Buffer tank w/ multiple aquastats

[Deering]

Hi All,

Wondering if I can tap into the collective brain trust here to help me puzzle my way through a controls question - my brain is pretty well cramped up on it.

I'm getting an 80-gallon pressurized buffer tank to help minimize short cycling of my Windhager pellet boiler during low-load periods and to help meet the morning domestic water spike load. The tank I'm getting is this one: http://thermomatrix.com/thermoproducts/coolfire-buffer-tank/ in the 80-gallon flavor.

My plan is to 'decouple' the boiler from the zone valves by having the boiler respond only to heat calls from the buffer tank, and have a dedicated pump between the boiler and the buffer tank. The zones have their own distribution pump and zone valves and controller, and will pull hot water from the buffer tank as needed - from the perspective of the zones, the buffer tank IS the boiler.

Here's my puzzle: The buffer tank comes with two sensor wells, high & low. I'd like to use sensors in both wells to control when the boiler turns on and when it turns off. When the tank is fully charged up and the boiler is off, both sensors will be reading close to the same temperature - the top sensor a bit higher of course. As the zones start to use hot water, the stratification boundary will move upward - as I understand it, if there's not too much churning from the water inlets, there will be a pretty clean temperature boundary that will move up and down the tank. That's why tall, 'skinny' tanks are preferred.

I want the pellet boiler to fire up when the upper sensor starts to drop in temperature - then I know that it will be running at least long enough to charge up nearly 80 gallons of water. If I have an aquastat connected to the upper sensor call for heat when the temp drops, say 10degF, that should work fine. But I don't want the boiler to stop until the lower sensor is also up to temperature. What I need is an aquastat that uses one sensor to 'make' and another sensor to 'break'. I've looked the the various Honeywell aquastats and the T775 controllers and none of them appear to do what I'm looking for, but I admit I'm a novice in this control logic stuff so I'm probably just missing the obvious. Maybe I need some sort of relay combination to make this happen?

I could try just going with a timing relay or adjustable high/low range on a single aquastat & sensor, but then I'd be relinquishing the advantage of actually knowing what's happening across the tank.

Any suggestions very appreciated.

 

[ewdudley]

One way is to use a DPST (or DPDT) relay set up to latch the call for heat on after the upper aquastat makes and until after the lower aquastat breaks. A Functional Devices RIB would do the trick.

But you'll need two aquastats unless there's such a thing an aquastat with two sensors that is in effect two aquastats in one. A pair of $50 Rancos would work.

Lower aquastat makes-on-fall, but relay is not energized until upper aquastat also makes-on-fall. One set of relay contacts makes the call for heat to boiler, the other set of relay contacts loops back around to latch the relay active until lower aquastat breaks-on-rise.

Upper aquastat can open when top of tank gets hot, but relay stays active because it is latched by lower aquastat.

 

[Deering]

Brilliant!! Thank you!

 

[DZL_Damon]

My 80 gallon buffer does not stratisfy as much as you would think. It's very similar to the unit you are looking at, but only 56" tall vs 72" and mine is 24" dia. vs 22". I have an HTP glass lined hot water tank with x3 2"npt holes except my top one is out of the center of the top, and I don't have the middle one like the brochure shows yours. I have boiler return from the bottom one and zone return from my middle. The difference in temperature from the top and bottom well only seems to be about 2 to 4*F when 1 of 4 zones is cycling at 1 gpm (but it will drop 40*F in an hour). However when the zone initially cycles on the lower well, it takes a larger dip than the top one. However, the top one drops soon enough too just not so drastically. In 30-40*F whether I can cycle my zone about once before my boiler calls to come on. I'm not using an aquastat, but rather an RTD temperature sensor that my boiler reads/trends. I can adjust the on/off point from work or my phone along with all the other parameters while I'm fine tuning the system. I'm using a 35*F differential currently. Right now I have my boiler target discharge at 175*F, the off point for my boiler (via the upper thermowell in my 80 gallon buffer) set at 190*F, and my on point is 155*F.

I've swapped them back and forth and it seems a tad more stable on the top well due to that initial drop when the zone cycles. I would just use a standard Honeywell aquastat with an adjustable deadband (usually adjusts 5-30*F below your set point) and you will be fine and real simple/less expensive.

 

[Deering]

Thanks for your observations, DZL_Damon. I suppose there's no reason why I couldn't try it out with a single aquastat first and monitor temps, then add more complexity if it seems warranted. I'd already have one of the necessary aquastats for the configuration that ewdudley spelled out.

I noticed your signature block...work at the Lab? I got a tour of that Petrokraft a couple years back and got a major case of envy. I hope it's working as well as I imagine it is.

 

[ewdudley]

As DZL points out you may not be able to maintain stratification in the buffer. And you may not even want to, depending on what the minimum return temperature of your pellet boiler is.

I'm running a 30 gallon buffer on my oil boiler; to maintain minimum return temperature and to lengthen each burn cycle. I have a few oversized cast iron radiators that dump slugs of cool water from time to time so I choose to mix buffer water with system return water to maintain minimum return temperature.

I've added a simple aquastat on the return port of the boiler, plus there's an L8124 'triple' aquastat that senses the upper water jacket of the boiler. The boiler runs according to its low limit (which acts more as its high limit), set to kick on at 160 and off at 175.

The buffer circulator runs when:

• External return port temperature low limit is satisfied AND system call for heat. OR
• External return port temperature low limit is satisfied AND oil burner is running. OR
• External return port temperature low limit is NOT satisfied AND oil boiler internal low limit is satisfied.

Load circulators are disabled when return port temperature falls below 150 degF and are re-enabled when return port temperature rises above 160 degF.

If the buffer does stratify well with high deltaT loads then a another technique would be to use a mixing valve to control return temperature and provide T-T to the boiler with a pair of buffer tank aquastats as described above.

 

[Deering]

My thinking on this has evolved exactly along those lines. If I install the aquastat in the lower sensing well, I can set the lower 'make' temperature to be above the 130F that my pellet boiler wants as a minimum return temp. This might take a bit of tuning since the pellet boiler will have a bit of lag time to fire up and come up to temperature before the circ pump engages, and while that's happening the buffer tank will be continuing to drop in temperature.

While that somewhat reduces the amount of energy available to draw out of the buffer tank, it eliminates the mixing valve and provides good boiler protection. 80 gallons is still large enough to prevent short-cycling in my system, even if I hold it at that higher temp.

 

[velvetfoot]

I like this idea. My 110 gallon tank is 6 feet tall. Now all I have to do is figure out this latching relay stuff.