Daily regime of solar controller during vacations
 

Hello.
I am interested in solar hot water controller's (delta T) regime of work during holidays.

I've read that if holiday lasts up to 3 days, it is recommended to keep the regime the same as on working day (pumps working during the day; pumps turned off during the night).

However, during holidays longer than 3 days when controllers are switched to "vacation" mode, they turn on the pumps during night and "send" the hot water from the storage tank to collectors, in order to convective cool the water.

What about during the day (on holidays)? Do pumps still work normally during the day, and circulate the water through the system, even though there is no hot water consumption?

My question is directed towards non-drainback systems, and systems where collectors can not be covered (with sheets...) during holidays.

Thank you for the reply.

The controller should have logic build into it so that your tank temperatures do not exceed a certain temperature. The reason for this is that the coolant in the lines will degrade at higher temperatures.

I'm not sure exactly how most controllers handle this, but temperature control IMO is the best way.

Thank you Daox,
As far as I am informed controllers will handle this in two ways: discharge an excess hot water from the storage tank and import a new cold one from the mains water pipe.
And during the night, it will send the water from the tank to collectors, so that it could be cooled down.

What confuses me is what is happening during a day, when controller is set to "vacation"? It will simply just loop the water (turn on the pumps), like on any working day (day when there is a hot water consumption) through the swh system? And that is it? It will loop the water (in order to prevent the stagnation), even though there is no hot water consumption (family is on vacation)?

This was the ancient way of dealing with excess solar heat. The old systems either had a heat dump (a radiator outside in the shade or nighttime collector cooling) or a quenching setup (dump hot water, replace with cold) to keep the collectors cool. This method is not so effective or efficient.
http://www.homepower.com/sites/defau..._Mehalic-6.jpg
Newer closed loop systems employ a "steamback" design. The manufacturers have learned how to build collectors that will not melt apart when they stagnate, AND that will hold pressure under stagnation conditions. The loop pump stops when the heat store has maxed out. If the coolant in the collector boils, the water boils first (fractional distillation) and forces the rest of the fluid into an expansion tank.
http://www.homepower.com/sites/defau...6_Ramlow-3.jpg
When this stagnation event occurs, the loop pump is locked out until the collector temperature falls below condensing temperatures. Normally the event only occurs on vacation days, as you mentioned.

Here is a discussion about your question regarding steamback design:
Steamback System Design - Heating Help: The Wall
You have to register to see the pictures, but IMHO It's worth the trouble in that particular discussion.

I'm not sure how this protects the glycol? Traditional glycol doesn't hold up to steam temperatures from what I know (which is perhaps wrong).

during a steamback event, only water steam is in the collector if it is plumbed so the fluid can easily be pushed out by the steam. Plumbing geometry for drainback designs will work for steamback. If the collector array is plumbed badly, some glycol will be trapped in low points and turn into nasty stuff when it boils. This is true for the classical drainback systems as well. You know the system is plumbed badly if it makes lots of rumbling sounds during a stagnation event for more than just a few seconds.

One example of this is evacuated U-tube collectors. No matter how the tubes drain, some fluid will be trapped in the bottom of the U and be forced to boil. Not a good choice for a glycol system at all. A much better choice would be heat pipes and header.