Sensor Location on DHW Tank for Zone Control

[Medman]

When I plumb in my storage tank this spring I am replumbing the rest of the system to match the 'simplest pressurized storage' diagram above.

My question is where on my DHW tank, which is a standard electric hot water tank, should I put the sensor which tells the zone valve to open?

If at the top, won't it call for heat too late, when the tank is mostly empty of hot water, or will the tank be non-stratified enough to trip the sensor after just some water has been removed?

 

[Nofossil]

When I plumb in my storage tank this spring I am replumbing the rest of the system to match the 'simplest pressurized storage' diagram above.

My question is where on my DHW tank, which is a standard electric hot water tank, should I put the sensor which tells the zone valve to open?

If at the top, won't it call for heat too late, when the tank is mostly empty of hot water, or will the tank be non-stratified enough to trip the sensor after just some water has been removed?

For simplicity's sake, I put my sensor in the aquastat well that's about halfway up my tank, alongside the aquastat probe. How are you planning on getting heat from the boiler into your electric tank? Plate HX and second circulator?

 

[Medman]

I am replumbing the system to match the 'simplest pressurized storage' system. I will have the sidearm HX as a zone, with the temp. sensor controlling the zone valve attached to the tank. I don't know if this setup will offer enough heat transfer to keep the tank hot though. If not, I will probably have to switch to a flat-plate or get a tank with a coil in it.

 

[Vtgent49]

ok, now if the EKO has controls to keep it from overheating, then why would you ever close the zone valve to stop the flow thru side arm from the tank? If it's open all the time, then the tank will never get too hot.

Of course you need a tempering valve.

I have a swing gate check valve to prevent reverse flow when the boiler is cooler than the tank.

Just a FYI, if the tank is 150 or so, and your cold in is about 55, then as you draw hot, and refill with cold the stratification will increase due to the extreme range of temps. This assumes your filler disperses properly and gently into the bottom of the tank.

 

[Nofossil]

Couple of suggestions:

1) The heat transfer rate of sidearm HX is very low, and could result in undesirably long recovery times. It may make more sense to plumb the sidearm in series with your supply or return manifold so that there's flow through it whenever the boiler is running.

2) Use a mixing valve on the DHW tank outlet so that you can safely heat the DHW above normal temps. This will be necessary if you use the approach above, but you could also use a much higher setpoint for controlling the zone valve/sidearm combination to stretch your DHW.

3) If you don't want to superheat the DHW, then a flat plate HX with a potable water circulator can accomplish a much more rapid heat recovery than the sidearm.

 

[Medman]

This is the way I have it plumbed now - in series, so that whenever the boiler is running the big circ pushes water to the house and there is flow through the sidearm. I have the plenum HX teed into this loop with two closely spaced tees and a small circulator. When the furnace calls for heat, the circ turns on and pulls from the loop.

In order to add pressurized storage in the boiler room, which is 140 ft. from the house, I wanted to use the 'simplest' method pictured above to avoid having a lot of extra components and to minimize standby losses. I would use the small circ on the near-boiler piping, the large circ would run to feed the zones on demand, and I would install two zone valves. Using this method means that I would only be sending water from the boiler/tank to the house when a zone calls for it. There would be no constant flow through the sidearm any longer, and this has been my issue.

I could continue to have flow run to the house with the sidearm in series and possibly install a three-way valve to direct water through the plenum hx when the furnace demands heat, but I am concerned about losses in the pipe, and charging the tank becomes more complicated.

Any thoughts?

 

[Medman]

In the image below, I don't have the oil boiler, and everything to the right of the storage tank is 140 feet away in the house. I want to use the setup below as it has the least components and charges/draws from the tank automatically.
I am looking to change to this system from a series system.

 

[Nofossil]

Thinking out loud:

Could you put the sidearm in-line and add a zone valve in parallel with the other zones to allow flow when no zones are calling for heat AND the DHW needs heat?

Don't have time to sketch it just now.....

 

[jdeere5220]

On your original question of where to put the temp sensor, your thinking is correct. If you put it at the top it won't react quickly enough. About a foot from the bottom seems about right. On my DHW there is a radiant-heating tap about 16 inches from the bottom, so I took out that plug inserted my well there. Works good, but I was lucky to have that tap.

 

[Medman]

NoFossil, that is essentially what I have now but without a zone valve; system water is flowing through the sidearm at all times that the temp is above 140*F. I want to replumb the system so that I am not flowing system water 140 ft to the house and 140 ft back when there is no call for heat, to reduce the line losses from the piping as it travels. I just don't know if what I want to do is possible with a sidearm. I may have to switch to a flat-plate hx or get an indirect DHW tank with back-up electric.
Even if there was a way to move a smaller amount of water through the sidearm (plumbed in series) continually, like 2 gpm, then increase the flow to the 8 gpm needed when the plenum hx requires heat.

