sensor problem?

12 Posts
May 23, 2012 05:26 pm
sensor problem?

My system has been installed and working for just over a year. In that year, I've needed to use my electric hot water heater for less than 12 hours. I have not had much luck with sensors, though. I'm pretty sure the one I have in the top of my tank is not reading correctly, and it's not very old.

The heat exchanger/storage are configured as follows ...
The heat exchanger is a tank of potable water, inside an outer jacket that holds the mixture that flows through the collector loop. The tank has a fixture on the top where the temperature sensor is supposed to go. The trouble, from the beginning, was that the "dip stick" mounted on the fitting that screws into the top of the tank was too long for me to remove it from the tank without drilling a large hole in the ceiling, and the sensor wouldn't fit into the hole at the top.
My first solution was to shorten the "dipstick" by cutting it off, inserting the sensor inside the tube, and running just the wire through the small hole at the top. It was a great challenge getting that tube resealed so I could stick it back into the tank with the sensor inside. After a month, or so, the sensor quit working because a tiny bit of water was seeping into the tube and the sensor ended up in a few inches of water at the bottom of the tube, eventually.
So, I opted to screw a submersible sensor into the top of the tank, even though it would be reading the temperature at the hottest point in the tank. In order to monitor the bottom of the tank, I attached a sensor to the collector loop output pipe at the bottom of the tank. That isn't ideal, either, but I get a good idea of what the top and bottom temperatures are, between the two sensors.
Well, lately, the top sensor has been reading about the same or lower than the bottom sensor. Normally, the top sensor reading rises pretty fast when the sun is shining and the system is running. Now, the bottom temperature overtakes the top temperature, and the circulating pump runs a lot longer than it should. In the middle of the day, if I'm not using any hot water, the pump might run for an hour at a time. That leads me to believe that the top sensor is reading low.
I don't have any alternate way of testing the temperature at the top of the tank. The water is under pressure, so I can't just unscrew the sensor and stick a thermometer in there. Based on common sense, it has to be reading wrong. I checked for air at the top of the tank. There is an air bleeder that is working properly, and I unscrewed the sensor a little to let some water drip out before I retightened it.

So, are these sensors that unreliable? Could it be failing? Or is there another explanation I haven't yet considered?
12 Posts
May 23, 2012 05:39 pm
Re: sensor problem? (update)

I got an idea, after I wrote the previous post. I have a spigot that will allow me to draw potable water directly from the tank (the house water goes through a mixing valve). I ran the spigot long enough for the water to get really hot and took out 16 ounces in a 32 oz measuring cup. I then added 16 oz of water at 80 degrees and checked the temperature with my darkroom thermometer (it only reads to 140F). The resulting temperature was 116 degrees. By my math, that means that the 80 degree water rose 36 degrees which should have taken the hot water down 36 degrees. Therefore, the  hot water started at 152 degrees. The sensor at the top of the tank reads 140. That means I have at least a 12 degree differential between the water coming out the nearby spigot and what the system thinks the maximum temperature is.

This is frustrating. It's hard to save any money on my utility bill when I keep having to put it back into new parts.
47 Posts
May 24, 2012 06:03 pm
Re: sensor problem?

Try disconnecting the sensor wires from the DTC and measure the resistance with an ohm meter.�  Compare it with the readings you see on the last page of this manual, .�  At 77F, you should see 10k ohms.�  At 140F, you should see 2,489 ohms.�  All of the temperature readings are listed in the doc.�  � That should give you a starting point to see if the sensor is reading correctly.
12 Posts
May 25, 2012 08:00 am
Re: sensor problem?

Thank you, for your time and expertise, Amy. You are the best. Here are the measurements I took this morning:

water temperature: 149F
sensor ohm reading (disconnected): 2560 ohms (137~138F)
When I hooked it back up with my 11K parallel resistor (actual value 10500 ohms), the Eagle II reads 150 degrees. The computed parallel resistance would be 2058 ohms, and that should read 149~150F, so the Eagle II appears to be reading correctly.

The good news is that my 11K resistor in parallel will provide a temporary solution. The bad news is that I'm going to have to RMA that sensor or buy a new one. Sad

BTW: I also wrapped the exposed part of the sensor in another inch of fiberglass insulation and that made no significant difference. It might have raised the reading by 1F, but the error is closer to 12F. The entire tank has an R30 wrap around it.

Thanks again, for your help.
462 Posts
May 27, 2012 10:21 am
Re: sensor problem?

Doug, which differential controller are you using? Does it provide for multiple sensor inputs? Even so, only one sensor will control the pump, the others will only give you temperature readings throughout the system.
 So when you say  you have the two  sensors in parallel, one at the top of the tank and one at the bottom, it makes me think that this is the problem, conflicting temperature (electrical) readings. Remember, current will divide when put through a parallel circuit depending upon the resistance of the load, in this case the sensor, and it is this current that the differential controller reads.

And when you say "Now, the bottom temperature overtakes the top temperature, and the circulating pump runs a lot longer than it should" , it definately sounds like the problem.

 Only one sensor will control the pump, so it's best to locate this sensor somewhere in the middle of the tank, or move it from the top to the bottom seasonally.
12 Posts
May 27, 2012 10:43 am
Re: sensor problem?

Thank you, Tom. I think you misunderstood my solution. I added a 10.5K resistor in parallel with the sensor to adjust it's reading up by around 10-12 degrees. That is just a temporary solution until I can replace the top sensor.

I am using an Eagle II controller, and there are 2 sensors, hooked independently. You can look at the video I posted and see them operating (back when everything was working right). I have one sensor at the top and one on pipe that exits the bottom of the tank (heat exchanger) to return to the collector. If I could have one in the middle of the storage tank, I would, but that's just not possible without fabricating something (which I may actually do, when it works it's way to the top of my priority list. One year into this, I've consistently had a surplus of hot water, so improving the system is not a priority).

As of right now, the system is working fine, with the parallel resistor installed to adjust the sensor reading upward. I'll acquire a new sensor after the holiday weekend.

Thank you, again, for your thoughts.
12 Posts
Jun 4, 2012 08:17 pm
Re: sensor problem?

The replacement sensor is installed. It appears the problem is resolved. We didn't have a lot of sun today, but the pump did run several times after I installed the replacement sensor and the readings all seem to be sensible.

I installed a 3rd sensor on the hot water pipe, leaving the tank. As long as I am using hot water, the reading on that sensor is comparable to the new one at the top of the tank.

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