Tank sensor locations

[free75degrees]

As far as I can figure there are two parts of a system controller that need a tank temperature reading: the part where we decide if the tank is hot enough to provide heat to the house (I'll call this the THH sensor for Tank Heat House), and the part where we decide if the tank is cool enough that we can keep heating it (i.e. the differential control that keeps coming up in various threads, I'll call this the TAH sensor for Tank Accept Heat). These two temperatures don't need to be from the same sensor and it may make sense to make them separate sensors. In my system I use the sensor at the top of the tank for the THH and the middle for the TAH. In another thread, someone mentioned that they are using a sensor on the tank output. I think this sensor location could be used for both the TAH and THH. However, I can't see how you would avoid a delay problem. For example, let's say we are charging the tank and the TAH sensor tells the controller to stop charging the tank because the output from the tank is higher than the input. The tank circ shuts off and now the TAH sensor starts to cool down. Since it cools down it will now cause the controller to start heating the tank again and the the TAH will heat up and shut down the circ again, etc, etc. It seems liek we'd get some nuisance cycling. How do people with their sensor on the output avoid this?

 

[WoodNotOil]

I find that temps are much more accurate in a well as opposed to strap-on. (With the exception of my well from the top of tank to the bottom which reads high) I think wells should be used unless they are not available. I also agree that ideally multiple sensors should be used for determining TAH and THH. In EPDM applications TAH has to be linked to max temp for the setpoint to prevent overheating the liner. So this would be top of tank. However, the differential could be determined from averaging several tank sensors to determine if the overall tank could benefit so long as the top has not reached setpoint.

To answer your question, I think one strap-on sensor on the output is not the best way to do this. However, people with devices limited to one tank sensor and strap-on applications probably find this an ok compromise that works most of the time. It is still far better than using an aquastat or have pumps run almost continuously as I have seen in some applications (and experienced myself before adding my controller). It is amazing how all that electricity adds up over time and the wild tank temp fluctuations you experience. A good control is really necessary in tank applications.

 

[chuck172]

The Tekmar 156 differential controller that I have uses a "source sensor" which should be strapped on the outlet pipe close to the heat source and the "storage sensor" which should be inserted in a well near the bottom of the tank.

 

[Nofossil]

The only temp that matters for THH is the top of the tank. Assume stratification - it only matters how hot the top is. If it's hot, you can heat the house.

In the simplest case, the only temp that matters for TAH is the bottom of the tank. If you're withdrawing water, adding heat, and returning it to the tank then you're adding heat to the tank. You might be destratifying it a bit, but the bottom tells you whether you can add heat. I'd be real careful to measure temp in the tank itself, not in nearby plumbing. That's probably why a well is suggested for this sensor.

There's some value in having an intermediate sensor or two to give you a sense of how 'charged' your tank is. With pressurized storage and good stratification, you'll have a sharp thermocline somewhere. Top and bottom temps won't give you a clue where it is.

 

[free75degrees]

The only temp that matters for THH is the top of the tank. Assume stratification - it only matters how hot the top is. If it's hot, you can heat the house.

agreed.

the only temp that matters for TAH is the bottom of the tank

If the bottom is at 120, the middle is at 140, and the top is at 160 with linear stratification in between and the tank input temp is 121, then we should not charge the tank because we will be removing heat (even though the bottom temp may go up). I think some kind of average temp is what we need. Better yet is to use the output temp... if the output is lower than the input then we can be sure that we added heat. If output is higher than input then we definitely removed heat. The problem with using the output is the problem that I mentioned in the original post.

 

[Nofossil]

If the bottom is at 120, the middle is at 140, and the top is at 160 with linear stratification in between and the tank input temp is 121, then we should not charge the tank because we will be removing heat (even though the bottom temp may go up). I think some kind of average temp is what we need. Better yet is to use the output temp... if the output is lower than the input then we can be sure that we added heat. If output is higher than input then we definitely removed heat. The problem with using the output is the problem that I mentioned in the original post.

I'll respectfully and mathematically disagree. If you're withdrawing water from the tank at 120 and returning it at 121, you're adding heat to the tank. Basic physics, no way around it.

There's a point of diminishing returns where the benefit of the tiny additional heat is outweighed by the loss of stratification, so you might elect to use a higher cutoff to avoid losing stratification.

In the simplest case, though, you could do a good job of charging just looking at bottom temperature. Actually, bottom temperature plus a delta would be a reasonable control strategy. I'd like a tank with a baffle that allows incoming water to find its level in order to preserve stratification.

 

[powerspec]

Why do we want stratification anyway? Most boilers have a dedicated circulation pump to eliminate stratification. If the temperature is not stratified in the storage tanks more heat can be stored. Am I missing something here?

 

[Nofossil]

Why do we want stratification anyway? Most boilers have a dedicated circulation pump to eliminate stratification. If the temperature is not stratified in the storage tanks more heat can be stored. Am I missing something here?

In storage, stratification is a good thing. Let's assume that you can put 180 degree water into your storage, and that you need at least 160 degree water to provide usable heat for your house. Let's also assume that your return temps are 140, for the sake of the argument.

