Tweaking the BioWin's Buffer Tank

[velvetfoot]

This is very interesting to me.

I think I have an even earlier model; it was a demo from one of the first batch that came over to the US. It doesn't have a feature to control the boiler circ pump; I had to rig my own. In addition to the controller that doesn't let the boiler pump run when the boiler temperature is not greater than 130, I installed a relay that runs the pump for 10 minutes after there is no more call for heat by the tank. (You're right, 20 minutes would be better).

I like your fan motor ploy. I was thinking the same thing, but didn't know where to tap. The other thing I was going to do with the information that the boiler is shut down (fan not running) is start the backup oil boiler if there is a call for heat at the same time, with a time delay. The thing is, if the boiler shut down on internal temperature instead of by the tank's controller being satisfied, the fan would also shut down, and then the oil boiler would come on. My current arrangement is the second stage of the Ranco controller that controls the pump for condensation (130) controls the kickoff of the oil boiler by turning it on if the pellet boiler is less than 120 for 20 minutes when there is a call for heat. It takes a while for the pellet boiler to get that cool, which I figure is safe and it seems to hit 120 in 20 minutes when the pellet boiler fires up from cold.

I don't know if this is helpful, but the thermowell in the boiler can hold another sensor, easily, which is where I've got one.

 

[Deering]

I like it!

I suppose you could always just have the pump run so long as the boiler is warmer than the tank lower temp, down to 130. Not sure how that would integrate with your oil boiler arrangement.

Seems to me you'd just want to make sure you power the pump from the boiler's 120V bus so you're not energizing the boiler when you think you have the power shut off. Might just want to have an electrician scope that one out before you throw the switch.

 

[velvetfoot]

I suppose you could always just have the pump run so long as the boiler is warmer than the tank lower temp, down to 130. Not sure how that would integrate with your oil boiler arrangement.

I like that idea. I could finagle some settings and see what running the pump for a while would do. A zone that turned on would affect things and might push cooler water to the zone? Also, there'd be less stratification with the pump running. I'm thinking of running the pump until the tank temps stabilize after turnoff and turning off the pump and see what happens with the boiler temp; what i've seen so far is that the boiler temp goes up over the top tank temp after it's stopped after ten minutes.

Seems to me you'd just want to make sure you power the pump from the boiler's 120V bus so you're not energizing the boiler when you think you have the power shut off. Might just want to have an electrician scope that one out before you throw the switch.

The pellet boiler pump is powered from the the pellet boiler circuit. I have an on-off switch wired in for each pump which is useful once in a while.

 

[PassionForFire&Water]

There should be a 10 minute post purge time delay installed.
After 10 minutes the boiler pump stops

 

[velvetfoot]

That's the way it is now. Do you think there's anything to be gained with a longer run time to extract some more heat?

 

[PassionForFire&Water]

the 10 minute post purge is now standard on all BioWINs and can not be changed.
I assume Windhager tested this and came to the conclusion that this is the most optimum post purge time.

 

[heaterman]

That's the way it is now. Do you think there's anything to be gained with a longer run time to extract some more heat?

In 10 minutes time the boiler and tank temp will equalize so there is nothing more that can be transferred.

 

[velvetfoot]

That's sort of not what I've been seeing.

I wonder if I should turn up my boiler pump? It's been on 3 (of 7).

The zone pumps are on pretty low, and I will tweak them if I need to when it gets cold.
I'm not sure what to do about the boiler pump though. I figure it'll transfer the heat from the boiler to the tank quicker and maybe I'll get a fuller heat load on the tank.
I turned it up to 7 this cycle and will see what happens.

 

[velvetfoot]

What's interesting is that the baseboards are getting warm even with no zone pumps running. As I recall, this happened before. There's no zone pumps and it must push some water up to the zones as well as going through the tank, though all the near boiler pipe is 1.25". I'll turn it back down again.

