What to use for open expansion tank?

[ED in CO]

I'm installing a "fireplace" boiler from New Horizons (through ebay). It is a Polish unit called Spectra.
http://www.cichewicz.com/files/1235465736/waterstove-manual.pdf
I plan to use a plate exchanger and I'm considering using the old 40 gal gas hot water heater for storage.
The fireplace side of the heat exchanger must be "open".
I have searched for an open expansion tank and cannot find anything in the USA like the "header tanks" used in the UK. Some diagrams show an "open expansion tank" that looks like a typical expansion tank (bladderless?) but I don't know how to make a closed tank "open".
The tank will be above the stove in a niche in the cathedral ceiling above a walk in closet. It appears that the tank should have a float valve to maintain the water level and be piped to accept any possible steam blow off from the top of the stove as well as for overflow. I hope someone has suggestions. I've searched and don't seem to find any particulars. In a recent post, someone used a water heater! But how does that incorporate a float valve to keep it full? Is there a commercial version of this type of tank to be found in the USA?
We also have a condensing gas boiler that I installed and radiant heat in the slab and fin style radiators in 2 bedrooms (that I plan to use as a dump zone).
Here is a diagram of a Spectra install with a boiler system. (in translation!) Notice that the upper left hand corner open expansion tank has no label? It is the mystery component.

 

[Nofossil]

I've thought about the same problem. If you don't have any leaks, a simple clear plastic sight tube running up the side of the tank and a manual fill valve should work. It shouldn't need a continuous supply of makeup water.

 

[maple1]

I think I would lean to trying an electric water heater. I wouldn't worry about the float thing & auto-fill makep water. Tie it to the boiler via the bottom drain valve, using a T to T in a sight glass setup using pex you can see through. T the top of the sight glass setup to the top of the tank via one of the top tank fittings, the other side of that T you can run up to a vent. Pipe in one of the other tank fittings to your water supply, valved so you can just turn a valve to add water if ever needed. Think that's more or less what hobbyheater did.

 

[ED in CO]

Thanks, I will look up hobbyheaters setup. I was trying to figure out what type of tank to use. So you think an old electric hot water heater would be the easiest to find and plumb? Would you size it the same as an expansion tank? The stove holds 14 gallons. RV tanks are 6 gallons and there are 10 and 20 gallon electric tanks that would fit on the upper shelf. Would the tank need to be open to the atmosphere-- have a hole in the top that is not sealed? Some tanks only have connections on the sides. By sight glass are you both referring to one of these? If things are working properly, filling manually should work fine and that simplifies the setup.

 

[Nofossil]

Yes - that's what I was thinking of. I think the top should be open to atmosphere, but through a long pipe. You don't want a continuous supply of fresh oxygen getting into your boiler water. I'd size it like an expansion tank and round up to the next convenient size.

 

[maple1]

It only holds 14 gallons?

Well, you certainly won't need a very big tank - 5 gallons would be lots. If you come across something bigger for cheap, no harm no foul.

There's a picture of my cabbaged up sight glass somewhere in my install thread, it's just a piece of partly translucent pex. The float has since sunk though. The install manual for my boiler specs that if an open expansion tank like that is used, it should be 2.5m above the top of the highest radiator to prevent oxygen saturation of the water.

 

[ewdudley]

For expansion purposes an open expansion cistern only needs maximum of 4% of system volume, I would guess a couple gallons if you go with the 40 gallon storage. But if the thing ever got away from you it would be nice to have a reservoir of water for the unit to work with to protect against boiling dry. A non-rusting tank material would certainly be preferable, even if you had a layer of mineral oil it seems like a steel tank would get pretty nasty over the years. Beer keg, soda canisters?

 

[ED in CO]

Yes, I just double checked the 14 gallons. Actually I seemed to have rounded up for memory's sake.
I wish there were a product that was just labeled "open expansion tank" instead of having to build my own.
I read Maple 1's install and I'm thinking you mean the pex that leads to the bottom of your expansion tank working as a sort of sight glass for the tank.
I think a keg would be just about the right size too. I just have to keep an eye out for something that will work.
Thanks for clarifying the concept of an open expansion tank. I'll probably have more questions as the project progresses!

More info on the stove for anyone who is interested.
There are a couple of different sizes of this stove. I'm pretty sure from the measurements of the stove that I have the 20-25 model.
Here are the specs from the website.

Power range of 20-25 kW (68242.84 btu/hr to 85303.55 btu/hr)
Fuel (wood) with a moisture content of 20%
Overall thermal efficiency of 75.6%
Water volume dm3 50 1.77 cubic feet
1.77 Cubic Feet = 13.2405194796 US Fluid Gallons
º C flue gas temperature> 250
Fire power emitted by convection kW 4 (13648.568 btu/hr) (not sure what this is or how it is different from the power range)
Max pressure bar 2 (2 Bars = 29.0075476 Psi)
Chimney under pressure (Vacuum Towers) Pa 20 (290.07547546041843 psi)
Recommended height of the chimney m 7-8 (22.96 to 26.25 ft.)
Recommended cross-section of the chimney cm2 400 (62 Sq. In)
Recommended diameter of the gas flue 200 (7.8 inches?)
The high efficiency of 75.6% has been confirmed by a certificate issued by the Institute of Thermal Technology in Lodz.

Here it is waiting to be installed! We are in the middle of a huge remodel, notice the ceiling is torn out. We bought a fixer upper. Had to tear out the slab and put in new so added radiant piping and a combi gas boiler. Doing most of the work ourselves.
Seemed like a natural to add a wood burning boiler. We heated with wood for over 20 years but with a system of ducts and a blower.
In the back corner are the pipes that lead to the utility room and there is a connection to the bedroom radiators at this point.
One of the pipes will also be an intake for outside air for the stove. The closet is behind the block wall although I might just build a tower in the back corner for the expansion tank. We are thinking of enclosing the stove in metal and stucco like a kiva, but would have a door to one side to access the plumbing.
I guess this would argue for an oversized tank if it was not in plain view.