apex to control water heater coils

[mth1993]

This weekend I am building a water tank look into my sump as shown in the DIY forms to reduce my electricity usage.

The system uses a small pump to circulate hot water from the heater tank through the coils in the sump and returns the cooler water to the heater tank. I am hopeful that this will save me a lot of electricity since my 2*500 w heaters are almost always on per the graphs of the apex.

Most people implement this with the normal heater control type logic.

The downside of this is is for some reason the water heater is out of hot water or the pump gets stuck on you can either cool or overheat the tank fairly quickly.

Once I know how long it takes to raise the tank a few degrees I will make sure that the pump never runs more than that every 30 min or so.

My challange is how do write the logic for if the water tank is out of hot water and to combine this with the logic for only running say 5 min every 30 min.

So my logic would be something like,

if need heat turn on pump for max of 5 min every 30 min, but if after in that 5 min the temp goes down turn off pump and let the electric heaters set 2 degrees below optimal temp turn on and heat while the water tank re-heats.

Sorry for the length but I wanted to provide all the details I could.

My real concern is I have two teens in the house who seem have a goal of using up all the hot water they can while I am still paying the bill.

Thanks

[aquamanic]

I think your issue with the pump getting stuck on is still there regardless of what you do. I think if it were me, what I would do is set the pump program to shut off if the temp rose to say 78. At 79 I would email myself and perhaps sound the audible alarm (in case this happens in the middle of the night). That way, if the pump is stuck on for some reason, at least you know about it.

As for the hot water tank running out (I too have a couple of teenage boys that measure shower times with calendars instead of watches) I would leave a heater or two in the tank but set them a degree or two lower than your program for the pump. Then if the hot water runs out, your heater(s) will turn on. Again, can do an email/sound alert.

I think trying to time this would be a mistake. You're in MN. What happens if the temp in the house drops rapidly, like in the spring time where the windows are left open over night but the outside temp drops to 40 (don't ask me how I know this can screw up your tank). Your timings wouldn't be worth anything at that point. Rather use the built in temp and outlet controls along with the alarm capabilities. I think you would be much happier with the results.

[mth1993]

Sorry I was not clear, I am planning on using the temp probe settings but setting a max that the pump can be on. In my example of running 5 min every 30 min max, if it only takes 3 min to heat the tank then the pump would shut off.

I am not sure if just having the heaters set 2 degrees low would counter the colling effect generated by the coil of cold water.

I was hoping to come up with some type of logic to handle this.

[Hawkdl2]

Complicated, but you could use a temperature gauge in the heater tank loop that would shut off the loop pump if the water tank had low temperature water. In tank heaters would be your backup so the reef would never get too low temp. Otherwise, I agree there is no easy fail safe mechanism for if (when) the loop pump fails in the ON position. Email alarms are always comforting.

One, expensive and complicated, option might be to install a solenoid and/or by-pass valve in the loop. In the even of an overtemp, the Apex would shut off the loop AND close the solenoid.

[aquamanic]

You could do it with the oscillate statement. So for your circulating pump:

OSC 0/5:00/25:00 Then ON
If Temp > 78 Then OFF

that will run 5 minutes every 30 min but only if the temp is < 78. However, if it takes more than 5 minutes to raise the temp, then you're out of luck because you limit the time to 5 min. In a cold house, your tank temp could continue to fall even though you're running every 30 minutes.

Also, if your outlet/pump sticks ON, you still have a problem. The controller thinks it's turned it off but it's still running (I assume that's the scenario you described).

Still too many chances to fail for my taste.

Once I know how long it takes to raise the tank a few degrees I will make sure that the pump never runs more than that every 30 min or so.

I don't believe the above statement will be true throughout the year. It's going to vary based on your inside temperature. I think that's why fundamentally I don't like the idea of trying to time a heater vs. letting a thermostat drive it.

[mth1993]

I like the idea of the Solenoid, I found this on Ebay for <$20.

New 1/2" 12VDC Solenoid Valve.
Connection pipe: 1/2"
Orifice diameter: 15
CV value: 4.6
Principe: kick pilot
Temperature: -5°C to 85°C
Voltage: DC12V
Work pressure: 0 to 5 bar
Max presure: 12 bar
Fluid: Air, gas, hot water, oil(<=20CST).
Control mode: Normally closed
Body material: Brass
Seal component: NBR

Do you think that would work? By using it the pump may burn up due to lack of flow but I would rather loose a $75 pump than over heat my tank.

One idea to solve the lack of flow for the pump is to have the return by the pump manifolded (not a word but you know what I mean) to not only get water from the .5" loop but from 3 *.25 lines from my kitchen, main bath, and master bath to provide them instant hot water.

[mth1993]

How about this for programming:

----Pumpx for loop------
OSC 0/5:00/25:00 Then ON
If Temp > 78 Then OFF
If Temp2 > 78 Then OFF
IF Temp < 76 Then OFF
IF Temp2 < 76 Then OFF

----Electric Heaters------
If Temp > 77 Then OFF
If Temp < 76 Then OFF
If Temp2 > 77 Then OFF
If Temp2 < 76 Then OFF

----Solenoid------
If PumpX On Then On
If Temp > 80 Then OFF
If Temp2 > 80 Then OFF

I may have to adjust the Temp and Temp2 values due to fluctuations in the two probes but above is for the concept.

[Hawkdl2]

If you also plan to use the pump to feed a home recirc system you will loose the ability, and redundancy, of controlling the heating to the tank by the pump and will have to reply only on the solenoid. That just gets you back to the single control point with no backup - if the solenoid fails to close your tank may overheat. The upside is solenoids usually fail in their "normally closed" state, so the likelihood of overheating the tank is probably pretty slim - when the solenoid fails, the loop will shut off.

I agree with Alan that it makes more sense to control the system primarily by temperature (loop and tank), not timings.