Location of temp sensor on HW tank

[Medman]

I have asked this question before, but I didn't get a clear answer.

Last year I ran my DHW sidearm continuously - as long as there was a fire in the boiler, the main circ ran, there was flow in the loop and through the sidearm.
This year, I am changing the configuration of the system to zone flow and the main circ will only run when the zones call for heat. I want to set up the DHW tank as a zone, so that loop water will flow through when the tank temp drops below a set temp.
Flow in my electric DHW tank is from bottom to top, fairly stratified. The tank is new and well insulated.

My thought is to put a 120* disc sensor about 1/3 up the tank, on the outside, under the insulation. Since the temp in the tank is always higher (at least 140*) when using the sidearm, the zone would only come on when the tank drops below 120* in the bottom third - leaving 2/3 of a tank of hotter water at the top.

Thoughts on this?

 

[stee6043]

I have asked this question before, but I didn't get a clear answer.
This year, I am changing the configuration of the system to zone flow and the main circ will only run when the zones call for heat. I want to set up the DHW tank as a zone, so that loop water will flow through when the tank temp drops below a set temp.

Perhaps I'm not reading this right - but are you suggesting you will only run your main circ when you have zones calling for heat? If you have a fire burning full-bore with no zones calling for heat aren't you concerned with rapidly overheating your boiler? I was under the impression that the main circ's in boilers should always be running when your boiler is up to operating temps. Perhaps I'm misinformed...

 

[Medman]

Sorry about that - take the "simplest pressurized system" diagram and cut it down the middle - boiler with all near-boiler piping, circs, and storage in shop, main circ, zone valves and heat loads in house. I only want to run main circ sending water to house when zones call, to minimize heat loss in the loop.

Wood circ will run sending water to storage whenever boiler is hot.

Hope this clarifies my needs.

 

[stee6043]

Ahhhh...makes much more sense.

Have you considered simply using an aquastat mounted towards the top of your sidearm to control the main circ? This would be less invasive than gutting your hot water heater to put a sensor on the tank itself. If you insulated around the sidearm I wonder if this might not be a pretty straightforward way to do it??

 

[Medman]

Wouldn't the location of an aquastat(or sensor) at the top of the sidearm cause short-cycling? I think the 'stat would see heat right away when the main circ came on, turning it off again.

I thought about putting the sensor at the tank outlet, on the top, but I was concerned that it would not trip early enough to prevent running out of hot water (given the tank is stratified and fills from the bottom) and would also short-cycle since heat from the sidearm loop will rise (and is returned) to the top of the tank.

Either the middle or lower location seems like a better bet to me, but the thermostat for the elec. elements is behind the top door, 2/3 of the way up the tank. Maybe I should use the same location.
In any case, I am going to test various spots for temps. I was hoping someone would have already tried this and could offer guidance.
Essentially what I am trying to do is use my elec. HW tank as an indirect tank, with an external HX instead of internal.
On an indirect HW tank, where is the sensor/stat located that tells the boiler/zone valve to send heat to the tank? Most appear to be a little below halfway up the tank.

 

[in hot water]

The bottom third of the tank is typically where a single aquastat is located. Think about a gas fired water heater, or an indirect tank. You are correct that it will stratify and be warmer at the top, adjust accordingly. It needs to be near the bottom so as cold water enters the tank it will fire up that pump, or zone, or valve.

I would highly recommend a good listed thermostatic mix valve on the water heater output. This would protect against any scald potential should something in you control of piping fail "on".

It also allows you to run the tank hotter, store more energy. Many solar guys run tanks at 160F or so.

hr

 

[stee6043]

Yeah, I guess what I was thinking was actually mounting the element of the aquastat so it is in contact with the part of your sidearm that has the DHW in it going "back in" to the water heater. In my case this would be where the pressure releif "used to be".

Or better yet...perhaps you do just the opposite and mount it on the bottom? Where the drain used to be. You could set the aquastat to run until the water coming out the bottom of your tank is 120-130 or so which "should" indicate it's 160+ at the top?? Just another thought. Perhaps you don't have the clearance between the tubing to pull this off?

I may not be helping you one bit...but at least I'm keeping your topic near the top! ha.

 

[Robby]

Different tank, similar problem. After mounting sensor for aquastat in various locations (down dip tube, on the outlet for coil), and not working the way I wanted, I cut a round hole in outer cover and slide sensor under insulation against inner tank. Works fine. I originaly did this as a trial, then intended to use heat paste. No need, works just fine. I use snap disks in other locations, never thought of using for hot water, but seems to me should be OK. They are very reliable. If in doubt use a couple of them.

Robby

 

[dogwood]

Medman, what would the range of temperature typically be from bottom to top in your tank? And if you located the aquastat at the bottom third of the tank as HR indicates is normally done, at what temperature would you want to turn the heat supply circ off if it came on at the 120 degee DHW temp? Assuming the appropriate mixing valve is in place and you are going to raise your tank's upper third temp higher than normal, if that is what you are going to do. Excellent questions you are asking by the way.

Mike

 

[Medman]

I keep the elec. thermostat set to 140* and the mixing valve on the outlet set to 120*. This is the recommended installation to prevent Legionnaires bacteria from forming in the tank. When using the sidearm last season I routinely had tank temps of 160-170*.
In winter, I turn the electric thermostat down or off entirely.
Robby, at what height did you end up placing the sensor?

 

[Robby]

My tank is a dual coil, 80 gal. The bottom coil is used for solar to heat water and top coil from boiler. Top coil is supplied by zone valve using 3/4 line from main manifold. When solar is producing enough heat the hot water rises, the top aquastat does not call for heat. It was recommended that I install the aquastat sensor on coil out line. No good. I cut a 1 1/4 hole with hole saw about 2" above coil return line (bottom line) and about 1/3 way around tank from coil outlet. By luck or good managment it works perfectly. Maybe some other location would be just as good.

One of my considerations was that when solar not working (prolonged rain,snow) I have less than 1/2 the tank storage so coil must be able to makeup hot water fast under high use.

Robby