Let's start with a tank that's 140 degrees top to bottom, and we'll dump some heat into it.

With stratification, we put 180 degree water into the top while the bottom remains at 140. We continue until the thermocline is halfway down the tank. Top half is 180, bottom half is 140. We have half a tank worth of usable hot water to heat our house. Everyone's happy.

Now, lets mix it up and de-stratify it. Same amount of heat energy, but now the whole tank is at 160. No usable heat. Sad family. Cold dog shivering by door.

 

[powerspec]

Good answer. But there is still only half the heat stored that there would be if the tank was not stratified. I see the storage not accepting heat (limit on top tank temp) when in fact there would be ample room for heat addition if mixing took place. In your example this is an acceptable situation if the water could be used in the load circuits from say 140F and up.
I am using a Benjamin CC500 oil/wood boiler integrated with my solar collectors and keep the boiler and solar storage tanks in constant mix so that whichever tanks are hotter will act as a heat souce to the cooler ones. I'm considering getting a EKO 40 or 60 but need to rethink how I handle the storage problem and the minimum return flow temp. Does your smaller EKO have a smoke problem when you reload? Is it indoor or outdoor? Does that EKO boiler have objectionable fan and combustion noise? I hate noise and I like a quiet house.

 

[Vtgent49]

Well, I need to respectfully disagree with No fossil (at my own risk! lol)

That 180 degree spec for baseboards, and for running oil boilers is a number that that may have been calculated using good science, when using oil. I can't pretend to tell you what science that was.

But, I can clearly state that using wood, non-gassifier, etc, that 160 degree water thru those same baseboards works just fine. Maybe not on an extreme cold day, and maybe the circ runs a bit extra. I'll go further to sat that 140 works also, and so does 120 or 100, or whatever depending on the outside ambient, solar gains, etc, etc.

So, I think we should lose the oil burner engineer specs, and start the thinking over. I.e. if I had 1000 gals, with 100 degrees at the top, and 75 at the bottom, I'd be happy to circulate that around. That would easily keep my 2200 sq ft warm on a day that was about 20F or so.

Yes, circ motor energy consumption adds to the equation, so it may not be worth it. So.... lets let it thermosiphon.... probpably another topic.

 

[Nofossil]

Good answer. But there is still only half the heat stored that there would be if the tank was not stratified. I see the storage not accepting heat (limit on top tank temp) when in fact there would be ample room for heat addition if mixing took place. In your example this is an acceptable situation if the water could be used in the load circuits from say 140F and up.
I am using a Benjamin CC500 oil/wood boiler integrated with my solar collectors and keep the boiler and solar storage tanks in constant mix so that whichever tanks are hotter will act as a heat souce to the cooler ones. I'm considering getting a EKO 40 or 60 but need to rethink how I handle the storage problem and the minimum return flow temp. Does your smaller EKO have a smoke problem when you reload? Is it indoor or outdoor? Does that EKO boiler have objectionable fan and combustion noise? I hate noise and I like a quiet house.

For any given amount of heat energy in a storage tank, there's more usable heat if it's stratified than if it's not. A tank that's all at 180 is more useful than a tank thats half 180 and half 140, but that's because it has more heat in it. To be fair, you HAVE to compare it to an unstratified tank that's all at 160. That's what you'd have if you dumped the same amount of heat into it without stratification. Limiting the amount of heat based i=on tank top temperature is a mistake - tank bottom is what should be used.

The EKO doesn't smoke much at all - none if I don't mess up or get carried away with adjustments. The fan is a bit noisy. Combustion is a low rumble Mine is in the basement and is not at all objectionable. We get more smoke from cooking disasters in our oven.

 

[WoodNotOil]

Here is a nice link about stratification in heat storage http://books.google.com/books?id=Es...&hl=en&sa=X&oi=book_result&resnum=4&ct=result . It is not a particularly light read. I will back Nofo up on this one. Unless you have slab radiant it is probably not practical to heat with water at low temps in the real world. Stratification makes the same amount of energy more usable in your heating system. If you can heat your house with 100* water, all the power to ya. Mine is no good under 140*, and even then it struggles at that temp in the really cold.

 

[DaveBP]

And my brethren and sistren let us not forget the other advantage of stratification.

Without stratification, as you put more heat into the tank, the return temp to the boiler goes up, right from the start ( OK, a little after the start ). Higher temps returning to the boiler means less delta T at the boiler, less efficient transfer of heat into the system water. If your firebox is still roaring because it's January and you don't want to get up in the wee hours so you loaded the sucker right full of your best, driest hickory, you don't want your return water temp to rise. The lower your return temperature the easier your boiler can put BTUs into your house. If your boiler output is set for 180F or 190F, if the return water is 160 or 170 it's harder for the boiler to transfer heat into that water than if the return temp is 140F.

So with a well stratified tank the boiler has an easier job shoving heat into storage and therefore your house. It can put BTUs into the tank at its maximum rate right up until the end of the burn (assuming the tank is matched to the boiler). With no stratification, it becomes gradually harder for the boiler to transfer heat to storage because it's trying to heat water that's becoming closer to the same temperature as the boiler is producing. It may have to lapse into idle/burn cycling. The world is not going to end because your boiler has to idle for a while. It's just about getting to the same result more efficiently.

