Tweaking the BioWin's Buffer Tank

[velvetfoot]

I've been tweaking the buffer tank's upper and lower temperature settings to try to get the most heat I can into the tank, and allow the most use of the tank. The BioWin's max internal and external setpoints are 176, but that can be overshot by some undetermined amount as the unit modulates down.

The tank controller I put together is latching, (thanks again, ewdudley), and the sensors are at the top and a little above the return opening at the bottom. With settings of 150/168 (top/bottom), I was able to eventually get a final reading of 181/174, with a boiler temp if 186. This was after the heat call from the tank was stopped, the circ pump run on 'purge' for 10 minutes, and the boiler shut down and in stand by. There was no call for heat from the zones, which is a big factor.

For the next cycle, I changed the settings to 145/170. Even if the controller doesn't initially see 170 before the boiler shuts down, it will after the boiler pump runs for a while in shutdown and then the pump will shut down after that 10 minutes I mentioned before. The issue could be if there are heat loads during that time with the boiler shutting down. Then, the controller might not see that temperature, and the pump would continue running until the boiler starts again on its own. We'll see how that works.

The fact that there's virtually no heat load is a factor now.

The boiler is protected by a thermomix valve and the boiler pump won't run unless its temp is above 130.

Any ideas/suggestions? I don't want to fly too close to the sun.

 

[maple1]

There's a bit of a catch-22 with storage tanks, in that the more heat you get into them the more potential there is for increased stand by losses from them. That might turn out to be negligible if the tank is very well insulated - or of no consequence if the tanks are in the heated space. It could be more of a factor if heating for DHW only in the summer.

 

[velvetfoot]

Well, I am spending more time in the basement nowadays.

 

[harttj]

I'm surprised you need a buffer tank. With a modulating pellet boiler seems it would not be needed. How many gallons does the boiler hold?


Seems adding storage defeats some of the benefits.

 

[ewdudley]

I'm surprised you need a buffer tank. With a modulating pellet boiler seems it would not be needed. How many gallons does the boiler hold?


Seems adding storage defeats some of the benefits.

Exactly. I believe heaterman offered a technique for setting the aquastats that would encourage the pellet boiler to operate continuously under modulation as the boiler was designed to do.

 

[maple1]

Would the storage tank acting as a hydraulic separator though add some benefits back in, and allow more opportunity for modulation?

Interesting stuff.

 

[ewdudley]

Would the storage tank acting as a hydraulic separator though add some benefits back in, and allow more opportunity for modulation?

Interesting stuff.

Seems to me you would want to circulate constantly through the buffer from the pellet boiler side in order to keep the changes in load as gradual as possible so that the pellet boiler can modulate smoothly in response to longer term changes in load.

As opposed to setting up a buffer for an oil buffer where you would want to let the buffer stratify so as to minimize short cycling.

 

[velvetfoot]

I'm surprised you need a buffer tank. With a modulating pellet boiler seems it would not be needed. How many gallons does the boiler hold?

Ideally, that could be true. The buffer tank serves double duty as a water heater: 11 gallons for dhw are in a 1.5" corrugated stainless coil and 108 gallons are buffer. The other thing is, it's shoulder season now and there's not that much of a load. I can see reduced load in the winter too because I have a wood insert that I'll use, but maybe not as much as before, with a good feature that the upstairs will now be heated better. I think those factors argue for a buffer tank here.

Exactly. I believe heaterman offered a technique for setting the aquastats that would encourage the pellet boiler to operate continuously under modulation as the boiler was designed to do.

Yes, I'm experimenting with different settings. Hence the topic. Getting the tank as hot as possible is probably generally good, despite standby loss. The emitters are baseboard, so they like warmer water. If I use heaterman's technique of lowering the external temp and keeping internal temp at max, I'm thinking the water temperature available from the boiler when directly heating the zones off the top of the tank will be less than the max of 176. Maybe less high a tank temperature achieved? The big plus is that when it is modulating down low like that, it's available for instant heat, and as long as it's firing to heat up the tank, it's ready to heat up a zone, and will run longer. It takes maybe 20 minutes for it to start churning out meaningful heat. 20 minutes for the buffer tank might not cut it when there's a shower or cold temperatures involved. (A reason to bump up the boiler min. temp.)

Would the storage tank acting as a hydraulic separator though add some benefits back in, and allow more opportunity for modulation?

I'm not totally clear on what a hydraulic separator is, but as I said, as long as the buffer tank is being heated, the more opportunity for the boiler to directly heat the zones, doing its modulation thing.

I really don't know what the best way to operate the system is. Maybe different settings for different times of year as well. Throw in the wood insert, and...

 

[velvetfoot]

Seems to me you would want to circulate constantly through the buffer from the pellet boiler side in order to keep the changes in load as gradual as possible so that the pellet boiler can modulate smoothly in response to longer term changes in load.

As opposed to setting up a buffer for an oil buffer where you would want to let the buffer stratify so as to minimize short cycling.

That's interesting. Froling brags about the stratifying qualities of this tank and expounds how it's a perfect match for their P4 pellet boiler.
This whole subject is interesting to me, anyway, and I can tweak things and experiment without really hurting anything.
I'm thinking along the same lines as well: it takes a good while before the pellet boiler comes up to temp, and if the buffer tank is set too low, it'll only put a small dent in the heating requirements when the boiler is warming up from standby.

