A Buffer tank issue?

[Alan Dunn]

Another valuable wealth of knowledge here from Dana.

I just lowered the Design Supply to 155F and supply differential to 10F. I never know what supply differential is. I always thought this is what I want the swt/rwt difference to be set at. I will keep an eye on that to see whether the tighter differential will short-cycle the boiler. It hasn't at 20F. Just slightly at the end of the cycle for the one smallest zone. Burner just stopped and let the pumps circulate until the thermostat satisfied on that zone.

Our boiler now comes on and stays on fairly long at low output heating range most of the time as shown here.

I say, this winter so far, the number of cycle per day is 10 or so according to the boiler statistics counter.

 

[Dana]

At ten cycles per day it's safe to say it's not short cycling, eh? (If we try hard enough maybe we can make that happen! )

The thermal mass of the buffer is definitely taking care of things while the whole-house load is less than the minimum-fire output. If we keep lowering the water temperature the calls for heat from the zones will be longer, and the odds of calls from zones overlapping each other in time increases. So even if it would short cycle with only one zone calling when the water temps are ultra-low, as long as two or more zones are calling it won't. With the water temps dialed-in the Reset Heating histogram would all cluster between the 10% to maybe the 30% bar, with maybe 1-2 hitting 50% during truly cold weather below 25F.

The greater the number of active zones, and the closer the reset curve gets dialed-in, the more the boiler can modulate with load rather than cycle. This guy has a radiant system on one of his places that runs without a thermostat, with the call for heat hard-wires as "always on". The outdoor reset is dialed in closely enough that it's always warm enough, and never too warm, with burns that run continuously for days or weeks in the dead of winter, simply modulating the burner up/down with outdoor temperature.

 

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[Alan Dunn]

At ten cycles per day it's safe to say it's not short cycling, eh? (If we try hard enough maybe we can make that happen! )

The thermal mass of the buffer is definitely taking care of things while the whole-house load is less than the minimum-fire output. If we keep lowering the water temperature the calls for heat from the zones will be longer, and the odds of calls from zones overlapping each other in time increases. So even if it would short cycle with only one zone calling when the water temps are ultra-low, as long as two or more zones are calling it won't. With the water temps dialed-in the Reset Heating histogram would all cluster between the 10% to maybe the 30% bar, with maybe 1-2 hitting 50% during truly cold weather below 25F.

The greater the number of active zones, and the closer the reset curve gets dialed-in, the more the boiler can modulate with load rather than cycle. This guy has a radiant system on one of his places that runs without a thermostat, with the call for heat hard-wires as "always on". The outdoor reset is dialed in closely enough that it's always warm enough, and never too warm, with burns that run continuously for days or weeks in the dead of winter, simply modulating the burner up/down with outdoor temperature.

One of coldest Octobers in 80 years draws to a close here today. Without running the two gas fireplaces, I bet the cycle count on the boiler would have been much higher.

After the adjustments (lowering Design Temp to 155F & differential to 10F), at steady state, the delta T near the boiler remains the same at around 4F only and doesn't take advantage of the cooler zone return water.
At a snapshot:
Near manifold SWT/RWT= 137F/107F
Near boiler SWT/RWT= 140F/136F
Boiler Target Temp=134F
Buffer tank = 139F
I wonder what Target Temp is targeting? SWT, RWT or buffer tank water temp?

Indoor Temp is optional on this boiler that we are not using. Always wonder how that functions if this indoor temp control is hooked up to a temp sensor? Is that a replacement to all the home wall thermostats?

With this 10F differential, the smallest zone short-cycles a lot. I think I am raising that up to 20F.

 

[Dana]

I wonder what Target Temp is targeting? SWT, RWT or buffer tank water temp?

The boiler is completely agnostic of the buffer's temperature as well as the temperature at the manifolds, and only targeting the SWT at the boiler.

The targeted SWT is the temperature is the current output temperature for the current outdoor temperature, calculated from your curve settings. If you're targeting 155F @ +25F outside, when it's +35F outside the target temp will be less than that, and when it's +15F outside the target will be higher (but never higher than what you specified as a maximum temperature.)

The supply differential defines the allowable bracket above & below the target temperature. When the zone radiation is too small to emit the minimum firing rate output at the current water temperature, there is "extra" heat going into the system causing the whole system's water temperature to rise. With a differential set to 10F the boiler turns off the burner when the temp rises to ~5F over the target temp, and doesn't re-fire until the temperature drops ~5F below the target temperature. With the differential set to 20F it will turn off when the system hits 10F over the target, and won't re-fire until the system temp is 10F cooler than the target.

With a 10F differential and the reported conditions...

Near boiler SWT/RWT= 140F/136F
Boiler Target Temp=134F

...I suspect the boiler had just turned off or would be imminently, since the measured temp at the supply output is a hair over the 5F overshoot from the target temp.