How about this: The diagram above has the sidearm mounted between supply and return with no valve, creating a loop. Boiler has Taco 007 mounted at return. When boiler is hot, 007 pushes water toward loop and tank. A majority of the flow will go into and through the tank, but some will flow through the loop into the sidearm even when the main circ is off. Just like resistors in parallel. Is there any way of calculating this? (Rt=(1/R1+1/R2)?

 

[Nofossil]

NoFossil, that is essentially what I have now but without a zone valve; system water is flowing through the sidearm at all times that the temp is above 140*F. I want to replumb the system so that I am not flowing system water 140 ft to the house and 140 ft back when there is no call for heat, to reduce the line losses from the piping as it travels. I just don't know if what I want to do is possible with a sidearm. I may have to switch to a flat-plate hx or get an indirect DHW tank with back-up electric.
Even if there was a way to move a smaller amount of water through the sidearm (plumbed in series) continually, like 2 gpm, then increase the flow to the 8 gpm needed when the plenum hx requires heat.

How about this: The diagram above has the sidearm mounted between supply and return with no valve, creating a loop. Boiler has Taco 007 mounted at return. When boiler is hot, 007 pushes water toward loop and tank. A majority of the flow will go into and through the tank, but some will flow through the loop into the sidearm even when the main circ is off. Just like resistors in parallel. Is there any way of calculating this? (Rt=(1/R1+1/R2)?

It can be calculated, but it's highly nonlinear and troublesome.

I'd say don't circulate to the house at all if there's no demand. If you put the sidearm inline at the house, the DHW will get heat any time any other zone asks for heat. I suggested the zone valve to allow for the possibility that the DHW might be the only heat demand, so you'd need a flow path. I'd expect that wouldn't happen often. In our case, it only happens during the shoulder seasons when the house itself might not need heat for days.

 

[Medman]

That's essentially what I had in mind. I just wonder if the sidearm will be able to supply enough heat to the DHW tank when demand is high.
As for my original question, I think I am going to mount the temp sensor 1/3 to 1/2 way up the tank, on the outside, under the insulation. This is a new tank insulated with closed-cell foam and it will stay hot for days once heated. I may have to play with the location a bit, using my infrared thermometer to find the best spot.

Thanks for all the input!

 

[Piker]

Medman,

I am in your exact situation. I also used the "simplest method." The only problem was that once the circs shut down when the storage is up to temp, you don't have any more DHW if you plumb a sidearm into the primary loop feeding the tanks.

Thus, I am zoning the DHW off of the load circ, just like any other heat load. I didn't want to use a plate heat exchanger because you could probably pave your driveway with our water... there's this black gunk that builds up in our pipes and eventually plugs them, so a brazed plate just isn't going to last. So here's what I did. I built a double sidearm heat exchanger. 1 1/2" copper on the outside 3/4" on the inside, and about 5'6" long.... times 2. I am going to put a stainless circ on the DWH side of the sidearm. This pump will be turned on and off by a close on rise strap on aquastat that will be located on the piping coming from the zone manifold. The zone itself will be turned on and off by a digital aquastat (of sorts) that will have a sensor probe in the DWH tank itself. To get the sensor in the tank, I will remove the PRV from the tank, put a short black iron nipple in it's place, attach a black iron "T" to the nipple. The "T" side of the nipple will now be fitted with the PRV... the end of the "T" will be fitted with a 3/4" male adapter to 1/2 sweat copper fitting. I will then attach a brass fitting that has 1/2" sweat on one side, and a 3/8 compression fitting on the other end for soft copper tubing. I can crimp and solder the end of a piece of 3/8 soft copper and shove it down into the tank through the 3/8 compression fitting for a well for the digital aquastat. This will allow me to expirament with different depths of the sensor for best results.

This is all a little expiramental. The close on rise aquastat should keep the DHW circ from coming on if the water in the storage tanks drops below what is usable for exchanging heat to the dhw tank... thereby preventing backward heat exchange from the DHW tank to the storage tanks when the electric elements kick on. The digital aquastat (actually a digital 110v fan/pump control that I will add a relay to so as to have a set of dry contacts for the 6-zone controller thermostat connection) should be sensitive and programmable enough that I shouldn't have to worry about the water in the tank getting to hot, thereby eliminating the need for a mixing valve on the DHW tank.

We'll see. I have never tried this before. I have all the stuff to pipe it up and get it running, and half of the work is done... finding time to finish is another issue, especially with the boiler season beginning to heat up.

Hope this helps.
cheers