 

[Deering]

10 minutes comes pretty close to equalizing temps between my boiler and the tank. 15 or 20 would probably be a little better. But what would really be the cat's meow would be to run the pump for 10 minutes, shut it off, wait for 30 minutes for the tank to stratify to allow cooler water to settle to the bottom/pump intake, then run the pump for another 5-10 minutes. After stratifying, my lower tank is often 15-20 degrees cooler than the boiler, say 150 vs the boiler's 170. I wanna suck every BTU outa that boiler that I can.

Velvet, could your zones be getting heat due to thermal siphoning? Do they warm up even when the boiler circulator is off? The hotter your tank is, the more pronounced that effect would be. With your tank as a hydraulic separator, they shouldn't see any effect from the boiler pump. A way to test this would be to valve them off while the boiler is running, then open them right after it stops and monitor the baseboards.

 

[velvetfoot]

No, they don't warm up when the boiler pump is off.

 

[Deering]

Hmmm. That shouldn't be happening. The water is going to take the easiest path back to the pump, and that's definitely through the tank. Unless there's some baffling or something inside the tank restricting flow through it...hmmm

 

[velvetfoot]

That's a good point, there is baffling in the tank, at least that's what the mfr. claims! I'll just turn it down some.

 

[Deering]

Out of curiosity, what speed are you running your boiler/tank circulator? You have a Caleffi, right? I just slowed mine down to the slowest setting and it seems like I'm getting longer run times. Maybe because the stratification is not being as disrupted by a high-speed jet of water.

 

[velvetfoot]

No, I have some B&G Ecocirc Varios for all the pumps. I currently have it on 3 of 7. No ghost flows. I notice way better stratification on extraction when the boiler pump isn't running. Charging, even on 3, the best I can do is something like 180 top and 174 bottom, after a ten minute purge and no zone pumps running. If a zone pump runs and the boiler pump is still running with the boiler off, you can see the boiler heat going directly to the zone. The bottom of the tank cools rapidly when heat extraction takes place, with the top remaing hot.

 

[Deering]

I don't know if this is helpful, but the thermowell in the boiler can hold another sensor, easily, which is where I've got one.

Say, where is that thermowell located?

I just had an issue this morning where I woke to my boiler sitting idle while my tank was down to 145 and the electric backup boiler was running. Nothing wrong with the pellet boiler. It just had an internal temp of 177, well above its max setpoint of 167, so it wasn't going to start until it cooled off below 167. Those darn Windhagers are pretty well insulated, so it might have taken another hour to cool while my electric handled the morning heating load. Not the arrangement I was hoping for when I installed a pellet boiler.

This goes back to our conversation about how long the pump should run post purge to extract heat - 10 minutes just isn't enough.

I'd like to figure out a way to have the boiler pump run whenever the top of the tank is warmer than the boiler. To do that I need a sensor in the boiler. Will need to figure out how to use the differential between the two temps to control a relay in the pump circuit. Seems like you might benefit from the same arrangement. Any ideas?

 

[velvetfoot]

Say, where is that thermowell located?

It's behind a panel in the back; the boiler sensor is in there and you can follow the wire.

The main thing is that the tank temp setpoint wasn't satisfied. In your control scheme, the boiler pump shuts down 10 minutes after the boiler goes to standby because of the internal max. In my scheme, the boiler pump would keep on running because the pump is keyed into the call for heat by the tank, which still hasn't been satisfied.

The boiler has internal and external setpoints. I don't quite understand what your settings are, like, what the 167? Internal, external, both?

Again, I think the main thing is that the tank temp setpoint wasn't satisfied. Perhaps you're cutting things a little too close.

What emitters do you have? I have baseboard, so I'd have a tendency for higher temps. I try to charge my tank to as high a temp as I can.

 

[Deering]

Both internal and external setpoints on my boiler are set to their max - 167. That's the Euro limit on the older Windhagers - the newer imports have a higher max for typical U.S. setups. (176?). I have my tank upper limit set at around 175 - the boiler will modulate above its 167 setting (why, I don't understand) - I've seen it as high as 183.

I have baseboards so higher temps are desired. In order to keep the boiler max temp below 167, I'd have to set my tank limits at around 160 or lower, which would not be suitable for my distribution system.

