Expansion tank placement

[Stlshrk]

We have the Woodshed building project well underway (pole barn style). The process of buying parts and components is also ongoing. Just a piece at at time here and there.

The boiler will be an Eko 40, which we will locate in the woodshed. We will have 500 gal of storage in the form of an old LP tank, also going in the woodshed. Our system will have two loops. The first one will go to the house to heat a DHW sidearm & to a water to air heat exchanger in our forced air system. The second loop will feed to the shop and another water to air heat exchanger.

To the question(s), is there a preferred location to put the expansion tank in a pressurized system?

High point of the system? If so that would be the water to air heat exchanger in the shop loop.

Close or far from the boiler?

Or, does it function better to locate it directly above the storage?

What about proximity to circs or other components?

Near or far from a relief valve?

Just fishing for pros & cons, what is working well for some, and what is not or has not for others.

Thanks

 

[steam man]

I believe if you check out www.heatinghelp.com they debate and answer that question. The short of it is I believe that the tank should be placed near the pump suction. This maintains a head on the pump suction. If you think about it, hot water will flash very easily under low pressure, i.e. the pump suction. If you want to prevent it, the tank will maintain the head on the pump suction and prevent this. There is more fluid dynamics involved (pressure-temperature relationship) but it makes sense. I've had this happen to me just recently on a situation pumping hot fluid with no head pressure. Placement in the suction side of your loop between the pump and any major pressure drop items should be ok. As far as the relief valve is concerned, there should always be one any place that could be a hydraulic lock, i.e any place that can be isolated where pressure can build up. I could tell you the most extreme stories on actual explosions from bad designs. I kind of think this should apply to expansion tank placement also but it isn't as much a safety consideration.

Mike

 

[Bob Rohr]

Just one boiler?

The tank placement establishes the PONPC. The one point in the system where the pressure developed by the circs cannot affect the system pressure. So you want to pump away from that location and also add your fill valve there.

Here is one idea with the wood boiler, 3 way boiler "body guard" temperature protection, and the piping to the loads with zone valves for zoning.

If multiple boilers or solar would be added I might suggest primary secondary to assure all the circs get along and each device gets proper flow under any condition.

I'd add an air removal device, like the Caleffi discal on the piping to the loads, and the air vent atop the boiler. Relief valve should stay on the boiler as per manufacturers placement.

A coin vent or small auto vent at the highest point at the air handler is a good idea. Use one with an iso valve built in for service or replacement.

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[Stlshrk]

Thanks Mike & Hot Rod

Yes, just one boiler ;-)

Mike - is the situation you describe with a pump problem any more likely working with a glycol mix than with just water and the only additives being anti-corrosives?

Hot Rod - My system will have a circ out of the bottom of the storage returning to the boiler. With the "body guard" temp protect, as it were. And, then a circ for the house loop and another for the shop loop which will both be plumed out of the top of the storage. I will plan to add the additional two air separators, for a total of three, and also place the exp tank where recommended. Will it matter if both of the circs are located downstream near their respective loads, but not past them? For instance, I'd like to have the circ for the shop load in the shop rather than the woodshed. That way I can control it with a thermostat in that building rather than burying all that control line in my ditches. Or, will increased distance from the PONPC negate the benefit of pumping from rather than to it?

 

[steam man]

My guess is using a glycol solution is going to have the effect of not flashing quite so easy. However, lower the pressure enough and it could flash. It has a boiling point.

Mike

 

[Stlshrk]

Thanks Mike,

I wasn't sure which direction it would impact the boiling point. It obviously lowers the freezing point, but if it raises the boiling point it may be something to consider for the benefit of that property as well.

 

[Bob Rohr]

glycol takes more pump power, especially when it is cold, and thick. Glycol manufacturers have conversion charts for different mix percentages. It can be quite a pump hit if you need to pump it when it is cold.

Typically 35-40% glycol is plenty. Glycol has a freeze point, where it becomes to thick to pump with a centrifucal pump, and a burst temperature where pipes can split. A 5F freeze solution could have a burst of minus 50. Again the manufacturer of the glycol have that data.

Glycol is an expensive, messy, maintenance prone fluid. Use it only when absoutely needed.

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[beavah]

I like mstr of sparks, he knows his stuff, Glycol in systems requires almost double in expansion tank sizing not to mention I like oversizing expansion tanks for wood system anyway. Listen to master sparks, every single glycol brand has different chactoristics, Absolutly use "8-way" boiler water treatment in all water systems. The stuff turns boiler water into champagne .Neutralizes ph scavenges o2, scale inhibitor etc

 

[rickh1001]

Master of Sparks,

I am a little slow on the uptake, so forgive me for not asking this sooner. Again, I am complete novice, but have been reading and studying. I finally got the PONPC in my head, after reading all of Holohan's books, and am just about ready to start tackling the plumbing after moving in our EKO 60 with 500 gal of storage (LP tank).

Holohan stresses the need to place the primary circulating pump after the boiler, but you are placing it on the boiler feed? I am guessing that since you place the expansion tank, fill valve and air scavenger just after the tank, that the entire storage tank is acting as a PONPC?

If I followed Holohan's diagrams, it seems I would want to place the pump just before the expansion tank/fill valve/air separator, all on the supply side of the pump. The bottom of the storage tank would then feed the boiler inlet, but with a 3-way valve to control the boiler inlet temp.

I like your diagram, but I guess I don't understand how it maintains the PONPC concept?

Sorry for my ignorance, which is vast and without end.

 

[Bob Rohr]

In this example I have two circs pumping to different directions. By putting the expansion tank at the buffer. I "create the PONPC for both circs to "pump away" from.

The boiler really doesn't care if I push cool return in the bottom, or pull the heated flow from the top. Although the circs do like to run in cooler fluid, since they are cooled by the fluid. so on the return to the boiler they operate a bit cooler. Not a huge deal, most are rated to something like 225f operating temperature.

Without the buffer tank and one circ I would pipe it like Dan shows with the air scoop, expansion, fill at the boiler discharge, and the circ just downstream of that point.

The main point is you never want the circ pumping right at the expansion tank, as the book explains.

The piping has changed a bit with mod con boilers. the manufactures prefer to see you pump into them on the return side so the pressure increase the circ provides is seen in the HX. It assures the pressure switches stay "made' and helps keep the HX from flashing. It has made expansion tank placement a bit of a head scratcher, however.

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