 

[mustash29]

If your boiler is on the left, and your loads on the right, a buffer / hyd separator is in the middle. It acts as buffer volume and also as a pri / sec piping concept.

http://www.taco-hvac.com/www.epa.gov/hydraulic_separator/index.html

 

[mustash29]

http://www.seekcg.com/wp-content/uploads/2013/02/MayJunepg38.40hires.pdf

 

[mustash29]

http://ctashrae.org/images/meeting/101112/hydronic_systems___dinner_session_presentation.pdf

 

[velvetfoot]

Hear is a nice writeup from Tarm:
http://blog.woodboilers.com/2013/05...013/05/using-pellet-boilers-with-thermal.html

 

[mustash29]

Very nice article.

 

[velvetfoot]

I moved the top sensor 1/3 of the way down the tank and changed the settings to 150 top/ 172 bottom.
It's good to have more heat to work with when the zones or dhw kick in.
The tank got to 179/172 at stop, and 180/176 after 10 minute heat purge.

 

[Deering]

I think a big part of the challenge you're having is the small amount of water you have in your DHW loop (11 gallons, right?). Once you deplete that 11 gallons, you have to rely on the thermal storage tank for all of your DHW. That forces you to keep the big thermal storage tank at a high temp, if I'm understanding your challenge. Ultimately this turns out to have a high lifecycle cost due to lowered system efficiency.

The simplest, but maybe most capital expensive, solution is to install an indirect water heater. On my system I have a 35 gallon system that has an integral heat exchange loop inside it. You could accomplish the same with an electric water heater and a flat plate heat exchanger, cheaper, but that adds another pump to the system. Plumb it as another zone in your system with its own zone valve or circulator. Then the indirect is your shower 'buffer' - you have enough HW for lots of showers and the thermal storage can be allowed to drop to a lower temp, with lower standby losses, and optimizing boiler modulation. It will still reheat the DHW tank, but perhaps at a slower rate. Think of it as adding more thermal mass to your system, allowing your boiler to cycle even longer at low modulation.

The downside with allowing your thermal storage to drop to a lower temperature is that the water supplied to your baseboards when there's a call for heat might be lower than desired for awhile. But that just means that the room heats up a bit more slowly until the boiler can catch up. Usually that's not as objectionable as a cold shower. If you have programmable thermostats for some spaces, you could program them to come on 15 minutes earlier.

The cheaper option would be to install low-flow shower heads. I tried out a 1.5 gpm Niagara head (available through Amazon for under $10) and I have to say that the quality of the shower experience was much better than I expected. I'd recommend starting with that.

 

[velvetfoot]

This is interesting. Our shower heads are low flow and there hasn't been any shortage of heat for the shower even when the tank gets below 130. I have the zone and boiler pumps set so they don't run under 130. The thing I didn't like was what I thought was the slow return from setback. I changed the setback return to a half hour earlier so then the heat is on for a half hour before I get up. Depending on how much energy is in the tank, it'll warm up the zones while the boiler starts.

I do have an indirect tank from the oil boiler, and I guess I could experiment with running that as well, but I already have that big well insulated tank with the coil in it too, and I'm leaning to not experimenting for a while, lol.

The other thing is, while it does store heat, it's nothing like a big boy tank like the wood burners use.

 

[ewdudley]

The simplest, but maybe most capital expensive, solution is to install an indirect water heater.

OP has a well insulated reverse indirect already.

 

[Deering]

OP? I have an Amtrol on my system.

 

[ewdudley]

(OP. Short for Original Poster, i.e., @velvetfoot)

 

[Deering]

This is interesting. Our shower heads are low flow and there hasn't been any shortage of heat for the shower even when the tank gets below 130. I have the zone and boiler pumps set so they don't run under 130. The thing I didn't like was what I thought was the slow return from setback. I changed the setback return to a half hour earlier so then the heat is on for a half hour before I get up. Depending on how much energy is in the tank, it'll warm up the zones while the boiler starts.

I get you now. Sorry for my misunderstanding above. First cup of coffee... Sounds like you're well on your way to tweaking the timing of the components.

 

[velvetfoot]

I do have the (Amtrol) indirect tank just sitting there though. I could maybe add a pipe or two and use it as a buffer tank for the cold well water, before it's heated.

 

[Deering]

Does the circulator between your boiler and pump run continuously as long as the tank is above 130, even if the boiler has been off for a substantial time?

 

[velvetfoot]

Does the circulator between your boiler and pump run continuously as long as the tank is above 130, even if the boiler has been off for a substantial time?

Not as planned. If there is no call for heat, the boiler pump does not run. I currently have the tank to start at 145, I think. However, if the boiler turns itself off because the internal temperature setting has been reached, then there still is a call for heat, and the boiler pump would run; I'm trying to avoid this.

 

[Deering]

In my case, after the boiler shuts down, it's designed to keep the circulator pump running until the internal boiler temp reaches 130. The idea being to pull the last remaining bit of heat out of the boiler. But if my thermal storage tank's lower temperature was 145, the boiler circ would just keep running and running because the boiler would never reach 130. I couldn't figure any way to change that. I have one of the older Windhagers. I think the newer ones were supposed to have a feature to set them up for thermal storage tanks, but mine didn't. Does yours?

The problem with this is that the boiler loses heat due to the big flue pipe running in it. And it can't be as well insulated as the storage tank, just due to all the necessary penetrations for pipes, augers and whatnot. So in essence it become another radiator that stays on continuously, or at least until it cools my tank down to below 130.

So how I got around that was by installing a time delay relay into the pump circuit. I have it actuated by the fan motor circuit. It runs for 10 minutes after the fan motor shuts off at shutdown. This pulls most of the heat out of the boiler, but doesn't drain my tank. A 20 minute relay would be better, but this is what I had, and the savings would be marginal.

The risk from a relay failure is fairly minor. If the pump shuts off the boiler would quickly reach its setpoint and turn off. Worst case scenario, the pressure relief would open. If the fan fails and stops, combustion quickly stops anyway.