Buffer Tank Piping Modifications: Dual to Single Direction Flow

[velvetfoot]

Man, my head is still spinning from mr.fixit's post, which seemed so easy at first, but made my head hurt as I tried to think about it more. Hopefully this will be more straightforward.

I plan to make the changes in the bubble marked 'new' in the diagram come summer. (Note: not all isolation valves shown.) It doesn't involve much piping and is reversible by closing a couple of valve. Comments?

Background: When I was first starting things up I had the boiler feed pumps set pretty high, which caused the first floor zone to heat up some since I guess flow went past the Taco flo-chk valve. There's a circ pump for each zone and they pump towards return. Since I turned down the speed, everything is fine. It's working fine, so why mess with a good thing, right? Fatal flaw, I guess. Writing about it also helps me think about it more too, I guess.

The way I look at my buffer tank is that it takes up excess heat generated by the pellet boiler, allows longer run times, longer times between starts, and therefore fewer starts. I'm still using a wood insert, so the 15kw boiler primarily warms up the second floor, dhw, and generally provides baseline comfort. While it modulates down to 30%, with the insert running, it's too much, so hence, the buffer tank.

Right now, when the boiler is running and the boiler feed pump is on, the loads get first crack at the supply water-excess goes to buffer tank via the top connection. When the boiler is off and a load comes on, flow is reversed through the top connection, and the loads are supplied by the buffer tank.

I propose to change the flow through the top connection to only one direction: out, supplying heated water to the zones. The boiler supply will be connected to a point 1/3 down from the top of the tank.

The buffer tank is a Froling Energy Tank. (broken link removed to http://www.woodboilers.com/images/stories/documents/energy-tank-rev5.pdf)
It supposedly has baffles built in to help with stratification. I don't notice much stratification going on when the boiler feed pump is running the way it is plumbed now, one connection, from the top. When the boiler feed pump is off, yes, stratification, for sure.

The way it is plumbed now, I get a nice heat fill, to around 180 at the sensors at the top and 1/3 down from the top. So, i'm happy there.

Even though the supply would be 1/3 down from the top, I think the supply to the zones would still be receiving the benefit of the hot water coming off a running boiler since it should go to the top.

What do you think.

Gotta go, ran out of time. Too cold for anything but a movies.

Thanks.

 

[ewdudley]

The way it is now the load pumps could push some flow through the boilers as well as through the buffer tank, but with your gentle Ecocircs an with the slight pressure of hot water rising in the buffer tank it looks like there isn't actually a problem. (Likewise there's a parallel flow through the loads when either boiler pump is running, but sounds like you've been there, done that.)

I think the conventional buffer tank strategy would be to pipe the supply from the boilers into the top of the buffer and then draw the space-heating loads from somewhere lower down so that there is a zone at the top that stays hottest for the DHW.

But your proposed change would certainly be valid as long as the buffer is always hot enough for DHW. Like you say, the supply entering in one third of the way down will rise right up to the top without much mixing.

(Like the single mixing valve feeding both boilers. That's just the kind of thing that would never occur to me.)

 

[velvetfoot]

Thanks very much for looking.

The way it is now the load pumps could push some flow through the boilers as well as through the buffer tank

I hadn't thought of that.

I've been very pleased with the DHW produced by this reserve indirect tank. I let the sensor 1/3 of the way down from the tank get down to 134 or so before boiler ignition.

Like you say, the supply entering in one third of the way down will rise right up to the top without much mixing.

I'm thinking that maybe with the supply entering from the side, 1/3 down the tank, I'll be working with the mysterious baffle design, instead of possibly against it pumping down from the top.

(Like the single mixing valve feeding both boilers. That's just the kind of thing that would never occur to me.)

Space was cramped, and it's not like that stuff is cheap.

 

[maple1]

What is your intent with the changes?

 

[velvetfoot]

'Fatal flaw, I guess.'
Maybe make the tank work more like it's supposed to, but it's working pretty well now.
So, I don't know, which probably contraindicates doing anything at all!

 

[ewdudley]

Maybe make the tank work more like it's supposed to

As you suggest in your OP, it's probably working exactly like it's supposed to. The only possible problems I can see would be if the load pumps over-pumped through the boilers, or if the boiler pumps over-pumped through the loads. If neither of those is happening more than likely it ain't broke.

 

[Karl_northwind]

I always treat the buffer tank as a hydraulic separator and air eliminator, and only pipe the boiler thru the tank, and pull all loads off the tank. seems to work well.

 

[velvetfoot]

I always treat the buffer tank as a hydraulic separator and air eliminator, and only pipe the boiler thru the tank, and pull all loads off the tank. seems to work well.

Yeah. My idea was to give the loads first crack at the water coming off the boiler when it's running. What I'm thinking of a negative is that If I change the piping, I might get a little less runtime out of the boiler since the boiler could shut down on temperature (that's the way I have it set up), while a load pump is still running-maybe, maybe not.

I'm still curious about how things would work piped like that, it wouldn't take that much work, would be reversible with valve alignment, but really, as others have said, it works okay now, maybe I should leave well enough alone. I'm wishy-washy though.