I like your approach. So you have a relay on the pump circuit. How are you powering the low voltage side of the relay? You're connecting the aquastat to a different terminal in the boiler - one that remains energized regardless of the boiler's internal temp? How does the boiler know there's a call for heat then?

 

[velvetfoot]

Are you sure about that? Did you go into the parameters menu to check? My boiler was originally set at 167. I thought I had an old one but both can be set to 176.

I went with 120 volt Rancos, mostly. The boiler has nothing to do with it. The call for heat comes from the tank, if the pellet boiler can it will start up, and the boiler pump will run only if the boiler temp is above 130.

 

[ewdudley]

Here's my control scheme for feeding a buffer tank with a solid fuel modulating boiler. It's coal boiler but I believe the constraints and goals are the same.

I use an aquastat near the bottom of the buffer to generate a call for heat (T-T) to the boiler. During the shoulder seasons I set it to minimum return temperature, 140 degF or whatever. Then I set the boiler supply temperature to whatever it takes to keep the house warm, starting at 150 degF in the fall and increasing to 180 degF as the season progresses. Then if the system starts to fall behind I start to raise the tank lower aquastat setpoint up from 140 degF.

If the return flow from loads can bring the bottom of the buffer below return temperature minimum then need another aquastat to disable load pumps until bottom of buffer recovers above boiler return temperature minimum. (In my case the control is simplified because I'm using a Taco 5000 series mixing valve to maintain minimum boiler return temperature, so no need to disable load flows if bottom of buffer tank goes below boiler return temperature minimum.)

 

[Deering]

Are you sure about that? Did you go into the parameters menu to check? My boiler was originally set at 167. I thought I had an old one but both can be set to 176.

Yeah, pretty sure. Been in parameters a bunch of times. It's 167 max. Your boiler must be a bit newer than mine. They made the switch shortly after I bought mine. I just need to add more baseboard or some panel radiators.

The boiler has nothing to do with it.

Then what are you using the sensor in the boiler temp well for? Or are you just saying that there's room for a second one?

I use an aquastat near the bottom of the buffer to generate a call for heat (T-T) to the boiler.

Maybe I'll just use the lower aquastat to control tank temp like you do, and use the upper to control the pump and see how that works out. I think the lower stat is going to dominate the tank control scheme anyway. If the upper stat falls below 167, it turns the pump on. The lower stat won't let the tank fall below 130 or so to protect the boiler during startup. Besides, I have a Caleffi 281 Thermobloc circ pump to provide redundant protection. The worst that can happen is that the pump runs for awhile with an idle boiler until the lower stat starts to call for heat.

 

[velvetfoot]

Then what are you using the sensor in the boiler temp well for? Or are you just saying that there's room for a second one?

That two stage Ranco senses boiler temperature for boiler protection, temp not less than 130 for running the boiler pump, as well as for when the oil backup comes on, temp <120 with a heat call for more than 20 minutes.

 

[velvetfoot]

More observations.
With an internal setpoint of 176, the BioWin shuts itself down after spending a fair amount of time at 181.
When a load comes on when charging the tank, the boiler temps drops, and not the tank, showing that the boiler is satisfying the load, as it should.
I am thinking now that arranging the controls so that the boiler runs until it shuts off by itself would be best, to more fully charge the tank and avoid situations that keep the boiler pump running. The pump will still run for 10 minutes after shutdown, and not under 130. With the more energetic, non-modulating nature of the oil boiler, this same philosophy may or may not work.

 

[bdud]

I have a Honeywell L4006B humidistat on the output pipe from the boiler. This turns on at a set temp and is wired to a separate input on my controller. I use this to control how long the circulation stays on for the buffer tank. I have not timed how long the pump stays on for, it depends on the temp sensed on the boiler outlet pipe, the buffer tank ultimately gets to a higher temp than the boiler turn off temp.

 

[velvetfoot]

humidistat

I know you mean aquastat.
So, if I understand it right, the boiler turns itself off based on its internal temperature setting, and then, after that event, the aquastat on the boiler supply pipe keeps the pump running until it (drops?) to a certain point?