The indoor sensor option doesn't replace the thermostats, but uses that input data to better adjust the target temperatures along the curve. It's not 100% clear, but it may in fact automatically fine-tune the curve for you to avoid having to program the design temp & target temp up/down to find the sweet spot. The verbiage in the manual states:

An optional indoor temperature feedback routine can be activated with the installation of an indoor sensor, connected to the designated TB2 contacts to automate adjustment of the outdoor reset routine.

(emphasis mine.)

If you have been using overnight temperature setbacks, don't. That forces the system to run at higher temperature to have reasonable recovery times. Ideally the water temps would always be low enough that NONE of the zones satisfy the thermostats quickly, which means almost always the radiation of 2 or more zones will be in play. That reduces the cycling even at lower temps and low differential. If the zones are all satisfying the thermostats within 15 minutes the water temps are too high.

The very low delta-T at the boiler is due to the high pumping rate on the primary loop. I can't tell from the pictures, but if there is a ball valve on the primary loop, try partially closing it while monitoring the boiler's reported flow rate, and bring the flow down to something between 2-3 gpm. That will double the delta-T at the boiler, lowering the entering water temp.

 

[Alan Dunn]

No it is never in any kind of setback at night.

I have a feeling this Target Temperature is looking at the buffer tank because the "Water Temp From:" parameter under zone heating is set at "Sec Loop" as stated in their concept buffer tank PDF drawing from P2 of the Programming Notes I attached.

It doesn't have a ball valve on the primary. Can I replace the primary 15-58 with another Alpha 2 like at the radiation side? This way, since they are the same pump, I can at least gauge and adjust the flow better?

I like to learn a bit more on the Indoor Temp Feedback feature and the manual has a limited coverage on it. It will be easy to hook it up. Any good literature out there that I can take a look?

Also, another thing about one setup parameter. Emitter type. It offers these choices: High Mass Radiant, Low Mass Radiant, Air Handler, Cast Radiant or Baseboard. Which one is more applicable to ours?

Many thanks for all the help

 

[Dana]

I can't read the attached picture, but I'm now understanding the target temperature is attempting to control the entering water temperature, not the supply temperature (?). If that is the case the curve needs to be set even lower.

Near boiler SWT/RWT= 140F/136F
Boiler Target Temp=134F

Note that the Target temp of 134F was being overshot by 0nly about 2F, and it was probably still burning. With a 10F differential and insufficient radiation calling for heat it would continue burning until the the entering water temp hit 139F. With a 20F differential it would continue burning until the entering water temp hit 144F.

I've never personally set up this series boiler, and I too am curious as to how using the indoor reset affects the setup. As yet I haven't done the research on that.

Regarding a second Alpha 2:

Remember, at the head loss of the boiler...

...at 6 gpm is about 2.5 feet...

....at 4 gpm it's 1.5 feet...

...at 2 gpm it's 0.5 feet.

That means the Alpha 2 is (like the 15-58) with only three fixed speeds going to be overkill for ultra-low pumping head even at it's lowest setting:

.

Even at the lowest speed you'll be looking at ~5 gpm @ 2 feet of head, which isn't a big change from where it is running right now.



You might be able to get there with a Taco VR1816 in fixed-speed mode at one of it's lowest settings:

At the lowest speed the VR1816 it will settle in around 3.5 gpm or about half what it's doing right now.

There may be better options out there that allow more flexibility at your ultra-low pumping head, but most systems with boiler head this low get pumped direct, not primary/secondary with a thermally massive buffer tank as the hydraulic separator. This is where having the additional pumping head of the radiation may be a help, and using just the Alpha, plumbing the tank in series. This is also why it's better to design it ahead of time rather than just hacking & tweaking.

In terms of selecting the radiation type, with the large water volume of the buffer tank the system behaves a bit more like a cast iron radiator system.

 

[Alan Dunn]

Ok Just an update. Haven't given up and got some help from IBC Inc. today.

The helpful support tech pointed me to a technical bulletin from Mar. of this year that "
IBC boilers can use a temperature sensor to call for space-heating (release 1.10.2). This feature can be useful in applications such as: Space-heating loads served by a buffer tank, where it is desirable to separate the burner cycles from the heating cycles.
The space-heating call for heat is tied to a temperature drop in the buffer tank, rather than a call for heat. The Heating Load from Sensor feature can reset the target temperature according to an outdoor temperature.
Note that the buffer tank ‘de-couples’ the secondary piping cycles from the tank heating cycles, so the boiler controller is only concerned with maintaining temperature in the buffer tank. Control of the zone pump or pumps, and wiring for any thermostats and zone valves, is independent of the boiler."

So what I am going to do is: (1) change 'Burner On From' to 'Temp. Sensor' from 'Thermostat' in the boiler's setup;
(2) remove/disconnect Tekmar zone controller's end switch from boiler;
(3) remove secondary Alpha 2 pump from boiler's wiring terminal and connect it with Tekmar.


Let's hope it works better in terms of getting some cooler water return temperature....