open tank debate

[ssfein]

To pick up on a Question on another thread and a debate I am having with a fellow do it yourselfer: My understanding of the general view of this forum is that there are two appproaches to heat storage in water 1- unpressurised tank with heat exchanger to create a pressurized boiler system in an open tank storage. 2- a pressurized tank creating a closed boiler and storage system. What I have seen pop up on this forum for the past few months but I have not seen fully addressed ( I may have missed it)------- is the open tank storage w/o heat exchanger and use corrosion and other additives to deal with the oxigination corrosion issue. Though I have never seen it done my friends contention is that for the 15' head of most 2 story houses you dont need a pressurised system to acomplish pumping to both floors. This is not just a point of interest as I have a EKO 25 on the way and I have to firm up my storage plan. By the way I have talked to Firestone and several other co. and none of them want to go near the idea of using EPDM for this kind of application where the temp would go over 165F.
Thanks

 

[free75degrees]

I think oxygen scanvenger chemicals would eventually get depleted and you would have to keep adding more to avoid rust. This would lead to excessive buildup of the oxygenated chemicals in the water.

 

[deerefanatic]

Oxygen Scavenger is NOT a good idea in an open system.. The stuff acts as a floc that coagulates the oxygen to itself then drops out... Eventually, it creates a nasty layer on the bottom of the boiler/tank/etc.......

A true anti-corrosion inhibitor is what is on order.. It keeps the metal from rusting when in the presence of Oxygen.. That's what I'm doing.

I'm using a product called CLT-550 by woodboilersolutions..... They are pretty knowledgeable on their product.... Dave at cozy heat sells their stuff.

 

[ssfein]

So does that mean that you are running an open tank and not a pressurized boiler?
SS

 

[deerefanatic]

Yes, open tank, with NO heat exchanger.......

It's an open system that thinks it's a pressurized system!

 

[Redox]

Yes, open tank, with NO heat exchanger.......

It's an open system that thinks it's a pressurized system!

Is your storage tank above the rest of the system? It would seem to me that it would be hard to keep the air out of the system otherwise.

Chris

 

[deerefanatic]

The actual storage tank is sealed and I have two open-top hot water heaters for "expansion tanks...... When I say unpressurized, it's not really true.. There's about 8-10 feet of downward head pressure from these elevated expansion tanks... but not real pressure (like 10-12 psi....)

The only reason I'm not trying to pressurize is this is an old milk tank that I got for free and I don't want to screw it up...

I also have a taco hyvent on top of the fitting on the top of the tank......

 

[Redox]

I hate to break this to you, but that is by definition a pressurised system. You may only have 3-4 PSIG on that milk tank, but that is real pressure and can cause a real catastrophic tank rupture. It may be strong enough to take it, but what are the consequences of a worst case scenario?

Chris

 

[ssfein]

So is there anyone who is running a real open tank system with no heat exchanger and a unpressurised boiler?
SS

 

[Grover59]

Yes I am, and I am not sure why there are not more people doing it, I suck the water out push it through a flat plate hx, and send it back to the tank, both dip tubes are under water. Last year I did not see any real problem with this set up I will be starting it up again soon.

Steve

 

[deerefanatic]

I hate to break this to you, but that is by definition a pressurised system. You may only have 3-4 PSIG on that milk tank, but that is real pressure and can cause a real catastrophic tank rupture. It may be strong enough to take it, but what are the consequences of a worst case scenario?

Chris

Well, the top of the expansion tanks are 8 ft above the top of my storage tank.. So maximum pressure is 8ft of head or 3.4 psi..... I really think that a round tank like that can handle 3.4 psi... In any case, the non-sealing manhole is the weak point as the only way I'm sealing it up is with RTV hi-temp silicone...... If the pressure gets to the point of doing tank damage, its going to blow that sealant out first.....

And as an extra note, if the sealant doesn't hold, my top fitting is welded into place in such a way that a dip tube could be attached to it to leave the tank open if necessary.......

 

[Redox]

I hate to break this to you, but that is by definition a pressurised system. You may only have 3-4 PSIG on that milk tank, but that is real pressure and can cause a real catastrophic tank rupture. It may be strong enough to take it, but what are the consequences of a worst case scenario?

Chris

Well, the top of the expansion tanks are 8 ft above the top of my storage tank.. So maximum pressure is 8ft of head or 3.4 psi..... I really think that a round tank like that can handle 3.4 psi... In any case, the non-sealing manhole is the weak point as the only way I'm sealing it up is with RTV hi-temp silicone...... If the pressure gets to the point of doing tank damage, its going to blow that sealant out first.....

And as an extra note, if the sealant doesn't hold, my top fitting is welded into place in such a way that a dip tube could be attached to it to leave the tank open if necessary.......

I'm not trying to start an argument, but have you thought of the consequences of such a failure? I'm guessing that you don't have a bulk milk tank in your basement, but I would hate to hear of a failure killing anything more than the grass outside your home. Those tanks aren't designed to handle pressure, even though they probably can.

