.
The temp guages I have ,  I don't believe are giving true values. Maybe due to location? I don't know.
Last night they were reading 112 F, this morning they read 84 F.
My shut-off for the cold feed to the roof is in the wrong location (it is the highest vertical valve in first post). It should be above the horizontal valve that you see above it. So I can't shut it off . This is where I'm thinking siphon is taking place, but not sure?
Thanks for your time.........artv

[GaryGary]

Hi Art,
I'd get a temperature reading right on the tank -- one of those inexpensive digital ones with the sensor on the end of a wire works well.  Just push it under the tank insulation.  This will give you a better idea what the tank temp is actually doing.  If you can have two sensors on the tank, that's even better as the tank may be stratifying.  Sometimes what looks like the tank cooling dowin is just the tank losing its temperature stratification.

If you have a shutoff valve for the floor loop, I'd turn that off for the tank temperature drop test as you may be thermosyphoning through the floor loop. 

How much collector area do you have?

Gary

[artv]

Hi Gary,...The collector is ~18sq/ft, I talked about it in another thread under Heating "collector question" it was the one about the big radiator that has 21 tubes in front and 21 behind.
You suggested cutting the back ones out ,and re-attaching to make the collector 6x6 ,instead of 6x3.
When you say "put two sensors on the tank " Do you mean one on each tank or two on just one tank?
Also are the digital sensors common to find and is there any form of electrically operated valves, I've been looking but can't seem to find one. Maybe I'm using the wrong terms when searching.
I already ordered a differential controller, It reads collector temp ,and tank temp, if the collector is hotter then it circulates.
I found the guy on your site, am still waiting for it to get here,this week I hope.
By 1:00 in the afternoon (if full sun) all the pipes  feel the same temp so I just circulate steady.
Thanks for the help......artv

[WindriderNM]

HF has a multi-meter with a temp probe for about $25

[WindriderNM]

Washing machines have electronic  valves Try looking for solenoid valves. I have scavenged several from various places over the years.

[artv]

Hi Windrider....thanks for the reply,...Originally I was going to use the washing machine valve but, I thought you applied a pulse to open, then another pulse to close. I didn't realize it needs a constant supply to stay open..so that's not any good to me.
I was wanting to have the valve open in the morning, and close at night, just have them on a timer.
By manually shuting off the collector on the roof, at night and turning it back on in the morning, I'm seeing 10F difference in stored temps.
Like I said before I don't think my readings are accurate but the difference between readings ,should be I would think.
I'll keep looking..........artv

[WindriderNM]

you could remove the spring and replace the solenoid with a permanent magnet, a + pulse would move it one way and a - pulse the other way.

[artv]

That's an interesting idea, but then I would have to figure a way to control which pulse it would recieve.
Also the valve I have only works on AC. This has me confused because the valve is just basically an electro-magnet.(isn't it?).
With the AC shouldn't the field be continously changing , making the valve open and close with the frequency.I don't know.
thanks for the help......artv

[WindriderNM]

The frequency of the ac is faster then the solenoid can move. It should work on dc. A circuit to produce + and - pulses could be made with an op-amp and some photo diodes or photo resistors or small photo cells ect. or photo cells connected to relays and diodes to get the dc for the solenoids.

[artv]

I'm not an electronics guy (although I appreciate the response) ...that may as well have been written in a foriegn language. lol
But I did try to get the valve to open with a fully charged truck battery ......nothing...........DC ,dosen't seem to activate the valve..
I thought I was starting to get a handle on this stuff ,but this has got me confused??
looks like more reading is required, I'll look up those things you said as well............Thanks for your help......artv

[WindriderNM]

Probably not enough voltage the valve is probably 120 volts and battery is 12 volts.