Check this site a few times a day for a while and you'll see the pattern. The clock is Finland time, 5 hours ahead of EST. You can see the return water temp to the boiler stays nice and low right until it's entirely full of the hottest water and then it feeds the hottest water continuously to heat his office until he boots the boiler again(Kontor is office, Ute is the outside temp) . If you're using baseboards, this is important. If you use radiant floors, it's just cool to watch.

http://www.elstyrteknik.se/pannrum/

This link is from Hansson, our Swedish member.

 

[free75degrees]

If the bottom is at 120, the middle is at 140, and the top is at 160 with linear stratification in between and the tank input temp is 121, then we should not charge the tank because we will be removing heat (even though the bottom temp may go up). I think some kind of average temp is what we need. Better yet is to use the output temp... if the output is lower than the input then we can be sure that we added heat. If output is higher than input then we definitely removed heat. The problem with using the output is the problem that I mentioned in the original post.

I'll respectfully and mathematically disagree. If you're withdrawing water from the tank at 120 and returning it at 121, you're adding heat to the tank. Basic physics, no way around it.

There's a point of diminishing returns where the benefit of the tiny additional heat is outweighed by the loss of stratification, so you might elect to use a higher cutoff to avoid losing stratification.

In the simplest case, though, you could do a good job of charging just looking at bottom temperature. Actually, bottom temperature plus a delta would be a reasonable control strategy. I'd like a tank with a baffle that allows incoming water to find its level in order to preserve stratification.

Maybe the difference here is that I am talking about an open tank with coil exchangers and I am guessing that you are talking about an open tank with a flat plate HX or a pressure tank. The bottom temp of 120 that I am talking about is not the temp of the water leaving the tank, it is the temp at the bottom of the tank. With a flat plate HX/open tank or with a pressure tank the bottom temp is essentially the same as the temp of the water leaving the tank. With a coil tank, if the temp at the bottom is 120 and the top is 160 and the water coming from the boiler is at 121, then the water from the boiler will be heated by the tank and leave the tank somewhere between 160 and 121. As the water flows through the coils from top to bottom it will be below the tank temp for most of the length of the coil since it starts at 121 and most of the tank is above 160. This means it will be taking heat from the tank for most of the length of the coil. It is not until the very end of the coil that it will be above the tank temp, so it will have very little coil length over which to give back the heat that it took out. Overall it will remove heat from the tank.

 

[DaveBP]

Pardon my rant of earlier this morning. I just got home from a bad night of chasing code bugs in somebody else's programming. Kinda punchy.

I did NOT understand you were talking coils in a pool but the aim of maintaining the highest delta T is still the essence of the thing. Unpressurized storage with flat plate or submerged coil heat exchanger is still most efficiently arranged with a counterflow pattern to the water flows. Heat running in opposite directions like traffic on a 2-lane road. If your tank is hottest at the top and your hottest boiler water enters the coil at the top it is hotter than that top water and can shed heat to it. And as the boiler water continues cooling down the coil toward the bottom of the tank it is surrounded by water that is still cooler than it is and so continues to shed heat all the way. If the water in the tank is all the same temperature, the boiler water will shed heat until it gets to that median temp in the tank and stop losing BTUs. So with the same temp water leaving the boiler the two arrangements give you different temp water returning to the boiler. More delta T with counterflow means more BTUs transferred for the same circulator electric bill.

Flat plates are arranged the same way. Hot goes in on opposite ends to maintain the highest delta T over the longest path. Flow rates and interface surface area can nudge the numbers, too. But they do it most efficiently with a counterflow pattern.

P.S. It's not that a stratified tank holds more heat. It holds it in a more usable form and makes adding or extracting heat from it more efficient.

 

[Vtgent49]

"With a coil tank, if the temp at the bottom is 120 and the top is 160 and the water coming from the boiler is at 121, then the water from the boiler will be heated by the tank and leave the tank somewhere between 160 and 121. As the water flows through the coils from top to bottom it will be below the tank temp for most of the length of the coil since it starts at 121 and most of the tank is above 160. This means it will be taking heat from the tank for most of the length of the coil. It is not until the very end of the coil that it will be above the tank temp, so it will have very little coil length over which to give back the heat that it took out. Overall it will remove heat from the tank."

I agree, any coil that is evenly spaced thru the depth of a tank, such as SSTS does it, is just wrong. If it's a heater coil, then it should spend ALL of it's time/distance at the bottom of the tank. If it's a heat gathering coil, then it should spend all of it's time at the top of the tank.

My sidearm can transfer a LOT of heat with just 3 feet of length/surface area. My solar DHW tank, a Vaughn, uses about 8 feet of pipe, finned, tightly wrapped, but it's ALL at the bottom of my 80 gal tank. It's transfer close to 100% at whatever speed I set my 3 speed Grundfos pump. So, those 90-100 foot coils I see people using, evenly spaced thru the depth of a tank, just make me MAD.

Both copper and the electricity to drive a pump are precious, right? That's why we are all hanging around this site, isn't it?

Sorry for the rant, lol.