Help with my faulty logic

[whywork]

Hi,

 

I have a "simple" program that does not behave the way I expected.

 

I have a Hot Water heater controlled by a 2477SA2 /ISY 4.05 in a "cabin" that is mostly OFF. I wanted use even less electricity, by having a program that made sure the hot water was off at night.

 

 

If

Status 'Hot Water' is On

And Time is 1:00:00AM

 

Then

Set 'Hot Water' Off

Wait 3 hours and 30 minutes

Set 'Hot Water' On

 

Else

- No Actions - (To add one, press 'Action')

 

 

What I expected - If I had turned on the hot water heater, it would turn off at 1 AM and then turn on at 4:30 AM. Also if the hot water heater was not ON, then nothing would change.

 

What happens - The hot water heater turns off at 1 AM and does NOT turn on at 4:30 AM

 

My questions:

 

1) Why does my program "not work"?

2) Is there a better way to accomplish my goal.

 

Thanks for your help and my education.

[MWareman]

1) Why does my program "not work"?

2) Is there a better way to accomplish my goal.

 

Because when your program turns off the hot water (then condition step 1), it no longer satisfies your run condition (hot water status becomes 'Off') - so the program aborts without ever getting to step 3.

 

You need 2 programs. One with your action and no condition - your then clause in the conditional program would simply run the then clause of your second 'action' program. That way - the wait won't get aborted once the heater turns off at 1am.

 

Michael.

[whywork]

DOH!

 

I should have been able to see that.

 

Thanks.

[oberkc]

if you simply want to turn hot water off at 1am and on at 430am, a simpler approach might be:

 

if

time is from 1am

to 430am (same day)

then

turn hot water off

else

turn hot water on

 

One benefit of this approach is that it is more tolerant of power failures during the period between 1 and 430.

[apostolakisl]

As far as the programming logic, I think that has been well addressed. However:

 

Turning your hot water tank off from 1am to 4:30 is not going to do any good. It won't save electricity/gas. The hot water tank won't turn on anyway if no one is using hot water. It takes many hours for a hot water tank to cool off enough before it will need to re-heat the water when no one is using it. And even if your tank was going to turn on during those hours, it would not use any more energy then as compared to the energy it would use at 4:30 to catch up. The energy savings from your program will be so small it will not be measurable.

 

Shutting off the hot water tank is only useful if you are not going to use it for at least 24 hours. In other words, when out of town.

 

There have been a lot of people who have looked at this issue and they all come to the same conclusion. Cocoontech.com forum has a bunch of threads on the subject.

 

The basic physics is as such:

 

1) Hot water tanks waste energy by leaking heat through the wall of the tank.

2) The rate of leaking heat is proportional to the difference in temp from the inside of the tank to the outside.

3) Hot water tanks don't lose heat fast enough for the inside temp to be appreciably different after just a few hours of non-heating. Thus the rate of loss is essentially unchanged by shutting down for a few hours.

4) If the tank is inside your house, and it is heating season, shutting off your tank makes almost no-difference in energy use at all for any amount of time. The only difference is if your furnace is more efficient than your tank.