Chris

 

[ssfein]

Steve if I understand the reason you are using a flat plate EX it is because you have a closed system boiler and an open tank system combined. Did I get that right? I am looking to see-If anyone is truly running a open system were there is no pressurization and no exchangers between the boiler and storage tank.
thanks
SS

 

[deerefanatic]

I hate to break this to you, but that is by definition a pressurised system. You may only have 3-4 PSIG on that milk tank, but that is real pressure and can cause a real catastrophic tank rupture. It may be strong enough to take it, but what are the consequences of a worst case scenario?

Chris

Well, the top of the expansion tanks are 8 ft above the top of my storage tank.. So maximum pressure is 8ft of head or 3.4 psi..... I really think that a round tank like that can handle 3.4 psi... In any case, the non-sealing manhole is the weak point as the only way I'm sealing it up is with RTV hi-temp silicone...... If the pressure gets to the point of doing tank damage, its going to blow that sealant out first.....

And as an extra note, if the sealant doesn't hold, my top fitting is welded into place in such a way that a dip tube could be attached to it to leave the tank open if necessary.......

I'm not trying to start an argument, but have you thought of the consequences of such a failure? I'm guessing that you don't have a bulk milk tank in your basement, but I would hate to hear of a failure killing anything more than the grass outside your home. Those tanks aren't designed to handle pressure, even though they probably can.

Chris

Oh yes, most definitely... NO the tank is NOT INDOORS!! It is outside in a purpose built shed that has the tank and all the pumps for all the zones as well as a control cabinet, expansion tanks, etc......

 

[WoodNotOil]

I am curious as to how a completely open system would supply heat to zones. Most houses have heating zones above where the boiler is and I was under the impression some pressure was required to move the water to that elevation. Do the pumps do all the work? How do they get/stay primed? I guess it would work, but would definitely want to see it done, before designing a system that way. I think pressure gives a larger margin for error in heating system designs and that is why there are so many ways to accomplish the same thing.

 

[deerefanatic]

Yes, that is true. In my system, the tank is above every zone on the place.... It's higher than the radiant floor of my shop (duh!) and the side arm in the milkhouse, and both zones of my house are located in the basement.........

-Matt

 

[jebatty]

...under the impression some pressure was required to move the water to that elevation. Do the pumps do all the work? How do they get/stay primed?

If the system is full of water, and the open expansion tank is the highest point in the system, then no pressure is needed to move the water, and pressure in the system does not further aid in the movement of the water. Water cannot be compressed. The circ pump is not a traditional pump; it circulates water by creating a pressure differential. As the pump operates, it creates a slightly higher pressure on the output side which immediately is equalized by water from the input side. Water simply flows through the pump and the system as a result of the pressure differential.

 

[ssfein]

So if you have a 2 story house the expansion tank that usually sits right above the boiler will now sit above the baseboard of the second floor? So how would you prevent the drain-back of the system to the storage tank if the expansion tank is open to the air? Just getting clear about it, this kind of system would eliminate the major expense of a heat exchanger and the issues of a pressurised storage tank.
SS

 

[jebatty]

The system would be filled with cold water to the point that some water was in the bottom of the expansion tank, and the tank would be sized to accept the total system expansion of water, with a drain to the outside if somehow this was exceeded. I use a 5% rule of thumb expansion factor between 32-200F, although actual is less. If total system is 50 gallons, an open air expansion tank would need to accept 2.5 gallons.

 

[Redox]

I think what SS is asking about is the pressure in the system at the highest point when the open tank is on the low side of the system. It would seem to me that the highest points will be in a vacuum and possibly break the siphon after the system is primed. Good question. Where are the experts? I'm just an interpreter, but would like to know the answer.

Chris

 

[BrownianHeatingTech]

I think what SS is asking about is the pressure in the system at the highest point when the open tank is on the low side of the system. It would seem to me that the highest points will be in a vacuum and possibly break the siphon after the system is primed. Good question. Where are the experts? I'm just an interpreter, but would like to know the answer.

You can't have an open tank below any zone, without using a heat exchanger.

If you want a strict open system, the storage tank would need to be in the attic, or up a hill behind the house, or something.

Joe

 

[Duetech]

When I put in my system I was told that non pressurized pumps died a lot quicker than pressurized pumps. I have found open pumps to be more expensive too. Even at Menards. Cave2k

 

[deerefanatic]

You don't have to buy special "open" pumps if you use anti-corrosion chemicals. The only differences is closed system pumps are cast iron where open system pumps are bronze.... Hence the increased price.....

Yes, you must have your expansion tank above the highest zone or it won't work.....

 

[Grover59]

Steve if I understand the reason you are using a flat plate EX it is because you have a closed system boiler and an open tank system combined. Did I get that right? I am looking to see-If anyone is truly running a open system were there is no pressurization and no exchangers between the boiler and storage tank.
thanks
SS

Yes you are right, I do have a pressurized boiler and open tank, I see what you mean.

Steve

 

[ssfein]

I am just trying to find out why we do what we do, obviously there are good reasons because pressurized systems are the standard. Early solar water heaters were drain back systems that were not pressurised but pumped several stories to the roof and carried very hot water to a tank that had a closed top. Could this kind of approach work. You would let the system drain back to the open storage tank and it would act as the expansion tank and at idle hold the majority of the systems water. I dont know how this would work re: pumps. it may be that on the drain back systems they were on the low end of the system so the would not loose their prime. So in this example that could be 2' off the ground at mid tank level. Anybody want to kick that one around?
thanks
SS